S. G. WILLIAMS. J^a, ._../-_£--. L ^.B OR. ,, i&mn^ Uttirmitg phatg THE GIFT OF ^On?!cvvvW o^ ^^»Sflc.\)i!ly\\\\Qcr^ K.Vbo^'5J5fc fe\p.\oo Cornell University Library arV15566 A high-school ascronomv: 3 1924 031 322 013 olin.anx Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 924031 32201 3 HIGH-SCHOOL ASTRONOMI IK -WHICH THE DESCEIPTIVE, PHYSICAL, AND PRACTICAL ARE COMBINED, WITH SPSCIAI. BSFERENCI. TO THE WANTS OV ACADEMIES AND SEMINARIES OF LEARNING. BY HIRAM MATTISON, A. M., X^TE PEOTESSOB OF NATUBAL PHILOSOPHY ' AND ASTRONOMY IN THE FAIXXT SEMINARY ; AUTHOB OF THE PBIMAEY ASTEONOMY ;. ASTEONOMICAI. MAPS J EDITOR OF BUEEITt's GEOGBAPHY OF THE HEAVENS, ETC., ETC. FEW YORK: PUBLISHED BY MASON BROTHER^ BOSTON: MASON & HAMLIN. PHILADELPHIA ; J. B. LIPPINCOTT & CO. CINCINNATI : W. B. SMITH & CO. 1863. 3 Kntered according to Act of Congress, In tne year 1853. FT EIEAM MATTISON, lit the Clerk's Office of the District Conrt of the United States for the SMitlitrii District of New Tort D.£- PREFACE. The design of this work Is to furnish a suitable text-book of Astronomy for academies and seminaries of learning. Though sub- Btantially a revised edition of the " Elementary Astronomy," so exten- sive and important have been the additions and improvements, as to justify the adoption of a nev? title, and to warrant the hope that it will not only be found eminently suited to its purpose, but that it may now go on for years without further revision or alteration. For juvenile learners, the " Primary Astronomy" may be preferred ; and for advanced classes, who wish to study the constellations in connection with Mythology, the " Geography of the Heavens" should be chosen in preference to all others ; but for all ordinary students, this intermediate work will be found sufficiently elementary on the one hand, and sufficiently extended on the other. The work is now divided in three parts. After an Introduction, whicli consists of Preliminary Observations and Definitions, and occu- pies twenty pages, Part First is devoted to the Solar System — the sun, planets, comets, eclipses, tides, &c. ; Part Second relates to the Sidereal Heavens — the fixed stars, constellations, clusters, and nebulae ; and Part Third to Practical Astronomy — the structure and use of instru- ments, refraction, parallax, &c. This department, so seldom intro- duced into text-books for schools, will be found especially interesting and valuable. Besides embracing all the late discoveries ia astronomy, under a strictly philosophical classification, the work is now thoroughly illus- trated, by the introduction of diagrams into the pages, in connection with the text ; and the adaptation throughout to the use of the black- board, during recitation, cannot fail to be appreciated by every prac- tical teacher. The variety of type affisrds an agreeable relief to the eye of the student, and at the same time distinguishes the main text (which ought, in all cases, to be thoroughly understood before it is passed) from (he less important matter, the more careful study of which may be left for a review. The suggestive topical questions at the bottom of the page complete the design. On the whole, the work is believed to be a decided improvement upon the works heretofore in use in this department of study ; and as such it is offered to the professional teachers of the country. New York, Jan. 1, 1853. ASTRONOMICAL WORKS /n the AuthoT^s Lihraryf and m&re or less consulted in the compilation of the following pages : A Oyde of Celestial Objects, for the use of Naval, Military, and Private Astronomers, &c. By Capt. Wm. Henbt Smtth, &c 2 vols. 8vo. London, 1S44 An Intro<^itcUon to Astronomy, in a Series of Letters from, a Preceptor ]to his Pnpil, &c. By John Bonntoastle, Professor of Mathematics, Ac. 1 vol. 8vo. London, 1822. An Introduction to tTie True AstronoTny; or, Astronomical Lectures read in tlie Astronomical School of the University of Oxford. By John Ksn.!^ M. D., F. E. 3-, &c. 1 vol. Bvo. Dublin, 1793. Astronomy Msplavned, upon Sir Isaac Newton's principles, &c., Ac By Jami^ Fee- GTJSON, F. E. 8. 1 -^ol. 4to. London, 1764. The Elements of Physical and Geometrical AstronoTny. By David Gregory, M. D., late Sullivan Professor of Astronomy at Oxford, &c. 2 vols. 8vo. London, 1726. Ash^onomy, in Five Books. By IEooer Long, D. D'., F. E. 8., &c., University of Cam- bridge. 2 vols. 4to, Canibriage (Eiig.), 1742. Astronomia Cat^olmia, &c., by Thomas Street; and A Series.of Odaervations on the Planets^ chiefly the Mopn, &c., by Dk. Edmund Hallet. 1 vol. 4to. London, 1716. AstronoTnical Lectures, read in the Public School at Cambridge (Eng). By Wiltjam "WmsTON, M. A., Professor of Mathematics, &c. 1 vol. 8vo. London, 1728. 77(.e Wondera of tlie Reavena ; apopular view of Astronomy, dec ByDtrNCAN Ekad- FOED. : 1 vol. royal 4to. New York, 1843. Popular Zectwres on Soimhce rmd Art, &c. By Dionysius Lardnee, F. E. S., tfcc, &c. 2 vols. 8vo. New York, 1846. dtdUfies of Astronomy. By Sik John F. "W". Heebchel, Bart, K. H., &c. 1 vol. 8vo. Philadelphia, 1849. Phenomena and Order of the Solar System^ and Views of the Architecture of &ie Hea/oem. By J. P. Nicuol, F. E. S. E., &c 2 vols. 12mo. New York, 1842. The PraeUcdl Astronomer., &c By Thomas Dick, LL.D. 1 vol. 12mo. New York, 1S46. Also, " Celestial Scenery," and " The Sidereal Heavens," by the same author. Tlie Planetary and Stellar Worlds. By Peop. O. M. Mitohel. 1 vol. 12mo.' New York, 1849. ., , , , An Elementary T^eatUte on Asta^onomnf., &c. By "William A Norton, A. M. 1 I'ol, 8vo. New York, 1845. An Jntroducifl,on to Astronmwy, &c. By Benson Olmsted, A. M. 1 vol. 8vo. New York, 1844. Also, Letters on Astronomy, and Idfe ) ^ CONTENTS INTRODUCTION- PEELIMINARY OBSEKVATIOKS AND DEJINITIONS. PAOE Chap. I. — Origin and History of the ScrawoE. Ptolemaic Theory of the Structure of the Universe . 12 The Oopernican System 13 II. — Definitions. Solids, Surfaces, Number of tlie Stars 168 Distances of the Stars IVO 11. DeSOKIPTION of the O0H8TELI.ATIONS. Nature, Origin, Classification 172 Visible in October, November, and December 174 " January, February, and March Ill " April, May, and June 180 " July, August, and September 183 III. — DODBLE, VaEIABLE, AND TeMPOEAKY StAKS, 8y Jbrm, and signi- fies sphere-like. Spheroids are of two kinds — OMate, ^ obi.a™^eoid. and Oblong or Prolate. An Oblate Spheroid "is a globe slightly flattened, as if pressed on oppo- site sides. This is a difficult figure to represent upon paper. Should the pupil fail to obtain a correct idea, the Teacher will be at no loss for an illustration. A Prolate or Oblong Spheroid is an elongated sphere. This figure, like an Oblate Spheroid, admits of various degrees of departure from the spbeiical form. It may be much or but slightly elongated, and the ends may be alike or otherwise. A common egg is an Oblong Spheroid. The Axis of a sphere is the line, real or imaginary, around which it revolves. The Poles of a sphere are the extremities of its axis, or the points where the axis cuts the two op- posite surfaces. The Equator of a sphere is an imaginary circle upon its surface, midway between its poles, the plane of which cuts the axis perpendicularly, and divides the sphere into two equal parts or hemispheres. Kinds of spheroids ! Describe each. What is the aiis of a sphere ? What> the '^oUa ? The equator t By what other name called ? What a Less Circle ? ' Jlendians ? DEFINITIONS. 19 The equator of a sphere is sometimes called a Great Circle., because no larger circle can be di'awn upon its surface. A Less Circle is one that divides a sphere into two unequal parts. GREAT AND LESS CIBOLES. The In tlie cut, the circles are represented in perspective. Treat Circle cnibrfices the middle of the sphere, where i1 diameter is included; while the Loss Circle pjisses around it MEKIDIAIf, between tlio Equator and the Poles, and is consequently " less" than the Equator. Meridians of a sphere are lines drawn from pole to pole upon its surface. 16. Lines and Angles. A Point is that which has no magnitude or extension, but simply position. " The common notion of a point is derived from the extremity of some slender body, such as the extremity of a common sewing-needle. This being perceptible to the senses, is a physical point, and not a Tnathematical poitii ; for, by the definition, a point has no magnitude." — Professor Perkins. A Hight Line is the shortest distance between two points. A Cwrve Line is one that departs con- tinually from a direct course. Parallel Lines are such as remain at the same distance from each other through- out their whole extent. A RIGHT LINE. CTJEVE LINE, Oblique Lines are such as are not paral- obuqde iiotb. lei, bur incline toward or approach each — other. "When two lines intersect or cut each other, the space included between them is called an Angle. 16. What is a point ? (Physical? Mathematical?) A right line? — a curve line ? — pari^el lines ? — an angle ? — kinds of angles ? Describe a right angle —an acute — an obtuse. 20 ASTKONOMrr. EIGHT AHQLB6. AOUTB AND OBTUSB ANGLES. Angles are of threp kinds — namely, tie lOgU Angle, the Acute Angle, and the Obtuse Angle. BigM Angles are formed when one right line intersects another perpendicu- larly, and the angles on each side ^re equal. An Acute angle is one that is less, and an Obtuse angle one that is greater, than a right angle. VI. Of TEIAlfGLES. A Triangle is a plane figure, bounded by straight lines, and having only three sides. Triangles are of six kinds — viz., Right-angled, Obtuse- cmgled, Acute-angled, Equilateral, Isosceles, and Scalene. A Eight-angled Triangle is one having one right angle. The parts of a Right-angled Triangle are the Base, the Perpendicular, and the . Hypothenuse. Rypofli&rmse^ from a Greek word, which signifies to eubtend or sbretch — a line sub- tended from the base to the perpendicular. OBTTJBE-ANGLED TEIANGLE. An Obiuse angled Triangle is one having an obtuse angle. EIGHT- ANGLED TEIANGLE. An Acute-angled Tria/rhgle is one having three acute angles. AN EQUILATEEAL TRIANGLE. An Equilateral Triangle Jiag all three of r its sides equal. EqiiAlateraiy from the Latin tBqv/as^ equal, and lateraUs^ from kt^Aie, side. 17. What is a triangle ? How many kinds? Describe (or draw) ariffht- ingled triangle. Describe its parte. (Hypothenuse ?) An obtuse ? Acute.! DEFrcanoNs. 21 TELUfGLE. An Isosceles Triangle has only two of its sides equal. The term Ismodes is from a Greefe word, signifying e^cd leffs ; hence a triangle witli two eqnal legs is called an Isosceles Triangle. A Scalene Triangle is one having no two sides equal. The terra Scalene is from the Greek skcUoTioe, and signifies obUque, vneaual (See obtuse and acute angled.) y*^18. ClKOLES AND ElJUIPSES. A Circle is a plane figure, bounded by a curve line, every part oit which is equally distant from a point within called the center. Concentric Circles are such as are drawn around a common center. The Circumference of a circle is the curve line which bounds it. The Diameter of a- circle is a rieht »"^™!k, gieoumfek- line passing through its center, and ter- minating each way in the circumfer- ence. The RadAus of a circle is a right line drawn fom its center' to any point in the circumference. The plural of radins is radii ; and as radii proceed from a common center, light, which proceeds from a luminous point in all directions, is said to radiate ; and the light thus dispersed is souaetimes called radiat/Ums or radiance. All circles, whether great or small, are supposed to be divided into 360 equal jjarts, called degrees / each degree into 60 equal parts, called minutes ; and each minute into 60 equal parts, called seconds. They are marked respectively thus: Degrees (°), minutes ('), seconds ("). Equilateral ? (Derivation ?) Isosceles 3 (Derivation ?) Scalene ? (Deriva- tion ?) 18. Wliatisaoirele! Concentric circles ? The Circumference ? Diameter ? Radios? (Plural, kJ ! '=< \ ^ u \V " \ / DKFINITIONS. 25 Tlie Torrid Zone is situated between thu fite zoots. the Tropics ; the Temperate, between the Tropics and the Polar Circles ; and the Frigid, between the Polar Circles and the Poles. Meridians are imaginary lines drawn from pole to pole over the earth's sur- face. Meridians cross the Equator at riglit angles ; and tlie plan^ of any two Meridians directly opposite each other would divide the earth into Eastern and Western Hemispheres, as the Equator divides it into Northern and Southern. We uiay imagine Meridians to pass through every conceivable point upon the earth's sur- face. They meet at the I'oles, and are furthest apart at the Equator. Lmigitude upon the earth is dis- tance either East or "West of any given meridian. A degree of longitude at the Equator comprises about 69^ miles, ,but is less and less as the meridians approach tlic Poles, at which points it is nothing. A degree of latitude is about 69^ miles on all parts of the globe. The First Meridian is that from which the reckoning of Longitude is commenced. On European charts and globes, longitude is usually reckoned from the Eoyal Ob- servatory at Greenwich, near London ; but in this country it is often reckoned irom the Meridian of Wasliington. It would be better for science, however, if all nations reckoned longitude from the same Meridian, and all charts and globes were constructed accordingly. A^ T onjiritude is tkurbstrial and celestiai. bphekbs. reckoned both East and West, the great- est longitude that any place can have is 180°. 20. The Celestiai, Spheee. The Celestial Sphere is the apj^a- rent concave sur- face of the hea- vens, surrounding the 6arth in all di- rections. The relation of the Terrestrial to the Celestial Sphere may be understood by the i.bove diagram, in which the stars surround the-earth in all directions, as Ihey seem to fill the whole celestial vault. 9 26 ASTEONOMT. EQUATOR OF TEIB MXATZNS, OK EquUfOCTlAL. The Axis of the Heavens is the axis of the earth pro- duced or extended both ways to the concave surface' oi the heavens. The Equator of the heavens^ or EqvAnoe- tial^ is the plane of the Eartli's equator extended to the starry- heavens. Declination is dis- tance either north or south of the Equinoc- tial. Declination fe to the heavens precisely what latitude is npon the earth. It is reckoned Irom the celestial eouator, both North and S&uth, to 90°, or to- the poles of the Tieavens. CeTestial Lati- tude can be explained ' better hereafter, and so with the terms , ZoMoGr &c. ^ Right Ascension is distance east of a given point, and is reckoned on the Equinoctial quite around the heavens. In one respect, Eight Ascension in the heavens is like longitade on tlie earth : they are both reckoned upon the equators of their respective spheres. But while longitude is reckoned both east and west of the first meridian, and can only amount to 180°, Eight Ascension is reckoned on^ (?aai«;or^, and consequently may amount to 860°, or the whole circle of the heavens. The principal diiferenee between Eight As- cension and Celestial Longitude is, that the former is reckoned on the Equinoctial, an^ the latter on the Ecliptie. BLE -§-HORIZOM The Sensible Horizon is that circle which terminates our view, or jvhere the earth and sky seem to meet. The Rational Horizon is an imaginary plane, below the visible horizon, and parallel to it, which, passing through the earth's cen- tei", divides it into upper and lower hemispheres. 1. These hemispheres are distiftgnished aa upper and lower vi\[h reference to the n4>- servcT only. 30. Celestial sphere ? (Eelabion to terrestrial ?) Axis of the heavens 3 Ec^uator of the Leavens ? Declination ? (Hov? illustrated by terrestria.' latitude ? How reekoned ? Its limits ?) Eight ascension ? (How resemblM lon^ritnde ? What difEercnoe I) Sensible horizon ? EationaJ ? £.vplaL!:. by DEFINITIONS. 27 om tlie 2. The sensible horizon is half the diameter of the earth, or about 4000 milos frt the rational ; sind yet so distant are the starS, that both these planes p.eeni to cut 1 celestial arch at the same point: and we see the same hemisphere of stars above the sensible horizon of any place that we should if the upper half of the eai-th were re- moved, and we stood on the rational horizon of that place. The Poles of the Horizon are two opposite points — one directly above, and the other directly beneath, us. The first is called the Zenith^ and the latter the Nadir, "The, points U]) and Down^ East and West^ are not positive and permanent directions, but merely rela- tive. 1. As the earth is a sphere, inhabited on all sides, jtp and pown, and east and the Zenith point is merely oppomte its c&titer^ and the west. Nadir tmoard its c^ter. 88 with the directions Up .^^^ r * the Bun. V 1 rri,- 2. We have not in every case been able to procure the astronomical symbol, laifi accounts for the blanks opposite several of the names. S. The names of the eighteen asteroids are included in braces. MYTHOLOGICAL HISTOET AND SYMBOLS. 30. Meecuet was the messenger of the ^^^ „, mekcuri gods, and the patron of thieves and dishon- est persons. His symbol denotes his ea3u- ceus, or rod, with serpents twined around it (5).* 1. MercTiry was represented as very eloquent, and skillful in in- ■ ; and qppiaining— as the god oT rhetoricians and orators. Paul aE ' Hence, T^en Paul and Barnabas visited Lystra, addressed the peo- ple, and wrought a miracle, they said, "The gods have come down to us in the likeness of men. And they called Barnabas Jupiter, and Paul MereuHus^ because he was iHie cMe^fspeaJcery 2. "The caduc&us of Mercury was a sort of wand or scepter, borne by Mercury as an ensign of quality and office. On medals, it is a sjTubol of good conduct, peace, and m-osperity. The rod represents power; the serpenis, ioisdom; and the two loings^ aiMg&nce and a(^mUyy — Encyclopedia. 8. The original form of this sign may he understood by the preceding cut, to which the present astronomical symbol ( Q ) beai'S but a slight resemblance. 31. Yenus was the goddess of' love and beauty, and her siffn is an ancient mirror or looking-glass (s), which she is represented as carrying in her hand. Anciently, mirrors were made of brass or sUver^ highly pol- ished, so as to reflect the image of whatever was brought before them. Hence it is said in the Book of Exodus, written fifteen centuries before Christ, that Moses "made the laver of brass^ and the foot of it of brass^ of the looking-glasses of the ^y6men," &c. For convenience, the ancient miiTors had a handle at- tached, -as represented in the cut, which very much resembles the si^ of the planet 32. The Eaeth (called by the Greeks Oe^ and by the Latins Terra) has two sym- bols — one representing a sphere and its eqnator (G), and the other (©) the four quarters of the globe. * All these symbols should be drawn in rotation upon the Blackboard, during recita- tion, by the Teacher, or some member of tho class. It will be well, therefore, for tlie student to observe each sign carefully, that he may be prepared to draw and eccplain it, if called upon. aries ? Secondaries ? Asteroids ? Orbits ? Comet and orbit ? Which self-luminous, and which opake ?) ' 29. How many primary planets ? How represented in astronomical works 3 Origin of names and symbols ? Repeat names. Draw symbols on black- board. (In what order arranged ? How asteroids designated ?) 30. Who was Mercury, in Mythology, -and what does his symbol denote ' (Eowwas he represented ? What Scriptural allusion? Describe his -^adu- beus. The meaning of its parts?) MYTHOLOGICAL HISTORY AJSTD SYMBOLS. 33 8FEA11 AND SHIIILD OF MA1». 33. Maes was the god of war, and his sign (i) represents an ancient shield or 'buckler ., crossed by a spear. Gunpowder was not known to the ancients, consequently they had no pistols, muskets, or cannon. Tlioy fought with shortswords and speai'S, and defended themselves with the shield^ carried on the left arm. A shield and spear wero, therefore, very appropriato emblems of wt^r. The original form of the sign of Mars ia pre- sented in the cut 34. Floea was the " qneen of all the flowere," and her symbol {y^) is a Jlower, the " Eose of England." 36. Clio was one of the Muses. Her sign {^) is a star, with a sprig of lawrel over it. 36. Yesta was the goddess oi fire., and her sign (S) is an altar., with &ftre blazing npon it. 37. Ibis was the beautifal waiting-maid of Juno, the qneen of heaven. Her symbol (/as) is composed of a semicircle, representing the radnhow, with an interior sta/r, and a base line for the horizon. " As an attendant npon Juno,'* says Prof. Hind, "the name was not inappropriate at the time of discovery, when Juno was traversing the 18th hour of right ascension, and was followed by Iris in the 19th." 38. Metis was the iirst wife of Jupiter, and the god^. dess of prudence and sagacity. Her symbol («,) is an eye (denoting wisdom) and a star. 39. Hebb presided over children and youth, and was cup-bearer to Jupiter. Her sign (s) is a cup. Hebe was celebrated for her beauty, but happening one day to stumble and spill the nectar, as she was serving Jupiter, she was turned Into an hosUer, and doomed to harness and drive the peacocks of the queen of heaven. 40. PAETHBNOPE'was OHO of the three Syrens— a sea nymph of rare beauty. They were all admu-able singers; hence a lyre (#) is her appropriate sign. 1. The three Syrens — ^Parthenope, Ligela, and Leucosia — were represented as dwell- 81. Venus and symbol ? (Ancient mirrors ? Scripture allusion ?) 32. The Earth — ancient name and symbols ? 83. Mars and symbol ? (Ancient mode of warfare ?) 34. Flora and sign ? 85. Clio and symbol ? 86. Vesta and her symbol ? sr. Iris and her sign ? (Prof. Hind's remark 2) 88. Metis and her sign 3 89. Hebe and her sign ? (Incident mentioned in note ?) 40. Parthenope and sign ) (What said of the Syrens ? Of the appro priateness of the name ?) 2* 34: ASTEONOMT. lug upon the coast of Sicily, and luring mariners upon tlie rocks of destmotion by their eiicbantiug songs. Hence whatever tends to entice or seduce to ruin is oft^n called a " syren song." 2. As this planet was discovered at the Naples Observatory, in Italy, it was quite ap- rropriate to name it after one of the Syrens, that Mythology located on the coast of a neighboring island. il. Egeeia was the counsuJor of Ifuma Pompilius. Symbol not yet agreed upon by astronomers. 42. AsTEiEA was the goddess of JiisHce, and her sign {iti) is a balance. Mythology teaches that Justice left heaven, during the golden age, to reside on the earth; but becoming weary with the iniquities of men, she returned to heaven, and commenced a constellation of stars. The constellations Virgo and Libra in the zodiac are representations of Astraea and her golden soajes. So the female figure, liohling a pair of soales, in the coat of arms of several of the United States, is a representation of Astrtea, and denotes Justice, 43. Irene was one of the Seasons. The planet was so named by Sir John Herschel, in honor of the peace pre- vailing in Europe at the time of its discoveiy (May, 1851). Its symbol (?^) is a dove, with an olive branch in lier mouth, and a star upon her head. 44. EuNOMiA was .another of the Seasons — a sister of Irene. (Symbol not ascertained.) 45. Juno was the reputed queen of heaven, and her sign ( 5 ) is an ancient mirror, crowned with a star — an emblem of beauty and power. 46. Ceees was the goddess of grain and harvests, and her sign (?) is a sickle. 47. Pallas (or Minerva) was the goddess of wisdom. and Qiwar. Her symbol ($) is the head of a spear. 1. The ancient PaXladiv/m was an image of Pallas, preserved in the castle of the city of Ti'oy ; for while the castle of the city of Minerva was building, they say this image fell from heaven into it, before it was covered with a roof. — Tooh^s Pam,tIieon, 2. To a similar ihble, respecting an image falling from heaven, the Apostle Paul al- ludes, Acts xix. 85 : — " Te men of Ephesus, what man is there that know^eth n H how that the city of Ephesus is a worshiper of the great goddess Plana, and of tlie image which fell down from Jupiter?" 41. Egeria and her symbol ? 42. Astrsea and sign ? (Mythological legend I Virgo and Libra ? Where else found ?) 48. Irene — ^by whom named, and why ? Symbol 3 44. Eunomia and symbol ? 45. Juno and symbol ? 46. Ceres and her symbol ? 47. Pallas and her symbol ? (Ancient Palladmim t Keputed origin ! Scriptural allusion to it ?) MYTHOLOGICAL HISTOKT AND ST1CB0L8. 35 SATURN, OR CHR0N09. 48. Htgela was the goddess of healthy and the daugh- ter of Esculapius, the father of the healing art. (Symbo) not ascertained.) Our modem word EygeUm, which eignifies the laws of health, &c., is derived from the goddess Hygeia. 49. JupiTEK was the reputed father of the gods — the king of heaven. His symbol (y) was originally the Greek letter ?, zeta (the same as our Z) — the initial of the Greek word^Zews, the name tor Jupiter. 60. Satuen — called by the Greeks Chronos — presided over time and chronology. His sign (^) represents a scythe. 1. Saturn was represented in MytholoCT" as an old man, with wings, hald excepting a fore- lock, with a scytlie in one hand, and an hour- glass in the other. The same figure is now used to represent time. 2. Our modern word chronology^ from clironoa, time, and logoUt discourse, signifiea tile science of keeping time, dates, &c. /'^ST. Uranus was the father of Saturn, and presided over astronomy. The symbol of this planet (W) consists of the letter H, with a planet suspended from the cross-bar, in honor of Sir William Herschel, its discoverer. This planet is popularly known by the name of Herschel, but astroilomers now almost universally call it tjranus. It bears this name in the British ^auticai AlmoTt-ao for 1851, with tlie full consent of Sir John Herschel, the son of the great discoverer. It was first called Georgntm Sidus, by Dr. Herschel, in honor of his royal patron. George IIL 52.. ISTeptukk was the god of the seas, but the symbol of the planet (¥) is composed of an L and a V united, with a planet saspended from the hair-line of the V, in- honor of Le Verrier, its discoverer. This planet was first called Le Verrier^ but is more generally known by the name of Neptune. 53. The Moon was called Zuna by the Eomans, and 48. Hygeia and symbol ? (Term liygeian ?) 49. Jupiter and his symbol ? 50. Saturn ? Greek name ? Symbol 3 (How represented in Mythology Word chroTwIogy ?) 51. Uranus and symbol ? (What other names, and why?) .52. Neptune and hia symbol ? (Former nainoV) 36 astkonom'j:. Selene by the Greeks. She is known by various 8,^01- bols, according as she is. new, half-grown, or full, thus : @. ©. O. ]. From Jjmut we'have our modem terms Vwnar and Vumacy ; tlie former of which sij;niftes pertaining to the moon, and the latter a disease anciently supposed to be caused by the moon, ^^ , ,_ . % Selme, in Mythology, was the daughter of EMoB, the Smi. Our English wonl seleiwgraplvy — a description of the moon^s surface — is ft-om Selene^ her ancient r.anie, and gra^^o, to describe. 54. The ScN — called Sol by the Romans, and HelioK by the Greeks — is represented by a shield or buckler, thus : 0'. ©> ©. As the large and polished bucklers of the ancients dazzled the eyes of their enemies, this in- strument was selected as an appropriate emblem of the sun. DISTANCES OF THE PLANETS. , (65. The distances of the planets from the sun, com- mencing with Mercury, and proceeding outward, are as follows; viz.: X^: Mercury . . . Venus .... tjy, Earth 95 Mars .... 145 The Asteroids, from 210 Jupiter .... 496 Saturn .... 909 Uranus .• . . . 1,800 Neptune . . . 2,862 37 millions, or to or 36,890,000 68,770,000 95,298,260 145,205,000 300,000,000 495,817,000 909,028,000 1,829,071,000 2,862,457,000 1. The first column of round numbers only should be committed to memory by tl e student Thdse should be well fixed in the mind, as it will greatly facilitate the pro- gress of the student hereafter. The family of Asteroids being less important, their dis- tances need not be learned in detail. The following table shows the distances of tlia several Asteroids from the sun : Flora .....209,826,000 Clio 222,378,000 Vesta 225,000,000 Iris 227,83^000 Metis 227,887,000 Hebe 231,089,000 Parthenope. 283,611,000 Egeria 244,940,000 Astrffia. 245,622,000 Irene 246,070,000 Eunomia 252,300,000 Juno 254.812,000 Ceres 263,718,000 Pallas. 264,256,000 Hygeia 300,822,000 58. The Moon — Latin and Greek names ? Symbols 1 (Words hmar am hinaoyf Who was ^Se&n* in Mythology I Selenography? Derivation!) 54. The Sun — Latin and Greek names ? Symbol, and why 3 55. ReheaTse, in round numbers, the diatanoes of the planets from the (■Substance of note Ist ? Objeot of note 2d ? Note 8d f Note 4th 11 DISTANCES OF THE PLANETS. 37 2. The comparative distances of the planets are represented in the cut, page 15, and - pear down to us. 83, What the ecliptic? (Ho-w cut the earth and snn ? Point out its courso in tihe heavens. "Why called the ecli^Ue f) >i4. What meant by above and hd&w the ecUptio ? (Remarks in note. J EOLIPTIO, ZODIAC, SIGNS, ETC. 61 85. The Poles of the JEdiptio are the extremities of an imaginary axis upon which the ecliptic seems to revolve. As the ecliptic and equinoctial are not in the same plane, their poles do not coincide, or are not in the same points in the bearens. The cause of this rariation will be ex- plained hereafter. 86f. The Zodiac is an imaginary belt 16° wide, viz., 8° on ea^^Ji side of the ecliptic, and extending from west to east quite around the heavens. In the heavens, it in- cludes the sun's apparent path, and a spac^of -eight de- grees south, and eight degrees north of it/?^ THB BCLIPTIO AND ZODIAC. ...0 — ^:i:7,f ^0--... ■■^-^-i:^ In this cut, the interior dotted circle represents the earth's orbit; the exterior the plcme of her orbit extended to the starry heavens. The dark lines each side of the ecliptic are the limits of the zodiac The earth is shown in perspective, largest near to us, and growing smaller as her distance is increased. The arrowB show her direction. 87. The great circle of the zodiac is divided into twelve equal parts, called signs. (These divisions are shown in the above cut, by the spaces between the perpendicular lines that cross the zodiac.) The ancients imagined the stars of each sign to represent some animal or object, and gave them names accordingly. On this account, they gave the name sodiao to this belt around the heavens ; not, as some have imagined, because it was- a sane, but from the Greek soon, an animal, because so many animals were represented within its limits. 85. The poles of the ooliptio ? (Do the poles of the ecliptic and the polea of heavens ooinoide ?) 86. What is the zodiac ? 87. How is the zodiac divided ? Idea of the ancients ? Origin of the name zodiae ? 62 ASTBONOMT. 88. The names, order, and symbols of the twelve signs of the zodiac are as follows : Aries (or the Eam) . . . . T Taurus (the EuU) b Gemini (the Twins). . . . n Cancer (the Crab) S Leo (the Lion) SI Virgo (the Yirgin) .... IE Libra (the Balance) — Scorpio (the Scorpion) . fll Sagittarius (the Archer) t Capricornus (the Goat) V3 Aquarius(the"W'aterman)™ Pisces (the Fishe^ . . . . K ANCIENT A8TE0L0GT. These names being from the Latin, their signification is added in parentheses, and should Oe understood by the pupil. In reciting, however, it is only necessary to give the ^rgt names — as Aries, Taurus, Oemini, &c By carefally observing these symbols, the stu- dent will detect a resemblance between several of them and the objects they represent. For instance, the sign for Aries represents his horns; so also with Taurufl, &c. 89. The ancients pretended to predict future events by the signs, aspects, &c. This art, as it was called, was denominated Astrology. Astrology was either natural or judicial. Nabwral Astrology aimed at predicting re- markable occurrences in the natural world, as earthquakes, volcanoes, tempests, and pestilential diseases. Jud/iciaZ Astrology aimed at foretelling the fates of individuals or of em- pires. "This science," says Webster, "was formerly in peat request, as men ignorantly supposed the heaven- ly bodies to have a ruling influence over the physical and moral world ; but it is now universally exploded by true science and philosophy." A iVagment of this ancient superstition, like the adjoining figure, may still be met with occasionally in the pages of an al- manac; and there are still persons to be found in al- most every community who think certain "signs" govern certain -portions of the human body, and that it is very important to do everything "when the sign is right" Impostors, also, are still taking advan- tage of tills credulity; and, professing to "tell for- tunes," as they call it, by the stars, impose upon and defraud the ignorant The stars have no more to do with our " destiny" than we have with theirs. .90. The order of the signs is from west to east around the heavens. Thus Aries, Taurus, Gemini, &c., around to Pisces. 88. Namet of the signs ! Symbols on blackboard. 89. What is astrology f How divided ? Define each. (Eemark of Web- ster ? Of the author V) 90. The order of the signs ? (Describe the cut. What said of Taiims ?) OELESTIAI, LATITUDE ASD LONGITUDE. 63 PBBPKNDIOUIAB VIBW OF THE BOLIPTIO. Into twelve equal parts, representing the twelve signs ; wbile tie osjaj^'which^he's'tars m eacli sign were supposed* to resemble is placed in that sign, and the symiol imme- diately opposite and within the sign. Bat the head of Taurus should point east instead or wfiSL CELESTIAL LATITUDE AND LONGITUDE. 91. Celestial Zongitude is distance east of a given 'p^t in the heavens, reckoned on the ecliptic. Begin- ning at the Vernal Ji'quinox, it is reckoned eastwai'd to 360°, or to the point whence we stai-ted. The pupil will consult the preceding cut, in which the longitude is marked for every ten degrees, B7 holding the book up to the south of him, the surface of the page will represent the plane of the ecliptic ; and the reckoning of 10, 20, 30, &c., from the top of *the cut eafttma/rdt will answer to the manner in which celestial longitude is reckoned eastward around the heavens. 92. Celestial Longitude is either Heliocentric or Geo- centric. The heliocentric longitude of a planet is its longitude as viewed from the sun ; and the geocentric^ its longitude as viewed from the earth. Geocentric is from ^^the earth, and Tcentron^ center; ftnd MUoavitric from heHoe, the sun, and Aea^roTt, center. 91. Celestial longitude ? Where begin to reoken ? Illnstr«ti by book. Point out order of reckoning in the heavens. 92. What is Jielioce^lSrio J-ongiinde ! Seoomtricf (JJ-OJivation rf terms 1 Illustrate by diagram.; \'' 54 ASTEONOMT. OEOOSHTBia AKD BELIOOBNTBIO LONGITUDB. In this cat, the planet B, when viewed from the earth at A, seems to be in the sign S) ; but when viewed from the sun, it appears to be in ir. Again : when iit 0, her apparent longitude from the earth is in ITL ; when from the sun, 6he appears to be in t. Thelearner will not only perceive the difference between geocentric and heliocenirio longitude, but will see why the latter more than the former indicates the true position of the planet It is an easy thing, however, if one is known, to deduce the other from it. MEAN AUD TEUE PLACES OF A PLANET. 93. The mean place of a planet is the place it would have occupied had it revolved in a cir- cular orbit, and with uniform velocity. The tfme place is that which it really occupies, revolving as it does in an elliptical orbit, and with unequal velocity. 1. In the cut, the dotisd ellipse represents the orbit of the plane^ and the points T T T, &e., its true 'place. In the circle or hypothetical orbit, the points M M, &c., indicate the Ttiecunplace of the planet MEAN AMlJ TKTJE PLACES OF A PLANET T T ..- A 83. What is meant by the m«a»^fcK!« of a planet? The inE0LIKATI0H' IN NOBTHZBK LATnUDE, 3. This cut shows how the student may measure tbo sun''8 declination wherever he may be located north of the equator. 'The shadows are such as are cast by objects during the year, about 45° north of the equator. On the 2Sd of December, when the sun has his greatest declination, the sliadow of the flag-staff extends north at 12 o'clock to the point C, where two boys are seen, having just "Briven down a stake. From this time to June 21st the shadow gradually shortens, till on that day it reaches tho point B, where another stake is driven. It then begins to elongate, and in six months is extended to C again. The point A is just half-way from B to in angula/r measurement, though the distances on the plain in the picture are very different. When the sun is on the equator, March 21st and September 28d, the shadow will reach only to A; and the angle A B and the top of the staff shows the nor'thsm, and A O and thQ top of the .staff the aouPtem declination. It will be found to be 2S^ 28' each way, as marked In the figure. 127. To what extent does the sun decline from the equinoctial north and south ? Why not more ? (SubstariGe of note 1 ? Note 2, and explain by diagram. JST ote 8, and diagram. What is a gnornon ?) EOTATION OF THE PLANETS UPON THEIK AXES. 69 4. The angle formed by the top and bottom of the pole and the point A will exactly corrGBpond with the latitude of the place where the experiment is made. 5. Let the students try this matter for "themselves. Select a level spot, and put up a stake, say ten feet high. Get an exact *' noon mark," or north and south line, whore the stake is driven, and at 12 o'clock, every fair day, put down a small stake at the end of the shadow. In this manner you will soou he able to measure the sun's declination for yourselves, to determine the latitude of the place where you live, and to understand how mariners at sea ascertain their latitude by the declination of the sun." 6. The ancients had pillars erected for the purpose of m^ing observations upon their shadows. Such a pillar is called a grtomon. /' EOTATION OF THE PLANETS UPON THEIE AXES. 128. The time, so far as known, of the revolution of the planets upon their respective axes, or, in other words, the length of their natural days, is as follows : Mercury . Venus Earth . . Mars . . h. m. 24: 5 23 21 24 00 24 87 Juno . Jupiter Saturn Uranus b. m. 27 9 56 10 29 9 30 These statistics are given upon the authority of Sir John F. "W. Herschcl, though he marks Juno and Uranus as doubtful. ' 129. The revolution of the earth upon its axis is the cause of the agreeable vicissitudes of day and night. PHILOaOPHT OP DAY AlTD KIQHT. How wisely adapted to the happiness of His creatures are all the works of 'God I Tho night prepares us for the day, and the day in turn prepares us to welcome the night; and in both instances the change ministers to the happiness of man and beast. And but for being carried around into the darkness of the earth's shadow, we should never have admired the dazzling firmament, as it declared the glory of God, and showed forth his handiwork- How beautiful the poetic allusion to the revealing power of night I Mysterious Night I when our first parent knew • Thee, from report divine, and heard thy name. Did he not tremble for this lovely frame. This glorious canopy of light and blue ? Tet, 'neath a curtain j obsemng the motion of spots fu^^^B^^m upon theii' surfaces, by the aid of tele- ^ t w^fe SI^M 1. In the cut we have an arc of the earth's orbit, and the ; earth revolving on her axis as she revolves around the snn. / i The arrows show the direction in both cases. / J 2. By holding the book up sonth of him, and looking at- i^ tentively at the cat, the student will understand why the sun " rises" or first appears in the east. It is because the earth revolves eastward. Thus the observer at A is carried round into the light, and sees the snn rise when he reaches B. Ai; TIME. ^^131. Time is duration measured either by natural or artificial means. The principal natiwal indicators of the lapse of duration are the revolution of the earth upon its axis, marking a natural day ; the change of the moon, denoting a lunar month ; and the cycle of the seasons, denoting a year. Time is measured a/rtificiaUy by clocks, watches, chronometers, dials, &c. ; the standard being the solar day still, which is divided artificially into 24 parts, called Jiours, and these again into Tninutes and seconds.