( R. C. CARPENTER. No. ..71- 111 Ss 113 WILili^ STREET, NEW YORK, T Wo too T frol cull T owi T eva Ini evei PAI DA Wll PE( STI JAI WO PO GH SM BO SM •n FROM Carpenter Estate Per V Init nex tot iltltaca, Neui ^ark dard Educational oUection of text- ce and literature, ibstruse and diffl- ny, maintains Its (pnole. ted States, and by i religious belief. 'orks, witli wMcli LLY'S Geog's. ish Grammar. iCRIBNER Writ- diumsi [ing. exts> ?D'S Latin. iries. 3ook. MusiC'BookB. Manual* i. lumesi details, n"iN. rs. Sabscriptioil, bks, with a view to Ialf tlie price an- j8 without expense The PuWisBers are prepared to make special and very favorable terms/or J!rs« intra- ductlon of any of the Natiosai, SEErss, and -will ftimish the reduced mtroamtory price-dist to teachers "whose application presents evidence of good faith. 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Cornell University Library value arV15577 ^ ^^^^ A fourteen weeks course in descriptive a " '^°^ 3 1924 031 olin.anx The original of tliis bool< 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/cu31924031322195 'h 1 . -r' ^ -3^T^./7^'«n^,;^,5 ^i ^.v>^si^i!^.ur^ h 1= V- i k'.v-i I f FOURTEEN WEEKS COURSE DESCRIPTIYE ASTRONOMY. BT J. DORMAN STEELE, Ph. D., PRINCIPAL OP ELMIKA FBEB ACADEMY, Aathor of " A Fourteen Weeks Course in Chemistry.' " Thje heavens declare the glory of God : and the firmament showeth hil -.ancly-work." Psalm xii. 1. NEW YORK: A. S. BARNES & Co, 111 & 113 WILLIAM STREET. BOSTON : WOOLWORTH, AINS WORTH & Co. 1870. IMPORTANT ANNOUNCEMENT, FOUETEEN WEEKS IN ALL THE SCIENCES: BY J. D. STEELE AND OTHERS. The unparalleled success of the first volume of this series, "Fourteen Weeks in Ohemistrt," has encouraged the pub- lishers to announce the following volumes on a like plan, comprising a complete course in Natural Science for those having hut a limited period to give to these branches. Th ey are especially adapted to Public and High Schools. I. — Fo'arteeii 'Weeks in Natural Philosophy. . .$1 50 II.— Fourteen Weeks iu Chemistry 1 25 III. — Fourteen Weeks in Astronomy (with chart). 1 50 IV.— Fourteen Weeks in Geology. In preparation. 1 75 V. — Steele's Ans. to Questions and Problems 1 50 VI. — Elements of Physiology. Jairvis 75 VII. — Object Lessons in Botany. Wood ] .5(1 VIII. — Chambers' Elements of Zoology ... 1 .',U TJiese boolis wiU be found, as soon as issued, at all the leading bookstores in the United States, or will be forwardeJ by the publishers by mail, postpaid, on receipt of price. A. S. BA