- The aboriginal tribes of this country all reckoned time by "moons," or montlis, as denoted by the moon's changes. 132. The motion of the earth upon its axis is the most regular of which we have any knowledge. It dues not vary one second in a thousand years. To this stability of the earth's motion upon her axis the prophet refere when he says; Thus saith the Lord, If ye can break my corenant of the day, and my covenant of the 180. Ib what direction do the planets rotate on their axes ? How ascer- tained ? (£x[>lain why the sun appears to rise in the east.) 131. What is time f What natural standards ? Artificial ? (How meas- ured byaborigines ?) 132. Whats^d of earth's motion on axis? (What reference to in Sorip- luies?? TIME. ' 71 night, and that there should not he day and night in their seasons, then may also my cOTOuant he hrolcen with David," &c.— Jeremiah xxziii. ao. 133. Time is of two kinds — Solar and Sidereal. A solar day is the time elapsing from the sun's crossing the meridian of any place, to his coming to the same me- ridian again. A sidereal day is the time intervening between the transit of a star across the meridian, to its coming to the same meridian again. 134. A solar day consists of 24 hours, at a mean rate, but a sidereal day is accomplished in 23 hours, 56 min- utes, and 4 seconds ; the solar day being near 4 minutes the longest. This slight difference of about 4 minutes daily, between solar and sidereal time, amounts to one wTwle day in every 365J. Owing to the revolution of the earth around the sun, and his apparent annual revo- luti(ni eastward among the stars, it requires 366 revolu- tiops of the earth, as measured by the fixed stars, to make 365-j days, as measured by the sun. 135. The cause of this difference in the apparent revo- lutions of the sun and stars, and consequent difference in the length of a natural day, as measured by the passage of a star or of the sun across the meridian, is this : The earth is constantly advancing in her orbit vrhile she re- volves on her axis, causing the sun to ap^pean- to move slowly eastward among the stars ; or, what is the same thing, the stars to appear to rise earlier and earlier every night, and one after another to overtake and pass by the sun. (See Artieie 119.) "When, therefore, the meridian is brought around to that point in the heavens where the sun was near 24 hours before, he is not there, but has moved a little eastward. But a star that, 24 hours before, was exactly behind the center of the sun in the distant heavens, will be found west of the sun, and will conse- quently cross the meridian before the sim does. The time required for the meridian to revolve from the stai to the sun constitutes the 3 minutes 56 seconds difference between solar and sidereal time. 183. Kinds of time ? Define each. 184. Length of solar day ? Sidereal? Difference? Arjount in year ? 185. State the cause of the diffurence in the time of the apparent revo'iitioo of the sun and stars. Illustrate by diagram. 72 ASTRONOMY. BOLAB AKD BXDEBBAL IIMII* K. O SIDEB gAL D^ Y _ J^Sk aoLAB D^■;f. - RftL ^ ■ J!a. SUN ON TBE MERIDIAN 1. To the man at A the Bun (S) is exactly on the meridian, or it is twelve oVIock, Boon. The earth passes on from B to D, and at the same time revolves on her axis. When she reaches £>, the man who has stood on the same meridian has made a complete revolution, as determined by the star G- (which was also on his meridian at twelve o'clock the day before) ; but the sun Is now east of the meridian, and lie must wait/our mvmiie9 for the earth to roll a little farther eastward, and brings the sun again over his north and south line. If the earth was not revolving around the sun, her solar and sidereal days would be the same ; but as it is, she has to perform a little more than one complete revo lution each solar day, to bring the son on the meridian. '.^ EQUATION OF TIME. 136. As the distant stars have no motion, real or ap- parent, around the ecliptic, and the earth's motion upon I it is uniform, it results that sidereal time is always exactly ' the same. ^r ■; A clock that keeps sidereal time is called a sidereal clock. One of these instruments Is almost indispensable in the observatory of the astronomer. 137. Solar time is constantly varjdng. !N^o two suc- cessive solar days are exactly of a length., The 24 hours given as the length of a solar day (134) is the amerage of all the solar days throughout the year. Hence it is called Tnea/n solar time. The time, as indicated by the transit of the sun across the meridian, from day to day, is called apparent tvme. 138. A well-regulated clock wiU keep mean sola/r time, and will vary from the appa/r&nt time (as indicated by a noon mark, or dial) to the amount of \Q\ minutes one way, and 14^ the other. The sun will at one time cross the meridian 16^ minutes before it is noon by the clock — the apparent time being 16^ minutes faster than mean or clock time ; while at another time it will be noon by the clock 14^ minutes before it is noon by the sun. 136. Is sidereal time always the same? Why must it be? (What is a sidereal clock ?) 137. What said of the variations of solar time ? What is meansoUir time f Apparent ? 188. What time do oomuion docks keep ? How much variation from sun ? How? EQUATION OF TIME. 73 139. The difference between apparent and mean solar time is called the Equation of Time. It is greatest about the 3d of November, when the clock is 16 minutes and 17 secondSv behind the sun. Four times a year — viz., April 15th, June 15th, September 1st, and December 23d ^ — the clock and sun will agree; or, in other words, mean and apparent time will be alike. 140. The inequality of the solar days depends upon two causes — ^the unequal velocity of the earth in her orbit (77, 78), and the inclination of her axis to the plane of her orbit (123). 141. If the earth's orbit were an exact circle, she would move with the same bqttal soi-ae Dim. velocity in all parts of it; and if she revolved with m(^ regularity upon her axis, ^ her solar days would be ,^ exactly of a length. ^ Let the circle in the a(^oining cutrep- ; resent the earth's orbit, and the projec- ^— tiona from the earth toward the eun a T pillar or gnomoji standing upon a given ^v- meridian. The cut will then show that ^? witli a circnlar orbit, and uniform motion V^ in it, and a regular rotation upon her ^T axis, ihe earth would bring the gnomon '"^r/ around toward the sun at regular inter- ^ , vals, both of distance in, her orbit, and *|^ of time. In.that case, all apparent solar ^^""^ days would be equal. " 142. As the orb't of the earth is elliptical, it requires more time for the earth to pass from the vernal equinox, through the aphelion, to the autumnal equinox, than it does Irom the autumnal equinox, through the perihelion, to the vernal equinox. The difference is about eight days — the sun being north- of the equinoctial about eight days longer than he is south of it. Hence the summers of the northern hemisphere are longer than the winters. 143. As the earth's orbit is an ellipse, and the earth 189. What this difference called ? When greatest ? When no difference I 140. What causes the inequality in the length of the solar days ? T41. What necessary in order that they may be equal? , (Illustrate by dia- gram and explanations.) ' *~ 142. What effect has the ellipticity of tUe earth's orbit upon the length of the eeasonf', north and south of the equator ? .,- ' 4 tmnSQITAL BOLAB DATS. D ^w^.^ eC p. «^ J9 fK A B0- - o ..._-^A *^- ■^ V ¥ c 74 ASTEONOMT. moves faster in some parts of it than in others, while its rotary motion is uniform, it follows that its orbitual ve- locity in longitude must sometimes be faster^ and • at others dower than its orbitual motion, thus caus- ing an inequality in the length of the solar days. From A to B in the adjoining cnt, the orbitual motion is slower than its mean rate, and the rotary n>otion gains nptm it Hence the gnomon is shown revolving too fast, and as pointing east of tlie snn, when the earth has per- formed her journey for a mean solar day. From B to A, the earth's motion in her orbit gains upon her rotary mo- tion, and thegwHoon is behind, orwest of the sun. At A anil B the clock and sun would ^rec. From A to I> the sun gains on the clock, till it gets 14i minutes ahead. From D to B this difference is diminished^ till at B the sun and clock agree. From B to C the clock gains on tbe sun, till the difference is ICJ minutes; and &om C to A this difference diniioisbes, till at A mean asd apparent time agree again, 144. The earth's perihelion is in n, and her aphelion in t ; the first of which she passes on the first of Janu- ary, and the latter on the 3d of July. We are conse- quently about three millions of miles nearer the sun Jan. 1, than July 3d. The natural effect of this variation would be, so fer m it bad any influence, to mod^ the cold and heat in the Northern Hemisphere, and to augment both in the Southern. For instance, our nearness to the sun in January would slightly soften our winter, while, at the same time, it sli^tly increased the heat of the summer south of the equator. Bo, also, our increased distance in Jaly would diminish tbe heat trf our smmner, and at the same time enhance the cold of the c€>rrespondfng winter in the Southern Hemi- Bphore. But the variation of 3,000,000 miles is so slight, when compared with the wholv Stance of tbe sun, that the change of temperature prodaced thereby is imperceptible;. \1tHE calendar, LEAP YEAE, OLD AND NEW STYLE, ETC. 145. The Julian calendar divided the year into 12 months, containing in all 366 days. But a full astro- nomical year, or the time requisite for the earth to re- volve irom one equinox around to the same equinox again, consists of 365d. 5h. 48m. 51s. Hence the Juhan 143. Explain the cause of this inequality ? (Ulnstrate by^dia^rora.) 144. Where are the perihelion and aphelion points, "and when passed^ When nearest, and how much ? ^What effect?) 145. Deficribe the Julian calenaar? An astronomical year? What dif ference 1 What effect 1 How corrected ? ETC. 75 year was near 6 hours, or one day in every four years, too short ; which, if left uncorrected, would in time com- pletely reverse the seasons, giving harvests in January, and snow in July. To prevent this constant falling be- hind, a correction was applied, by adding one day to February every fourth year. Hence it is called Bissex- tile or Lecup Year. 146. But one whole day added for every four years was 44m. 36s. too much. Fi'om a. d. 325 to 1582 this excess amounted to about 10 days ; so that the civil year was thus Hiuch ahead of the astronomical. In 1582, Pope Gregory XIII. applied a further correction, or re- formed the Julian calendar. To make the civil and as- trondinical years agree, so that the vernal equinox would happen on the 21st of March, as it did 125Y years before, Gregory resolved to strike out of the civil year the 10 days it had gained, and ordered that the 5th of October should be called the 15th. This reformed or corrected calendar is called the Gregorian calendar. 147. To prevent the civil year from running ahead of the astronomical again, in the lapse of centuries, by the 11m. 12s. which it exceeded the astronomical, it was pre- scribed that at certain convenient periods the intercalary day of the Julian period should be omitted. Thus the centennial years 1700, 1800, 1900, are, according to the Julian calendar, bissextiles ; but on these it was ordered that the intercalary day should not be inserted, inserted again in 2000, but not inserted in 2100, 2200, 2300 ; and 80 on for succeeding centuries. 148. The Gregorian or reformed calendar was adopted as soon as promulgated, in all Catholic countries ; but in England, the "change of style," as it was called, did not take place till September, 1752. Eleven nominal days were then struck out, and the 3d of September was called the 14th. At the same time, the time of the beginning 146. Was the calendar then correct ? Why not ? What result ? Whc corrected? When? How? What this reformed calendar called ? 147,. What further correction necessary ? How effected ? 148. Was the Gregorian calendar at once adopted ? When ir England ! How then adopted ? What other change at the same tJrae ? What effect in 76 A6TE0N0MY. of the civil year was changed from the 25th of March to January Ist, as it now stands. The year 1752^, which was to have begun on the 25th of March, was made to begin on the Ist of January preceding ; so that for dates falling between the 1st of January and the 25th of March, the number of the year is one greater by the New than by the Old Style. And as the intercalary day was omit- ted in 1800, there is now, for all dates, 12 days difference between the old and new styles. Kussia is now the only Christian country in which the Gregorian calendar is not used. TIME, AS AFFECTED BT LONGITTJDE. 149. As the sun's crossing the meridian of any place determines it to be 12 o'clock, apparent solar time, at that place, it is evident NOON that 12 o'clock comes sooner to places east on the earth's surface, and later to places west. 1. Let the adjoining cut represent the earth, the arrows indicating the direc- tion of her revelation, and the sun beii^ on the meridian at XII. at the top. It ^ Vlllji will then be day over all the ligbt por- o ^^\ tion of the globe, and nightover all the ^iil shaded portion. On the meridian exactly under the sun it is just XII. o'clock noon ; while at the meridian on the opposite side of the earth it isjust 12 o'clock at night, or midnight, w hen the light and shade meet on the riglit, it is Vf. o'clock morning; and directly opposite on the left, is VI. o'clock even- ing. 2. Observe that when it is XII. at A, It is I. o'clock at B, II. o'clock at G, &c., while it is only XI. o'clock at D, X. o'clock at E, IX o thns showing how it is that time is earlier east, and later west of any given meriaian. 150. Every 15° of longitude upon the earth's surface makes an hour's difference in the time. If east of tlie given meridian, it vrill be an hour ean^lier / if west^ an hour later. rcckoninff years of time ? "What the di£Eerenca.^now between Old and Keie style, and why ? What calendar used in Russia ? 149. What effect has the longitude of a place upon its time 1 (Diagram, aud explain?) 150. What difference of longitude is required to make an hour's difference hi time? When earlier? When later f (How demonstrated 1 When 6 TIME, AS AFFECTED BY LONGITTTDE. 77 1. If the sun passes through 860<3 every 24 hottrs, he must pass over 15° each hour, as 360° -^ 24 = 150. Hence every 15° must make an hour's difference in the time ; and when it is sunrise, or 6 o'clock, solar time, in New York city, it will be noon, or 12 o'clock, 90° east of New York, and niidnigfht 90° west of it 2. Taking the circumference of the earth at 25,000 miles, the sun passes over 1041§ miles every hour at the equator; for 25,000 miles -^24 equals 1041g miles. And if 1041 § miles be divided by 60, the number of minutes in kn'hour, it gives about 1?^ miles as the space over wliich the sun travels at the equator every minute. Every 17i miles, therefore, east or west, will make one minute's difference in the time. As we recede from the equator north or south, the meridians approach each other, and a degree of longitude becomes less and less to the poles. ' 8. A person leaving Boston with the exact time will find, on reaching Albany, about 8° west of Boston, that his watch is some 12 minutes aliead of the Albany time; and on reaching Buffalo, about 5° further west, that it is some S2 minutes ahead of the true time at Buffiwo. So in traveling from Buffalo to Boston, the Alhiiny and Boston time will be found to be the same extent ahead of the Buffalo time. Hence conductors on railroads, running their trains by time, set their watches iVom Albany to Buffalo by some standard agreed upon — as, for instance, Syracuse time— and reject all other local time, be it faster or slower. 151. As every 15° upon the earth's surface makes an hour's diflference in the time, it is easy to convei't degrees into time, or time into degrees. By this means, a mari- ner having the time at the place whence he sailed, and the- time where he is, from observing when the sun crosses the meridian, can ascertain, from the difference between his standard and local time, his distance east or west of the port whence he sailed, or, in other words, his longitude. 1. Time is converted into degrees by multiplying the hours by 15 for the degrees, and adding one-fourth of the minutes to the product; for every minute of time makes ^% and every second of time \' in longitude. 2. On the other hand, degrees of longitude are converted into time by dwiding them by 15 forthe hours, and multiplying the remainder, if any, by 4 for the minutes, &c. 152. The rotation of the planets upon their respective axes has caused them to swell out at their equators, and contract at their poles — thus assuming the form of oblate spheroids (page 18). 1. "When fluids are left free to yield to the influence of attraction, as mutually existing between their paiticles, they invariably assume a spherical fofm. Hence water, in fall- ing from the ck)uds, takes tne form of spherical drops ; and melted lead, thrown from the top of a shot-tower, takes a spherical form, and cooling in the air on iis passage , down, remains perfect little globes, called sJiot. % A 8o?id sphere would never become oblate by revolution. It might burst, from its powerful centrifugal tendency, as grindstones sometimes do in manufactories of cut- lery ; hut it must be Jk^iS^ or at least soft and yielding, in order to become oblate by revolution. o'clock in New York, what time 90° east ?— 90° west? How many miles does the sun pass over in an hour at the equator ? Per minute ? How deter- mined ? How north and south of equator ? At 45th degree ? What differ- ence from Boston to Albany and Buffalo ? From Buffalo to Boston ? Henco what practice ?) 151. Can time be converted into degrees, and degrees into time? How useful in navigation? (How convert time into degrees? Degrees into time ?) 152. Effect of rotation upon figure of planets ? (Note 1 ? 2. Solids ? S. What does oblateness indicate ? 4. Proof from Scriptures ? Eemark ?) 78 ASTRONOMY. f the planets, then, Beems to indieste two things ; First, that they i or plastic state ; and, secondly, that they began to revolve while 3. The oblateness of t were all once in a fluid ^ __.,_, in that state, or before any part of them had become solid, like our continents and islands. -4. So far as the earth is concerned, we are taught in the Holy Scriptures — the beat and most accarate of all books — that the earth and water of our globe were once so mixed, that the whole appeared as a " void" of " waters ;" and that they were afterward separated into " earth" and " seas" by the Almighty Creator. (Bee Genesis i., 2, 9, 10.) Thus we see that true science and the Bible are always in harmony with each other. 153. The difference between the polar and equatorial diameters of the planets,' so far as known, is as follows : the Earth, 26 miles ; Mars, 25 ; Jupiter, 6,000 ; and Saturn, 7,500. The oblateness of Jupiter and Saturn is as plainly visible through a telescope, as the difference in the following figures is to the eye of the student OBIGIHAL FORM. PRESENT AFFEABANOE. The plain line in the middle figure shows the original form, and the dotted line its present form. The difference is the change produced by its rotation. When measured by the proper instruments, it is found, in the case of Jupiter, to amount to about tt of his average diameter; and that being 89,000 miles, y^^ is but little less than 6,000. 154. As Mercury and Yenus rotate in about the same time of our globe, and their sidereal years are only 88 and 225 days respectively (72), it follows that Mercury has but 88 natural days to his year, and Yenus only about 225 to hers. But the natural day of Jupiter being only 10 hours long, and his year equal to about 12 of ours (11 years 317 days), he must have 10,397 natural days in one of his years. So Saturn's year, consisting of 29 years 175 days of our time, will allow him to rotate on his axis about 25,000 times ; or, in other words, will allow of 25,000 natural days in each of his years. The year of tTranus being equal to 84 years and 27 days of 153. State the difference of equatorial and polar diameters of planets ? (Eemark respecting Jupiter and Saturn ?) 154. How many natural days has Mercury in his year ? Venus ? Jupiter ? How 80 many ? Saturn ? Uranus ! (Demonstrate.) AS AFFECTED BT LONGITUDE. 79 our time (71), and his diurnal revolution 9^ hours (128)) it follows that he has 92,683 natural days in his year. 29 years 175 days = 10,700 days of our time; X24=a58,240 honrsH-lOi hours, th» time of Saturn's revolution, ^24,594^^^, the number of days in his year. So 84 years, 87 days, the periodic time of Uranus = 86,687 days, or 880,488 hours ; which J- 9i hours, the time of the planet's diurnai revolution == 92,683, the number of natural days in his year. / 155. As going froTn the earth's center is to ascend ' (page 27), and the equator of an oblate ffpheroid is fur- ther from the center than the poles, it follows, that the earth being an oblate spheroid, we must ascend some- what in going from either pole to the equator. A river, therefore, running for a great distance toward the equator, would actually ascend ; or, in other words, run up hill — the centrifugal force generated by the earth's motion driving the water on toward the equator. The Mississippi is said to be higher at its mouth than it is some thousands of milea north of it If its bed conforms at all to the general figure of the earth, this must cer tainly be the case, as may be demonstrated by the aid ^ . „„„ „..„„,„., of the annexed dikgram. Let A B represent tlhe polar, ^■^™« kukkiko dp hili. and C D the equatorial diameters. The entire differ- O ence between them is 26 miles, or 13 miles on each ^^^ side. The two circles represent this difference. Now ,^^ as the earth's circumference is 25,000 miles, the dis- ^ tance from the poles to the equator (being one- / fourth of that distance) must be 6,250 miles ; and in / that 6,250 miles the ascent is IS miles, or over two / miles to every 1,000 toward the equator. The Mis- ^J sissippi runs from the 50th to the 30th degree of 1 north latitude inclusive, or 21 degrees; which, at \ 69y miles to a degree, would amount to about 1,500 \ miles. If, then, it runs a distance equivalent to 1,500 \ miles directly south (in a winding course of about '^^^ 8,000), theory requires that it should be about three ^^ miles higher at its moulh than it is 1,500 miles directly north. There is some philosophy, therefore, in saying that if a river runs for a great distance from either pole toward the equator, it must run up hill. 156. Shoiil(i the earth cease to rotate upon its axis, the waters about the equator wouM at once rush toward the poles, flooding them to the depth of 6^ miles, and reced- ing from the equator to the same amount. So far as the solid portions of the earth would permit, it would at once become a perfect sphere. (See page 17, and also Art. 153 and note.) 157. It has already been stated (77), that the orbits of all the planets were ellipses ; but they are not all alike eccentric. The orbit of Mercury is quite elliptical, while 155. What curious fact follows from the earth's oblateness? (What in stance ffiyen ? Illustrate by diagram.) 156. What would be the effect should the earth cease to rotate ? 80 AS'HEONOMY. that of Venus is nearly a circle. The student should observe that the eccentricity is not the deviation from a circle, but the distance from the center of an ellipse to either foci (see page 23 and cuts). The eccentricity of the orbits of the principal planets is as follows: Milei. Mercury 7,000,000 Vcmis 492,000 Earth 1,618,000 Mars 13,500,000 Vesta 21,000,000 .Tuno 64,000,000 HileB. Ceres 21,000,000 Pallas 64,250,000 Jupiter 24,000,000 Saturn 49,000,000 Uranus 85,000,000 Neptane PEEOESSION OF THE EQUINOXIS. PEKOESSION OF THE EQUHSTOXES. 158. The equinoctial points have already been defined (125) as two points in the earth's orbit where the equi- noctial or celestial equator (20) cuts the sun's center. They are in opposite sides of the ecliptic, or 180° apart (see 119 and cut). The vernal equinox is the point from which both celestial longitude and right ascension are reck- oned (20 and 91) ; but not being marked by any fixed ob- ject in the heavens, it is reached just when the sun comes to be exactly over tlie earth's equator, or in the equinoctial 169. But it is found by long and careful observation that the earth reaches the equinoctial point about 22 minutes and 23 seconds ear- lier every year than on the year preced- ing. This is equal to 50^" of arc in the ecliptic. In this manner the equinoc- tial points are slowly receding westward, 157. What said of the orbits of Mercury and Venus ? Of eooentrioity ? 158. Are the equinoctial points marked by any fixed object in the heavens How know when reached ? 159. Are they stationary or not 1 Beached how much earlier annually ? PRECESSION OF THE EQUINOXES. 8l or fallmg back upon the ecliptic, at the rate of 50J" a year, or 1° every 71f years. This would amount to 30°, or one whole sign in 2,14:0 years, and to the entire circle of the ecliptic in 25,868 years. This very interesting phenomenon may ba explained by the preceding diagram. Let tlie point A represent the vernal equinox, reached, for instance, at 12 o'clock on the 20th of March. The next year the sun will be in the equinoctial 22 minutes 28 seconds ear- lier, at which time the earth will be 50i" on the ecliptic, back of the point where tlio sun was in the equinoctial the year before. The next year the same will occur again ; and thus the equinoctial point will recede westward little by little, as shown by the small lines from A to B, and from to D. It is in reference to the stars going forward, or seeming to preoecU the equinoxes, that the phenomenon was called the Precession of the Equinoxes, But in reference to the motion of the equinoxes themselves, it is rather a recession. 160. The cause of this wonderful motion was unknown, until Newton proved that it was a necessary consequence of the I'otation of the earth, combined with its elliptical figure, and the unequal attraction of the sun and moon on its polar and equatorial regions. There being more matter about the earth's equator than at the poles, the former is more strongly attracted than the latter, which causes a slight gyratory or wabbling motion of the poles of the earth around those of the ecliptic, like the pin of a top about its center of motion, when it spins a little obliquely to the base. 161. One marked effect of this recession of the equi noxes is an increase of longitude in all the heavenly bodies. As the vernal equinox is the zero or starting point, if that recedes westward, it increases the distance between it and all bodies east of it ; or, in other words, increases their longitude to the amount of its recession. Hence catalogues of stars, and maps, showing their lon- gitude, need to be corrected- at least every 50 years, othei-wise their longitude, as laid down, will be too little to indicate their true position. Allowing the world to have stood at this d^te (1853) 5,857 years, the equinoxes have receded already thrcfugh about 75° of longitude. At the same time the constellations have gone forward How ranch in angular "measurement ? Eevolving which way I At what rate? How long for 1°? For,SO°? For the whole circle of the eclipti. ? (Illustrate by diagram.l 1 60. Cause of recession ? Who discovered ? 161. Effect of recession upon longitude? Explain how effected. Sisri^ and constellations ? 4* «2 ASTEONOMT. eastward, and left the signs which bear their names. Hence the sign Aries actually covers the constellation Pisces. 162. Another effect of the recession of the equinoxes is, that it gives to the pole of the earth a corresponding revolution around the pole of the ecliptic in 25,868 years. Lot the lino A A in the figure represent the plane of the ecliptic; BB, the poles of the ecliptic; CO, the poles of the earth ; and D D, the equinoctial. £ £ is the obliquity of the ecliptic. The star C at the top represents the pole star, and the curve line passing to the right fW>m it may represent the circular orbit of the north pole of the heavens around tbe north pole of the ecliptic 163. This gyratory motion of the north pole of the heavens, while it keeps at the distance of 23° 28' from the pole of the ecliptic, will cause it to change its place in the heavfcns to the amount of 46° 56' in 12,934 years; thus alternately approaching toward and receding from the stars, at every i-evolution of the equinoxes around the ecliptic. Thus the place of the pole is in constant but very slow motion around the pole of the ecliptic. 164. The Nutation of the earih's axis is another sniall and slow gyratory motion, by which, if subsisting alone, the pole would describe among the stars, in the period of about 19 years, a minute ellipse, having its longer axis equal to 18", and its shorter about 14" ; the longer axis pointing toward the pole of the ecliptic. It is on account of these varied motions shifting the point from which longitude and right ascension are reckoned, and also the pole of the heavens, that it becomes necessary, in de- 162. What other effect of recession ? (Ilktstrate by diagram.) 163. What effect upon the apparent distance of the stars from the north pole of the heavens ? 164. What is J^-uiaiinnf What meant by epocA, and wliy necessary to state! TELESCOPIO VIEWS OF THE PLANETS MEEOUET. 83 scribing the place of a star or planet, by any of these standards, to state the epoch or time, and also whether it be mean right ascension — i. e., right ascension after hav- ing been corrected for the recession of the equinox, the zero point. 165. The Colwres are two great cii-cles crossing at the poles of the ecliptic at right angles. One passes fhrough the equinoxes, and is thence called the Equinoctial Colure ; the other passes through the solstices, and is called the Solstitial Golure. They are to the heavens what four meridians, each 90° apart, would be to the earth. CHAPTER 111. , TEIiESCfOPlO VIEWS OP THE PLANETS. 166. By the aid of telescopes, we discover myriads of objects in the heavens that are entirely invisible to the naked eye ; while objects natui*ally visible are immensely magnified, and seem to be brought much nearer the ob- server. Tbis impression of nearness is an intellectual conclusion drawn l^om the fact of the increased distinctness of the object ; as we judge of the distance of objects, in a great measure, by their dimness or distinctness. MBECUEY. 167. Under favorable circumstances. Mercury is visible to the naked eye, but yet is seldom seen, owing to his nearness to the sun. During a few days in March and April, and August and September, he may be seen for several minutes in the morning or evening twilight, when 165. What are the colwres ? Describe. 166. Effect of the telescope upon vision ! Upon distant objects ? (Why appear nearer ?) 167. Can Mercury be seen by the naked eye? Is he often seen! Why not % When may ne be seen ? How appear ? ,. 84. asteonCmy. his greatest elongations (99) happen in those months. He appears like a star of the third magnitude, with a pale rosy light. See 104 and note. 168. Through a telescope, Mercury exhibits different phases in different parts of his orbit, similar to those pre- sented, by the moon in her revolution around the eartli. The German astronomer, Schroeter, discovered numerous mountains upon the surface of Mercury, one of which he estimated to be nearly 11 miles in hight. By observing these at different times, he determined the diurnal revo- lution of the planet to be 24h. 5ra. 28s. But these obsei-va- tions have not been confirmed by any other astronomer. Tlie apparent angular diameter of Mercury varies from 5" to 12", according to his position with respect to the earth (56 and 80). So far as is known.he is not attended by any satellite. VENUS. 169. When favorably situated, Venus is one of the most conspicuous members of the planetary system, and is a most brilliant object even to the naked eye. Her color is of a silvery white, and, when at a distance from the sun, either east or west, she is exceedingly bright and beautiful. When nearest the earth, her apparent di- ameter is 61", which is greater than that of any other planet, owing to her being so much nearer than Jupiter or Saturn. Under a telescope, Yenus exhibits all the phases of the moon, as she revolves around the sun. The cause of this phenomenon is, that we see more of her enlightened side at one time than at another ; and the same is true of Mercuiy. 1. The tclcscopid npp6nraiir.o of Venus, at different points in lier orbit, is represented In the following llgui'e. At iS nnd W she has her greatest eastern and western elonga- 168. How appear through telescope? What said of Schroeter ? What conclusion from observing the spots ? Confirmed by others, or not i An- gular diameter of Mercury ? Why vary ? Has he a satellite ? 169. What said of Venus ? Her apparent diameter? Why greater than that of Jupiter ? How appear through telescope ? Came of her phases ? (Describe phases when east of the sun — west. What prediction before the discovery of tlie telescope ?) TELESCOPIC VIEWS OF VENUS. 85 Won, and Is stationary; while her positions opposite the words "direct" and "retro- grade" represent her at her conjunctions. The spots on the face of the sun represent Venus projected upon his disk, in a transit, the arrow indicating her direction. TELESCOPIC PHASES OF VKNU5. ^---■—7^="-— -F^ 0=E£BH!iH5;iT mt^^^^^umm p' ---sis; ^^S |ii§^-^^5=i ^^^BM ^-d. J 3=~:r==.-=fltT*lifflli^^=^ StPHil 2. Before the discoveryof the telescope, it was asserted that if the Copernican theory were true, Mercury and Venus would exhibit different phases at different times ; and as -those phases could not be seen, it was evident that the theory was false. But no sooner had Galileo directed his small telescopes to these objects, than he found them exhibiting the very appearances required by the Copernican theory, its opponents themselves being judges. 170. Besides the phases above mentioned, a close in- spection of Venus will reveal a variety of spots upon her surface. These are supposed to be the natural divi- sions of her surface, as continents, islands, &c.. Schroeter measured several moimtains upon this planet, one ol which he estimated at over twenty miles in hight. There is evidence of the existence of an atmosphere about this planet, extending to the distance of about three miles. SPOTS SEEN UPON THE SURPACE OP TENUS. lYl. "Were a person situated upon one of the exterioi planets, at a distance from our globe, it would exhibit phases like Mercury and Venus, in its annual revolution ; and the continents, islands, and seas would appear only as ^ots upon her surface, assuming various forms, ac- cording to the position from which they were viewed. 170. Wtat else seen upon Venus? Whfit supposed to be? Seliroelur's measurements ? Has Venus an otmosphere ? 171, How would our globa appear it viowod from a distaneo ? 86 ASTEONOaiY. DISTANT TELESCOPIC VIEWS OF THE EABTH. 2. 8. Above w6 have foiir different views of our own globe. No. 1 is a view of the Northern Hemisphere; No. 2, of the Southern ; No. 3, of the Eastern Continent ; No. 4, of the Western. A common terrestrial globe will present a diiferent aspect from every new position from which it is viewed ; as the earth most in her appearance to the in- habitants of other worlds. MAES. 1T2. Mars usually appears like a star of the second magnitude, of a reddish hue. "When in opposition, or nearest to the earth, he appears quite brilliant, as we see his disk fully illuminated. His apparent diameter is then about 18" ; whereas, when on the opposite side of the ecliptic, or in conjunction with the sun (80), it is only 4". He exhibits slight phases, and his surface seems to be variegated with hill and vale, like the other planetary bodies. " Upon this planet," says Dr. Herschel, " we discern, with perfect distinctness, the outlines of what may be continents and seas." When it is winter at his north pole, that part of the planet is white, as if covered with ice and snow ; but as summer returns to his north- ern hemisphere, the brightness about his north pole dis- appears. 173. The general ruddy color of Mars is supposed by Sir John Herschel to indicate " an ochery tinge in the general soil, like what the red sandstone districts on the earth may possibly offer to the inhabitants of Mars." Others suppose it to indicate the existence of a very dense atmosphere, which analyzes the light reflected from the planet. When the sunlight passes through vapor or clouds in the morning or evening, tho different rays of which it is composed are separated, and the red rays only pass to the 172. Usual appearance of Mars 1 When brightest, and why ? Apparent diameter ? Cause of great variation ? Phases 1 Hersohel's remark 1 Spot at north pole % 178. Supposed causes of his coUr t (Note.) THE ASTEEOIDS. 87 e&Tth, giving to the clonds a gorgeona crimson appearance. In a similar mannor it is supposed that tho atmosphere of Mars may give him his crimson line. TELZSCOPIO APFBABANOES 07 MARS. 1. The right-hand figure represents Mars as seen at the Cincinnati Observatoiy, August 5, 1845. On the 30th of tho same month he appeared as represented on the left. The middle vieio is from a drawing by Dr. Dick. 2. Just ^ast of the " Seven Stars," or Pleiades^ the student will find another group called the B'yades; one of which, called Aldebaran, is of a reddish cast, and somewhat "esembles the planet Mars. "When Mars is in opposition, however, at his nearest point tons, and with his enlightened side toward us, be appears much larger and brighter than Aldebaran. 174. As the periodic time of Mars is only 1 yr. 322 days (71), his motion eastward among the stars will be very rapid, as in that time he must traverse the whole circle of the heavens. His rate of motion being about 1° for every two days, or one whole sign in 57 days, it will be easy to detect h;s eastward progress by observing his change of position with reference to the fixed stars, for a few evenings only ; and by marking his place occa- sionally for two years, we may track him quite around the heavens. THE ASTEROIDS. 175. The Asteroids are invisible except through tele- scopes, though Yesta was once seen by Schroeter with the naked eye. Few of them present any sensible disks, even under the telescope. They have a pale ash-color, with the exception of Ceres, which is of a reddish hue, resembling Mars. A thin haze or nebulous envelope has been observed around Pallas, supposed to indicate an extensive atmosphere ; but no spots or other phenomena have ever been detected. " On such planets," says Sir John Herschel, " giants might exist; and those animals which on earth require the buoyant power of water to counteract their weight might 174. What said of the eastward motion of Mars ? How detected ? Eate ? 175. Are the asteroids visible to naked eye ? Schroeter ? How appear ■jnder telescope ? Ceres S Pallas ! (Kemarks of Sir John Herschel !) 88 ASTEONOMT. TELESCOFia yiSW OF JUFITEB. there be denizens of the land. A man placed on one of these planets might spring with ease to the bight of 60 feet, and sustain no greater shock in his descent than he does on the earth from leaping a yard." See 66 to 67, and notes. JTJPITEE. 176. To the naked eye, Jupiter appears like a fine bright star of the first magnitude. His apparent di- ameter varies from 30" to 46", according to his distance from the earth. His color is of a pale yellow. " Under a telescope, his ob- latenesB is plainly perceptible (as shown at 135), and his disk is seen to be streak- ed with curious helts, running parallel to his equator, as shown in the cut. 1. The number of belts to be seen upon the disk of Jupiter depends very much upon the power of the instrument through which he is viewed. An ordinary telescope will show tho two main belts, one each side of his equator ; but those of greater power ex- hibit more of these curious appendages. X)r. Herschel once saw his whole disk covered with small belts. 177. These belts sometimes continue without change for months, and at other times break up and change their forms in a few hours. They are quite irregular, both in form and a/pparent density ; as both bright and dark spots appear in them, and their edges are always broken and uneven. They are supposed by some to be openings in the atmosphere of the planet, through which its real body is seen ; while others think they may be clouds^ thrown into parallel strata by the rapid motion of Jupi- ter upon his axis. The spots in the belts are thought to 176. Jupiter to naked oyo? Apparent magnitude? Cause of variation ? Color ? I'igure ? Belts ? (Number of belts ? Ordinarily ? As seen by Hersclicl ? What view in tiie out ?) 177. Are these belts permanent and regvlar ? What supposed to be ! What said of spots in the belts ? What ascertained by observing spots ' (What said in note 7) sATUKir. 89 be caverns or mountains, or, at least, something perma- nently attached to the body of the planet. It was by watching these that the rotation of the planet upon hia axis was ascertained. One of these spots, first observed in 1665, disappeared, and reappeared rc^larly in the same form for more than forty years; showing conclusively that it was something per- manent, and not a mere atmospherical phenomenon. 178. In examining Jupiter with a telescope, frona one to four small stars will be seen near, h,im, which, on examination, will be found to accompany him in hia eastward journey around the heavens, and to revolve statedly around him. These are the moons of Jupiter, of which we shall speak more fully under the head of Secondary Planets. The writer once saw all four of these satellites at once, and very distinctly, through a common ship telescope, worth only twelve or fifteen dollars. They were first seen by Galileo with a telescope, the object-glass of which was only one inch in diameter! If the student can get hold of any such instrument whatever, let him try it upon Jupi- ter, and sec if he cannot see from one to four small stars near him, that will occupy different positions at diiferent times. 179. As the periodic time of Jupiter is 11 years 317 days (71), his rate of motion eastward through the fixed stare is about 30° a year. Still, this motion can soon be detected, and in 12 years we may watch his progress quite around the heavens. The writer has watched this planet from the constellation Aries, west of the " Seven Stars," till he passed that group, and onward through a , n, o, &c., to 'i% his present po- sition (1853). In five years (1858) he will getaround to Aries again, where lie was seen m 1846 ; and thenceforward will perform the same journey again every twelve years, SATUEN. 180. This planet. is plainly visible to the naked eye, appearing like a star of the third magnitude, of a pale bluish tint. His average angular diameter is about 18". By the aid of the telescope, he is found not only to be oblate, and striped with belts, and attended by satellites Jike Jupiter, but to be encircled by a suite of gorgeous rings, which renders him one of the most interesting objects in all the heavens. 178. What else discovered about Jupiter ? What are tliey ? (Eemark in note ?) 179. Jupiter's rate of motion eastward ? Is it easily^etected ? (Eemark mnote? where was Jupiter in 1846 ? InlSoS? Where «om.^ Wherein 1858 ?) 180. Natural appearance of Saturn? Angular diameter? Appearance through telescope? 90 ASTBONOMT. 181. The oblateness of Saturn (15) is distinctly vip ble through good telescopes (as shown in the cut), while the body of the planet is of a Uad color, and the rings of a nilvery white. They may be compared to concen- tric circles (18) cut out of a sheet of tin. They are broad, flat, and thin, and are placed one within the other directly over the equator of the planet, and revolve with him about his axis, in the same direction, and in the same time (128). They are estimated to be about 100 miles in thickness. 182. These rings are solid matter, like the body of the planet. This is proved by the fact that they invariably cast a strong shadow themselves upon the body of the planet, and frequentl}' exhibit the planet's shadow very distinctly upon tiieir own surfaces. It is also evident that they are wholly detached from the planet, as the fixed stars in the distant heavens beyond have been seen through the opening in the rings, and between the planet and the first ring. The a^oining cut is an ex- eellent representation of Sa- turn as seen througli a tele- ncope. Tlie oblateness of the planet is easily perceptible, and his shadow can be seen upon the rings back of the planet The shadow of the rings may also be seen run- ning across his disk. The writer has often seen the opening between the body of the planet and the interior ring as distinctly as it appears to the student in the cut. Un- der very powerful telescopes, these rings are found to be again subdivided into an indefinite number of concentric circles, one within the other, though this Is considered doubtfiil by Sir John HersclieL y/ 183. As our view of the rings of Saturn is generally an oblique one, they usually appear elliptical, and never circular. The ellipse seems to contract for about 1\ j^ears, till it almost entirely disappears, when it begins 181. Oblateness ? Color ? Kings — ^what like ? How situated ? Wiiat motion 2 Thickness ? 182. What said of subttoMce of the rings ? What proof? What evidence that tliey are detached ? (Remark of author as to seeing satellites ? Ee- speeting rings ? Opinion of Hersehel ?) 183. What the general apparent figure of tlie rings? Why elliptical i TBLESCOPIO VIEW OF SATUEH. SATHEK. 91 to expand again, and continues to enlarge for 7^ years, when it reaches its maximum of expansion, and again begins to contract. For fifteen years, the part of the rings toward us seems to be thrown wp, while for the next fifteen it appears to drop helow the apparent center of the planet; and while shifting from one extreme to the other, the rings become almost invisible, appearing only as a faint line of light running from the planet in opposite directions. The rings vary also in their inclina- tion, sometimes dipping to the right, left. and at others to the TELEBCOPIO PHASES O? THE BINOS OF SATUBN. The aboTe is a good Tepresentation of the various inclinations and degrees of expan- sion of the rings of Saturn, during his periodic journey of 80 years. 184. The rings of the planet are always directed more or less toward tue eartii, peependioulae view opthe eikgs of satubit. and sometimes exactly toward us ; so that we never see them perpen- dicularly, but always either exactly edge- wise, or obliquely, as shown in the last figure. Were either pole of the planet exactly toward us, we should then have a perpendicular view of the rings, as shown in the adjoining cut. 185. The various phases of Saturn's rings are ex- plained by the facts that his axis remains parallel to it- self (see following cut), with a uniform inclination to the What periodic variation of expansion ? Of inclination ? When nearly in- visible! 184. How are the rings situated with respect to the earth % How would they appear if either poS of Saturn were toward us ? 92 ASTRONOMY. •plane of his orbit (122), whicli is very near tlie ecliptic (108) ; and as the rings revolve over his equator, and at right angles with his axis, they also remain parallel to themselves. The revolution of the planet abont the earth every 30 years (72) must therefore bring first one side of the rings to view, and then the other — causing all the variations of expansion, position, and inclination which thQ rings present, BATirBN AT DIFFEKEHT POINTS US HIS OBBIT. 1. Hore observe, first, that the axis of Satnrn, like those of all the other planets, remains permanent, or parallel wWi itself; and as the rin^ are in the plane of his equator, and at right angles with his axis, they also must remain parallel to themselvejs whatever position the pianet may occupy in its orbit 2. This being the case, it is obvious tnat while the planet is passing from A to E, the sm will shine upon the under or south side of the I'ings; ana while he passes from £ to A again, npon the upper or north side; and as it requires about 30 years for the planet to traverse these two semicircles, it is plain that the alternate day and night on the rings will he 15 years each. 8, A and E are the eguinoctial^ and C and G the solstitial points in the orbit of Saturn. At A and E the ringsare edgewise toward the sun, and also toward the earth, provided Saturn is in opposition to the sun. To an observer on the earth, the rin^ will seem to expand from A to C, and to contract fi'om to E. So, also, from E to G, and from G to A. Again : from A to E the front of the rings will appear a^ove the planet's center, and from JE to A below it 4. The rings of Satm-n were invisible, as rings, from the 22d of April, 1S48, to the 19th of January, 1849. He came to his equinox September T, 1S4S ; from which time to February, 1S56, his rings will continue to expand. From that time to June, 1S68, they will contract, when he will reach his other equinox at E, and the rings will be invisible. From June, 1863, to September, 1870, they will again expand ; and from September, 1870, to March, 1S77, they will contract^ when he will be at the equinox passed Septem- ber 7, 1848, or 29^ years before. 5. Tlie writer has often seen the rings of Saturn in different stages of expansion and contraction, and once when they were almost directly edgewise toward the eai-th. At that time (January, 1849), they appeared as a bright line of light, as represented at A and E, in the above cut 185, What is the cause of these varying phases, &c. ? (Explain by dia- gram. When rings invisible ? When at his equinox ? How long rings ex- pand? Contract? When rings next invisible? Expansion again? Con- traction ? At what point then ? Author's observations ?) SATtlEN. 93 186. The dimensions of the rings of Saturn may be stated in round numbers as follows : Miles, Distance from the body of the planet to the first ring 19,000 Widthof interior ring 17,000 Space between the interior and exterior rings 2,000 Width of exterior ring 10,500 Thickness of the rings 100 These statistics, as given tiy Sir Jplin Herschel, are as follows : Exterior diameter of exterior ring 40"'095 := 176,418 miles. Interior do 8iy'-2S9 = 155.2T2 " Exterior diameter of interior ring 84"'475 = 161,690 Interior do 26" -668 = 117,839 £qu.ttorial diameter of the body 17" '991 = 79,U'ii) Interval between the planet ana interior ring 4"'839 = 19,090 Interval of the rings 0"'408= 1,791 Thickness of the rings not exceeding 250 187. The rings of Saturn serve as reflectors to reflect the light of the sun upon his disk, as our moon reflects the light to the earth. In his nocturnal sky, they must appear like two gorgeous arches of light, bright as the full moon, and spanning the whole heavens like a stupen- dous rainbow. In the annexed cut, the beholder is supposed to be situated some 30° north of the equator of Saturn, and looking directly south. The shad- aw of the planet is seen travelling: up the arch as the night advances, while a Mew Moon is shown in the west, and a Fuft Moon in the east at the same time. 188. The two rings united are nearly 13 times as wide as the diameter of the moon ; and the nearest is only y^^th as far from the planet as the moon is from us. 1. The two rings united are 27,500 miles wide ; which -;- 2,100 the moon^s diame- ter r=7 12 A . So 240,000 miles, the moon's distance -r- 19,000 the distance of Saturn's in- terior ring = 12 1-^. 2. At the distance of only 19,000 miles, our moon would appear some forty times as brge as she does at her present distance. How magniiicont and inconceivably grand, then, must these vast rings appear, with a thousand times the moon's magnitude, and only one-twelfth part of her distance 1 186. State the distances and dimensions of his rings, beginning at the hody of the planet, and passing outward. (Wliat additional statistics from Iler- bchel?) 187. What purpose do the rings of Saturn serve ? How appear in his evening sky ? 188. Width of two rings, as compared with moon ? Distance ? (Demon- strate botli. How would onrmoou appear at tlie distance of Saturn's rings 3'' NIGHT SCENE UPON SATUBN. 94 ASTRONOMY. 189. Besides the magnificent rings already described, the telescope reveals eight satellites or moons, revolving around Saturn. But these are seen only with good in- struments, and under favorable circumstances. On one occasion, the writer saw fire of them at onco, with a six-inch refractor mann- bctnred by Mr. Henry Fitz, of New York ; bnt the remaining three he haa never seen. For a further description of these satellites, see chapters on the Secondary Planets. 190. The periodic time of Saturn being nearly 30 years (72), his motion eastward among the stars must be very slow, amounting to only 12° a year, or one sign in 2^ years. It will be easy, therefore, having once ascer- tained his position, to watch his slow progress eastward year after year. Saturn is now (October, 1852) about 15° west of the seven stars, and consequently will pass them eastward early in 1854. UEANTJS. 191. Uranus is scarcely ever visible except through n telescope; and even then we see nothing but a small round uniformly illuminated disk, without rings, belte, or discernible spots. His apparent diameter is about 4", from which he never varies much, owing to the small- ness of our orbit in comparison with his own. Sir John Hersohel says he is without discernible spots, and yet in his tables he lays down the time of the planef s rotation (which could only be ascertained by the rotation of spots upon the planet's disk), at 0^ hours (12S). This time is probably given ijn tlte authority of Bchroeter, and is marked as doubtful by Dr. HerscheL 192. The motion of Uranus in longitude is still slower than that of Saturn. His periodic time being 84 years 27 days, his eastward motion can amount to only about 4° 17' in a whole je&v. To detect this motion requires instruments and close observations. At this date (185.3) Uranus has passed over about \ of his orbit, since his discovery in 1781 ; and in 1865_will have traversed the whole circuit of the heavens, and. reached the point where Herschel found him 84 years before. 189. What else seen about Saturn? When seen? (Observations of the autlior.) 190. Motion of Saturn eastward ? Eate ? 191. Hovi' Uranus seen? How appear through telescopes? Apparent diameter ? Why so small, when so miteh larger than Venus » Why so little variation ? (Eemark respecting spots.) 193. What said of Uranus' apparent motion? Eate" per year? In 185S, how far since discovered ? When made a complete revolution since 1781 ? THE SOLAE SYSTEM IN MINIATUEE. 96 193. Uranus is attended by several satellites — four at least, probably five or six. sir William Herschel reckoned six, though no other observer has conflruied this opinion : and even his son, Sii' John Herschel, seems to consider the existence of si:E satellites quite doubtful NBPTDNB. 194r. Neptune is a purely telescopic planet, and his immense distance seems to preclude all hope of our coming at much knowledge of his physical state. A single satellite has been discovered in attendance upon him, and the existence of another is suspected ; but il others exist, they are as yet undetected. 195. On the 3d of October, 1846, Mr. Lassell, of Liverpool, England, supposed he had discovered a ring about the planet, similar to the rings of Saturn ; but this supposition has not yet been confirmed by the observa- tions of other astronomers. 196. The periodic time of Neptune being 164 years 226 days, his motion in longitude amounts to only about 2° 10' per year; and yet this slow motion of about 21" per day is easily detected, in a short time, by the aid of the proper instruments. It is by this motion, as well as by the disk which it exhibits under the telescope, that the object was first distinguished from the fixed stars, and recognized as a planet. THE SOLAE SYSTEM IN MINIATITEE. 197. Choose any level field or bowling-green, and in its center place a globe two feet in diameter, to represent the sun. Mercury may then be represented by a mus- tard-seed, at the distance of 82 feet ; Venus by a, pea, at the distance of 142 feet ; the earth also by a pea, at the distance of 215 feet. A large pin's head would repre- sent Mars, if placed 327 feet distant ; and the Asteroids may be represented by grains of sand, ^>?m ^00 to 600 193. Attendants of Uranus ? How many ? (Keniark m note !) 194. How Neptune seen ? What attendant? Sus.i»ii!i«.u! 195. Supposition of Lassell ? Is it confirmed ? 196. Motion of Neptune per year ? Why so slow ? (^-- "t hp WIST£B BAYB. the rays of light fall wpon it — a circumstance that greatly affects the amount of light received by any given por- tion of its surface. Hence we have summer in the northern hemisphere in July, "when the earth is farthest from the sun ; and winter in January, when she is near- est the sun (144). Though nearer the sun in January than in July, still, as the northern hemisphere is then inclined from the sun, his rays strike its surface obliquely ; less light falls upon the same space than if its contact was more direct, and it is consequently cold. But in July, the rays are more direct — -the northern hemisphere being inclined toward the sun — and it is summer, notwithstanding we are three millions of miles further from the sun than in January. 1. The comparative amount of light received in the northern hemisphere in July and January may be illustrated by the accompany- ing figure, in which the rays of light at dif- ferent seasons are represented to the eye. In January, they are seen to strike the northern hemisphere obliquely, and consequently the same amount of light is spread over a much greater sur- face. In July, the rays fall almost perpendicularly upon us, and are much more intense. Hence the variations of temperature which constitute the seasons. 2. If the student is not perfectly clear as to Twv) the north pole is turned first tmnard and then/com the sun, he will need to be guarded against the vulgar idea that the earth's axis " wabbles," fis it is called. By consulting 119 to 121, and the cuts, it will be seen that the very permanency of a plan- et's axis, combined with its periodic revolution, gives the beautiful and ever welcome ciianges of the seasons. How simple, and yet how elfcctual, this Divine mechanism I 201. As the inclination of the axis of a planet and the length of its periodic time determine the extent and character of its zones, and the length of its seasons, it follows that where these are known, we have a reliable clew to the seasons of a planet, even though we have neither visited nor heard from it ; and as we do not know the inclination of the axis of Mercury, we have no knowledge of his seasons. zoo. What governs the general temperature of the planets ? The tem- perature of particular zones ? What result from this last ? Why not warm- est in January, &o. ? (Illustrate by diagram.) How are the poles shifted to and/roTO the sun ? Do the poles "wabble I" , , o 201. How ascertain the character of the seasons of distant planets 1 faea- Bons of Mercury ? 5 98 ASTRONOMY. 202. The seasons of Yenus are yery remarkable. So great is her inclination (122), that her tropics tall within 16° of her poles, and her polar circles (as if to retaliate for Ihe trespass upon their territory), go up to within 15° of her equator. Thus the torrid and frigid zones over- lap each other, and the temperate zone is altogether annihilated. The period of Venus being but 225 days (T2), the sun declines in that time from her equinoctial to within 15° of one pole ; then back to the equinoctial, and to within 15° of the other pole, and again back to the equinoctial. The effect of this very great inclination is to give eight seasons at her equator every 225 days. In her short period of 225 days, the sun seems to pass A-om her nortlicm solstice through her equioox to her southern solstice, and back to the point from which he started. "When he is over one of her tropics, it is winter not only at the other tropic, but also at her equator ; and as the sun passes over from tropic to tr&pic, and back again every 225 days, making spring at the equator as he approaches it, summer as he pa:?se9 over it, autumn as he declines f^om it, and winter when he reaches the tropic, it "feUgws that at her equator Venus has etgla seasons in one of her years, or in 225 of our da^fc-' Her seasons, therefore, at her equator, consist of only about four weeks of our time, or 2S^ days ; and, from the heat of suoitner to the cold of winter, can be only about 56 days. At her tropics, she has only four seasons of 56 days each. 203. The polar inclina^vn^ Mm-s being 28° 40' (122), his torrid zone must be ^° lO' from his poles — ^leaving only 32° 40' for the width of his temperate zone. But as his year consists of 68T days, his four seasons must consist of about 172 days each, or nearly twice the length of the seasons of our globe. 204. So slight is the inclination of the axis of Jupiter to his orbit, that he has but a narrow torrid zone, and small polar circles. As his orbit departs from the plane of the ecliptic only 1° 46' (108), and his axis is inclined to his orbit only 5° 3', it foftows that his axis is nearly perpendicular to the ecliptic. The sun never departs more than 5° '%' from his equator ; and still, as his peri- odic time is about 12 years (72), he has alternately six years of northern and six of southern declination. His narrow torrid zone and small polar circles leave very ex- 202. Seasons of Venus 3 Where her tropics ? Polar circles ? Temperate zone ! Sun's declination upon her ? Its effect ? (Substance of note f) ) 203. Zones of Mars ! Length of seasons, and why ? / 204. Zones of Jupiter, and why 1 Describe his climate. Seasons 3 Days •nd nights 3 Poles ! " SEASONS OF THE DIFFERENT PLANETS, ETC. 99 tensive temperate zones. In passing from his equator to his poles, we meet every variety of climate, from the warmest to the coldest, with but slight variations in any latitude, from age to age. His days and nights are al- ways nearly of the same length, as the sun is always near his equinoctial. His poles have alternately six years day and six years night. 205. The polar inclination and zones of Satwrn differ but little from those of Mars ; but his seasons are greatly modijBed by the length of his periodic time. This being about 30 years, his four seasons must each be about 7-^ years long ; and his polar regions must have alternately 15 years day and 15 years night. The rings of Saturn, which lie in the plane of his equator, and revolve every 10} hours, are crossed by the sun when he crosses the equinoctial of the planet. During the southern declina- tion of the sun, which lasts fifteen years, the south side of the rings is enlightened, and has its summer. It has also its day and nigbt, by revolving in a portion of the planet's shadow. W hen the sun is at the southern tropic, it is midsummer on the south side of the rings, as the rays of light then fall most directly upon them. As the sun approaches the equator, the temperature .decreases, till he crosses the equinoctial, and the long winter of fit- teen years begins. At the same time, the north side of the rings begins to have its spring ; summer ensues, and in turn it has fifteen years of light and heat. The influ- ence of these wonderful rings iipon the climate of Saturn must be very considerable. During the winter in each hemisphere, they cast a deep shadow upon some p6rtion of his surface during the day ; and in the summer, these immense reflectors so near the planet, and so bright in the sunlight, must contribute greatly to the light, if not to the warmth, of his summer evehings. The poles of Saturn are alternately 15 years in the light, and 15 years in darkness. 206. Of the inclination of the axes of Uranus and 205. Zones of Saturn, and why 3 Length of seasons ? Rings — how en- lightened ? Influence upon climate ? Polar days and nights ! 100 ASTEONOMT. Neptune, respectively, we have no knowledge, and con sequently can form no opinion respecting their tropics, polar circles, zones, &c. - If not too much inclined, like Venus, they have but four seasons in thevr yea/r, which would make each season of Uranus 22 years and 9 days long, and each season of Neptune 41 years and 56|^ days long ; as these periods are, respectively, one-fourth of the periodic time of the planet (72). Tbns we see that tropics, polar circles, zones, and seasons are not peculiar to our globe, bat areanecessary result of an inclined axis, and a revolntion aronnd the sun. Tho cam^es which produce our seasons are known to be in operation in other planetary worlds, and it would be unreasonable to deny that tho effect was there also, DISCOVEKT OF THE DIFFERENT PLANETS. 207. The old planets, as they are called, viz., Merowry, Yenus^ Ma/rs, Jwpit&r^ and Saturn, have been known as planets, or " wanderers," from the earliest ages. Ura/nus was discovered by Sir William Herschel, March 13th, 1781. Ifeptvme was demonstrated to exist iefore it had been seen, by M. Le Verrier, of France, August, 1846 ; and first seen by Dr. Galle, of Berlin, Sept. 23, 1846. 208. The discovery of Neptune is probably one of the greatest achievements of mathematical science ever recorded. By comparing the true places of Uranus with the places assigned by the tables, it was found that he was not where his known rate of motion required him to be ; and after making all due allowance for the attraction of Jupiter and Saturn (65), by which pertur- bations would be produced, it was found that there was evidently the effect of some other body, exteriar to the orbit-of Uranus, the attraction of which body helped to cause the perturbations of Uranus. From this effect, produced by an unknown and invisible world, lying far Dut beyond the supposed boundaries of the solar system, not only was the existence of its cause demonstrated, but its direction, distance, mass, and period were proxi- mately ascertained. 205. What said of the seasons of Uraiius and Neptune ? Probable length of former 3 Latter ? (Remark in note ?) 20r. What said of the " old planets ?" Of Uranus? Neptune? 208. Describe the discovery of Neptune. Perturbations ? Tables, &c. ? (Describe snccessive steps in detail, what said of Mr. Adams 9) DISCOVERT OF THE DIFFERENT PLANETS. 101 1. On the evening pf the 28d of September, 1S46, Dr. G-alle, one of the astronoznerfl of the Boyal Observatory at Berlin, received a letter from Le Terrier, of Paris, request- ing him to employ the great telescope at his command in searching for the supposed new planet, ana giving its position, as ascertained by calculation, as 825° 52'8' of geocen- tric longitude. Dr. Galle, taking advantage of the very evening on which he received Le Terrier's letter, soon discovered an object resembling a stjir of the eighth magnitude, near the spot indicated by Le Terrier, as the place of the new planet On consulting an accurate star chart, it was found that no such star was there laid down, and observations were at once commenced, with a view to detecting any change of place. In three hours time, it was seen to have moved ; and by the next evening at eight o'clock, it was found to have retrograded more than four seconds of time (see 97 and cut) — a circumstance which proved it to be much nearer the earth than the fixed stars, and consequently a planet — the very planet which had caused the unaccountable irregularities of Uranus. The geocentric longitude of the planet, at midnight-, September 28, 1846, was 325° 52"8' ; which was less than 1° from the place assigned to it by Le Terrier ! The reason why Le Terrier wrote to Dr. Galle was, that the former haid no suitable telescope for con- ducting the search in which he was so deeply interested. 2. It is worthy of remark that Mr. Adams, of St John's College, Cambridge. Eng- land, had also calculated the place, &c., of the new planet, and had arrived at results similar to those reached by Le Terrier; but as the latter h&Apublialbed his conclusions first, the honor of the discovery is generally accorded to Le Terrier. 209. The Asteroids have all been discoTered daring the present century, and most of them since 1847. And to the number now known, it is not improbable that others, will be added from time to time. The following table will show the date, Ac, of the discovery of the several astcroi f object which is more remote. The moon at A fills the same angle that is ! • filled by the sun at B. • ; 2, This fact may serve to illustrate the comparative influence of things t \ present and future upon most minds. The little moon may eclipse the sun ; ! j or even a dime, if held near enough to the eye, will completely hide all his ■ ! glories from our view. So in naorals and religion. The " things which are ■ : Been and temporal" are too apt to hide ftom our view the more distant but \ \ superior glories of the life to come. ; , 220. The density of the moon is only about j j two-thirds that of the earth, and her swrface xjtli ; i as great. The light reflected to the earth by her, ! j at her full, is only g^^^o o ■ott P^rt as much as we i i receive on an average from the sun. ij 221. The daily apparent revolution of the ■^'j: moon is from east to west, with the sun and .' ^^\ stars ; but her real motion around the earth is ^> ^ ■■' from west to east. Hence, when first seen as a " new moon," she is very near, but just east of the sun ; but departs further and further from him eastward, till at length she is seen in the east as a full moon, as the sun goes down in the west. ^2. The moon performs a sidereal revolution around the earth in 27d. Th. 4:3m. ; and a synodio in 29d. 12h. 44m. The sidereal is a complete revolution, as measured by a fixed star ; but the motion of. the earth eastward in 219. Moon's diameter, as compared with that of the sun' With sun's apparent diameter ? Why appear so near of a size ? (Illustrate by diagram. Eellection of the author?) 220. Density of the moon ? Her light? 221. Her daily apparent motion ? Eeal motion? How traced? 222. What is her sidereal revolution ? Her synodio ? What difference ? Why? (Illustrate by diagram.) 5* 106 ASTKONOMT. her orbit gives the sun an apparent motion eastward among the stars (119), and renders it necessary for the moon to perform a little more than a complete revolution each month, in order to come in conjunction with the sun, and make a synodic revolution. SISEBXAJ. AKD STHODIO SSTOLtmOHB OF THE UOOK, f^\ SIDEREAL REVOLUTION 27J-0AYS B/* "~ :~- %-:::^W^ , o9toff(-?. C'-. V; ^.vo^:?!?^-^ •■■■..X.y s^NO?!?. SUN_AND Moon IN COMJUNCT ION-MEW MOON,' rfcii. 1 -^, mo : k:^ y DAILY PfiOGRBSS OP THE MOON EASTWARD. 1. On the right, the earth is shown in her orbit, revolving around the sun, and thd moon in her orbit, revolving around the earth. At A, the sun and moon are in con- jttncHon, or it is 2^ew Moon. As the earth passes from D to E, the moon passes around from A to B, or the exact point In her orbit where she was 27J days before. But she is still west of the sun, and must pass on from B to C, or 1 day and 30 hours longer, before she can again come in conjunction with him. This 1 day and 20 hours constitutes the difference between a sidereal and a synodic revolution. 2. The student will perceive that the difference between a sidereal and synodic revo- lution of the moon, like that between solar and sidereal time, is due to the same cause- namely, the revolution of the earth around the sun. Bee 135. 223. The daily angular motion of the moon east- ward is 13° 10' 35". Her average hourly motion is about 32,300 miles. This motion may be detected by watching her for a few hours only ; and by mark- ing her position, with ref- ence to the stars, from night to night, her daily journeys will appear .pro- minent and striking. The estimate of 18° 10' 85" is made for a sidereal day of twenty-four liomrs. In the ahove cut, the daily progress of the moon may be traced from her conjunction or " change" at A on the right, aroond to the same point again. This being a sidereal revolution, requires only 27^ days. 223. Daily angular motion eastward ? How detected 3 (For what day is this estimate made ?) THE MOON. 107 224. In her journeyings eastward, the moon often seems to run over and obscure the distant planets and stars. This phenomenon is called an oo- cultation. The adjoining cut represents the new moon as just about to obscure a distant star, by passing be- tween us and it In 1850, she occulted Jupiter for three revolutions in succession— viz., Jan. 3.0th, Feb. 27th, and March 26th. Through a telescope, the moon is seen to be constantly obscuring stars that are invisible to the naked eye. They disappear be- hind the moon's eastern limb, and in a short time reappear from behind her western ; thus distinctly exhibiting her eastward motion. 225. Though the moon's orbit is an ellipse, with res- pect to the earth, it is, in reality, an irregular curve, always coTica/oe toward the sun^ and crossing the earth's orbit ■?'^^-,iW""*-. every 13° nearly. --k^^'* * ®'k'- . 1. If the earth stood still in her orbit, the ( ® ® ) moon would describe just such a path in the .'-''f'j^ •■-^'■■' • ecliptic as she describes with respect to the /,«» ^\ earth. 2. If the earth moved bat slowly on her way, the moon would actually retrograde on the eclip- tic at the time of her change, and would cross her own path at every revolution, as shown in tlie adjoining figure. But as the eM*th advances some 46 millions of miles, or near 100 times the iiameter of the moon's orbit, during a single lu- nation, it is evident that the moon's orbit uever can return into itself, or retrograde, as here rep- resented. 5 MOON 8 OltBIT ALWAYS CONOATB TOWARD THE SUN. B-^ A_ 8. That the lunar orbit is always concave toward the sun, may be demonstrated by the above diagram. Let the upper curve line A B represent an arc of the earth**s orbit, equal to that passed through by the earth during half a lunation. Now the radius and arc being known, it is found that the chord A B must pass more than 400,000 miles within the earth. But as the moon departs only 240,000 from the earth, as shown in the figure, it follows that she must describe the curve denoted by the middle line« which is concave toward the sun. 224. What are occultaUons ? How produced ? (Are they frequent ? Are planets ever peoulted? Describe process.) 225. What'is the form of the moou's orbit with respect to the 6arth ? The sun ? (Ho'j*^ if the earth were stationary ? If moving slowly ? Domons strate her orbit to be concave, &c. Draw orbit for complete lunation, ano^ describe her relative motion.) 108 ASTEONOMT. 4. This subject may be still farther illustrated by the following out, representing THE MOON'S PATH DUBIXG A COMPLETE LUNATION. C B MOON'S PATH. Here the plain line represents the earth^s orbit, and the dotted one that of the moun At A the moon crosses the earth's track 240,000 miles hehind her. She gains on the earth, till in seven days she passes her at B as a FvU Moon, Continuing to gain on the earth, she crosses her orbit at C, 240,000 miles ahead of her, being then at her Third Quarter. From this point the earth ^ins upon the moon, till seven days afterwar,d she overtakes her at D as a JHfew Moon. From D to E the earth continues to gain, till at E the moon crosses 240,000 dehind the earth, as she had done four weeks before at A. Thus the moon winds her way along, first within and then without the earth ; always gaining upon us when outside of our orbit, and falling behind us when within it. 5. The small circles in the cut represent the moon^s orbit with respect to the earth, which is as regular to us as if the earth had no revolution around the sun. 226, The Kioon never retrogrades on the ecliptic, or returns into her own path again ; but is always ad- vancing with the earthj at the rate of not less than 65,700 miles per hour. 1. The moon's orbitual velocity, with respect to the earth, is about 2,800 miles per hour. When out- side the earth, as at B, in the last figure, she gains 2,300 miles per hour, which, added to the earth's ve- locity, would give 70,300 miles as the hourly velocity of the moon. When toitMn the earth's orbit, as at D, she loses 2,300 miles per hour, which, subtracted from 68,000 miles (the earth's hourly velocity), would leave 65,700 miles as the slowest motion of the moon in space, even when she is falling behind the earth. ' 2. Could we look do^-n perpendicularly upon the ecliptic, and see the paths of the earth and moon. we should see the latter pursuing her serpentine course, first within and then outside our globoj somewhat as represented by the dotted line in the annexed figure. Her path, however, would be concave toward the sun, as shown on the preceding page, and not convex, as we were obliged lo represent it hero In so small a diagram. / 227. That the moon is opake, like the rest of the plan- /ets, and shines only by reflection, is obvious, from the* fact that we can see only that part of her upon which the sun shines ; and as the enlightened portion is some- times toward and sometimes from us, the moon is con- stantly varying in her apparent form and brightness. These variations are called her phases, 226. At what rate does the moon advance with the earth ? Moon*s or- bitual velocity, with respect to the earth ? Slowest motion ? (Ulustrate the moon's course.) 227. What proof that the moon is opako ? What meant by her phases t THE MOON. 109 228. The cause of the moon's phases — ^her waxing and waning — is her revolution around the earth, which ena- bles us to see more of her enlightened side at one time than at another. OATTBB OF TIDE MOON^S FHASSa €^- FIRSTpR. . €' €>--::>€ GIBSOUS ^- f/ ^A ^^1-^-^ "^ \i \ \ oh^S:, ^^ ^,-— OBBOUSf ) Gl ■) / 4 \ 1. This cut represents the ii.oon revolving eastward aroond the earth. In the outside circle, she is represented as she would appear, if viewed from a direction at right angles with the plane of her orbit. The side toward the sun is enlightened in every case, and she appears like a half moon at every point. 2. The interior suit represents her as she appears when viewed from the earth. At A it is New Moon ; and if seen at all so near the sun, she would appear like a dark globe. At B she would appear like a crescent, concave toward the east At C, more of her enlightened side is visible; atD, still more; and atE, the enlightened hemisphere is fully in view. We then call her a J?'«^^ Jtfofwi. From E around to A again, the dark portion becomes more and more visible, as the luminous part goes out of view, till she comes to her change at A. When at D and F, the moon is said to be gibbous. S. If the student will turn his book bottom upward, and hold it south of him, he will see wJli/ the crescent of the old moon at H is concave on the west, instead of the east, like the new moon, and why she is seen before sunrise, instead of just after sunset. 229. The cusps of the moon are the extremities of the crescent. Her syzygies are two points in her orbit 19<5° apart, where she is new and full moon. (See positions 1 and 3 in the last cut.) The quad/ratwres are four points 90° apart (like 1, 2, 3, and 4 in cut) ; and her octants eight points 45° apart (like A, B, C, &c., in the cut). 230. The moon is said to change when she comes in conjunction with the sun, and is changed from Old Moon to iSfew Moon. 228. Cause of phases ? (Illustrate.) 229. What are the ^ €. 'JC .t)' 234. What are these rude figures supposed to be ? (Note.) 285. What interesting fact established by watching the moon ! What Celow the snn at new moon, as represented in the figure, and her shadow may fall above the north pole orbelo^tbe south. At such times, then, there can be no solar eclipse. 2. On the right, the mqon is shown at her fall, both above and below the earth's shadow, in which case there can be no lunar eclipse. was largest? If brought nearer ? How if planets smallest ? How affected by distance? (How, then, 'with planets nearest the sun? More remote' Does any primary throw its shadow out to the next eyterior planet ?) *249. At what time of the moon do solar eclipses always occur ? Lunar * Why? 250. Why not two eclipses every lunar mouth? (lUuBtrate.) ECLIPSES OF THE SUN AITD MOON. 119 LUHAB EOLIPSB. SOLAS ECLIPSE. 251. Eclipses of the sun always come on from the west, and pass over eastward ; while eclipses of the moon come on from the east, and pass over westward. This is a necessary result of the eastward motion of the moon in her orbit. 1. In the right hand cut, the moon is seen re- volving eastward, throwing her shadow upon the earth, and hiding the western limb of the sun. In some instances, however, when the eclipse is very slight, it may first appear ou the northern or southern limb of tlie sun — that is, the upper or lower side; but even then its dii'echon must be from west to east It will also be obvious from this figure, that the shad- ow of the moon upon the earth must also trav- erse her surfece from west to east; conse- quently the eclipse will be visible earlier in the west than in the east 2. On the left, the moon is seen strilting into the eaifth's shadow from the west, and having her eastern limb first obscured. By holding the book up south of him, the student will see at once why the revolution of the moon east- ward must cause a solar eclipse to proceed from west to east, and a lunar eclipse from east to west To locate objects and motions correctly, the student should generally imagine himself looking to the south, as we are situated north of the equinoctial. The student should bear in mind that nearly all the cuts in the book are drawn to represent a view from northern lati- tude upon the earth. Hence by holding the book Qp south of him, the cuts will generally afford an accurate illustration both of the posi- tions and motions of the bodies represented. 252. Eclipses can never take place, 'except when the moon is near the ecliptic ; or, in other words, at or near one of her nodes. At all other times, she passes above or below the sun, and also above or below the earth's shadow. It is not necessary that she should be exactly at hei node, in order that an eclipse occur. If she is within 17° of her node at the time of her change, she will eclipse the sun ; and if within 12° of her node at her fall, she will strike into the earth's shadow, and be more or less eclipsed. These distances are called, respectively, the solar and lunar ediptic limits. 251. What is the direction of a solar eclipse? A lunar? Why this dif ference ? 252. Where must the moon be, with respect to the ecliptic and her nodes, in order to an eclipse ? What meant by ecli2>tic limits ? Name the distance of each, respectively, from the node. (Illustrate.) L20 ASTEONOMY. Tliis sabject may be onderstood hy consalting the following flgnre : THE UOON CRASGISQ AT DIFFEBENT DIBTANOEB FBOU HES H0DB9. 1. Let the line EE represent the ecliptic, and the line the plane of the moon's orbit The light globes are the sun, and the dark ones the moon, which may be iniag- (ned ae much nearer the stadeut; hence their apparent diameter is the same. 2. Let the point A represent fJie node of the moon's orbit Now if the change occur when the moon is at B, she will pass below the sun. If when at C, she will just touch his lower limb. At C, she will eclipse him a little, and so on to A ; at which point, if the change occurs, the eclipse would be central, and probably total 3. If the moon was at G, H, I, or J, in her orbit, when the change occurred, she would eclipse the upper or ncH-thern limb of the son, according to her distance from her node at the time ; but if she was at K, she would pass above the sun, and would not eclipse him at all The points C and J will represent the Solar ScUptic lArmts. 253. All parts of a planet's shadow ar6 not alike dense. The darkest portion is called the urnhra^ and the partial shadow \kiQ penvmbra. UMBBA A2TD PEKUMBBA OF THE EABTH AND UOON. Penwmbra is from the Latin -Dwie, almost, and -MOT&ra, a shadow. In this cut the earth's umbra and penumbra will be readily found by the lettering ; while A is the um- bra, and B B tlie penumbra, of the moon. The iatter is more broad than it should be, owing to the nearness of the sun in the cut as it never extends to much over half the earth's diameter. The student will see at once that solar eclipses can be total only to persons within the umbra; while to all on which the penumbra fells, a jwrtion of the sun's disk will be obscured. 25-i. The average length of the earth's umbra is about 860,000 miles ; and its hreadth^ at the distance of the moon, is about 6,000 miles, or three times the moon's diame^r. As both the earth and moon revolve in elliptical orbits, both the above estimates are subject to variations. The length of the earth's umbra varies from S42,21T to 871,262 miles ; and its diameter T7here the moon passes it varies from 5,2S5 to 6,365 miles. ^ 255. The average length of the moon's umbra is about 239,000 miles. It varies from 221,148 to 252,638 miles, _ 253. What is iheumbra of the earth or moon? The penimibra ? (Deriva- tion ? Within which are solar eclipses total ?) a54. The average length of the earth's shadow ? Breadth at the moon'i« distance ? (Do they vary? Why?) ECLIPSES OP THE SUN AND MOON. 121 according to the moon's distance from the sun. • Its greatest diameter, at the c^istance of the earth, is 170 miles ; but the jpenumbra may cover a space on the earth's surface 4,393 miles in diameter. 256. "When the moon but just touches the limb of the sun, or the umbra of the earth, it is called an appulse. (See D and G, in the first cut on the opposite page.) A partial eclipse is one in which only part of the sun or moon is obscured. A solar eclipse is partial to all places outside the umbra ; but within the penumbra, where the whole disk is obscured, the eclipse is said to be total. A central eclipse is one taking place when the moon is exactly at one of her nodes. If lunar, it is total, as the earth's umbra is always broad enough, at the moon's distance, if centrally passed, to obscure her whole disk. But a solar eclipse may be central and not total, as the moon is not always of sufiicient apparent diameter to cover the whole disk of the sun. In that case, the eclipse would be annula/r (from anmil/us, a ring), because the moon only hides the center of the sun, aad leaves a bright ring xmobscured. FBOQKESS OF A CENTRAL EOLIPSS. GtAag oS*. Annular. 257. It has already been shown (5G) that the apparent magnitudes of bodies vary as their distances vary ; and as both the earth and moon revolve in elliptical orbits, it 255. Average length of the moon's umbra ? Does it vary? Why? Great- est diameter at the earth's surface ? ^ Of penumbra ? 256. What is an o^^^ise .^ A _pa/'^wj:Z eclipse? A total? Acenttalf Arc all central eclipses total ? Why not ? What called then ? Why ? 257.. Why are some central eclipses total, and others partial .and .ii'.miliir i (Diagram.) G 123 ASTEONOMT. follows that the moon and svm must both vary in their respective apparent magnitudes. Hence sonie-' central eclipses of the sun are total, while others are partial and annular. VOTAZ. AN1> ANmrLAB EOXJFSES OT rBB SIHT. 1. At A, the earth is at her ctiphsiion^ and the sun beiire at his most dfstaat pornt, wi^i have his le&st apparftHt ma^itude. At the same time, the moon is in perigee, and ap- pears li*rffer-thAa usual. If, therefore, she pass centrally over the sun's disk^ theeclipse will he total. 2. At B, this order is reversed. The earth is at her pert!ieUony £wid the- inmiw in a/pogee; so tlwit the smi appears larger, and the moon smaMer tlian aswal. If. then, a central eclipse occur under these circumstances, the moon will not be largo enongfh to eclipse the whole of the sun, hut will leave a ring, apparently around herselt nnob- Bcured. Such eclipse will he amwdar. /^258. As the solar ecliptic's limits are farther from tli,e 'moon's nodes than the lunar, it results that we have more eclipses of the sun than of the moon. There tnaj be seven in all in one year, viz., five solar and two lunar ; but the most usual number is four. There can never be less than two in a year ; in which case, both must be 'of the sun. Eclipses both of the sun and moon recur in nearly the siame order, and at the same intervals, at the expiration of a cycle of 223 lunations, or 18 years of 365 days and 15 hours. This cycle is called the Period qf the Eclipses. At,, the expiration of this time, the sun and the moon's nodes will sustain the same relation to each other as at the beginning, and a new cycle oi eclipses begins. 259. In a total eclipse of the sun, the heavens are shrouded in darkness, the planets and stars become visi- ble, the tempcKature declines, the animal tribes become agitated, and a general gloom overspreads the landscape. Such were the effects of the great eclipse of 1806. In a lunar eclipse, the moon begins to lose a portion of lier 258. Which kind of eclipses is most frequent ? Why I The greatest number in a year ? How many of each ? Least number, and which ? UsnaJ number 3 What said of the order of eclipses ? Time of cycle ? 259. Describe the effects of a total ecUpse of the sun. The process of a •nnar eclipse t ECLIPSES OF THE SUN AND MOON. 123 light and grows dim, as she enters the earth's penumbra, till at length she comes in contact with the umbra, and the real eclipse begins. 260. In order to measure and record the extent of eclipses, the apparent diameters of the sun and moon are divided into twelve equal parts, called digits ; and in predicting eclipses, astronomers usually state which " Umh" of the body is to be eclipsed — the southern or northern — ^the time of the first contact, of the nearest approach of centers, direction, and number of digits eclipsed. PITB DIGITS ECLIPSED. . TWELVE DIGITS. "261. The last mvnulm' eclipse visible in the United States occuiTed May 26, 1854. The next total eclipse of the sun will be August 7, 1869. ' ' Some of the ancients and all barbarous nations formerly regarded eclipses with amazement and fear, as supernatu- ral events, indicating the displeasure of the gods. Colum- bus is said to have made a very happy use of this supersti- tion. When the inhabitants of St. Domingo refused to allow him to anchor, in 1502, or to furnish him supplies, he told them the Great Spirit was offended at their conduct, and was about to punish them. In proof, he said the moon would be darkened that very night ; for he knew an eclipse was to occur. The artifice led to a speedy and ample supply of his wants. 262. Eclipses can be calculated with the greatest pre- cision, not only for a few years to come, but for centuries 260. How are eclipses measured and recorded 1 261. When the next annular eclipse visible in *liis country ? The next total ? How have the ignorant and superstitious regarded eclipses ? Anec- dote of Columbus ? 124 ABTEONOMY. and ages either past or to come. This fact demonstrates the truth of the Oopernican theory, and ilhistrates the order and stability that everywhere reign throughout the planetary regions. CHAPTER VII. SATELLITES OF THE EXTBRIOK PLANETS TBLESCOPIO VIEWB OF THE MOOI78 OF JUPITBB. 263. JupriEE is attended by four satellites or moons. They are easily seen with a common spy-glass, appear- ing like small stars near the primary. (See adjoining cut, and note at 178.) By watch- ing them for a few evenings, they will be seen to change their places, and to occupy dif- ferent positions. At times, only one or two may be seen, as the others are either between the observer and the planet, or ieyond the primary, or eclipsed by his shadow. 264. The size of these satel- lites is about the same as our moon, except the second, which is a trifle less. The first is about the distance of our moon ; and the others, respect- ively. , about two, three, and five times as far off. 4th. COMPARATIVE DISTANCES OP JFPITBE'S U00N8. 81 2cL 1st 262. What said of the calculation of eclipses ? What does tliis demon- strate and illustrate ? 268. How many moons has Jupiter ? How seen ? Why not all seen at once ! 264. Their size ? Distances ? Periods ? Why so rapid ? SATELLITES OF THE EXTERIOR PLANETS. 125 Their periods of revolution are from 1 day 18 hours to 17 days, according to their distances. This rapid mo- tion is necessary, in order to counterbalance the power- ful centripetal force of the planet, and to keep the satel- lites from falling to his surface. The magnitudes, distances, and periods of the moons of Jupiter are as follows : Diameter in milee. Distance. Periodic times. l8t 2,500 259,000 1 day 18 hours. 2d 2,068 414,000 S "12 " 8d 8^877 647,000 7 "14 " 4th 2,890 1,164,000 17 " " 265. The orbits of Jupiter's moons are all in or near the plane of his equator ; and as his orbit nearly coin- cides with the ecliptic, and his equator with his orbit, it follows that, like our own moon, his satellites revolve near the plane of the ecliptic. On this account, they are sometimes between us and the planet^ and sometimes beyond him, and seem to oscillate, like a pendulum, from their greatest elongation on one side to their greatest elongation on the other. 266. Their direction is from west to east, or in the direction their primary revolves, both upon his axis and in his orbit. From the fact that their elongations east and west of Jupiter are nearly the same at every revolu- tion, it is concluded that their orbits are but slightly elliptical. They are supposed to revolve on their re- spective axes, like our own satellite, the moon, once during every periodic revolution. 267. As these orbits lie near the plane of the ecliptic, they have to pass through his broad shadow when in opposition to the sun, and be totally eclipsed at every revolution. To this there is but one exception. As the fourth satellite departs about 3° from the plane of Jupi- ter's orbit, and is quite distant, it sometimes passes above or below the shadow, and escapes eclipse. But such escapes are not frequent. 265. How are their ortita situated ? How satellites appear to move ? 266. Direction of secondaries 3 Form of orbits 1 How ascertained 3 What motion on axes ? 267. What said of eclipses ? Of fourth satellite 3 Of solar eclipsca upon Jupiter 3 Number of solar and lunar 3 11* 126 ASTEONOMT. These moons are not only often eclipsed, but they often eclipse Jupiter, by throwing their own dark shadows upon his disk. They may be seen like dark round spots traversing it from side to side, causing, whenever that shadow falls, an eclipse of the sun. Altogether, about forty of these eclipses occur in the system of Jupiter every month. 268. The immersions and emersions of Jupiter's moons have reference to the phenomena of their being eclipsed. Their enbrcmce into the shadow is the immersion / and their coming out of it the em,ersion. KOLZPSES OT JXrPXTEBS UOOITB, BMBS8I0NB, ETC. \ 1. The above is a perpendicular view of the orbits of Jupiter's satellites. His broad shadow is projected in a direction opposite the sun. At O, the second satellite is suifer- ing an im/mersion, and will soon be totally eclipsed ; while at D, the first is in tlie act of ffmersi&n. and will soon appear with its wonted brightness. The other satellites are seen to cast their shadows oif into space, and are ready in turn to eclipse the sun, or cut off a portion of his beams from the face of the ]3rimary. 2. If the earth were at A in the cut the immersion, represented at C, would bo iu- Tisible ; and if at B, the emersicm- at D could not be seen. So, also, if the earth were exactly at F, neither could be seen ; as Jupiter and all his attendants would be directly beyond the sun, and would be hid from our view. 269. The system of Jupiter _may be regarded as a miniature representation of the solar system, and as fur- nishing triumphant evidence of the truth of the Co'per- nican theory. It may also be regarded as a great natu- ral clock, keeping absolute time for the whole world ; as the immersions and emersions of his satellites furnish a uniform standard, and, like a vast chronometer hung up in the heavens, enable the mariner to determine his lon- gitude upon the trackless deep. 268. What are the imrnersiona and emersiime of Jupiter's moons? (Are the immersions and emersions always visible from the earth ? Why not ! lUustrato.) 269. How may the system of Jupiter he regarded 3 What use made of in naviga'ion? (Illustrate method. Much used?) SATELLITES OF THE EXTERIOR PLANETS. 127 By Ions a"^ Cftrefiil observations upon these satellites, astronomers have been able to construct tables, showing the exact time when each immersion and emersion will take place, at Grepnwicli Observatory, near London. Now suppose the tables fixed the time for acertain satellite to be eclipsed at 12 oVlock at Greenwich, but we find it to occur at 9 o'clock, for instance, by our local time : this would show that our time was three hours behind the time at Greenwich; or, iu other words, that we were tlu-ee hours, or 45°, xcest of Greenwich. If our time was ahead of Greenwich time, it would show that we were east of that meridian, to the amount of 15° for every liour of variation. But this method of finding the longitude is less used than the " lunar method" (Art. 245), on ac- count of the greater difficulty of making the necessary observations. 2T0. By observations upon the eclipses of Jupiter's moons, as compared with the tables fixing the time of their occurrence, it was discovered that light had a pro- gressive motion, at the rate of about 200,000 miles per second. 1. This discovery may be illustrated by again referring to the opposite cut. In the year 1675, it w^as observed by Koemer, a Danish astronomer, that when the earth was nearest to Jupiter, as at E, the eclipses of his satellites took place 8 minutes 18 seconds sooner than the mean time of the tables ; but when the earth was farthest from Jupiter, as at F, the eclipses took place S minutes and 13 seconds Zate/* than tiie tables predicted the entire difference being 16 minutes and 26 seconds. This diiference of time he ascribed to the progressive motion of light, which he concluded required 16 minutes and 26 seconds to cross the earth's orbit from E to F. 2. This prom-ess may be demonstrated as follows: — ^]6m. 26s. = 9S6s. If the radius of the earth's orbit be 95 millions of miles, the diameter must be twice that, or 190 mil- ; lions. Divide 190,000,000 miles by 986 seconds, and we have 192,697^ ^{j^ miles as the progress of light in each second. At this rate, li^ht would pass nearly eight times around the globe at every tick of the clock, or nearly 500 times every minute I f SATURN. V 271. The moons of Satuni are eight in number, and are seen only with telescopes of considerable power. The best time for observ- satellites of sathrn. ing them is when the planet is at his equinoxes, and his rings are nearly invisible. In January, 1849, the author saw five of these satellites, as represented in the adjoining cat The rings appeared only as a line of light, extending each way from the planet, and the satellites were in the direction of the line, at different distances, as here represented. 272. These satellites all revolve eastward with the rings of the planet, in orbits nearly circular, and, with the exception of the eighth, in the plane of the rings. Their mean distances, respectively, from the planet's cen- 270. What discovery by observing these eclipses? (Illustrate method. Diagram. Demonstration.) 271. Number of Saturn's moons ? How seen ? Best time ? 272. How revolve ? Shape of orbits ? How situated ? Distances ? Periodd ? 128 ASTEONOMT. ttr are from 123,000 to 2,366,000 miles ; and their pe- riods from 22 hours to 79 days, according to their dis- tances. The distances and periods of the satellites of Saturn are as follcws; DiBtaQce in miles. Periodic times. 1st 123,000 day 22 hours. 2d 158,000 1 " 8 " 3d 196,000 1 " 21 " 4th 251,000 2 " IT " Distance in miles. Periodic times 5th 851,000 4 days 12 hours. 6th 811,000 15 " 22 " 7th.... 3,866,000 79 " 7 " COMPAEATPnB DISTANCES OP THE UOONS OF BATUBN 2Y3. The most distant of these satellites is the largest, supposed to be about the size of Mars ; and the remain- der grow smaller as they are nearer the primary. They are seldom eclipsed, on account of the great inclination of their orbits to the ecliptic, except twice in thirty years, when the rings are edgewise toward the sun. The eighth satellite, which has been studied more than all the rest, is known to revolve once upon its axis during every periodic revolution ; from which it is inferred that they all revolve on their respective axes in the same manner. 1. Let the line AB represent the „„ plane of the planet's orbit, O D his ^^^^^ »^ SATtiEN-NO eclipses. axis, and E F the plane of his rings. "t The satellites being in the plane of the rings, will revolve around the shadow of the primary, instead of passing through it, and being eclipsed. 2. At tile time of his equinoxes, how- ever, when the rings are turned toward the sun (see A and £, cnt, page 92), they must be in the center of the shad- ow on the opposite side ; and the moons, revolving in the plane of the rings, must pass through the shadow at every revolution. The eighth, however, may sometimes escape, on account of bis departure from the plane of the rings, as shown in the cut. UKANUS. 274. Uranus is supposed to be attended by six secon- daries. Sir Wm. Herschel recorded that he saw this number, and computed their periods and distances ; and on his authority the opinion is generally received, though 273. Size? Eclipses of ? When' Wliy not oftoner ? (Illastrate.) 274. Satellites ojt Uranus ? Upon what authority ? Distances? Periods! Situation of orbits? Form? Direction in revolution i Eemarls of Dr. Herschel ? NATTJEK AND CAUSE OF TIDES. 129 no other observer has ever been able to discover mort) than three. They are situated at various distances, and revolve in from 1 day and 21 hours to 117 days. Their orbits are nearly perpendicular to the ecliptic, and they revolve iachward, or from east to west, contrary to all the other motions of our planetary system. Their or- bits are nearly circular, and they are described by Dr. JSerschel as " the most diflScult objects to obtain a sight of, of any in- our system." The distances and periods of the system of TJronaa, as laid down by Dr. Herschel, are as follows : Distance in miles. Periodic times. 1st 224,000 5days21 hours. Zd 296,000 8 " IT " 8d 340,000. + 10 " 23 " Dietsnce in miles. Periodic limes. 4th 890,000 ISdaysll hours. 5th 777,000 83 " 2 " 6th.... 1,556,000 IIT " 17 " NEtTUNE. 275. Neptune is known to be attended by one satel- lite, and suspected of having two. Professor Bond, of Cambridge, Mass., states that he has at times been quite confident of seeing a second. The mean distance of the known satellite from its primary is 230,000 miles, or near the distance of our moon. Its period is only 5 days and 21 hours. We have here another illustration of the great law of planetary motion explained at 74. So great is the attractive power of Neptune, that to keep a satellite, at the distance of our moon, from faUingto his surface, it must revolve some five times as swiftly as she revolves around the earth. The centripetal and centrifugal forces must he balanced in all cases, as the laws of gravitation and planetary motion, discovered by Newton and Kopler, (Extend to and prevail among all the secondaries. CHAPTER VIII. NATUKE AND CAUSE OF TIDES. 276. Tides are the alternate rising and falling of the waters of the ocean, at regular intervals. Flood tide is when the waters are rising; and ebh tide, when they are 275. What said of Neptune's secondaries? Eemark of Prof. Bond ? Dis- tance and period of the known satellite 3 (Eemark in note.) 276. What are tides ! Flood and ebb tides? High and low? How often do they ebb and flow! 130 A8TE0N0MT. falling. The highest and lowest points to which they go are called, respectively, high and low tides. The tides ebb and flow twice every twenty-four hours — i. e., we have two flood and two ebb tides in that time. 277. The tides are not uniform, either as to time or amount. They occur about 50 minutes later every day (as we shall explain hereafter), and sometimes rise much higher and sink much lower than the average. These extraordinary high and low tides are called, respectively, spring and nea/p tides. 278. The cause of the tides is the attraction of the sun and moon upon the waters of the ocean. But for this foreign influence, as we may call it, the Vaters having found their proper level, would cease to heave and swell, as they now do, from ocean to ocean, and would remain calm and undisturbed, save by its own inhabitants and the winds of heaven, from age to age. In this figure, the earth is represented as Burronnded by water, in a state of rest or equilibriuiu, as it would be were it not acted upon by the sun aud mooD. 279. To most minds, it would seem that the natural effect of the moon's attraction would be to produce a single tide-wave on the side of the earth toward the moon. It is easy, therefore, for students to conceive how the moon can produce oiie flood and one ebb ^^^ nm^wAvt tide in twenty-four hours. 1. In this cat, the moon is shown at a distance above the earth, and attracting the waters of the ocean, so as to produce a high tide at A. But as the moon makes her apparent westward revolution around the earth but once a day, the simple raising of a flood tide on the side of the earth toward the moon, would give us but one flood and one ebb tide in twenty-four hours ; whereas it is known that we have two of each, ^ 2. "The tides," says Dr. Herschel, " are a subject on which many^ persons find a strange difficulty of conception. That the moon, by her attraction, should heap up the waters of the ocean under her, seems to many persons very natural. That the same cause should, at the same time, heap them up on the opposite side of the earth (viz., at B in the figure), seems to many palpably absurd. Tet nothing is more true." 280. Instead of a single tide-wave upon the waters of 277. Are the tides uniform ? What variation of time ? As to amount ? What are thesa extraordinary high and low tides called ? 278. The cause of tides 3 How but for this influence ? 279. What most obvious effect of the moon's attraction ! (Substance of note 1 ? Eemark of Dr. Herschel ?) NATTJEE AND CAUSE OF TIDES. 131 TWO TIDE-WAVEA the globe, directly under the moon, it is found that on the side of the earth directly opposite there is another high tide ; and that half way between these two high tides are two low tides. These four tides, viz., two high and two low, traverse the ocean from east to west every day, which accounts for both a flood and an ebb tide ef ery twelve hours. In this cut, we have a representation of tlic tide-waves as they actually exist, except that their liigUt, as compared witii tlie magni- twde or the earth, is vastly too great It is designedly exaggerated, the better to illustrate the principle under consideration. "Wliilo the moon at A attracts the waters of the ocean, and produces a high tide at B, we see another high tide at C on tlie opposite side of the globe. At the same time it is low tide at D and E. 281. The principal cause of the tide-wave on the side of the earth opposite the moon is the difference of the moon's attraction on different sides of the earth. If the student well understands the subject of gravitation (65), he will easily perceive how a dilTerence of attraction, as above described, would tend to produce an elongation of the huge drop of water called the earth. The diameter of the earth amounts to about vj^th of the moon^s distance; so that, by the rule (69), the difference in her attraction on the side of the earth toward her, and the opposite side, would be about y , th. The attraction being stronger at B (in the last cut) than at the earth's center, and stronger at her center than at 0, would tend to separate these three portions of the globe, giving the waters an elongated form, and producing two opposite tide-waves, as shown in the cut X 282. A secondary cause of the tide-wave on the side of the earth opposite the moon, is the revolution of the earth around the common center of gravity between the earth and moon, thereby generating an increased centri- fugal force on that side of the earth. The center of gravity between the earth and moon is the point where they would exactly balance each other, if connected by a rod, and poised upon a fulcrum. OENTEE OF GRAVITY BETWEEN THE BA.ETH AND MOOiV. This point, which, according to Ferguson, is about 6,000 miles from tlie earth's center, lb represented at A in the above, and also in the next cut 280. How many tide-waves are there on the globe, and how situated ? 281. State the principal canae of the wave opposite the moon ? (Demon- strate by diagram.) 282. What other cause operates with the one just stated to produce the tide-'wave opposite the moon ? (^What is the center of gravity between the earth and the moon ? Where is it situated ? Illustrate the operation of this secondary cause. Diagram.) 132 ASTEOHOMY. BEOONDABT OAUBE OF HIGH TIDE OPPOSITE THE UOON. 1. The point A represents the center of gravity between the ear.h and moon ; and ai is this point which traces the regular curve of the earth's orbit, it is representuj iji the arc of that orbit, while the earth's center is 6,000 miles one side of it. Now tiic law of gravitation requires that while both the moon and earth revolve around the sun, they should also revolve around the common center of gravity between tliem. or around the point A. This would give the earth a tMrd revolution^ in addition to that around the sun and on her axis. The small circles show her path around the center of gi-avity, and the arrows her direction. 2. This motion of the earth would slightly increase the centrifugal tendency at B, and thus help to raise the tide-wave opposite the moon. But as this motion is slow, corresponding with the revolution of the moon around the earth, the centrifugal force cbuld not be greatly augmented by such a cause. 283. As the moon, which is the principal cause of the tides, is revolving eastward, and comes to the meridian later and later every night, so the tides are about 50 minutes later each successive day. This makes the in- terval between two successive high tides 12 hours and 25 minutes. Besides this daily lagging with the moon, the highest point imE-wAVEs behini> the moos. of the tide-wave is found to be _.... S) -..^ about 45° behind or east of the ..-^ \ ~" --^ moon, so that high tide does not /•' i "^ occur till about three hours after ' ■ the moon has crossed the merid- ^^st. ian. The waters do not at once ^^^^(V yield to the impulse of the moon's ^^^^3 attraction, but continue to rise ^^^^C after she has passed over. In the cut, the moon is on the meridian, but the highest point of the wave is at A, oi 45° east of the meridian ; and the corresponding wave on the opposite side at B ia equally behind. 284. The time and character of the tides are also affected by winds, and by the situation of difierent places. Strong winds may either retard or hasten the tides, or may increase or diminish their hight ; and if a place is situated on a large bay, with but a narrow opening into the sea, the tide will be longer in rising, as the bay has 283. What daily lagging of the tides ? Interval between two suooesaivo high tides ? What other Tagging ! Cause of this last ? 284. What modification of the time and eharaoter of the tidas ? NAT0EE Airo CAUSE OF TIDES. 1J3 to fill up through a narrow gate. Hence it is not usually- high tide at New York till eight or nine hours after the moon has passed the meridian. 286. As both the sun and moon are concerned in the production of tides, and yet are constantly changing their positions with respect to the earth and to each other, it follows that they sometimes aoi against each other, and measurably neutralize each other's influence ; while at other times they conibme their forces, and mutu- ally assist each other. In the latter case, an iinusually high tide occurs, called the Spring Tide. This happens both at new and full moon. OATTSE OF SPSnra TIDES. 1. Here the 6un and moon, being in conjunction, unite their forces to produce an ea- traordinary tide. The same effect follows when they are in opposition ; so that we have two spring tides every month — namely, at new and full moon. 2. If the tide-waves at A and B are one-third higher at the moon's quadrature than usual, those of and D will be one-third lower than usual. 286. Although the sun attracts the earth much more powerfully, as a whole, than the moon does, still the moon contributes more than the sun to the production of tides. Their relative influence is as one to three. The nearness of the moon makes the difference of her attraction oh different sides of the earth much greater than the difference of the sun's attraction on different sides. It must not be forgotten that the tides are the result not so much of the attraction of the sun and moon, as a whole, as of the difference in their attraction onditferent sides 283. Do the sun and moon always act together in attracting the waters ? Why not ? How affect each other's influence ? Effect on the tides ? What are SpriTig Tides f When do they occur ? (Illustrate by diagram the cause pf spring tide, when the sun and moon are in conjunction.) 286. Comparative influence of sun and moon in the production of tides ? Why moon's influence the greatest ? (Substance of note ? Demonstration.) 12 134 ASTEONOMY. I & _e-4^ '*' \ >n6.\ jehCA. Bields Comet has a -\ .fiod of 64 years. That (J .'^SS (then first noticf with care, , and iden jcd as the same that tiad appeared in 1456, 1531, and 1607) has a period of about 76 years. It is called Hal- ley's Comet, after Dr. Halley, who determined its periodic time. The great comet of 1680 has a periodic time of 570 years, so that its next return to our system will be in the year ENCKE S COMET. 800. Velocity of comets » tTniform or not ? Comet of 1472? Of 16S0 ? Z0\. Temperature f Comet of 1680? Supposed cause of their tails ? 302. Periods? Eooke's? Biela"s ? Halley's? That of 1680? Snppoeed periods of others ? Opinions of Prof Nichol and Dr. Hersoliel 3 142 ASTEONOMT. 2250. Many are supposed to have periods of thousands of years ; and some liave their orbits so modified by the attraction of the planets, as to pass off in parabolic curves, to return to our system no more. Prof. Nichol is of opinion that the greater number visit our system bnt once, and then fly oiT in Jiearly straight lines till they pass the center of attraction between the solar system and the fixed stars, and go to revolve around other suns in the far distant heav- ens. Sir John Herschol expresses the same sentiment. '-^'' 303. The distances to which those comets that return must go, to be so long absent, must be very great. Still their bounds are set by the great law of gravitation, for were they to pass the point " where gravitation turns the other way," they would never return. But some, at least, do return, after their " long travel of a thousand years." What a sublime conception this affords us of the almost infinite space between the solar system and the fixed stars. ORBIT OP HALLBT^S COUBT. 304. The perihelion distances of the various comets that have appeared, and whose elements have been esti- mated by astronomers, are also exceedingly variable. While some pass very near the sun, others are at an im- mense distance from him, even at their perihelion. Of 137 that have been particularly noticed, 30 passed between the sun and the orbit of Mercury. 44 between the orbits of Mercury and Venus. 34 " " Yenus and the earth. 23 " " the earth and Mars. 6 " " Mars and Jupiter. 808. Distances to which they go ? Eemark respecting the law of gravi- i/ation ? Wliat specimen of orbit given ? 80i. What said of perihelion distances? How many noticed? Where did OF COMETS. 143 The orbit of Encke*a comet is wholly within the orbit of Jupiter, while 'that of Biel.i's extends but a short distance beyond it^ The aphelion distance of Halley^s comet Is 8,400 millions of miles, or 550 millions of miles beyond the orbit of Neptune. orhits of several comets. But these are all comets of short periods. 305. The numJter of ^^- ■-■■.. comets belonging to, or \^ '•■.. ;,, that visit the solar system, / encke's \ is very great. Some have / RALLEY'svfv^ ,... ^■■■■::::^%^ estimated them at several ""? -f;?;..,..^,-^ / \ aiillions. When we con- j ^'--0% ^^-. ■■-' I ^ider that most comets are \ f®- ■'"•"•.■■*■ '\ ] seen only through tele- \ \ ./ \ / scopes — an instrument of \ )/^ ,V' comparatively modern \ ^1 ' ■•■^^, date — and that, notwith- .■•■'■"■--.# '■^%«-' .■•■ standing this, some 450 ,..-•'' '" are mentioned in ancient annals and chronicles, as having been seen with the naked eye, it is probable that the above opinion is by no means extravagant. It is supposed that not less than 700 have been seen at different times since the birth of Christ. The paths of only about 140 have been deter- mined. The extreme difEculty of ODserving comets wnose nearest pome is beyond the orbit of Mars, is supposed to account for the comparatively small number that have been seen without that limit ; and the proximate uniformity of the distribution of their orbits over the space included within the orbit of Mars, seems to justify the conclusion, that though seldom detected beyond his path, they are nevertheless equally distributed throngh all the spaces of the solar heavens. Eeasoning uponithis hypothesis. Professor Arago concludes that there are probably s&ven milUons of comets that belong to oi visit the solar system. y^OQ. The directions of comets are as variable as their forms or magnitudes. They enter the solar sj'stem from all points of the heavens. Some seem to come up from the immeasurable depths below the ecliptic, and, having doubled " heaven's mighty cape," again plunge down- ward with their fiery trains, and are lost for ages in the ethereal void. Others appear to come down from the > zenith of the universe, and, having passed their peri- they pass ? (What samples given in cut ? Where does the orbit of Encko's comet lie? OfBiela's? Of'Halley'a?) 305. The number of comets? What estimate? Why probably correct? How many supposed to have been seen since the birth of Christ ? (Why so fpwseen? How supposed to be distributed ? What conclusion of Arago ?) 806. Direction of comets? (Remark of late writer?) 144 A8TE0N0MY. helion, reascend far above all human vision. Others again are dashing through the solar system, in all possible directions, ajjparently without any prescribed path, or any guide to direct them in their eccentric wanderings. In- stead of revolving uniformly from east to west, like the planets, their motions are direct, retrograde, and in every conceivable direction. It is remarked by a late writer, that the average vndmiaiiona of al! the planes in which the comets now on record have been found to move. Is about 90°. This he re- gards ns a wonderful instance of the goodness oi Providence, in causing their motions to be performed in a manner least likely to come in contact with the earth and the other planets. 307. Of the physical nai/wre of comets, little is known. That they are, in general, very light and vapory bodies, is evident from the fact that stars have sometimes been seen even through their densest portions, and are gene- rally visible through their tails, and from the little attrac- tive influence they exert upon the planets in causing perturbations. While Jupiter and Saturn often reta/rd and delay comete for months in their periodic revolutions, comets have not power, in turn, to hasten the time of the planets for a single hour ; showing conclusively that the relative masses of the comets and planets are almost in- finitely disproportionate. Such is the extreme lightness or tenuity of cometaiy bodies, ttiat in all probability the entire mass of the largest of them, if condensed to a solid substance, would not amount to more than a few hundred pounds. Sir Isaac Newton was of opinion, that if the tail of the largest comet was compressed within the space of a cubic inch, it would not then be as dense as atmospheric air! The comet of 1770 got entangled, by attrac- tion, amonff the moons of Jupiter, on its way to the sun, and remained near them for f(»ir Tnoniks ; yet it did not sensibly affect Jupiter or his moons. In this way the orMta of comets are often entirely changed. 308. Comets were formerly regarded as harbingers of famine, pestilence, war, and other dire calamities. In one or two instances, they have excited serious apjjre- hension that the day of judgment was at hand, and that they were the appointed messengei-s of Divine ^vrath, hasting apace to burn up the world. A little reflection, however, will show that all such fears are groundless. The same unerring hand that guides the ponderous planet 307. Physical nature of comets ? What proofs of their light and vapory character f (What said of theu- probable mass ? Opinion of Newton ! What said of the comet of 1T70 ? What effect on orbits ?) 30S. How comets formerly regarded ? Why no fears of collision ? (What istimate of "chances I" 1 OF COMETS. 145 iQ its waj'', directs also the majestic comet; and where infinite wisdom and almighty power direct, it is almost profane to talk of collision or accident. Kven those who have calculated the "chances" of collision — as if chance had any thing to do among the solar bodies — have concluded the chances of collision are about as one to 281,000,000 — i. a, like the chance one would have in a lottery, where there were 231,000,000 black balls, and but one white one ; and where the white ball must be produced at the first drawing to secure a prize. 309. Were a collision actually to take place between a comet and the earth, it is not probable that the former would even penetrate our atmosphere, much less dash the world to pieces. Prof Olmsted is of opinion that in such an event, not a particle of the comet would reach the earth — that the portions encountered by her would be arrested by the atmosphere, and probably inflamed ; and that they would perhaps exhibit, on a more magnifi- cent scale than was ever before observed, the phenomena of shooting stars or meteoric showers. The idea, there- ^ ibre, that comets are dangerous visitants to our system, nas more support from superstition^than from reason or /science. / / The air is to us what the waters are to fish. Some fish swim around in the deep, ' ' while others, like lobsters and oysters, keep on the bottom. So birds wing the air, while men and beasts are the " lobsters" that crawl around on the bottom. Now there is no more probability that a comet would pass through the atmosphere, and ii^ure us upon the earth, than there is that a handful of ^^ or vapor thrown down upon the sur face of the ocean, would pass through and kill the shell-fish at the bottom. 310. After all that is supposed to be known respecting comets, it must be admitted that they are less under- stood than any other bodies belonging to our system. " What regions these bodies visit, when they pass beyond the limits of our view ; upon what errands they come, when they again revisit the central parts of our system ; what is the difference between their physical constitution and that of the sun and planets ; and what important ends they are destined to accomplish in the economy of the universe, are inquiries which naturally arise in the mind, but which surpass the limited powers of the human understanding at present to determine." 800. What probable effect in caae of collision ? Prof. Olmsted's opimon ? (Bemork respecting the air, fish, lobsters, &c. ?) 810. Are we as well acquainted with comets as with other bodies of oar system 3 What inquiries suggested ? How answered ? ^^ 7 146 ASTEONOMY. CHAPTER X. OF THE BUN. THE SUN AND THE UOON 9 ORBIT. 311. Of all the celestial objects with which we art acquainted, none make so strong and universal an im- pression upon our globe as does the sun. He is the great center of the solar system — a vast and fiery orb, kindled by the Almighty on the morn of creation, to cheer the dark abyss, and to pour his radiance upon surrounding worlds. Compared with him, all the solar bodies are of inconsiderable dimensions ; and without him, they would be wrapped in the gloom of interminable night. 312. The fc/rm of the sun is that of an oblate sphe- roid, his equatorial being somewhat greater than his polar diameter. The mean of the two is 886,000 miles. He is 1,4:00,000 times as large as the mighty globe we inhabit, and 500 times as large as aU the planets put together. Were he placed where the earth is, he would fill all the orbit of the moon, and extend 200,000 miles be- yond it in every direction. It would take 112 such worlds as ours, if laid side by side, to reach across his vast diameter. 1. The vaet znasinitiide of the smi may be iBferred from the fact, that when rising or set- ting, he often appears larger than the largest building, or tbe tops of the largest trees. Now If the angle filled by him at the distance of two miles is over 100 feet across, what must it be at tbe distance of 95 millions of miles? 2. "Were a railroad passed through the sun's center, and should a train of cArs start &OIQ one side, and proceed on at the rate of 30 loiles an boar, it would vequire 8:^ years 811. Describe the sun. How oompare with the rest of the system ? 812. What is his ibim? Diameter? Mass, as compared with our>lobe? With all other bodies of the system ? With moon's orbit ? (What sensible evidence of the vast magnitude of the sun 2 Illustration from railroad ' Demonstration as to its comparison with moon's orlttt ?) OF THE SUN. 147 to cross over uis diameter. To traverse his vast circamfei'enoe, at the same rate ol speed, would require nearly 11 years. 8. The menu distance of the moon from the earth's center is 240,000 miles ; conse- quently the diameter of her orbit, which is twice the radios, is 480,000. Subtract this from 886,000, the sun's diameter, and we have 406,000 miles left, or 208,000 miles on each Bide, beyond the moon's orbit. 313. By the aid of telescopes, a variety oi spots have been discovered upon th'e sun's disk. Their number is exceedingly variable at diiferent times. From 1611 to 1629, a period of 18 years, the sun was never found clear of spots, except for a few T ■'■•-r-v ^ -I ^ nr^ . SPOTS OK THE SUN. days in December, 1624. At other times, twenty or thirty were frequently seen at once ; and at one period, in 1825, upwards of fifty were to be seen. Prof. Olmsted states that over 100 are sometimes visible. From 1650 to 1670, a pe- riod of 20 years, scarcely any spots were visible ; and for eight years, from 1676 to 1684, no spots whatever were to be seen. For the last 46 years, a greater or less number of spots have been visible every year. For several days, during the latter part of September, 1846, we could count sixteen of these spots, which were dis- tinctly visible, and most of them well defined ; but on the 7th of October following, only six small spots were visible, though the same telesco])ewas used, and circum- stances were equally favorable. The sun is a difficult object to view through a telescope, even when the eye is pro- tected in the best manner by colored glasses. In some cases (as in one related to the author by Professor Caswell, of Brown Univei"Sity), the heat becomes so great as to .'ipoil the eye-pieces of the instrument, and sometimes the eye of the observer is irrepa- rably injui'ed, 314. The solar spots are all found within a zone 60° wide — *. «., 30° each side of the sun's equator. They are generally permanent, though they have been known to 813. View of sun's surface through telescopes ? Number of spots seen ? Are they always to be seen? How from 1611 to 1629? In 1825? Prof. Olmsted's statement? How from 1650 to 1670? From 1676 to 1684 ? In 1846 ? (What said of difficulties of observing ?) 814. where are these spots situated ? Are they permanent 3 What mo- ^^^^^- s ■'i^Si> 148 AfiTEONOMT. break in pieces, and disappear in a very short time. They sometimes break out again in the same places, or wliere none were perceptible before. They pass from left to right over the sun's disk in 13 days, 15 hours, and 45 minutes ; from which it has been ascertained that he performs a sidereal revolution on his axis, from west to east, or in the di- „ ' - ^. ^ BtDEBBAL AND STNOSIO BBTOLUTIONB OF THE SUN. rection of all the planets, every 25 e / days, 7 hours, and * ®^... ^ 48 minutes. ~"^^---^-?.?.-!oj, 1. His apparent or aynodio revolution requires 27 days 7^ boars ; but tbis is as mucb more than a complete revolution upon bis axis, as tbe eartb baa ad- vanced in ber orbit in 25 days 8 ^^ri" bours. Let S represent tbe sun, *ffi3'C and A the earth in her orbit When she is at A, a spot is seen upon the disk of the sun at B. The sun revolves in the direction of tbe arrows, and in 25 days 10 bonrs tbe spot comes round to B again, or opposite tbe star E. This is a nidereal revolution. 2. During tliese 25 days 8 hours, the earth has passed on in ber orbit some 25°, or nearly to 0, which will require nearly two days for the spot at B to get directly toward, the eartb, as shown at D. This last is a synodic revolution. It consists of one cnm-' plete revolution of tbe sun upon his axis, and about 27° over. /315. Of the nature of these wonderful spots, a variety /of opinions have prevailed, and many curious theories have been conetructed. Lalande, as cited by Herschel, suggests that they are the tops of mountains on the sun's surface, laid bare by fluctuations in his luminous atmos- phere ; and that the penumbras are the shoaling declivi- ties of the mountains, where the luminous fluid is less deep. Another gentleman, of some astronomical knowl- edge, supposes that the tops of the solar mountains are exposed by tides in the sun's atmosphere, produced by planetary attraction. To the theory of Lalande, Dr. Herschel objects that it is contradicted by the sharp termination of both the in- ternal and external edges of the penumbrse ; and ad vances as a more probable theory, that " they are the tion have they I What conclusion from it? (What revolution is this? What time required for a synodic revolution ! lUnstrnte.) 815. What are these spots supposed to be ? Lalande ? &o. Dr. Herschel's remark? Prof Olmsted? Prof. Wilson? Experiments of Prof. Henry ? OF THE StTN. 149 dark, or, at least, comparatively dark, solid body of the sun itself, laid bare to our view by those immense fluc- tuations in the luminous regions of the atmospnere, to ■which it appears to be subject." Prof. Olmsted supports this theory by demonstrating that the spots must be " nearly or quite in contact with the body of the sun." In 1773, rrof. "Wilson, of the University of Glasgow, ascertained, by a series of observations, that the spots were probably " vast eaxa/vations in the luminous matter of the sun ;" the nuclei being their bottom, and the um- brae their shelving sides. This conclusion varies but little from that of Dr. Herschel, subsequently arrived at. In a series of experiments conducted by Prof. Henry, of Princeton, by means of a thermo-electrical apparatus, applied to an image of the sun thrown on a screen from a dark room, it was found that the spots were perceptibly colder than the surrounding light surface. 316. The TTiagnitude of the solar spots is as variable as their number. Upon this point, the second cut pre- ceding gives a correct idea, as it is a pretty accurate rep- resentation of the sun's disk, as seen by the writer on the 22d of September, 1846. In 1799, Dr. Herschel ob- served a spot nearly 30,000 miles in breadth ; and he further states, that others have been observed, whose diameter was upward of 45,000 miles. Dr. Dick ob- serves that he has several times seen spots which were not less than ^ of the sun's diameter, or 22,192 miles across. It is stated, upon good authority, that solar spots have been seen by the naked eye — a fact from which Dr. Dick concludes that such spots could not be less than 50,000 miles in diameter. The observations of the writer, as above referred to, and represented in the cut, would go to confirm this deduction, and to assign a still greater magnitude to some of these curious and interest- ing phenomena. 317. The axis of the sun is inclined to the ecliptic 7-^°, 816. Wliat said of the size of the solarepots? Dr. Heraohel's observa- tions ? Dr. Dick's ? The writer's 3 150 ASTEONOMT. or, more accurately, Y° 20'. This is but a slight deviation from what we may call a perpendicular ; so that, in rela- tion to the earth, he may be considered as standing up and revolving with one of his poles resting upon a point, just half his diameter below the ecliptic. As the result of the sun's motion upon his axis, his spots always appear first on his eastern limb, and pass off or disappear on the west. But though the direction of the spots, as viewed from the earth, is from east to west, it only proves his motion to coincide with that of the earth, which we call from west to east; as when two spheres revolve in the same direction, the sides toward each other will appear to move in opposite directions. During one-half of the passage of the spots across the sun's disk, their apparent motion is accelerated / and during the remainder, it is reta/rded. This apparent irregularity in the motion of the spots upon the sun's surface, is the necessary result of an equable motion upon the surface of a globe or sphere. W hen near the eastern limb, the spots are coming partly toward us, and their angular motion is but slight ; but when near the center, their angular and real motions are equal. So, also, as the spots pass on to the west, it is their angular motion only that is diminished, while the motion of the sun upon his axis is perfectly uniform. 318. The figure of the sim affects not only the appa- rent velocity of the spots, but also their forms. When first seen on the east, they appear narrow and slender, as represented in the cut, page 147. As they advance westward, they continue to widen or enlarge till they reach the center, where they appear largest ; when they again begin to contract, and are constantly diminished, till they disappear. 319. Another result of the revolution of the sun upon an axis inclined to the ecliptic, and the revolution of the 817. How is the sun's axis situated ? What sadd of the direction of the epota ? Of their rate of motion ? 318. Of the cause of this irregularity ? What variations in the forma of the solar spots ? Cause ? 819. What other result of the sun's revolution about an inclined axis? (Illustrate by diagrams.) OF THE SUN. 151 earth around him, is, that when viewed from our mov- able observatory, the earth, at different seasons of the year, the direction of the spots seems materially to vary TABIOTJS BIEBOTIONB OF TUE 80I.Aa SFOTB. March. June. September December. 1. Let E F represent the piano of the ecliptic In March, the spots describe a curve, whicli is convex to the south, as shown at A. In June, they cross the sun's disk in nearly straight Hues, but incline upward. In September, they curve again, though in theopposite direction; and in December, pass over in straight lines, inclining (ioiwi- wara. The figures B and D show the inclination of the sun's axis. 2. The cause of this difference in the direction of the solar spots will be ftilly nnde> stood by the following diagram : SOLAIt SPOTS OBSERVED FROM DIFFERENT POINTS. JONEe^SBB MARCH .■*; Let the student imagine himself stationed upon the earth at A, in March, looking upon the sun in the center, whose north or upper pole is now inclined toward Jmn. The spots will then curve dozonward. Three months afterward — viz., in June — the earth will be at B ; when the sun's axis will incline to the left, and the spots seem to pass upward to the right In three mouths longer, the observer will he at C, when the north pole of the sun will inclino/rom Aim, and the spots seem to curve upward ; and in three months longer, he will bo at D, when the axis of the sun will incline to 1M right, and the spots seem to incline downward. 320. Of the physical constitution of the sun, very lit- tle is known. When seen through a telescope, it is like a globe of fii-e, in a state of violent commotion or ebu- litiori. La Place believed it to be in a state of actual combustion, the spots being immense caverns or craters, caused by eruptions or explosions of elastic fluids in the interior. 820. What said of the physical constitution of the sun? La Place's opin- ion ? Most probable opinion ? 152 ASTEONOMT. Tte most probable opinion is, that the body of the sun is opake, like one of the planets ; that it is surrounded by an atmosphere of considerable depth ; and that the light is sent off from a luminous stratum of clouds, float- ing above or outside the atmosphere. This theory accords best with his density, and with the phenomena of the solar spots. / 321. Of the temperature of the sun's surface, Dr. Her- schel thinks that ^t must exceed that produced in fur- naces, or even by chemical or galvanic processes. By the law governing the diffusion of light, he shows that a body at the sun's surface must receive 300,000 times the light and heat of our globe ; and adds that a far less quantity of solar light is sufl5cient, when collected in the focus of a burning-glass, to dissipate gold and platina into vapor. The same writer observes mat the most vivid flames disappear, and the most intensely ignited solids appear only as black spots on the disk of the sun, when held between him and the eye. From this circumstance he infers, that liowever dark the body of the sun may appear, when seen through its spots, it may, neverthe less, be in a state of most intense ignition. It does not, however, follow, of necessity, that it mvst be so. The contrary is, at least, physically possible. A jperfeGtly refledn/oe canopy would effectually defend it from the radiation of the luminous regions above its atmosphere, and no heat would be conducted downward through a gaseous medium increasing rapidly in density. The great mystery, however, is to conceive ho\v so enormous a conflagration (if such it be) can be kept up from age to age. Every discovery in chemical science here leaves us completely at a loss, or rather seems to remove further from us the prospect of explanation. If conjecture might be hazarded, we should look rather to the known possibility of an indefinite generation of heat by friction, or to its excitement by the electric discharge, than to any actual combustion of ponderable fluid, whether solid or gaseous, for the origin of the solar radiation. 821. Sun's temperature ? Dr. Herschel's idea ? What reasomTig againal nis opinion ? Wnat mystery ? THE ZODIACAL LIGHT. 153 ZODIAOAL LIGHT. 322. The Zodiacal Light is a faint nebulous light, re- sembling the tail of a comet, or the milky way, which seems to be reflected from the regions about the sun, and is distinguishable from ordinary twilight. Its form is that of a pyramid or cone, with its base toward the sun, and inclined slight- ly to the ecliptic. It seems to surround the sun on all sides, though at various depths, as it may be seen in the morning preceding the sun, as well as in the evening following him ; and the bases of the cones, where they meet at the sun, are much larger than his diameter. 323. Of the natfure of this singular phenomenon, very little is positively known. It was formeiiy thought to be the atmosphere of the sun. Prof. JSTichol says : " Oi this, at least, we are certain — the zodiacal light is a phe- nomenon precisel}' similar in kind to the nebulous atmos- pheres of the distant stars, &c." Sir John Herschel re- marks that it is manifestly of the nature of a thin len- ticulai'ly-formed atmosphere, surrounding the sun, and extending at least beyond the orbit of Mercury, and even of Venus. He gives the apparent angular distance of its vertex from the sun, at from 40° to 90° ; and the breadth of its base, from 8° to 30°. It sometimes ex- tends 50° westward, and 70° east of the sun at the same time. 324. The form of this substance surrounding the sun, and which is sufficiently dense to reflect his light to the S22. What is the zodiacal light ? Its form ? When seen ? 823. Nature of this light ? Former opinion ? Prof. Niohol's remark ? Dr. Hersohel's ? Its extent from the snn ? 824. Form of this light ? How situated with respect to sun's axis, &o. ? (Illustrate by diagram.) 7* 154 ASTEONOMT. LIGHT. 1 Pf^^^M S^^^^^^ss^^^^^^^ 'J^-'iMMii' T |"|' 1 '\- 1,1^^—^^ ^^^^^^SaSaTiS^^^^^^^a^S ■ earth, seems to be that of a lens ; or rather that of a huge wheel, thickest at the center, and thinned down to an edge at the outer extremities. Its being seen edgewise, and only one-half poem, eitent, em., of the zodiaoai. at a time, gives it the'ap- pearance of two pyramids with their bases joined at the sun. It is an interest- ing, fact, stated by Prof. Nichol, that this light or nebulous body lies in the plane of the sun's equator. A line dravn through its transverse diameter, or from one apex of the pyr# mids to the other, would cross the axis of the sun at right angles. This fact would seem to indicate a revolution of this curious sub- stance with the sun upon his axis. Let A, in the above cut, represent the sun, B B his axis ; then G will represent the extent, and D D the thickness of this curious appendage. 325. In regard to its atmospheric character. Dr. Dick observes that this opinion now appears extremely dubi- ous ; and Prof. Olmsted refers to La Place, as showing that the solar atmosphere could never reach so far from the sun as this light is seen to extend. Another class of astronomers suppose this light, or rather the substance reflecting this light, to be some of the original matter of which the sun and planets were composed — a thin nebulous substance in a state of con- densation, and destined either to be consolidated into new planetary worlds, during the lapse of coming ages, or to settle down upon the sun himself as a part of his legiti- mate substance. This theory will be noticed again when we come to speak of Nebulae and Nebulous Stars. 326. After all the observations that have been made. a25. Eomark of Dr. Dick respecting its atmosplierio cliaracter ? Olmsted and La Place? What other theory ? 155 and the theories that have been advanced, it must be ad- mitted that the subject of the zodiacal hght is but imper- fectly understood. Prof. Olmsted supposes it to be a nebulous body, or a thin vapory mass revolving around the sun ; and that the meteoric sTwwers which have oc- curred for several yeai-s in the month of ITovember, may be derived from this body. This is the opinion of Arago, Biot, and others. The best time for observing the zodiacal light is on clear evenings, in the months of March and April. It may be seen, however, in October, November, and De- cember, before sunrise ; and also in the evening sky. THE suit's motion IN SPACE. 327. Although, in general terms, we speak of the sun as the fixed center of the system, it must not be under- stood that the sun is absolutely without motion. On the contrary, he has a periodical motion, in nearly a circular direction, around the common center of all the planetary bodies ; never deviating from his position by more than twice his diameter. From the known laws of gravita- tion, it is certain that the sun is affected in some measure by the attraction of the planets, especially when many of them are found on the same side of the ecliptic at the same time ; but this would by no means account for so great a periodical motion. 328. In addition to the motion above described, the sun is found to be moving, with all his retinue of planets and comets, in a vast orhit, around some distant and hitherto unknown center. This opinion was first ad- vanced, we think, by Sir William Herschel ; but the honor of actually determining this interesting fact be- longs to Struve, who ascertained not only the direction of the sun and solar system, but also their velocity. 826. Is this subject well understood as yet? Prof. Olmsted's theory I When the best time for obsorving the zodiacal light? 827. Is the sun really stationary ? What motion % How alfected by plan- ets « 828. What other motion ? Who first advanced tlie opinion that lie had Buoh a motion ? Who demonstrated it ? Toward what point is the sun and 156 ASTKONOMY. The point of tendency is toward the constellation Her- cules, right ascension 259°, declination 35°. The ve- locity of the sun in space is estimated at 8 miles per second, or 28,000 miles per hour. Its period is about 18,200,000 years; and the arc of its orbit, over wiiich the sun has traveled since the creation of the world, amounts to only about ^tnfot'i P^^* ^^ ^^^ orbit, or about 7 minutes — an arc so small, compared with the whole, as to be hardly distinguishable from a straight line. 329. With this wonderful fact in view, we may no longer consider the sun as fixed and stationary, but rather as a vast and luminous planet, sustaining the same rela- tion to some central orb that the primary planets sustain to him, or that the secondaries sustain to their primaries. Nor is it necessary that the stupendous mechanism of nature should be restricted even to these sublime propor- tions. The sun's central body may also have its orbit, and its center of attraction and motion, and so on, till, as Dr. Dick observes, we come to the great center of all to the THEONE OF GoD ! FrofeBSor Midler, of Dorpat, in BaBsiO} has recently announced as a discovery that the star Alcyons^ one of the seven stars, is the center around which the sun and soiax system are revolving. CHAPTER XI. MISCKLLANBOUS REMARKS UPON THE SOLAK SYSTEM. NEBULAE THEOET OF THE OEIGIN OF THE SOLAR SYSTEM. 330. It was the opinion of La Place, a celebrated French astronomer, that the entire matter of the solar system, which is now mostly found in a consolidated Bolar system tending ? Its velocity ! Period of revolution ? Amount of its progress since the creation of the world ? 829. How, then, should the sun be considered ? How extend the analogy ! What further recent discovery, and by whom ? 880. State the " nehnlar theory" of the origin of the solar system? Who first started this theory ? OEIGIN OF THE SOLAE SYSTEM NEBULAE THEOET. 157 state, in the sun and planets, was once a vast nebula or gaseous vapor, extending beyond the orbits of the most distant planets — that in the process of gradual conden- sation, by attraction, a rotwry motion was engendered and imparted to the whole mass — that this motion caused the consolidating matter to assume the form of various concentric rings, like those of Saturn ; and, finally, that these rings collapsing, at their respective distances, and still retaining their motion, were gathered up into plan- ets, as they are now found to exist. This opinion is sup- posed to be favored, not only by the fact of Saturn's revolving rings, but by the existence of the zodiacal light, or a resisting medium about the sun ; and also by the character of irresolvable or planetary nebulae, hereafter to be described. 331. To this theory, however, there are many plau- sible, if not insurmountable, objections. ia.) It seems to be directly at variance with the Mosaic account of the creation of the sun, moon, and stars. The idea that the sun and all the planets were made wp, so to speak, out of the same general mass, not only throws the creation of this matter back indefinitely into eternity, but it substitutes the general law of attraction for the more direct agency of the Almighty. The crea- tion spoken of in the Bible thus becomes not the origi- nating of things that did not previously exist, but the mere organization or arrangem&nt of matter already existing. (J.) The supposed consolidation of the nebulous mass, in obedience to the general law of attraction, does not of itself account for the rota/ry motion which is an essen- tial part of the theory. Under the influence of mere ai> traction, the particles must tend directly toward the cen- ter of the mass, and consequently could have no tendency to produce a rotary motion during the process of conden- sation. (e.) The variation of the planetary orbits from the 831. What said of it? State the first objection named? The second? Third? Fourth! Fifth? What remark added by the author ? 14 158 ASTRONOMY. filane of the sun's equator contradicts the nebular theory. f the several primary planets were successively thrown off from the general mass, of which the sun is a part, they could not have been separated from the parent body till they were near the plane of its equator. Now, as the sun is assumed to be a part of the same mass, re- volving still, the theory would require that the portions now separated from him, and called planets, should still revolve in the plane of his equator. But instead of this, it is found that some of them vary from this plane to the amount of near 42°. {d.) This theory assumes n^t only that the primary planets were thrown off from the parent mass by its rapid revolution, but that the primaries, in turn, threw off their respective satellites. These, then, should all revolve in the plane of the planetary equators respect- ively, and in the direction in which their primaries re- volve. But their orbits not only depart from the plane of the equators of their prirharies (Jupiter's satellites excepted), but the moons of Uranus actually have a retrograde or iackwa/rd revolution. (e.) If the sun and planets are composed of what was originally the same mass, it will be necessary to show why they differ so materially in their physical natures — why the sun is self-luminous, and the planets opake. Bat we have not room to discuss the subject at length in this treatise. It is but justice, however, to say, that men eminent for learning and piety have advocated the nebula/r theory., in the belief that it is perfectly con- sistent with the Mosaic account of creation. But^ the writer is frank to state, that while he acknowledges the force of some of the considerations urged in its sup- port, he has not yet seen reason for adopting this theory of the origin of the solar system. "Through faith we understand that the worlds were framed by the word of God [not by the law of gravitation}, so that things which are seen were not made of things which do appear [or of pre-existing matter]." — Heb. xi. 3. J\ 332. Upon the supposition that the sun and planets were created as they are, by the direct act of God, an ■VTEEE THE ASTEROIDS ORIGIN ALLY ONE PLANET ? 159 inquiry at once arises as to the probable extent of the creation recorded by Moses. Does it include the whole universe ? or is it to be understood as appj-icable only to the solar system ? Upon this point our only light is, that "in the beginning God created the heavens and the earth" — that he not only made the isun and moon, but that " he made the stars also ;" and that when these were spoken into being, God had " finished" his work. (See Genesis, 1st chapter.) " Thus the heavens and the earth were finished, and all the host of them." It seems most probable, therefore, that the Mosaic creation includes the whole material universe — that when God " laid the foun- dations of the earth," and the " heavens were the work of his hands," he " made the worlds also ;" that is, they were then all " fi-amed by the word of God." WERE THE ASTEROIDS ORIGINAI-LT ONE PLANET ? 333. Some very curious speculations have been enter- tained by astronomers in regard to the origin of the Asteroids. As in the case of the recently discovered planet, Neptune^ the existence of a large planet between the orbit of Mars and Jupiter was suspected before the asteroids were known. This suspicion arose mainly from the seeming chasm that the absence of such a bodj"- would leave in the otherwise well-balanced solar system. The prediction that such a body would be discovered in the future stimulated the search of astronomers, till at length, instead of one large planet, eighteen small ones have, one after another, been discovered. 334. From certain peculiarities of the asteroids, it has been considered highly probable that they are the frag- ments of one large planet, which has been burst asunder by some great convulsion or collision. The grounds of this opinion are as follows : 332. What other interesting question started ? What light npon this sub- ject ? What most probable ? 883. What curious speculation respecting the asteroids ? What suspicions before any of them were discovered ? 884. What opinion respecting the origin of the asteroids ? State the grounda of this opinion in order. 160 ASTRONOMY. {a.) The asteroids are much smaller than .»ny of the othei" primary planets. (b.) They are all at nearly the same distance from the sun. (c.) Their periodic revolutions are accomplished in nearly the same time. The difference of their periodic times' is not greater than might i-esult from the supposed disruption, as the parts thrown forward would have their motion accelerated^ while the other pai-ts would be thrown hack ox retarded ; thus changing the periodic times of both. (of.) The great departure of the orbits of the asteroids from the plane of the ecliptic is supposed to favor the hypothesis of their having been originally one planet, the assumption being that the explosion separating the ori- ginal body into fragments would not only accelerate some portions and retard others, but would also throw them out of the plane of the original orbit, and in some cases still further from the ecliptic. (e.) Their orbits are more eccentric than those of the other primaries. Although the fables show the eccen- tricity of Uranus's orbit as greater iu miles than that ol even Juno or Pallas, yet- when we consider the difference in the magnitude of their orbits, it will easily be seen that his orbit is less elliptical than theirs. (/.) The orbits of Ceres and Pallas, at least, cross each other., This, if we except, perhapspthe orbits of some of the comets, is a perfect anomaly in the solar system. 335. From all these circumstances, it has been con- cluded that the asteroids are only the fragments of an exploded world, which have assumed their present forms since the disruption, in obedience to the general laws oi gravitation. This theory, first advanced by Dr. Olbers, is favored by Prof. ISTichol, Dr. Brewster, Dr. Dick, and others ; while Sir John Herschel observes that it may serve as a specimen of the dreams in which astronomers, like other speculatoj-s, occasionally and harmlessly in- 835. Who was the author of this theory ? What distinguished astrono- •ners favor it S What says Sir John Hersohel ? Remark of Dr. Dick ? Opin- ion and roijiarlcs of the author? AEE THE PLAJTETS m HABITED 1 161 dulge. Dr. Dick remarks that the breaking up of the exterior crust of the earth, at the time of the general deluge, was a catastrophe as tremendous and astonishing as the bursting asunder of a large planet. In view, how- ever, of the harmony and order that everywhere reign throughout the planetary regions, directing the pathway and controlling the destiny of every world, it is hard to believe cither that one world has been so consti'ucted as to esyplode, like a vast bomb-shell, and scatter its frag- ments over the regions of its former pathway ; or that He who guides even the erratic comet has allowed a pon- derous world to get so off its track, as to dash itself to pieces against its fellow worlds. AEE THE PLANETS INHABrrED BY EATTOITAL BEINGS ? 336. Upon this interesting question, it must be ad- mitted that we have no positive testimony. The argu- ment for the inhabitedness of the planets rests wholly upon analogy, and the conclusion is to be regarded only in the light of a legitimate inference. Still, it is remark- able that those who are best acquainted with the facts of astronomy are most confident that other worlds as well as ours are the abodes of intellectual life. Indeed, as Dr. Dick weU remarks, it requires a minute knowledge of the whole scenery and circumstances connected with the planetary system, before this truth comes home to the understanding with full conviction. 337. The analogies from which it is concluded that all the primary planets, at least, are inhabited by rational beings, are the following : (a.) The planets are all solid hodies resembling the earth, and not mere clouds or vapors. (&.) They all have a spherical or sfh&roidal figwre^ like our own planet. (c.) The laws of gravitation, by which we are kept upon the surface of the earth, prevail upon all the other 336. What other qnestiou proposed ? What admission ? Nature of the evidence of the inhabitedness of the planets 2 What remarkable fact ? Re- mark of Dr. Dick ? 837, State the principal points of analogy between our globe and the othei 14* 162 ASTBONOMT. / planets, as if to bind races of material beings to their sur- laces, and provide for the erection of habitations and other conveniences of life. It is very remarkable, how- ever, that those planets whose bulks are such as to indi- cate an insupportable attractive force, are not only less dense than our globe, but they have the most rapid daily revolution ; as if, by diminished density, and a strong centrifugal force combined, to reduce the attractive force, and render locomotion possible upon their surfaces. {d) The magnitudes of the planets are such as to af- ford ample scope for the abodes of myriads of inhabit- ants. It is estimated that the solar bodies, exclusive of the comets, contain an area of 78,000,000,000 of square miles, or 397 times the surface of our globe. According to the population of England, this vast area would afford a residence to 21,875,000,000,000 of inhabitants; or 27,000 times the population of our globe. le.) The planets have a dmirnaZ revoluUon around their axes, thus affording the agreeable vicissitudes of day and night. Not only are they opake bodies like our globe, receiving their light and heat from the sun, but they also revolve so as to distribute the light and shade alternately over each hemisphere. There, too, the glo- rious sun arises, to enlighten, warm, and cheer ; and there " the sun-strown firmament" of the hjore distant heavens is rendered visible by the no less important blessing of a periodic night. {/.) All the planets have an cmnual revolution round the sun ; which, in connection with the inclination of their axes to their respective orbits, necessarily results in the production of seasons. {g.) The planets, in all probability, are enveloped in ai/nwsjpheres. That this is the case with many of them is certain ; and the tact that a fixed star, or any other orb, is not rendered dim or distorted when it approaches their margin, is no evidence that the planets have no at- mosphere. This appendage to the planets is known to vary in density ; and in those cases where it is not de- planets. Substance ? Forms ? Gravitation ? Magnitude ? Days and nights? Seasons^ Atmospheres V Moons? Mountains? &u. AEE THE PLAITETS INHABITED? 163 tected by its intercepting or refracting the light, it may be of a nature too clear and rare to produce such phe- nomena. /"(/i.) The principal primary planets are provided with inoons or satellites^ to afford them light in the absence of the sun. It is not improbable that both Mars and Yenus have each, at least, one moon. The earth has one ; and as the distances of the planets are increased, the number of moons seems to increase. The discovery of six around Uranus, and only one around ISTeptune, is no evidence that others do not exist which have not yet been dis- covered. (i) The surfaces of all the planets, primaries as well as secondaries, seem to be variegated with hill amd dale, mountain amd jpTmn. These are the spots revealed by the telescope. (_;'.) Every part of the globe we inhabit is destined to the support of animal life. It would, therefore, be contrary to the analogy of nature, as displayed to us, to suppose that the other planets are empty and barren wastes, utterly devoid of animated being. And if ani- mals of any kind exist there, why not intelligent beings ? 338. If other worlds are not the abodes of intellectual life, for what were they created ? What influence do they exert upon our globe, especially those most remote? There are doubtless myriads of worlds beyond our system that will never even be seen by mortal eye, and that have no perceptible connection with our globe. If, then, they are barren and uninhabited islands in the great ocean of immensity, we repeat, for what were they ci-eated ? The inquiry presses itself upon the mind with irresistible force. Why should this one small world be inhabited, and all the rest unoccupied ? For what purpose were all these splendid and magnificent worlds fitted up, if not to be inhabited ? Why these days and years — this light and shade — these atmospheres, and seasons, and satellites, and hill and dale? 838. What difficulty on the supposition that the planets are not inhab- ited! 164 ASTEONOMT. 339. To suppose all these worlds to be fitted up upon one general plan, provided with similar conveniences as abodes for intellectual beings, and yet only one of thenj to be inhabited, is like supposing a rich capitalist would build some thirty fine dwellings, all after one model, though of difierent materials, sizes, and colors, and pro- vide in all for light, warmth, air, &c. ; and yet, having placed the family of a son in one of them, allow the remaining twenty-nine to remain unoccupied forever! And as God is wiser than man, in the same proportion does it appear absurd, that of the twenty-six planetary temples now known to exist, only one has ever been occu- pied ; while the remainder are mere specimens of Divine architecture, wheeling thi-ough the solitudes of immen- sity ! The legitimate and almost inevitable conclusion, therefore, is, that our globe is only one of the many worlds which God has created to be inhabited, and which are now the abodes of his intelligent ofispring. It seems irrational to suppose that we of earth are the only intel- ligent subjects of the " Great King," whose dominions border upon infinity. It is much more in keeping with sound reason, and with all the analogies of our globe, to suppose that " Each revolving sphere, a seeming point, Which through night's curtain sparkles on the eye, Sustains, lil^e this our earth, its busy millions." 340. The fact that we neither see, nor hear, nor Tiea^ from the inhabitants of other worlds, is no evidence that such inhabitants do not exist. It would have been premature in Columbus had he concluded, when he saw land in the distance, that it was uninhabited, simply be- cause he could not hear the shout of its savages, or see them gathered in groups upon the beach. So in regard to the distant planets. Our circumstances forbid oui- knowing positively that they are inhabited ; so that the absence of that knowledge is no argument against the inhabitedness of other worlds. 389. What illustration ? Conclusion 1 Poetry? 840. What said of the objection that we neither aee, hear, nor Jiear from the inhabitants of the other worlds ! ARE THE PLANETS INHABITED? 165 34:1. It may be thought that the extremes of heat and cold on some of the planets must be fatal to the idea of animal life, at least. But even this does not follow. Upon our globe, some animals live and flourish where others would soon die from heat or cold. And some ani- mals, having cold blood, may be frozen, and yet live. So in other worlds. He who made the three Hebrews to live in the fiery ftimace, can easily adapt the inhabit- ants of Mercury to their warm abode. And of the exte- rior planets we have only to say : " Who there inhabit must have other powers, Juices, and veins, and sense, and life, than ours ; One moment's cold, like theirs, would pierce the bone, Freeze the heart's blood, and turn us all to stone 1" Adaptation is a law of the universe ; and this at once obviates every difficulty in regard to the temperature of the planets, which might otherwise be urged as a reason why they were not inhabited. 841. Objection drawn from extremes of temperature? Poetry! What great law answers every such objection ! PART II. raE SIDEREAL HEAVENS. CHAPTEE I. THE FIXED STARS CLASSIFICATION, NUMBER, DISTANCE, ETC. 342. The sidereal heavens embrace all those celestial bodies that He around and beyond the solar system, in the region of the fixed stars. The fixed stars are distinguished from the planetary bodies by the following characteristics : {a.) They shine hy their own light, like the sun, and not by reflection. (5.) To the naked eye, they seem to twinkle or scintil- late ; while the planets appear tranquil and serene. (c.) They maintain the same general positions, with respect to each other, from age to age. On this account, they are c&WeA. fixed sta/rs. (d.) They are inconceivably distant; so that, whei viewed through a telescope, they present no sensible disk, but appear only as shining points on the dark concave of the sky. To these might be added several other peculi- arities, which will be noticed hereafter. 343. For purposes of convenience, in finding or refer- ring to particular stars, recourse is had to a variety of artificial methods of classification. 342. What parts of the book have we now gone over ! XJpoti what do we now enter ? What is meant by the sidereal heavens 2 How are the iixed stars distinguished from planetary bodies ? 848. What are constellations ? Their origin ? THE FIXED STABS CLASSIFICATION. 167 First, The whole concave of the heavens is divided into sections or groups of stars of greater or less extent. Tlie ancients imagined that the stars V7ere thrown toge- ther in clusters, resembling different objects, and they consequently named the different groups after the objects which they supposed them to resemble. These clusters, when thus marked out by the figure of some animal, person, or thing, and named accordingly, were called constellations. 344. Secondly, The stars are all classed according to their magnitnides. There are usually reckoned twelve different magnitudes, of which the first six only are visible to the naked eye, the rest being telescopic stars. These magnitudes, of course, relate only to their apparent brightness ; as the faintest star may appear dim solely on account of its immeasurable distance. "The method by which stars of different magnitudes are distinguished in astronomical charts is as follows : STABS OP DIFFEEENT MAGNrTtTDBS. 1 2 3 i 6 £789 10 11 12 ■>jC:;^;v>j^,^:>>^.:.;^ii " It mnst be observed " says Dr. Herschel, " that ttns classification into magnitudes is entirely arbitrary. Of a multitude of bright objects, diflFering, probably, intrinsically both in size and in splendor, and arranged at unequal distances from us, one must of necessity appear the brightest ; the one next below it brighter still, and so on." 345. The next step is to classify the stars of each con- stellation according to their magnitude in relation to each other, and without reference to other constellations. In this classification, the Greek alphabet is first used. For instance, the largest star in Taurus would be marked (a) Alpha ; the next largest (^) Beta ; the next (7) Gamma, &e. When the Greek alphabet is exhausted, the Roman or English is taken up ; and when these are all absorbed, recourse is finally had to figures. As Greek letters so frequently occur in catalogues and maps of the stars, and on the celestial globes, the G-reek alphabet is here inserted, for the benefit of those who are not 844. How classified by magnitudes ? (Eemark of Dr. Hersohel ?) 345. Next step in classifying ? How conducted 1 Greek letters ? (Bepsa the alphabet.) 168 ASTRONOMY. (cqiisinted with it; bat us the capitals are seldom used for designaflng the staia, the small characters only are given ; THE.GEEEK ALPHABET. a Alpha a V P Beta b i r Gamma 1 Delta JT t Epsilon e short P ? Zeta z £ 1 Eta elohg r e Theta th u I Iota i K Kappa Lamnda k 1 X c Mu m w Na n Xi X Omicron short Pi p Eho r Sigma s Tau t TJpsilon u ph Chi oil Psi /ps Omega o long 346. To aid in finding particular stars, and especially in determining their numbers, and detecting changes, should any occur, astronomers have constructed cata- logues of the stars, one of which is near 2,000 yeare old. Several of the principal stars have a specific name like the planets — as Si/rius, Aldebaran, Segulus, &c. ; and clusters of stars in a constellation sometimes receive a specific;, name, as the Pleiades and Hyades in Taurus. ' 347. The stars are still further distinguished into double, triple, and quadruple stars, binary system, vari- able stars, periodic stars, nebulous stars, &c.; all of which will be noticed hereafter. NUMBER OF THE FIXED STABS. 348. The actual Tvwmher of the stars is known only to Him who "telleth the number of the stars, and calleth them all by their names." The powers of the human mind are barely sufficient to form a vague estimate of the number near enough to be seen by our best tele- scopes, and here our inquiries must end. The number of stars, down to the twelfth magnitude, has been estimated as follows : 846. What further methods for finding particular stars ? 847. How are the stars still further distinguished I 848. Number of the stars? Of each magnitude? Number visible to naked eye ? Additional seen through telescopes ? Total ? Remarks of Herschel and Olmsted ? NUMBEK Olf THE FIXED STARS. 169 Visible to tlio naked eye. Ist magnitude 2d " 3d " 4th « 5th " . 1 6th " 18 52 177 376 1,000 4,000 Total 5,623 Yisible only through telescopea 7th magnitude 26,000 8th 9th 10th 11th 12th 170,000 1,100,000 7,000,000 46,000,000 300,000,000 Grand total, 354,301,623 Of these stars, Dr. Ilerschel remarks that from 15,000 to 20,000 of the first seven magnitudes are already regis- tered, or noted down in catalogues ; and Prof. Olmsted observes that Lalande has registered the positions of no less than 50,000. 349. The reason why there are so many more of the small stars than of the large ones is, that we are in the midst of a great cluster, with but few stars near us, the num Der mcreasing as tne nifmbbb of staes op bach magnitude. circumference of our view is enlarged. (See second cut, page 28, and also the adjoining.) Let tiie central star represent the sun (a star only among the rest), with the solar system revolving between him and the first circle. Tiie IS stars in Si^ace 1st will appear to be of the first m^nitnde. on acconnt of their nearness^ and they are thus few be- cause they embrace but a small part of the entire cluster. The stai-s of space 2d will appear smaller, being more distant ; but as it embraces more space, tliey 'will be more numerous. Thus as wo advance IVom one circle to another, the apparent magnitude constantly diminishes, bnt the num- ter constantly increases. Tlie large white circle marks the limit of our natural vision. Even this cut fails to present fully to the eye the cause of tlie rapid in- crease in nlimbers, for we can only show the surface of a aid section of our firm.iment of stars, which exhibits the increase in a plane only ; whereas our sun seems to be im- bedded in the midst of amagniacent cluster (like a single apple in the midst of a large tree richly laden with fruit), the stars of which we view around us in every direction. 34SI. Why so many more of small stars than of the larger ? (lUiistrato by diagram. Does this convey a complete idea of the position of the snn, with reference to tlie fixed stars ? Why not ? What does nis position more nearly resemble ?) 8 170 ASTEONOMT. 350. If we suppose that each of these STms is accom- panied only by as many planets as are embraced in our solar system, we ha,vejvme thousand miUiona of worlds in our fii-mament. No human mind can form a concep- tion of this number ; but even these, as will hereafter be shown, form but a minute and comparatively insignifi- cant portion of the boundless empire which the Creator has reared, and over which be reigns. " Lo, these are parts of his ways ; but how little, a portion i& heard of Him? but the thunder of his power who can under- stand." (Job xxvi. 14.) DISTANCES AND MAGNTTUDES OF THE STABS. 351. It has been demonstrated that the nearest of the fixed stars cannot be less than 20,000,000,000 ifwewty hillioni) of miles distant ! For light to travel over this space, at the rate of 200,000 miles per second, would re- quire 100,000,000 seconds, or upwards of three years. What, then, must be the distances of the telescopic stars, of the 10th and 12th magnitudes ? " If we admit," says Dr. Herschel, " that the light of a star of each mag nitnde is half that of the magnitude next above it, it will follow that a star of the first magnitude will require to be removed to 362 times its distance, to appear no larger thap one of the twelfth magnitude. It follows, therefore, that among the countless multitude of such stars, visible in telescopes, there must be many whose light has taken at least a thousand years to reach us ; and that when we observe their places, and note their changes, we g^e, in fact, reading only their history of a thousand years' date, thus wonderfully recorded." Should such a star be struck out of existence now, its light would continue to stream upon us for a thousand years to come ; and should a new star be created in those distant regions, a thousand years must pass away before its light could reach the solar system, to apprise us of' its existence. 850. What supposition and conclusion ? Scripture quotation ? 851. Distances of. the nearest stars ? Time for light to travel over thia apaee ? Suppositions and conclusions of Dr. Herschel 3 MAGNITUDE OF THE STAES. 171 352. From what we have already said respecting the almost inconceivable distances of the fixed stars, it will readily be inferred that they mugt be bodies of great magnitude, in order to be visible to us upon the earth. It is probable, however, that " one star differeth from another" in its intrinsic splendor or " glory," although we are not to infer that a star is comparatively small be cause it appears small to ns. 353. The prevailing opinion among astronomers is, that what we call the fixed stars are so many suns and centers of other systems. From a series of experiments upon the light received by us from Sirius, the nearest oi the fixed stars, it is concluded that if the sun were re- moved 141,400 times his present distance from us, or to a point thirteen billions of miles , distant, his light would be no stronger than that of Sinus ; and as Sirius is more than twenty billions of miles distant, he must, in intrinsic magnitude and splendor, be equal to two suns like ours. Dr. Wollaston, as cited by Dr. Herschel, con- cludes that this star must be equal in intrinsic light to nearly fourteen suns. According to the measurements of Sir "Wm. Herschel, the diameter of the star Vega in the Lyre is 38 times that of the sun, and its solid con- tents 54,872 times greater! The star numbered 61 in the Swan is estimated to be 200,000,000 miles in di- ameter. 354. Sir John Herschel states, that while making ob- servations with his forty-feet reflector, a star of the first magnitude was unintentionally brought into the field of view. " Sirius," says he, " announced his approach like the dawn of day ;" and so great was his splendor when thus viewed, and so strong was his light, that the great astronomer was actually driven from the eye-piece of his telescope by it, as if the sun himself had suddenly burst upon his view. 852.. What inference from the great distance of the stars ? What probst- bility as to the real magnitude of the stars ? 358. The prevailing opinion among astronomers ? Conclusions from ex- periments with Sirius ? Magnitude of Kfl^ffl? Of No. 61 in the Swan ? 354. Incident stated by Dr. Herschel ? (Relative light of the stars of the first six magnitudes ?) 172 ASTEONOMT. Accordlneto Sir Vm. Hersohel, the relative light of the stars of the flrst six magnl. tades 1b as jollows ; Light of a star of the average Ist magnitude 122 S >i " 2a " 23 " sa " 12 " " " 4th " 6 •' " " 6th " 2 " « " 6th " 1 CHAPTER II. DESCRIPTION OF THE CONSTELLATIONS. 355. Although this work is designed particularly to illustrate the mechanism of the heavens, as displayed in the solar system, we are desirous of. furnishing the learner with a sufficient guide to enable him to extend his inquiries and investigations not only to the different classes of bodies lying beyoiid the limits of the solar system in the far off heavens, but also, to the constella- tions, as such. For this purpose, we shall here furnish a brief description of the principal constellations visible in the United States, or in north latitude ; by the aid of which, the student will be able to trace them, with very little difficulty, upon that glorious celestial atlas which the Almighty has spread out before us. If the student will be at the trouble to identify the constellations by the aid of these descriptions, and without the aid of charts, it will givo him a practical familiarity with the heavens which can be acquired in no other way. Indeed, this exercise is indispen- sable to a competent knowledge of sidereal astronomy, even where maps of the constel- lations are used. Let all students, therefore, embrace every favorable opportunity for looking up the constellations. Those who wish to study their mythological, history will consult the author's edition of the " Geograply!/ of Vl6 HewomA^ by E. H. Burritt — ^the most reliable and popular work upon this subject in the English language. 356. Of the nattire and origin of the constellations we have already spoken, at 343. Their formation has been the work of ages. Some of them were known at least 3,000 years ago. In the 9th chapter of Job, we 855. Principal design of this text-book ? What further object ? What done for this purpose ? (Substance of note ?) 356. What said of the /ormation of the constellations? Antiquity! Scripture allusions ? UESCEIPTION OF THE COKSTELLATIONS. 173 read of " Arcturus, Orion, and Pleiades, and the cham- bers of the south ;" and in the 38th chapter of the same book, it is asked, " Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion ? Canst thou bring forth Mazzaroth in his season ? or canst thou guide Arcturus with his sons ?" 357. The constellations are distinguished into ancient and modern. According to Ptolemy's catalogue, the ancients had only 48 constellations ; but being found convenient in the study of the heavens, new ones were added to the list, composed of stars not yet made up into hydras and dragons, till there is now scarcely stars or room enough left to construct the smallest new con- stellation, in all the spacious heavens. The present num- ber, according to the catalogue of the Observatory Royal of iParis, is 93. 358. The constellations are further divided into the Zodiacal, Worthern, and Southern. The zodiacal con- stellations are those which lie in the sun's apparent path, or along the line of the zodiac. The nou'thern are those ■which are situated between the zodiacal and the north pole of the heavens ; and the southern, those which lie between the zodiacal and the south pole of the heavens. They are distributed as follows — viz., 12 zodiacal, 35 northern, and 46 southern. This division is convenient for reference; but in tracing the constellations in the heavens, or upon a map, it is better to begin with those that are on or near the meridian, and proceed eastward, taking northern and southern together, so far as they are in view. And where classes in astronomy are organized during the fall months, it will be fonnd advantageous to begin with the constellations that are in view at seasonable hours during those months. 359. In consequence of the eastward motion of the earth in its annual revolution, the constellations rise ear- lier and earlier every night ; so that if an observer.were to watch the stars from the same position for a whole year, he would see each constellation, in turn, coming to the meridian at midnight (or at any. other hour fixed 357. How are the constellations classified ? How many of eacn } In all ? 858. How further classified ? Describe each. How many of each ? (What said in note ?) 359. What said of the rising of the constellations? How proceed in de- scribing and tracing ? 174 ASTEOITOMY. upon), till he had seen the whole panorama of the heav- ens. Beginning, therefore, with the constellations that are on or near the meridian at 9 o'clock, on the 15th ot November, and going eastward, we shall now proceed with our description of the constellations. OCTOBER, NOVEMBEE, AND DECEMBER. 360. Andromeda. — Almost directly over head, at 9 o'clock, on the 15th of November^ may be seen the con- stellation Andromeda. The figure is that of a womcm in a sitting posture, with her head to the southwest. Andromeda may be known by three stars of the second magnitude, situated about 12° apart, nearly in a straight line, and extending from east to west. The middle star of the three is situated in her girdle, and is called Mvrach. The one west of Mirach is AlpJierat, in the head of Andromeda; and the eastern one, called Al- maak, is in her l^t foot. The star in her head is in the equinoctial colure. The three largest stars in this con- stellation are of the second magnitude. Hear Mirach, are two stars of the third and fourth magnitudes, and the three in a row constitute the girdle. This constellation embraces 66 stars, of which three are of the 2d magnitude, two of the 3d, and the rest small. About 20 from v, at the northwestern extremity of the flrdle, is a remarkable cluster or nebula of very minute stars, and the only one of the ind which is ever visible to the naked eye. It resembles two cones of light, joined at their base, about §° in length, and jO in breadth, 361. Pegasus {the Flymg Horse). — ^The figure is the head and fore parts of a Twrse, with wings. The three principal stars are of the 2d magnitude — viz., Algenib, about 15° south of Alpherat, in Andromeda; MarlcaJ), about 18° west of Algenib ; and Sheat, 15° north of Markab. These three, with Alpherat in Andromeda, form what is called the Square of Pegasus. The head of the figure is to the southwest, almost in a line with Algenib and Markab, and about 20° from the latter. * 860. Constellations on the meridian, in what months taken np ? An^ droriMda — where situated ? Figure ? Position ? How known ? Name principal stars. (How many stars in constellation? What cluster, and where ?) 861. Figure of Pegasus? Principal stars? How situated? Forming what ? How the horse situated ? His head where DESCRIPTION OF THE CONSTELLATIONS. 175 362. Pisces (the Fishes) consists of two fishes, distin- guished as the northern and western, connected bj an irregular line of stars. The Western Fish is situated directly south of the square of Pegasus — is about 20° long, with its head to the west. It includes a number of small stars, just south of Pegasus. The Northern Fish is about the same size, with its head near Mirach in Andromeda, and its body extending to the nprth. This, also, includes small stars only, and is by no means conspicuous. The flexure or ribbon, uniting the tails of the northern and western fishes, extends eastward trom the latter, from star to star, till it comes opposite the former, when it turns to the north, taking several small stars in its way, till it joins the northern fish. 363. AQTTAKros (the Water-bearer) is represented by the figure of a man in a reclining posture, with his head to the northwest. Its four largest stars are of the third magnitude. It is situated directly south of the head of Pegasus, and from 5° to 30° north of a star of the first magnitude, in the southern fish. Three of the principal stars of Aquarius are near each other in the water-pot which he holds in his right hand. 364. Pisces Austealis (the Southern Fish) is situated directly south of Aquarius. Its largest star is FomaV- haut, of jthe 1st magnitude, which constitutes the eye of the fish. The body extends westwaj-d about 20°. 365. Geits (the Crane) is situated directly south of the southern fish, with its head to the north. It is composed of a few stars only, of the fourth magnitude. As it is 45° south of the equinoctial, it appears low down in the south to persons situated in the Middle or Eastern States. ^366. The Phcenix is about 25° east of the Crane. It 862. Describe Pisces. The Western Fiah ? The Northern ? Flexure I Z^Z. Yigura of Aquarius f Largest stars ? Situation and extent? Fur- ther description. 364. Pisces Australis — largest star ? Situation of figure ? 365. fiSras— how situated ? Where ? Composition ? 866. Situation of the Plumia f Principal stars ? 176 ASTEONOMT. has two stars of the 2d magnitude, about 12° apart east and west. The most western of these, in the neck of the bird, is about 25° southeast of Fomalhaut, in the South- em Fish. The other stars of the figure ai-e of the 3d and 4:th magnitudes. 367. Cassiopeia (the Queen). — About 30° northeast of Andromeda is Cassiopeia. The figure is that of a woman sitting in a chair, with her head from the pole, and her body in the Milky "Way. Its four largest stars are of the 3d magnitude. 368. Peeseus (the King). — ^Directly north of the " seven stars," and east of Andromeda, is Perseus. The figure is that of a man with a sword in his right hand, and the head of Medusa in his left. Algol, a star of the 2d magnitude, is about 18° from the Pleiades (or seven stars), in the head of Medusa ; and 92 northeast of Al- fol is AlgeTiib, of the same magnitude, in the back of 'erseus. It embraces four other stars of the third mag'- nitude, besides many smaller. 369. MusoA (the Fly) is about 12° south of Medusa's head. It is a very small constellation, embracing one star of the 2d magnitude, two of the 3d, and a few smaller. 370. The Teiangles include a few small stars, about halfway between Musca in the southeast, and Mirach in Andromeda in the northwest. Its two principal stars are of the 3d magnitude. 371. Aeies (the Ram). — The head of Aries is about 10° south of the Triangles. It may be known by two stars about 4° apart, of the 3d and 4:th magnitudes. The most northeasterly of the two is the brightest, and is called Arietis. The < back of the figure is to the north, and the body extends eastward almost to the Pleiades. 867. Where is Gassiopda f Figure ? Situation ? Largest stars 3 868. Perseus — figure 1 Two principal stars 3 Names 3 Situation ? ULag- nitude? 869. Where is Jfesaas.^ Size? Composition? 370. The Tricmgles — where ? Principal stars ? 871. Where is' Aries? How known? Which of two prindpal stars brightest? Name? How figure situated ? Extent? DESOEIPTION OF THE CONSTELLATIONS. 177 372. Cetus (the Whale). — Directly southeast of Ari- etis, and about 25° distant, is Menka/r^ a star of the 2d magnitude, in the mouth of Ceturi. This is the largesi constellation in the heavens. It is situated below oi south of Aries. It is represented with its head to the east, and extends 50° east and west, with an average breadth of 20°. The head of Cetus may be known by five remarkable stars, 4° and 5° apart, and so situated as to form a regular pentagon, or five-sided figure. About 40° southwest of Menkar, is another star in the body ot the figure, near which are four small stars nearly in a row, and close together, running east and west. Passing eastward, we next take the constellations that are on the mei-idian in T JANUARY, FEBEUAET, AND MAECH. 373. Taueus (the Bull) will be readily found by the 'seven stars or Pleiades, which Ije in his neck. The largest star in Taurus is Aldebarcm, in the Bull's eye, a star of the first magnitude, of a reddish color, somewhat resembling the planet Mars. Aldebaran, and four other stars in the face of Taurus, compose the Hyades. They are so placed as to form the letter Y. 374. Oeion lies southeast of Taurus, and is one of the most conspicuous and beautiful of the constellations. The figure is that of a man in the act of assaulting the bull, with a sword in his belt, and a club in his right hand. It contains two stars of the first magnitude, four of the second, three of the third, and fifteen of the fourth. Be- telguese forms the right, and Bellatrix the left shoulder. A loose cluster of small stars forms the head. Three small stai'S, forming a straight line about 3° in length, constitute the ielt, called by Job " the hands of Orion." They are sometimes called the Three Kings, because they point out the Hyades and Pleiades on the one hand. 372. Getus — what star pointed out ? Size of constellation ? Situation ? Extent ? How know its liead f What otlier star pointed out ? What cou- Btellations next described in order ? .878. Tatirue — how found ? Largest star ? Hyades ? 37i. Orion — situation? Cliaraoter? Figure? Composition! 178 ASTEONOMT. and Sirius on the other. A row of very small stars runs down from the belt, forming the sword. These, with the stars of the belt, are sometimes called the Ell and Yard. Mintika, the northernmost star in the belt, is less than J° south of the equinoctial. Sigel, a bright star of the first magnitude, is in the left foot, 15° south of Bellatrix ; and Sa/i^h, of the third magnitude, is situ- ated in the right knee, 8^° east of Rigel. 375. Leptjs (the Hare) is directly south of and near Orion. It may be known by four stars of the third mag- nitude, in the form of an irregular square. Zeta, of the fourth magnitude, is the first star, situated in the back, and about 5° south of Saiph in Orion. About the same distance below Zeta are the four principal stars, in the legs and feet. 376. CoLUMBA JSToACHi (Noah's Dove) lies about 16' south of Lepus. It contains but four stars, of which Phaet is the brightest. It lies on the right, a little higher than Beta, the next brightest. This last may be known by a small star just east of it. 377. Eeidands (the Elver Po) is a large and iiTegular constellation, very difiicult to trace. It is 130° in length, and is divided into the Twrthern and southern streams. The former lies between Orion and Cetus, commencing near Eigel in the foot of Orion, and flowing out westerly in a serpentine course, near 40°, to the Whale. 378. Canis Majoe (the Greater Dog) lies southeast of Orion, and may be readily found by the brilliancy of its principal star, Sirius. This is the largest of the fixed stars, and is supposed to be the nearest to the solar sys- tem. 379. Aego Navis (the Ship Argo) is a large and splendid constellation southeast of Sirius, but so low down in the south that but little of it can be seen in the United 875. Where is Lepm f How known ''. Describe. 876. Columba NoacTii — situation? Composition? 877. Describe Eridwnue. Length ? Division i Situation ? 878. Where is Ownin Mujar situated ? How found ? What of Sirhis f • 879. Describe Avgo ^'avis. Wliere situated ? Priucipal stars, and wiiere ? DESOEIPTION 01" THE CONSTELLATIONS. 179 States. It lies southeast of Oanis Major, and may be known by the stars in the prow of the ship. Ma/rkeb^ of the fourth magnitude, is 16° southeast of Sjrius. Naos and y, still further south, are of the second magnitude, and Canopus and Miaplacidus of the first. 380. Canis MmoE (the Lesser Dog) is situated about 25° northeast of Sirius, and between Canis Major and Cancer. It is a small constellation, having one star, Procyon, of the Ist magnitude, and Oomelza, of the 2d. 381. MoNocEKOS (the Unicorn). — A little more than half way from Procyon to Betelguese in Orion, are three stars in a row, about 4° apart, and of the ith magnitude. They extend from northeast to southwest, and constitute the face of Monoceros. His head is to the west, with Canis Minor on his back, and his hind feet about 25° southeast of Procyon. It is a large constellation, with but few stars, and those mostly small. 382. Htdea (the Water Serpent).— About 20° east of Procyon are four stars of the fourth magnitude, situated about 4° apart, and so as to form a diamond; the longer axis running east and west. These constitute the Toead of Syd/ra, which points to the west. The figure extends to the south and east more than 100°, taking in an ir- regular line of stars of the 3d and 4th magnitudes. The largest star is about 15° southeast of the head. It is of the 2d magnitude, and is called AlpJia/rd. 383. Cancer (the Crab) is the least remarkable of the zodiacal constellations. It is situated about 15° north of the diamond in Hydra. It has no stars larger than the 3d magnitude, and is distinguished for a group of small stars called the Nebula of Cancer, which is often mis- taken for a comet. A common telescope resolves this nebula into a beautiful assemblage of bright stars. 384. Gemini (the Twins) may be known by two bright 880. Camia Minor — ^where ? Describe. 881. Where is J&i?joc«ros ? How situated ? Composed?- Character? 382. Where is the head of Hydra f How formed ? Extent and position > Largest star? 888. Describe Cancer. Situation ? Composition ? For wlial JistiD guished ? 180 ASTRONOMY. stars of the 2d magnitude — one in the head of each figure. They are about 5° apart ; the northeasterly one, and the brightest of the two, being about 25° due north of Procyon. This is Pollux y and the other one is called Castor. The bodies of the Twins extend from Castor and Pollux about 15° to the southwest, or toward Betel- guese, in the right shoulder of Orion. " This conetellation," says Dr. Adam Clark, " was deemed propitious to mariners ;" and on this account, the ship in which SL Paul sailed f^om Alexandria (Acts xxviii. 11) had the sign of Castor and PoUox, 385. Heeschel's Telescope covers two stars of the 5th magnitude, near each other, and about 10° north of Cas- tor ; and one other star of the same magnitude, about 10° northwest of the two first named. It is a small affair to immortalize Herschel's grand telescope. 386. The Ltnx is situated between Gemini and Can- cer on the south, and the Pole in the north, the head being to the northwest. It has no stars larger than the 4th magnitude, and these are in two pairs — the first 15° northeast of Cancer, and the other 30° north of it. It is a loose and tame constellation, with nothing striking or peculiar by which it may be identified. 387. Camelopaedalis (the Camelopard) extends from Perseus to the Pole. This, too, is a tame and uninterest- ing constellation, with but few stars in it, and those of the 4:th magnitude, or less. The hind feet of the' figure touch the Milky Way, and the head is composed of two stars of the 5th magnitude, 5° and 10° from the Pole star, toward the " dipper" in the Great Bear. We now pass eastward to constellations that are on the meridian in APEIL, MAT, AND JUNE.^ 388. Uesa Majob (the Great Bear) is one\of the most conspicuous in the northern heavens. It may be known SSi. Gemini — how known ? Names and situation of principal stars ? Of figures? (Note.) 885. Herschel's Telescope — wiiere? Character? 886. Situation of the J/yrKa ? Position ? Character ? 887. Position of Oomudopardalis / Extent ? Character ? Where the feet ? The head, and how composed? What range of constellations next de. scribed ? DE8CEIPTI0N OF THE CONSTELLATIONS. 181 by the figure of a large dipper^ which constitutes the ninder part of the animal. Ijiis dipper is composed of seven stars. The first, in the end of the handle, is called BenetnasJi, and is of the 2d magnitude. The next is Misa/r, known by a minute star almost touching it, called Alcor. Mizar is a double star. The third in the handle is Alioth. The first star in the bowl of the dipper, at the junction of the handle, is Megrez. Passing to the bottom of the dipper, we find Phad and Mer(M£, while Duh/ie forms the rim opposite the handle. Merak and Dubhe are called the Pointers, because they always point toward the Pole star. The head of the Great Bear Ues far to the west of the Pointers (apparently east when seen ielow the Pole), and is composed of numerous small stars ; while \h&feet are severally composed of two small stars, very near to each other. Megrez in Ursa Major, and Caph in Cassiopeia, are almost exactly opposite each other, on different sides of the Pole star, and about equally distant from it. They are both in the equinoc- tial colure. 389. Leo (the Lion). — About 55° southwest of the Pointers is Hegulus, a star of the 1st magnitude, in the breast of Leo. This star is situated directly in the ecliptic. The Thead of the figure is to the west, the back being to the south. North of Kegulus are several bright stars, in the form of a sickle, of which Eegulus is the handle. Denehola is a bright star of the 2d magnitude, in the Lion's tail, about 25° northeast of Eegulus, and 35° west of Arcturus. 390. Leo Minoe (the Lesser Lion) is a small cluster of stars, of which one is of the 3d, and others of the 5th magnitude, about half way between Eegulus and the Pointers. The head of the figure is northwest, and the principal stars form the body in the east, and the fore paws, which are extended to the west. 388. Desorite Ursa Mcyar. How known? Names of principal stars? Which are the Pointers f What said of Megres and Coph ? B3S. Where is Begulm? In what constellation ? How situated? Magni- tude? How Leo placed ? Where is the swife / How constituted ? Whero is Jjenelola ? 390. Deaoribe Leo Minor. Where and how situated ? 16 182 ASTRONOMY. 391. Coma Beeenices (Berenices' Hair) is a beautiful cluster of small stars, about 20° northeast of Denabola, and half way from Leo Minor to Arcturus. It has but one star as large as the 4tli magnitude. 392. CoK CoKOLi (Charles's Heart) is a bright star of the 3d magnitude, about 12° north of Coma Berenices. The figure includes several other stars, east and west, of the 5th magnitude. 393. Bootes (the Bear-driver) is directly east of Coma Berenices. The figure is that of a man, with his head toward the Pole, and Arcturus, a star of the 1st magni- tude, in the left knee. The other stars are of the 3d and 4th magnitudes. Three small stars, forming a triangle, and situated 15° northeast of Arctui-us, mark the right hand of the figure ; while two stars of the 3d and 4th magnitudes, and still further north, mark his shoulders. The head is marked by Nekha/r, another star of the 3d magnitude. 394. YiKGO (the Virgin) lies directly- south of Coma Berenices and Bootes. The figure is that of a woman with wings, with her head to the west, near Denabola in Leo ; and her feet about 40° to the east. Spioa, the prin- cipal star, is of the 1st magnitude, about 35° southwest of Arcturus. 395. Ckatee (the Cup) is composed of six small stars 30° west of Spica. The largest is of the 4th magnitude. 396. CoEvis (the Crow) is still nearer, being only 15° southwest of Spica. It has two stars of the 3d magni- tude, and three of the 4th. 39Y. LiBEA (the Balance) is about 25° east of Spica. It has two stars of the 2d magnitude, about 10° apart, which, with two others of the 3d magnitude southeast of 891. Coma Berenices — character? Situation? 892. Gor OoroU — principal star ? Situation ? 893. Where is Bootes ? Figure ? Position ? Principal stars ? 894. Where is Virgo situated ? Figure ? Position ? Principal stars ? 895. Orater — ^how situated ? Largest star ? 896. Gttrms — where ! Composition ? 897. Where is Libra t Composition ? DESCRIPTION OF THE CONSTELLATIONS. 183 them, form a small quadrilateral figure. Its few remain- ing stars are at the east, and of the 4th magnitude. 398. Centaueus (the Centaur) is a fine compact con- stellation about 30° south or southeast of Spica. It has nine stars of the 3d magnitude, mostly in the head of the figui'e. It is too low in the south to be visible in the United States, except when near the meridian. y JULY, attgust, and septembee. 399. Uesa Minoe (the Lesser Bear) is composed of a few stars near the north pole of the heavens, and mostly of the 3d and 4th magnitudes. The back of the figure is toward the pole, with its head to the west. The Fole ^tar, of the 2d magnitude, is in the extremity of its tail. 400. Deaco (the Dragon) is an ii-regular serpentine con- stellation, embracing a large circuit in the polar regions. He winds round between the Great and Little Bear, and, commencing with the tail, between the Pointers and Pole star, is easily traced, by a succession of bright stars extending from west to east. Passing south of Ursa Minor, around nearly to Cepheus, it returns westward, and terminates in four stars, which form the head, near the foot of Hercules. These four stars are 3°, 4°, and 5° apart, so situated as to form an irregular square ; the two upper ones, Mamis and Jiasfaicm, being the brightest, and both of the 2d magnitude. 401. Heecuijss (the Giant) is a large, but not very striking or conspicuous constellation. The figure is that of a giant, with a large club in his right hand, and a hydra in his left. The nead of the figure is to the south, and the whole is composed of stars from the 2d to the 4th magnitude. This constellation is thickly set with stars, the largest of which is called HasalgetM, in the head 898. Describe Centaurus. Position? Composition? 399. Vrm Mmor — position ? Principal star ? 400. Draco — position? How traced? Where head? How composed! Form what ? 401. Hercules — ^figure? Situation? Composition? Principal star? Num- ber of stars ? .184 ASTEONOMY. of the figure, and is of the 2d magnitude. It has nine stars of the 3d magnitude, and 19 of the 4th. 402. CoEONi BoEEALis (the Northern Crown) is about 15° west of the middle of Hercules. Its princij^al star is Alphacca, a bright star of the 2d magnitude, about 20° northeast of Arcturus. About the same distance, directly east of Arcturus, is a small group of stars, which constitute the head of the Serpent. 403. ScoEPio (the Scorpion) is one of the most interest ing and splendid of the constellations. It is situated about 45° east of Spica, adjoining Libra. The head of the figure is composed of five stars — one of the 2d, and the others of the 3d magnitude — forming an arc of a cir- cle convex to the west. The largest of these five stars is in the ecliptic, and is called Oraffias. About 9° south- east of Graffias is Anta/res, a star of the 1st magnitude, in the body of the figure, and of a reddish color. A number of bright stars of the 4th magnitude extend to the southeast into the Milky "Way, and then curve around to the east and north, forming the tail of Scorpio. 404. Lepus (the Wolf) consists of a small group of stars, about 15° southwest of Antares. The head of the figure is to the north. 405. SEEPENTAErns (the Serpent-bearer) is a large but uninteresting constellation, between Scorpio on the south, and Hercules- on the north. The figure is that of a man grasping a serpent, the head of which has already been described (402). The folds of the serpent may be traced by a succession of bright stars extending for some dis- tance to the east. The principal star in Serpentarius is of the 2d magnitude, and is called lias Alhague. It is situated in the head of the figure, and within 5° of Ka- salgethi, in the head of Hercules. The feet of the figure 402. Coroni Morealis — ^location? Principal star? What other group of stars mentioned ! 403. Describe Scorpio. Situation ? Composition ? Largest star in hoa'l ' What other large star? Position and composition of tail ? 404. Zepus — composition? Position? 405. Serpmtafius — situation ? Figure ? Principal star ? Situation ? DESOKIPTION OF THE CONSTELLATIONS. l^O rest upon Scorpio, and the right shoulder touches the Milky Way. 406. Lyea (the Harp) is a small constellation 15° east of Hercules. Its principal star is Vega, of the 1st mag- nitude, one of the brightest stars in the northern hemi- sphere. It has two stars of the 3d magnitude, and sev- eral others of the 4th. 407. CyGNUs (the Swan) is situated directly east of Lyra. Three bright stars, which lie along the Milky Way, form the body and neck of the Swan, running northeast and southwest ; and two others, at right angles, in a line with the middle one of the three, constitute the wings. These five stars form a large cross. Arided, in the body of the Swan, is a star of the 1st magnitude, and the remaining ones of the constellation are of the 3d and 4th. , 408. The Fox and Goose is located just south of Cyg- nus, with the head to the west. It is a small constella- tion ; the two principal stars of which, of the 2d magni- tude, form the head of the Fox. Most of the figure is in the Milky Way. 409. Aquila (the Eagle) is still south of Cygnus and the Fox. It is conspicuous for three bright stars in its neck, of which the central one, Altair, is a brilliant white star of the first magnitude, just east of the Galaxy. 410. Dblphinus (the Dolphin) is a beautiful little clus- ter of stars, 15° northeast of the Eagle. It may be known by four principal stars in the head, of the 3d magnitude, arranged in the figure of a diamond, and pointing northeast and southwest. A star of the same magnitude, about 5° south, makes the tail. 411. Antonids lies directly south of Aquila, his head being near Altair, and the body and feet to the south- west. Two stars of the 3d magnitude constitute the right 406. Zyra — situation ? Principal star ? Wliat others ? 407. (%/grms — situation? Composition? 408. Fox and Goose — location ? Position of figure ? 409. Aquila — where ? For what oonspiouous ? 410. Desoriba JJelpliirms. How known? 411. ^afoniiM— situation? How placed ? Compositiou? 16* 186 ASTEONOMT. arm, and several smaller ones make the bow and arrows held in his hand. 412. Sagittaeius (the Archer) lies next to Scorpio, and may be known by three stars in the Galaxy, arranged in a curve, to represent the iow of the archer. The central star is the brightest, and has a bright star directly west of it, forming the head of the a/rrow. The head and chest of Sagittarius are just east of the Milky Way, be- tween the tail of Scorpio and the head of Capricornns. 413. Capeicoenus (the Goat) is situated about 20° northeast of Sagittarius. The' head of the figure is to the west, and is composed of two bright stars, of the 3d magnitude, and about 4° apart. There is a smaller star between them, and several stiU smaller close around them. 414.»Ceux (the Cross) is a brilliant little constellation, but too far south to be visible to us at the north. It con- sists of four principal stars — namely, one of the 1st, two of the 2d, and one of the 3d magnitude. Besides these, there are several fine constellations about the south pole of tlie heav- ens, as the Alta/i\ the Peacock^ Charleses Oak^ &c. ; but as they cannot be traced f^om the latitudes in \f bich this book will be used, it is thought not Important to describe them. 412. Sagittarius — where ? How known ? 413. Oapricornus — where? Position of figure I Composition? 414. On/x — describe. Composition ? (What said of south circumpolor constellations ? Names ? Why not described ?) DOUBLE STABS. 187 CHAPTER III. »OUBLE, VARIABLE, AND TEMPORAKT STARS, BINARY SYSTEMS, ETC 415. Maht of the stars which, to the naked eye, ap- pear single, are found, when examined by the aid of a telescope, to consist of two or more stars, in a state of near proximity to each other. These are called BoubU Stands. When three or more stars are found thus closely connected, they are called Triple or Multvple Sta/rs. 416. Double and triple stars are supposed to be consti- tuted in two ways — ^flrst, by actual contiguity; and secondly, where they are only near the same line of vision, one of the component stars being far beyond the other. In the former case, they are said to \)q physically double, from the belief that they are bound together by attraction, and that one revolves around the other ; while in the latter case, they are considered as only optically double. 6TAEB OPTICALLY DOTTBLB. Appatent positions. True posiliom. A B Here the observer on the left sees a large and small star at A apparently near toge- ther — the lowest star being much the smallest. But instead of their being situated aa they appear to be, "with respect to each other, the true position of the smaller star may be at B instead of A ; and the diiference in their apparent magnitudes may he wholly owing to the greater distance of the lower star. Upon this subject Dr. Herschel remarks, that this nearness of the stars to each other, in certain cases, might be attributed to some accidental cause, did it occur only in a few Instances ; but the frequency of this companionship, the extreme closeness, and, in many cases, the near equality of the stars so conjoined, would alone lead to a strong suspicion of a more near and Intimate relation than mere casual juxtaposition. 415. What said of double, triple, and multiple stars ? 416. How are they supposed to be constituted? How distinguished? (Illustrate by diagram. Eemark of Dr. Herschel ? How many specim'ens of double stars given ?) 18S ASTEONOMY. The following will convey to the stndent an idea of the telescopic appearance of some of tbo double stars; 6PRCIMENS OF DOVBLB BTABB. 417. A is a double star in Ursa Minor, commonly known as the Pole star. It consists of a star of the 2d, and another of the 9th magnitude, situated about 18" apart, or about four times the diameter of the larger star. They are both of a silvery white. It requires a pretty good telescope to show this star double ; hence it is considered a very good test object by which to ascer- tain the qualities of a telescope, especially of the low- priced refractors. The writer has often seen the companion of the Pole star distinctly, with a six-inch refracting telescope, mannfactured by Mr. Henry Fitz, New York. 418. B is a view of the double star Castor, in the Twins. The stars are of a greenish white, of the 3d and 4th magnitudes, and about 5", or two diameters of the principal star, apart. This also is considered a good test object. Through ordinary telescopes, the stars seem to be in contact ; but with those of higher power, they appear fairly divided. These stars constitute what is called a Bina/ry System. 419. C is a representation of Miza/r, the middle star, in the tail of the Great Bear. It may be seen double with a good spy-glass. The stars are both of a greenish white, of the 3d and 4th magnitudes, and about 14' apart. Mizar has sometimes been seen without a com,- panion, and at other times it has been known suddenly to appear. The companion is not Aloor, near Mizar, and visible to the naked eye, but a telescopic star. at. What is Fig. A in the out ? How composed ? Color ? How seen ? (Eemark of author in note ?) 418. Fig. B — color? Magnitudes? Distance apart? Further remark? 419. Fig. C— how seen? Color? Magnitude^ Distance? Additional remarks ? BINAET AND OTHEK SYSTEMS. 189 420. D is a view of the double star Mintalca, the mid- dle star of the three forming the belt of Orion. The component stars are of the 4th and 8th magnitudes — the largest of a reddish hue, and the small one white. They are about 10" apart, or four times the diameter of the largest star. 421. E is a view of Higel, in the left foot of Orion. The components are of the 1st and 9th magnitudes, and about 10" apart. Their color is a yellowish white. 422. F is a view of the bright star Yega, in the Lyre. Its companion is a star of the 11th magnitude, situated about 40" distant. This is a close test object for an ordi- nary telescope. 423. The nurnber of double stars has been variously estimated. Sir "William Herschel enumerates upwards of 500, the individuals of which are within 30" of each other. Professor Struve of Dorpat estimated the num- ber at about 3,000 ; and more recent observations fix the number at not less than 6,000. The great number of the double stars first led astronomers to suspect a physical connection by the laws of gravitation, and also, a revolio- tion of star around star, as the planets revolve around the sun. EINAUT AND OTHEK SYSTEMS. 424. By carefully noting the relative distances and angular positions of double and multiple stars, for a se- ries of years, it has been found that many of them have their periodic revolutions around each other. Where two stars are found in a state of revolution about a com- mon center, they constitute what is called a Binary Sys- tem. These, it must be remembered, are the double and multiple stars, which appear single to the naked eye. Sir W . Herschel noticed about 50 instances of changes in the angular position of double stars ; and the revolu 420. Fig. D — desorite. Magnitude ? Color ? Distance ? 421. Fig. E — place J Components? Distance? Color? 422. Fig. F — companion? 423. Number of double stars ? Led to what ? 42t. Motions of double stars ? What are binary iysUmn ? 1 90 ASTEONOMT. tiop of some eighteen of these is considered certain. Their periods vary from 40 to 1,200 years. 425. The star numbered YO in the Serpent-bearer is a binary system. The periodic time of the revolving star is about 93 years. In the course of its revolution, the two stars sometimes appear separated, sometimes very near together, and at other times as one star. They are of the 5th and 6th magnitudes, and of a yellowish hue. 426. The star |, in the left hind paw of Ursa Mc^or, is one of these stellar systems. The revolution of its component stars began to be noticed in 1781 ; since which time they have made one complete revolution, and are now (1853) some fourteen years on the second. Of course, then, their periodic time is about 58 years. Their angular motion is about 6° 24' per year. Dr. Dick Bnpposes these stars to be some 200,000,000,000 miles apart; and npon th( supposition tliat thu smaller revolves around the latter, computes its velocity to be nol less than 2,471,000 miles every hour. This would be 85 times the velocity of Jupiter ■and 23 times the velocity of Mercury — ^the swiftest planet in the solar system. 427. The star y in Yirgo is another of these systems. It has been known as a double star for at least 130 years. The two stars are both of the 3d magnitude, and of a yellowish color. The late E. P. Mason, of Yale College, estimated its period at 171 years. More recent observa- tions and estimates by Madler give a period of 145 years. 428. "To some minds, not accustomed to deep reflec tion," says Dr. Dick, " it may appear a very trivial fact to behold a small and scarcely distinguishable point of light immediately adjacent to a larger star, and to be in- formed that this lucid point revolves around its larger attendant ; but this phenomenon, minute and trivial as it may at first sight appear, proclaims the astonishing fact, that Sims revolve arownd suns, and systems a/rowrm systems. This is a comparatively new idea, derived from our late sidereal investigations, and forms one of the 425. Describe 70 Ophiachi? 426. What specimen described ? Period ? Yearly angular motion ? (Dr. Dick's remarli !) 427. What other binary system ? How long known ? Components ? Period'! 428. Quotation from Dr. Dick. BINAKT AND OTHER SYSTEMS. 191 most sublime conceptions which the modern discoveries of astronomy have imparted. 42D. "It undoubtedly conveys a very siiblime idea, to contemplate such a globe as the planet Jupiter — a body thirteen hundred times larger than the earth — ^re- volving around the sun, at the rate of twenty-nine thou- sand miles every hour ; and the planet Saturn, with its rings and moons, revolving in a similar manner round this central orb, in an orbit five thousand six hundred and ninety millions of miles in circumference. But how much more august and overpowering the conception of a sun revolving around another sun — of a sun encircled with a retinue of huge planetary bodies, all in rapid mo- tion, revolving round a distant sun, over a cu'cumference a hundred times larger than what has been now stated, and with a velocity perhaps a hundred times greater than that of either Jupiter or Saturn, and carrying all its planets, satellites, comets, or other globes, along with it in its swift career ! Such a sun, too, may as far exceed these planets in size as our sun transcends in magnitude either this earth or the planet Yenus ; the bulk of any one of which scarcely amounts to the thirteen-hundred- thousandth part of the solar orb which enlightens our day. 430. " The further we advance in our explorations of the distant regions of space, and the more minute and specific our investigations are, the more august and as- tonishing are the scenes which open to our view, and the more elevated do our conceptions become of the gran- deur of that Almighty Being who 'marshalled all the starry hosts,' and of the TivultvpUcity and variety of ar- rangements he has introduced into his vast creation. And this consideration ought to serve as an argument to every rational being, both in a scientific and a reli- gious point of view, to stimulate him to a study of the operations of the Most High, who is ' wonderful in coun- sel, and excellent in working,' and whose wOrks in every 429. What farther remarks ? 480. Continue quotation. (What table ? Note ?) 192 ASTEOHrOMY. part of his dominions adumbrate the glory of his perfec- tions, and proclaim the depths of his wisdom, and the greatness of his power." The following table shows the periodic times of the principal binary systems, so far as known : BINAKY SYSTEMS. Names. 11 Coronse .... ^ Cancri .... g Ursse Majoris TO Ophiuchi 61 Cygni . . y V irginis Castor. . tf Coronse . y Leonis. . Period in years. 43-40 55-00 58-26 93-00 452-00 145-00 286-00 145-00 1200-00 Names. u Leonis ...... I Bootes a Hercules . . . b Ursae Majoris c " " p Ophiuchi . . . b " . . . c " ... 13 Coronse . . . . Period in " years. 82-533 117-140 31-468 58-262 61-464 73-862 80-340 92-870 608-450 The Btudent should here be reminded that these are not systems of planets revolving around suns, but of *mw revolvi^ a/rov/nd atm ; and tliat their component stars may not only bo as far apart as our sun and Sirins, but that they are probably each the center of hia^wn planetary system, like that which revolves around our central orb. /431. Besides the revolutions of these double stars around each other, they are found to have a proper mo- tion together in space, like that which our sun has around the great central Sun. Upon this subject Sir John Her- schel observes, that these stars not only revolve around each other, or about their common center of gravity, but that they are also transferred, without parting company, by a progressive motion common to both, toward some determinate region. The two stars of 61 Cygni, which are nearly equal, have remained constantly at the same, or very nearly the same,, distance of 15 , for at least 50 years past Meanwhile, they have shifted their local situation in the heavens,. in this interval of time, through no less than 4' 23" — the annual proper motion of each star being 5".8; by which quan- tity (exceeding a third of their interval) this system js every year carried bodily along in some unknown path, by a motion which, for -many centuries, must be regarded as ui^iform and rectilinear. Among stars not doable, and no way differing from the rest in any other obvious partjcular, // Cassiopeiaa is to he remarked as having the greatest proper motion of any yet ascertained, amounting to 8".74 of annual displacement 431. What other motion of th« stars ? Dr. Herschel ? (Specimen in note ? Motions ? What star named as having the greatest proper motion of anv yet known?) STAES OF VARIOUS COLOKS. 193 432. But though motions which require whole centu- ries to accumulate before they produce changes of ar- rangement, such as the naked eye can detect, are quite sufficient to destroy that idea of mathematical fixity which precludes speculation, yet are they too trifling, so far as practical applications go, to induce a change o^ language, and lead us to speak of the stars, in common parlance, as otherwise than fixed. 433. Most of the double, triple, and multiple stars are of various colors, beautifully contrasting with each other. other suns, perhaps, With their attendaDt mooos Communicating male and female light, (Which two great sexes animate the world,) Stored in each orb, perhaps, with some that live." It is probable, however, that, in most cases, this variety of colors is merely compUmenta/ry, in accordance with that general law of optics which provides that when the retina is under the influence of excitement, by any bright colored lights, feebler lights, which, seen alone, would produce no sensation but of whiteness, shall for the time appear colored with the tint complimentary to that of the brighter. Thus, if a yellow color predominate in the light of the brighter star, that of the less bright one in the same field of view will appear blue ; while, if the tint of the brighter star verge to crimson, that of the other will exhibit a tendency to green, or even appear as a vivid green, under favorable circumstances. 434. This first law of contrast is beautifully exhibited by 1 Cancri — ^the latter by y Andromedse; both fine double stars. If, however, the colored star be much the less bright of the two, it will not materially afiect the other. Thus, for instance, i\ Cassiopeise exhibits the beautiful combination of a large white star, and a small one of a rich ruddy purple. It ia by no means, however, intended to say, that in all snch cases one of the colors Is a mere effect of contrast; and it may he easier suggested in words than conceived In 482. Why called " fixed stars," if in motion ? 433. What said of the color of double stars ? Quotation from Milton ? Cause of this variety of colors ? 434. Specimens of complimentary colors ? ( 4je they all complimentary !) 17 1P4 ASTKONOMT. Imii^natlon what variety of inmnination Ubo awns — a red and a green, or a yellow an*! a blue one — must affwd a planet circulating about either, and what charming contrasts and " grateful vieisairades" — a red and a green day, for instance, alternating with a wUiln one and with darlcncBS — might arise from the presence or absence of one or other, or both, above the horizon. Insulated stars of a red color, almost as deep as that of blood, occur in many parts of the heavens ; but no green or blue star, c^any decided hue, bae^ we believe, ever been noticed nnassociatod with a companion brighter than itsell VAEIABLE OE PERIODICAL STAES. 435. Yariable stars are those which undeigo a regular ^periodical increase and diminution of lustre, amount- ing, in some cases, to a complete extinction and revival. These variations of brilliancy, to which some of the fixed stars are subject, are reckoned among the most remarkable of celestial phenomena. Some of them pass through their successive changes with great rapidity ; while in other cases, their brilliancy is increased or diminished gradually for months. Toe time occupied by one of these stars, in passing through all their different phases, is called its period. 436. One of the most remarkable of these variable stars is the star Omiaron, or Mira in the Whale. Its period is about 332 days, during which time it varies Irom a star of the 2d magnitude to complete invisibility. It appears about twelve times in eleven years^ — remains at its greatest brightness aboat a fortnight ; being then, on some occasions, equal to a large star of the 2d magni- tude. It then decreases for about three months, when it disappears. In about five months, it becomes visible again, and continues to increase during the remaining three months of its period. Its increase of light is much more rapid than its de- crease. It increases from the 6th to the 2d magaitude in 40 days, continues thus brilliant 26 days, and then fades to the 6th magnitude again in 66 days. Hence it is above the 6th magnitude for 1S2 days, and below 200 days of its period. 4S&. What are variaik eiarg f How regarded ? What (Efferenoe ? Whn4 their p»i4od f 436. What remarkable sample dBsoribed! Period? Amount of variation ' Progress of variation.? VARIABLE OE PERIODICAL STARS. 195 437. Another remarkable periodic star is that called Algol, in the constellation Perseus. It is usually visible as a star of the 2d magnitude, and such it continues for the space of 2 days 14 hours, when it suddenly begins to diminish in splendor ; and in about 3| hours, it is re- duced to the 4th magnitude. It then. begins again to increase, and in 3^ hours more is restored to its usual brightness ; going through all its changes in 2 days 20 hours and 48 minutes, or thereabouts. Through all its successive changes, this star shines with a white light, while the color of all the other variable stars is red. 438. The cav^e of these periodic variations in the brightness of some of the stars is not known. Some suppose them to be occasioned by opake bodies revolv- ing around them, and cutting off a portion of their light from us. Speaking of the sudden obscuration of Algol, mentioned above. Dr. Herschel remarks that it indicates a high degree of activity in regions where, but for such evidences, we might conclude all lifeless. 439. "I am disposed," says Dr. Dick, "to consider it as highly probable, that the interposition of the opake bodies of large planets revolving around sucn stars may, in some cases, account for the phenomena. " It 13 true that the planets connected with the solar system are so small, in comparison of the sun, that their interposition between that orb and a spectator, at an immenbe dis- tance, would produce no sensible effect. But wc have no reason to conclude that in all other systems the planets are formed in the same proportions to their central orbs as ours ; but from the variety we perceive in every part of nature, both in heaven and earth, we have reason to conclude that every system of the universe is, in some re- spects, diiferent from another. There is no improbability in admitting that the planets wliich revolve round some of the stars may be so large as to bear a considerable propor- tion (perhaps one-half or one-third) to the diameters of the orbs around wiiichthey re- volve ; in which case, if the plane of their orbit lay nearly in a line of our own vision, they would, in certain parts of their revolutions, interpose between our oye and the stars, so as to hide for a time a portion of their surfaces from our view, while in that part of their orbits which is next to the earth." 4J:0. Others, again, are of opinion that those distant suns have one luminous and one opake or clouded hemi- sphere ; and that their variations may thus result from a revolution upon their axes, by which they would present us alternately with their full and their diminished luster. 487. What other Bpecimen ! Usual appearance ? Period ? Peculiar color ? 438. Cause of these variations ? Supposition ? "Or. Herschel ? 439. Dr. Dick's opinion ? (Eeasoning in note 1) 410. What other hypothesis stated 3 196 ASTBONOMT. Another theory is, that these stars are moving with inconceivable velocity in an immense elliptical orbit, the longer axis of which is nearly in a direction toward the eye, and the shorter axis of which would be imper- ceptible from our system. In such case, the star would appear alternately to approach and recede ; now looking in upon our quarter of the universe for a few days, and then rushing back into immensity, to be seen no more by human eyes during the lapse of years or of ages. 441. " Whatever may be the cause" says Mr. Abbott, " the fact of these variations is perfectly established, and the contemplation of the stupendous changes which must be occurring in those distant orbs overwhelms the mind with amazement. "Worlds vastly larger than our sun sud- denly appear, and as suddenly disappear. Now they blaze forth with most resplendent brilliancy, and again they fade away, and often are apparently blotted from existence. These worlds are unquestionably thronged with myriads of inhabitants ; and the phenomenon which to us appears but as the waxing or waning luster of a twinkling star, may, to the dwellers on these orbs, be evolutions of grandeur, such as no earthly imagination has ever conceived. But these scenes, now veiled from human eyes, will doubtless all be revealed, when the Christian shall ascend on an angel's wing to the angel's home." TEMPOEAET STAES. 442. 1 emporary stars are those which have appeared from time to time in different parts of the heavens, blaz- ing forth with extraordinary luster, and, after remaining for a while apparently immovable, died away, and lelt no traces of their existence behind. Some writers class them among the periodical stars, while others notice them under the head of " New and Lost Stars." A star of this kind, which appeared in the year 125 441. Kemarts of Mr. Abbott? 442. What are temporary ataraf How classed? Mrst noticed? What other instaiiorj '( TEMPOEAET STABS. 197 B. c, led Hipparchus to draw up a catalogue of the stars the earliest on record. In a. d. 389, a similar star ap peared near the largest star in the Eagle, which, after remaining for three weeks as bright as Venus, disap- peared entirely from view. M3. On the 11th of November, 1572, Tycho Brahe, a celebrated Danish astronomer, was returning, in the evening, from his laboratory to his dwelling-house, when he was sui-prised to find a group of country people gazing upon a star which he was sure did not exist half an hour before. It was then as bright as Sirius, and continued to increase till it surpassed Jupiter in brightness, and was visible at noonday. In December of the same year it began to diminish ; and in March, 1574, had entirely disappeared. This remarkable star was in the constellation Cassio- peia, about 5° northeast of the star Caph. The place where it once shone is now a dark void ! 444. This star was observed for about 16 months, ana during the time of its visibility its color exhibited all the different shades of a prodigious flame. _^" First it was of a dazzling white, then of a reddish yellow, and lastly of an ashy paleness, in which its light expired." " It is im- possible," says Mrs. Sumerville, " to imagine any thing more tremendous than a conflagration that could be visi- ble at such a distance." 445. In reference to the same phenomenon. Dr. Dick observes, that " the splendor concentrated in that point of the heavens where the star appeared must have been, in reality, more than equal to the blaze of twelve hundred thousand worlds such as ours, were they all collected into one mass, and aU at once wrapped in flames. iN^ay, it is not improbable, that were a globe as large as would fill the whole circumference of the earth's annual orbit to be lighted up with a splendor similar to that of the as. What other remarkable iostauce described? By whom 3 When! In what constellation ? Position 3 ■14.4. How long observed ? Appearance ? Mrs. Snmerville ? 445. Sr. Dick's remarks i 198 ASTEONOMT. sun, it would scarcely surpass in brilliancy and splendor the star to which we refer." 446. Eev. Prof. VmcE, who has been characterized as " one of the most learned and pious astronomers of the age," advances the opinion, that " the disappearance of some stars may be the destruction of that systeni at the time appointed by the Deity for the probation of its in- habitants ; and the appearance of new stare may be the formation of new systems for new races of beings then called into existence to adorn the works of their Creator." 447. La Place, whose opinion upon such subjects is always entitled to consideration, says : " As to these stars which suddenly shine forth with a very vivid light, and then immediately disappear, it is extremely probable that great conflagrations, produced by extraordinary causes, take place on their surface. This conjecture is confirmed by their change of color, which is analogous to that presented to us on the earth by those bodies which are set on fire, and then gradually extinguished." 448. Dr. John Mason Goode, author of the Book of Nature^ &c., seems to have entertained opinions similar to those already expressed. "Worlds and systems of worlds," says he, " are not only perpetually creating, but also perpetually disappearing. It is an extraordinary fact, that within the period of the last century, not less than thirteen sta/rs^ in different constellations, seem to have totally perished, and ten new ones to have been created. In many instances, it is imquesti enable, that the stars themselves, the supposed habitations of other kinds or orders of intelligent beings, together with the different planets by which it is probable they were sur- rounded, have utterly vanished, and the spots they occu- Cied in the heavens have become blanks. "What has efallen other systems will assuredly befall our own. Of the time and rnanner we know nothing, but the fact is incontrovertible; it is foretold by revelation; it is in- scribed in the heavens ; it is felt through the eai-th. Such is the awful and daily text. "What, then, ought to be the comment ?" 446. Prof. Vinoe's remarks? 447. La Place's I 448. Dr. Goode's ' OLUSTBBS OF STAKS AND NEBULA. 199 CHAPTER IV. OliUSTKRS OF STABS AND NEBUL^ TELESGOPIO TIBW OF TUB FLEUDBS. 449. In surveying the concave of the heavens in a clear night, we observe here and there groups of stars, forming bright patches, aa if drawn together by some cause other than casual distribution. Such are the Pleiades and Hyades in Taurus. These are called Clusters of Stars. The luminous spot called the JBee Hive, in Cancer (visi- ble to the naked eye), is somewhat similar, but less definite, and requires a moderate telescope to re- solve it into stars. In the sword-handle of Perseus is another such spot or clus- ter, which is also visible to the naked eye, but which requires a rather better telescope to resolve it into dis- tinct stars. When fairly in view, however, it is one of the most splendid and magnificent spectacles upon which tb'i »ye can rest. " O -what a confluence of ethereal fires, From worlds unnumber'd down the steep of heareD, Stream to a point, and center on my sight." 450. Many of these faint and compact clusters have 'jeen mistaken for comets, as through telescopes of mod- 4i9. Clusters? Specimens? 450. What mistake respecting ? What like ? How known that they are not comets ? 200 ABTEONOMT. BOUin> OLUSTEB IN CAPBICOEN. erate power they appear like such. Messier has given a list of 103 objects of this sort, with which all who search for comets ought to be familiar, to avoid being misled by their similarity of appearance. That they are not comets, is evident from their fixedness in the heavens, and from the fact, that when we come to examine them with in- struments of great power, they are perceived to consist entirely of stars, crowded together so as to exhibit a defi- nite outline, and to run up to a blaze of light in the cen ter, where their condensation is usually the greatest. 451. Some of these clusters are of an exceedingly rough figure, and convey the idea of a globular space filled full of stars, insulated in the heavens, and consti- tuting in itself a family or so- ciety apart from the rest, and subject only to its own internal laws. It would be a vain effort to attempt to count the • stars in one of these clusters. They are not to be reckoned by hundreds ; and on a rough calculation, grounded on the apparent inter- vals between them at the bor- ders, and the angular diameter of the whole group, it would appear that many clusters of this description must contain, at least, from ten to twenty thousand stars, compacted and wedged together in a round space, whose angular diameter does not ex- ceed eight or ten minutes, or an area equal to a tenth part of that covered by the moon. 452. Some of these clusters have a very irregular out- line. These are generally less rich in stars, and especi- ally less condensed toward the center. They are also less definite in point of outline. In some of them, the stars are nearly all of a size ; in others, extremely different. It is no uncommon thing to find a very red star, much 451. What said of the/orm of these clusters ? Stars in each ? Appnreui diameter ? 452. What further respecting forms ? Character of irregular clusters i NEBULA. mi RICH CLTJSTEK IN BJJBKNIOES' IIAIB. brighter than the rest, occupying a conspicuous ..L'lation in them. 453. It is by no means improbable that the individual stars of these clusters are suns like our own, the centers of so many distinct systems, and that their mutual dis- tances are equal to those which separate our sun from the nearest fixed stars. Be- sides, the round figure of some of these groups seems to indicate the existence of some general bond of union, of the nature of an attractive force. This is one of tli& most gorgeous clusters In all the heavens. Sir John Herschel pro- nounced it the most magnificent object he had ever beheld. It is about 6' in diameter, and contains a countless throng of stars, that scarcely ever fail to elicit a burst of sur- prise and astonishment from the beholder! Who can gaze upon such a scene, and not for a time forgot earth, in tbe rapt contempla- tion of the distant glory? " There's not a scene to mortals given, That more divides the soul and clod, Than yon proud heraldry of heaven- Yon burning blazonry of God," A similar cluster, though somewhat different in form, may be found between S and !?, In Hercules. This, too, is a most magnificent object Under favorable circumstances, It may be seen witli the naked eye ; and by the aid of telescopes, it is easily resolved Into myriads of stars. " It is, indeed, truly glorious," says Smyth, " and enlarges on the eye by studious gazing.". " Perhaps," says Prof Nichol, " no one ever saw it, for the first time, through a telescope, without uttering a shout of wonder." NEBULA. 454. The term JVehulce is applied to those clusters of stars that are so distant as to appear only like a faint cloud or haze of light. In this sense, some of the clus- ters heretofore described may be classed as nebulae ; and, indeed, it may be said of all the different kinds of nebu- lae, that it is impossible to say where one species ends, and another begins. 453. What said of individual stars in clusters ? Of round figure of some clusters ? (What speoimen in out ? What said of it ? Angular diameter ? Effect of seeing ? Poetry? What other similar cluster 3 What said of it () 454. VfhsABia Mbulmr How differ from olustan* 3 9* >? 202 ABTEONOHT. 455. Hesolvahle Nefmlm are those clusters, the light o whose individual stars are blended together, when seei. through a common telescope ; but which, when viewed through glasses of sufficient power, can be resolved into distinct stars. 456. IrresohabU N^lm are those nebulous spots which were formerly supposed to consist of vast fields of matter in a high state of rarefaction, and not of distinct stars. But it is doubtful whether any nebulae exist which could not be resolved into stars, had we telescopes of sufficient power. " About the close of last year," says Dr. Scoresby, in 1 846, " the Earl of Rosse succeeded in getting his great telescope into complete operation ; and during the firat month of his observations on fifty of the unresolvable nebulae, he succeeded in ascertaining that 43 of theiu were already resolvable into masses of stars. Thus is confirmed the opinion, that we have only to increase the power of the instrument to resolve all the nebulae into "stars, and the grand nebular hypothesis of La Place into splendid astronomical dream." DOOSLK 1IIBUI.& 457. Nebulae of almost eveiy conceivable shape may be found in the heavens. Some are round — others elliptical. Some occur sin- gly, while others are double, or seem to be connected together. The specimen here shownisjnthe Greybound. The two nebulse are elliptical, as shown, and are so united as to stand perpendicularly to each other. 458. Annula/r JVebuloe are those that exhibit the fonn of a ring. Of these, but few specimens are known. One of the most striking may be found about 6° helow Misa/r, 455. What are resolvable nebulse? How when seen through powerful telescopes ! 456. Irresolvable nebulee ? Are any nebulEe really irresolvable ? Remarks from Dr. Scoresby i 457. What further description of nebulee ? Specimen ? 458. What are ommiZo)" nebulce ! Are they common 9 What epeoimou ii cut 1 Describe It. NEBTJLiE. 203 Great Bear. It ANKITLAB NBBtTL^ the middle star in the tail of the consists of a large and bright globular nebula, surrounded by a double ring, at a considerable distance from the globe; or rather a single ring divided through about two-fifths of its circum- ference, and having one portion turned up, as it were, out of the plane of the rest.* A faint nebu- lous atmosphere, and a email round nebula near it, like a satellite, com- pletes the figure. 459. Another very conspicuous nebula of this class may be found half-way between /3 and 7, in the Lyre, and may be seen with a telescope of moderate power. It is small, and particularly well defined, so as, in fact, to have much more the appearance of a flat oval solid ring, than of a nebula. The space within the ring is filled with a faint hazy light, uniformly spread over it, like a fine gauze stretched over a hoop. 460. " JPlanetcm/ JVebiilcB,'" says Dr. Herschel, " are very extraordinary objects. They have, as their name imports, exactly the appearance of planets — round or slightly oval discs— in some instances quite sharply ter- minated, in others a little hazy at the borders, and of a light exactly equable, or only a very little mottled, which, in some of them, approaches in vividness to that of the actual planets. Wnatever be their nature, they must be of enormous magnitude." 461. Stellar Ifeiulm, or Nebulous Stars, are such as present the appearance of a thin cloud, with a bright star in or near the center. They are round or oval- 459. What other annular nebulae ? Describe. 460. Planetary nehalx I Describe. 46i. Stellar nebulae ? Kemarks of Professor Mitchel? 204 ASTEONOMT. STELLAB NEISITLM. shaped, and look like a star with a bun- around it,^ or a candle shining through horn. " It was an object of this kind," says Prof. Mitchel, " which first suggested to Sir W. Herschel his great theory of the formation irf suns out of a nebulous fluid. He thought it impossible to ac- count for the 'central location of stars, surrounded by nebulous matter, in any way except by sup- posing this to be a sort of atmos- phere attracted to, and sustained in its spherical form by, the power of the centfal body. I have examined specimens of these objects, and always with increasing wonder. Their magnitude must be enor- mous, as the stars are certainly not nearer than other stars ; and yet the circular halo around them is of a diameter easily measured, and proves them to have a circumference perhaps greater than the entire orbit of Neptune." ^ „ „ GREAT NEfitTLA. IN ORION. 462. One of the most remark-, able nebula in all the heavens may be foimd around the mid- dle star in the sword of Orion. It is easily seen with a common telescope. It is shaped like the head of some animal — alish,-il)r instance — witli its mouth open. Near the inner surface of this month are four stars, ranged in the form of a trapesium. It requires a good telescope to see four stars ; but, with powerful instruments, six are visible, instead of four. 462. Doseribo tlio nebula of Orion? Where situated? Shape? What stars in it? NEBULA. 205 468. The sim is considered by astronomers as belong- ing to this class of nebulous stars; and the Zodiacal Lvgltt (322 and 325) has been regarded as of the nature of the gaseous matter with which the nebulous stars are surrounded. It is supposed that if we were as far from the sun as from the stellar nebula, he would appear to us onlj as a small and nebulous star ! 464. Until recently, the most powerful instruments have failed to reveal any thing like distinct stars, as com- posing the body of the remarkable nebula in Orion. Both theHerschels regarded it as positively irresolvable; or, in other words, as composed of nebulous fluid or un- organized matter. But it has recently been seen to be composed of distinct stars, both by the monster telescope of Lord Eosse, and the great refractor of Cambridge, near Boston. 465. The magnitude of this nebula must be beyond all human conception. " If," says Mr. Smyth, " the parallax of this nebula be no greater than that of the stars, its breadth cannot be less than a hundred times that of the diameter of the earth's orbit ; but it| as is more probable, it is a vast distance beyond them, its magnitude must be utterly inconceivable." ■466. Prof. MitcJiel observes, that in case light be not absorbed in its journey through the celestial spaces, the light of the nebula of Orion cannot reach the eye in less than 60,000 years, with a velocity of twelve millions of miles in every minute of time! And yet this object may be seen from this stupendous distance, even by the naked eye ! "What, then, must be its dimensions ? Here, indeed, we behold a universe of itself too vast for the imagination to grasp, and yet so remote as to aj)pear a taint spot upon the sky." 467. The number of such nebulous bodies is unknown 463. Kemarlis respecting the sun ? 464. How the nebula in Orion regarded ? Wliat recent discovery ? 465. Its probable magnitude ? Keraark of Smyth? 466. Prof. Mitchel's observations respecting its distance and dimensions ? 467. What said of the number of nebidous bodies in the heavens ? Where T.oat abundant ? Hersohel's catalogue i Various fonns ! 208 ASTRONOMY. perhaps we should say innumerable. They are especially abundant in the Galaxy or Milky Way. Sir W. Her- Bchel arranged a catalogue, showing the places of two thousa/nd of these objects. They are of all shapes and sizes, and of all degrees of brightness, from the faintest milky appearance to the light of a fixed star. 468. Sta/r Dust is a name given to those exceedingly faint nebulous patches that appear to be scattered about at random in the far-distant heavens. It is barely visible through the best telescopes, and seems to form a sort of back-ground, far beyond all stars, clusters, and nebulae, resolvable or irresolvable. 469. " The nebulae," says Sir John Herschel, " fur oish, in every point of view, an inexhaustible field of speculation and conjecture. That by far a larger share of them consist of stars, there can be little doubt ; and in the interminable range of system upon system, and firrtMugent upon firmament, which we thus catch a glirapse8:^44eimagination is bewildered and lost " 470. It is a geBSKral belief amopg astronomers that the material universe^ consists of distanct clusters, separated from each other by innumerable chasms : that the fixed stars by which we" are surrounded constitute one great cluster — the sun being a star with the rest, and appearing as he does to us, solely on account of, our nearness to him ; that the nebulae are far beyond our cluster, like so many distinct continents i^ the.-bous^leg§^cean of immensity. 471. Could we leave our system, and pass outward toward the fixed stars, they would doubtless expand to the dimensions of suns as we approached them, while our own central luminary would dwindle to a glimmering star. Reaching the frontier of the cluster, and plunging off into the awful solitudes of space, toward the distant nebulae beyond, we should see them also expand as we drew near, while our vast firmament of stars seemed~to 468. What is meant by star dust f Where supposed to be situated ? 469. Hersohel's remark respecting the nebulae ? 470. What the prevailing opinion among astronomers, as to the structure of the universe ? 471. What ima^finary journey and scenery described by the author t NEBULiE. 207 be gathering into a compact group ; till at length, enter- ing the bosom of the distant nebulae, we should find our- selves surrounded by new and strange constellations ; and if we saw our own firmament at all, should see it only as a faint annular nebula, far beyond the reach ol all unassisted vision. 472. The great stellar cluster in which the sun and solar system are imbedded is supposed, in its form, to resemble a double convex lens, with the sun and solar system near its center ; aud by being viewed edgewise from our central position, to give us the phenomenon of the Milky Way, OBSjLT ETBBTTIiA. OF THE BOI.AB BYSTXU. The above is an edgewise view of thegreat stellar cluster, in the midst of which the solar system is placed, as drawn by Sir "William Herschel. Its figure was ascertained by gauging the space-penetrating power of his telescope, and then " sounding the heavens, * to ascertain the distance through the cluster, in all directions, to the open void. The nebulse lie in distinct and independent islands, far beyond the limits of our cluster. Let the student imagine the sun to be one of the stars near the middle of the lens- shaped cluster, of which the above is an edge view, with the planets revolving close around it I^ then, he look out upon the surrounding stars, the number visible, and their distinctness, will depend upon the direction in which he looks. If toward the thin part of the cluster (either up or down in the cut), fewer stars will be seen, while they will be comparatively distinct But if the view be toward the edge of the cluster, instead of the sides (or horizontally, in the cut), there will be seen beyond the large stars, and fadinc; away to an indistinct and mingled light, a nmnberless host of stare ; and this zone of distant stars will extend quite around the heavens. Such is tlie Galaxy or Milky Way. The zone of milky light is the light of the stars in the remote edge of the great cluster. The opening in the left end of the figure is a split in the clu^iter, and constitutes the division seen in the milky way, extending part way around the heavens. See cut page 203. The vast apparent extent of the Galaxy, as compared with other nebulse, is supposed to be justly attributable to its comparative nearness. WorQ we as far from the solar system as from the nebulse in tbe Lyre, the Milky Way would doubtless appear as an annvlar nebula no larger than that. It may therefore with propriety be called " the great nebula of the solar system." 473. Sir W. Herschel estimated that 50,000 stars passed the field of his telescope, in the Milky Way, in a 472. .Supposed form of our own stellar cluster I Philosophy of Galaxy (Why apparently so large ? How appear at a great distance f) 478. Stars in Milky Way ? Mutual distances ? Character of efich star ! 208 ASTEONOMT. single hour! And yet the space thus examined was hardly a point in the mighty concave of our 6wn "sun- Btrown firmament." "What an idea is here conveyed to the mind, of the almost boundless extent of the uni- verse ! The mutual distances of these innumerable orbs are probably not less than the distance from our sun to the nearest fixed stars, while they are each the center of a distinct system of worlds, to which they dispense light and heat. 474. "Were the universe limited to the Great Solar Cluster, in the midst of which we are placed, it would be impossible to conceive of its almost infinite dimen- sions ; but when we reflect that this vast and glowing zone of suns is but one of thousands . of such assem- blages, which, from their remoteness, appear only as fleecy clouds hovering over the frontiers of space, we are absolutely overwhelmed and lost in the mighty abyss of being ! 475. And here we close our rapid and necessarily im- perfect survey of the Sidereal Heavens. And while the mind of the student is filled with awe, in contemplating the vastness and majesty of creation, let him not forget that over all these Jehovah reigns — that " these are but parts of his ways ;" and yet so perfect is his knowledge and providence in every world, that the very hairs of our heads are numbered, and not a sparrow falls without his notice. And while we behold the wisdom, power, and goodness of God so gloriously inscribed in the heav- ens, let us learn to be humble and obedient — to love and serve our Maker here — that we may be prepared for the still more extended scenes of another life, and for the society of the wise and good in a world to come. 474. Magnitude of our own duster? What in comparison with all otliorsi 4T5. Remarks in closing paragraph ? Moral reflections 1 PART III. PRACTICAL ASTRONOMY. CHAPTER I. PROPERTIES 1 LIGHT. 476. Practical AstroTwniy has respect to the mecms employed for the acquisition of astronomical knowledge. It includes the properties of light, the structure and use of instruments, and the processes of mathematical calcu- lation. In the present treatise, nothing ftirther will be attempted than a mere introduction to practical astronomy. In a work desired for popular use, mathematical demonstrations would be out of place. Still, every student in astronomy should know how telescopes are made, upon what laws they depend for their power, and how they are used. It is for this purpose mainly that we add the following chapters on Practical Astronomy. 477. Light is that invisible ethereal substance by whrch we are apprised of the existence, forms, and colors of material objects, through the medium of the visual organs. To this subtile fluid we are especially indebted for our knowledge of those distant worlds that are the principal subjects of astronomical inquiry. 478. The term ligJit is used in two different senses. It may signify either light itself, or the degree of light by which we are enabled to see objects distinctly. In this last sense, we put light in opposition to darkness. But 476. Parts of the book gone over ? Subject of Part III. ? Of Chapter I. ? What is practical astrmomij f ( How far discussed in this treatise ?) 477. Define light. For what indebted to it? 478. Dift'erent senses in which the term is used ? What is darltness ? Can It be dark and light at the same time ? Is there any place without light ? (Quotation from Dick ?) 210 ASTEONOMT. it should be borne in mind that darkness is merely the absence of that degree of light which is necessary to human vision ; and when it is dark to us, it may be light to many of the lower animals. Indeed, there is more or less light even in the darkest night, and in the deepest dungeon. " Those unfortunate individuals," says Dr. Dick, " who have been confined in the dark- est dungeons, have declared, that though, on their first entrance, no object could 1)e per- ceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive mice, rats, and other animals that infested their cells, and likewise the walls of their apartments ; which shows that, even in jsuch situa^ tions, light is present, and produces a certain degree of influence." 479. Of the nature of the substance we call light two theories have been advanced. The first is, that the whole sphere of the universe is filled with a subtile fluid, which receives from luminous bodies. an agitation; so that, by its continued vibratory motion, we are enabled to per- ceive luminous bodies. This was the opinion of Des- cartes, Euler, Huygens, and Franklin. The second theory is, that light consists of particles thrown off from luminous bodies, and actually proceeding through space. This is the doctrine of Newton, and of the British philosophers generally. Without attempting to decide, in this place, upon the relative merits of these twi by* potheses, we shall use those terms, for convenience sake, that indicate the actual passage of light from one body to another. 480. Light proceeds from luminous bodies in straight lines, and in all directions. It will not wind its way through a crooked passage, like sound ; neither is it con- fined to a part of the circumference around it," As the sun may be seen from every point in the solar system, and far hence into space in every direction, even till he appears but a faint and glimmering star, it is evident that he fills every part of this vast space with his beams. And the same might be said of every star in the firmament 481. As vision depends not upon the existence of light merely, but requires a certain degree of light to emanate from the object, and to enter the pupil of the eye, it is obvious that if we can, by any means, concentrate the 479. What theories of the ruLi/wre of light, and by whom supported respect- ively ? (Eemark of author ?) 480. How light proceeds from luminous bodies ! (Radiations from sun and Btars!) 481. How improve vision, and why ? (Animals !) EEFBMjnON OF LIGHT. 211 light, BO that more may enter the eye, it will improve our perception of visible objects, and even enable us to see objects otherwise wholly invisible. Some nnimals have tho power of adapting their eyes to the existing degree of light The cat, horse, &c., can see day or night; while the owL that sees well in the niEhtsees poorly in the day-time. 482. Light may be turned out of its course either by reflectwn or refraction. It is reflected when it falls upon the highly polished surface of metals and other intrans- parent substances ; and refracted when it passes through transparent substances of different densities. LIGHT EEFKAOTEn BT WATEE. KEFEACnON OF LIGHT. 483. Whenever light passes from a rare medium to one more dense, and enters the latter obliquely, it inva- riably leaves its first direction, and assumes a new one. This change or bending of the rays of light is what is called Refraction. The term refract is from the Latin re, and frango, to break ; and sigaifles the break Ingof the natural course of the raya. 484. As air and water are both transparent, but of different densities, it follows that, when light passes obliquely from one to the other,,* it will be refracte^. If it pass from th^ air into the water, it will be re- fracted wward a per- pendicufar. Here the ray A C strikes the water perpendicularly, and passes directly through to B without being refracted. But the ray D strikes tlie water at C obliquely ; and instead of passing straigtit _ _ ._ _ through to E, is refracted at C, B F E and reaches the bottom of the water at F. I^ therefore, a person were to receive the ray into the eye at F, and to judge of the place of the object from wliich the lijjht emanates from the direction of the ray G F, lie would conclude that he saw the object at G, unless he made allowance for the refraction of the light at C. -\ AIR 'X ==^ ■ ^seJ 1 '- s, ) 482. How light turned out of course ? 4SS. What i» refraciion ? How produced? (Derivation of term '^roflt/) 48i. How refracted by air and water ? (Illustrate by diagram.) 212 ASTEOMOHT. 4:85. When lieht uoht pbockkdiho feojj watd, passes obliquely ^ ^ i'rom a denser to a rarer medium, as from water into air, it is refracted from a perpendicular to- ward a horizontal. Here the lamp A shines up throiigli water into air. The ray that strikes the surface per- pendicularly passes on to B ■without being refracted; but the other rays that leave the water obliquely are refracted toward a horizontal directiou, in proportion to their distance ftom the perpendicular; or, in other words, in propor- tion to the obliquity of their contact with the surface of the water. 486. In consequence of the refraction of light toward a horizontal direction, in passing from water into air, a pole, half of which is in the water, seems bent at the surface, and the lower end seems nearer the surface than it really is. For the V '"• " EPFECT OF REFEACTIOW. same reason, the bottom of a river seems higher, if seen obliquely, than it really is ; and the water is always deeper than we judge it to be. In this cut, the oar, the blade of which is in the water, seems bent at the smface of the water. The rays of light passing from the part under water to the surface at D, are refract- ed toward a horizontal direction at that point, and received into the eye of the observer at B, who, judging of the position of the immersed portion of the oar from the direction of the rays D B, locates the blade of the oar at G; thus reversing the etfoct illustrated at 484. 487. The refracting power of different transparent substances depends mainly upon their density. Water refracts more than air, glass more than water, and dia- mond most of all. But the angle of incidence, or the obliquity of the contact of the rays with the denser sub- 485. How -when light passes from denser to rarer mediums ? (Diagram.) 486. Effect of refraction upon objeots seen under water ? (Diagram.) 487. Upon what does the refracting power of different transparent media epend ? EEFEACTION OF LIGHT. 213 EETKOT of BEFBAOnoiT. stance, has also much to do in determining the amoimt of refraction. ^ 488. By the aid of re- fraction, we may see ob- jects that are actually ie- hind an opake or intrans- parent body. Here the piece of money at A, at the bottom of the cup, would be invisible to the beholder at B, if the cup was empty, as the light from the money would pass from A to C; but when the cup is filled with water, the light is refracted to B, and the beholder sees the money appa- rently at D. 489. By the law of refraction, light has been found to consist of a combination of colors. By passing a beam of light through a triangular piece of Bint glass called a prism^ it is seen that some parts of the light are more refrangible than others, so that the light is analyzed, or separated into its component parts or elements. BETBAGTION BY A PBISU. Let a ray of light from the sun be admitted through a hole in the window shutter, A nto a room from which all other light is excluded ; it will form, on a screen placed a lit- tle distance in front, a circular image, B, of white light. Now, interpose near the shut- ter a glass prism. 0, and the light, in passing tlirough it, will not only be refracted in the same direction, both when it enters the prism and when it leaves It, but the several rays of which white light is composed will be separated, and will arrange in regular order on the screen, immediately above the image B, which vrill disappear. The violet ray, it will be seen, is most refracted, and the red least ; the whole forming on the screen an elongated image of the sun, called the solar spectrv/m. — Johnston, 488. What other effect of refraction ? - (How illustrated ?) 4 9. What discovery by refraction ? (How made ?) 214 ASTEONOMT. 490. It is the refraction of the clouds that gives the sky its beautiful colors morning and evening ; and the refracting power of the rain-drops produces the beautiful phenomenon of the rainbow. ATMOSPHBEICAL EBFEACTION. ^ 491. The refracting power of the atmosphere produces many curious phenomena. Sometimes ships are seen bottom upwards in the air, single or double. At other times, objects really below the horizon, as ships or islands, seem to rise up, and to come distinctly in view. 492. A very important eflPect of refraction, as it relates to astronomy, is, that it more or less affects the apparent places of all the heavenly bodies. As the light coming from them strikes the atmosphere obliquely, and passes downward through it, it is refracted or bent toward the earth, or toward a perpendicular. And as we judge of the position of the object by the direction of the ray when it enters the eye, we place objects higher in the heavens than they really are. ATHOSFHERIOAL BEFRACTION. Let A, in the cut, represent the earth ; B, the atmosphere ; C C, the visible horizon ; and the exterior circle the apparent concave of the heavens. Now, as the liirht passes from the stars, and strikes the atmosphere, it is seen to curve downward, hecanse it stril;es the atmosphere obliquely; and the air increases in density as we approach the earth. But as the amount of refraction depends not only upon the density, but also upon the obliquity of the contact, it is seen that the refraction is greatest at the horizon, and gradual]}' diminishes till the object reache^s the zenith, when there is no obliquity, and the refraction wholly ceases. The dark lines in the cut show the true, and the dotted the apparent positions. 490. What other effects of refraction ? 491. Atmospherical refraction? Effects on terrestrial objects? 482. Upon apparent places of stars, &o. ? (Diagram. What said of exag- geration t) ATM03PHEKI0AI, KEFEACTION. 215 In the cut, the depth of the atmosphere, as compared with the globe, is greatly exag^ gorated. Even allowing it to be 60 miles deep, it is only ^'^th of the semi-diameter of the globe, which is equal to only about -\;th of an inch upon a common IS-lnch globe. But it was necessary to exaggerate, in order to illustrate the principle. , , 493. The amount of displacement of objects in the liorizon, by atmospherical refraction, is about 33', or a little more than the greatest apparent diameter of either the sun or moon. It follows, therefore, that when we see the lower edge of either wppa/rently resting on the horizon, its whole disk is in reality below it ; and would be entirely concealed by the convexity of the earth, were it not for refraction. 494. Kefraction sometimes causes the sun and moon to appear elongated horizontally, when near the horizon, and seen through a dense atmosphere. The rays from their lower limb being refracted more than those from the upper limb, on account of coming to us through a lower and denser portion of the atmosphere, the lawer portion seems higher in proportion ; or, in other words, the perpendicular diameter of the object seems the shortest. It is then called a Tiorizontal moon. 495. Another effect of refraction is, that the sun seems to arise about three minutes earlier, and to set about three minutes later, on account of atmospherical refraction, than it otherwise would ; thus adding about six minutes, on an average, to the length of each day. The atmosphere is said to he so dense about the North Pole as to bring the sun above the horizon some days before he should appear, according to calculation. In 1596, some Dutch navigators, who wintered at Nova Zembla, in latitude 76°, found that the sun be- gan to be visible 17 days before it should have appeared by calculation ; and Kepler computes that the atmospheric refraction must have amounted to 5^, or 10 times as much as with us. 496. The twilight oi raoTomg and evening is produced partly by refraction, but mainly by reflection. In the morning, when the sun arrives within 18° of the horizon, his rays pass over our heads into the higher region oi the atmosphere, and are thence reflected down to the earth. The day is then said to ddlon, and the light gradually increases till sunrise. In the evening, thip *493. Amount of displacement of celestial objects by refraction 3 What follows ? 494. What effect upon apparent form of moon, &0. ? 495. On length of days? (How about North Pole 8) 496. Cause of twUiglitf (Note.) 216 ASTEONOMT. process is reversed, and the twilight lingers till the sun is 18' below the horizon. There is thus more than an hour of twilight both morning and evening. In the arctic regions, the sun is never more than IS© below the horizon; bo that the twilight continues during the whole night 497. In making astronomical observations, for the pur- poses of navigation, &c., allowance has to be made for refraction, according to the altitude of the object, and the state of the atmosphere. For this purpose tables are constructed, showing the amount of refraction for every degree of altitude, from the horizon to the zenith. EEFEACnON BY GLASS LENSES. 498. A lens is a piece of glass or other transparent substance, of such a form as to collect or disperse the rays of light that are passed through it, by refracting them out of a direct course. They are of different forms,' and have different powers. MixvA xjMiv^ vii^^ v/ ^ LENSES OF DIFFBEENT FOEMS. In the adjoining cut, we havB an edgewise a B o D b / view of six different lenses. A is a plamc-coiwex, or half a double con- vex lens ; one side being convex, and the other plane. B is a plano-conca/oe ; one surface being con- cave, anil the other plane. C Is a doiible-convex lens, or one that is bounded by two convex surfaces. D is a double-cotiotvoe lens, or a circular piece of glass hollowed out on both sides. E is a concaw-cotwex Ions, whose curves differ, but do not meet, if produced. F is a mmiscuSf or a concavo-convex lens, the curves of whose surfaces meet 499. A double-convex lens converges parallel rays to a point called the foGus ; and the dis- tance of the focus de- pends upon the degree of convexity. In the first of these cuts, the lens is quite thick, and the focus of the rays is quite near; but the other being less convex, the focus is more remote. LIGHT BEFBACTED B7 LENSES. 497. Wliat allowat Ji. for refraction ? Tables ? 498. What is a lens ? (Draw and describe different kinds ?) 499. Befracting pow ZSr A. TUBE, SBFBAOTXNG TBLBSOOPE UOUNTED ON . A is the ol^ect-glasa, B the ^e-pMce^ and G the place where the tube in which the eye-piece is set. slides in and out of the large tube, to adjust the eye-piece to the focal distance. By placing the lenses in a tube, the eye is easily placed in the focus, and ttl9 object-glass directed toward any desired object. 518. The object-glass of a telescope is usually pro- tected, when not in use, by a brass cap that shuts over the end of the instrument ; and the eye-pieces, of which there are several, of diflFerent magnifying powers, are so fixed as to screw into a small movable tube in the lower end of the instrument, so as to adjust them re- spectively to the fo- cus, and to the eyes of different observ- ers. Such telescopes usually represent ob- jects in an inverted position. The adjoining cut represents the simplest form of a mounted refractor. The object-glass is at A, where the brass cap may be seen covering it B is the small tube into wiiich the eye-piece is screwed, and which is moved in and out by the small screw C. Two eye-pieces may be seen at D — one short one, for astro- nomical observations; and a long one, for land objects. For viewing the sun, itis-necessary toadd a screen, made of colored glass. At E, a bolt goes into a socket in the top of the stand, in which it turns, allowing the telescope to sweep 518, How object-glass protected! What said of eye-pieces? ^vtil, Illation ?) (Cat and 224 ASTEONOMT. KToond the horizon ; while the joint, connecting the saddle in which the telescope reatft with the top of the holt, allows it to be directed to any point between the horizon and the zenith. But such stands answer only for comparatively small instruments, 519. Refracting telescopes are mounted in various ways. So important is it that they should not shake oi vibrate, that, in most observatories, the stand rests upon heavy mason-work in no way connected with the build- ing, so that neither the wind nor the tread of the ob- server can shake it. They are then furnished with a double axis, which allows of motion up and down, or east and west ; and two graduated circles show the pre- cise amount of declination and right ascension. They are then furnished with clockwork, by which the tele- scope is made to move westward just as fast as the earth turns eastward ; so that the celestial object being once found, by setting the instrument for its right ascension and declination, or by the aid of the Finder — a small telescope attached to the lower end of the large one — it may be kept in view by the clockwork for any desirable length of time. A telescope thus furnished with right ascension and declination circles is called an Equatorial, or is said to be eguatarially movm^ted, because it sweeps east and west in the heavens parallel to the equator. 520. The object-glasses of telescopes are not always made of a single piece of glass. They may be made of two concavo-convex glasses, like two watch crystals, with their concave sides toward each other, or with a thin double concave glass between them. They are thus double, or triple ; but when thus constructed, the whole is called a lens, as if composed of a single piece. Lenses have also been formed by putting two concavo-convex glasses together, and filling the space between them with some transparent fluid. These are called JBmiow lenses, from Prof. Barlow, their inventor. 521. As a prism analyzes the light, and exhibits dif- ferent colors, so a double-convex lens may analyze the 519. How refractors mounted, and why ? When equatorial, and why ? 520. How ohjeotr-glaases made ? What a lens f A Barlow lens ? 521. What is s,a Achromatic ie\^co^6% (Derivation of term ?) DIFFERENT KINDS OF TELESCOPES. 225 light that falls near its ciicumference, and thus represent the outside of the heavenly bodies as colored. But this defect is remedied hj using disks made of different kinds of glass, so as to correct one refraction by another. Re- fracting telescopes thus corrected are called Achromatio telescopes. Achr obtain the true altitude, and thence the latitude. 541. Parallax ? True place of a celestial body ? Apparent ? When par- allax greatest ? Least? Called what, and why ? (Diagram? What objects greatest parallax ?) 542. Effect of parallax? How obtain true altitude ? (How differ from re fraction ? How then obtain true altitude ?) "^iiS ASTEONOMr. 543. The principles of parallax are of great import- ance to astronomy, as they enable us to determine the distances of the heavenly bodies from the earth, the mag- nitudes of the planets, and the dimensions of thfeir or- bits. The sun's horizontal parallax being accurately known, the earth's distance from the sun becomes known ; and the earth's distance from the sun being known, that of all the planets may be known also, because we know the exact periods of their sidereal revolutions, and, according to the third law of Kepler, the squares of the times of their revolutions are proportional to the cubes of their mean distances. Hence, the first great desideratum in astronomy, where measure and magnitude are concerned, is the determination of the true parallax. At a coancil of astronomers assembled In London some years since, from the most learned nations in Europe, the 6un*s mean horizontal parallax was hettled, ss tlie result of their united observations, at 0° 0' 8".5776. Now the value of radius, expressed like- wise in seconds, is 206264".8; and this divided by 8".5776, gives 24047 fnr the distance of the sun from the earth, in semi-diame{ers of the latter. If we take the eqinUofial semi-diameter of the earth as sanctioned by the same tribunal, at {7924-i-2=;)8962 miles, we shall have 24047X8962=95,278,869 miles for the sun's true distance. 544. The change in the apparent position of the fixed stars, caused by the change of the earth's place in her revolution around the sun, is called their annual paral- lax. So immense is their distance, that the semi-annual variation of 190,000,000 of miles in the earth's distance, irom all those stars that lie in the plane of her orbit, makes no perceptible diflTerence in their apparent magni- tude or brightness. The following cut will illustrate onr meaning: ^ •-.0 A •■#- -> -» Let A Tei»resent a fixed star in the plane of the earth's orbit, B. At C, the earth ia 190,(lUU,(iOU (if miles nearer the stiir than it will be at D six months afterward; and yet this semi-annual variation of 190,000,000 miles in the distance of the star is so small a fraction of the whole distance to it^ iis neither to increase or diminibh its apparent brightness. 643. Use of parallax? How employed ? (Note?) 544. "WJiat meant by earth's an.»M*a2j3araZ^^ Effectof variation of eartli'j distance on the fixed stars? (Diagram.) MISCKLLANIA. 239 FABALLAX OF TUB 6TA£^ \ 545. It is only those stars that are situateu near the axis of the earth's orbit whose parallax can '>e measured at all, on account of its almost imper- ceptible quantity. So distant are they, that the variation of 190,000,000 miles in the earth's place causes an apparent change of less than 1' in ^X , the nearest and most favorably situ- / \ ated fixed star. / \ Let A represent the earth on the 1st of January, and B / \ a Btar observed at that time. Of course, its apparent place / \ in the more distant heavens will be at C. But in six / N. months the earth will he at D, and the star B will appear to be at E. The angle ABD or OBE will constitute the parallatic angle. In the cut, this angle amounts to _ -. ... about 48°, whereas the real parallax of the stars is less D,7( ^ OA than ^'fjth of one degree, or 7;7y'^f,th part this .imount "--- '^ Lines approaching each other thus slowly would appear parallel ; and the earth's orbit, if filled with a globe of fire, and viewed i^om the fixed stars, would appear but a point of light 1' in diameter I MISCELLANIA. 546. The Atmosphere is an. elastic gas, which sur- rounds the earth on every side. It is supposed to be from 40 to 60 miles in hight, growing more rare as we ascend, and is kept around the earth by attraction. 647. Wind is air put in motion by heat, causing bodies of air to rise from the earth's surface, and other air to rush in to supply its place. The velocity of the wind ranges from 5 to 100 miles an hour. 548. Clouds are collections of vapor suspended in the air. They range from two miles to half a mile in hight, according to their density and weight. They serve to screen us from the oppressive heat of the sun, and to convey water from the rivers and oceans, and pour it down in showers upon the earth. 549. liain is water condensed, or collected into drops by attraction, and falling from the clouds. Scdl is drops 645. What stars have perceptible parallax? Amount? (Diagram, and explain.) 546. What is the atmosphere ? Extent? How kept around the earth ! 547. Wind ? How put m motion ? Velocity 3 548. Clouds? Uses? 549. Rain? Hail? Snow? 24:0 ASTEOKOMT. of rain frozen on its way to the earth ; and Snow is par- ticles of clouds frozen before being condensed into drops. 550. Lightning is electricity passing from one cloud to another, or between the clouds and the earth ; and Thwnder is the sudden shock given to the atmos]5here by the passage of the electricity through it. 561. The Aii/rora Borealis, or Northern Light, is a reddish unsteady light sometimes seen in the north. It is supposed to be caused by electricity passing through the upper regions of the atmosphere, about the North Pole. 652. " Shooting Stars''' are meteors that shoot down- ward toward the earth, like stars falling from their spheres. They are usually seen one' at a time, and only in the night, but sometimes fall in showers, and no doubt fall in the day time, though invisible^"^ From 3 o'clock in the morniDg, November 13, 1S3S, till daylight, the whole heavens were filled with these fiery particles and streaks of light darting downward from the sky. These meteors, no doubt, come from regions beyond the limits of the atmosphere, and arc ificnited by their rapid passage through it. Their origin and nature are as yet matters oi" inquiry and speculation. 553. Aerolites, or Meteoric Stones, are masses of stone or iron that have fallen from the sky at various periods, and on almost every part of the globe. They are often found after the explosion of large meteors, sometimes while they are yet hot. A large meteor exploded over Gabarras counly. North Carolina, a few years since, several pieces of which were picked up the next day. One piece, weighing 19 lbs., had struck a large pine tree lying on the ground, and had gone through it, and into the earth, to the depth of three feet. In some cases, large masses of iron have fallen. In Becemher, 1795, a stone weighing 51 lbs. fell in Yorkshire, England. The writer has a piece of an aerolite that weighed 90 lbs., that fell in New Jersey. A large mass of meteoric*i^on may he seen in the museum of Tale College. ~~P^ 550. Lightning and thunder? 551. Aurora Borealis ? 552. "Shooting stars?" How seen? (What shower mentioned? Dia- tance from which they come ?) 55S. Aerolites ? (What instances of their fall cited?)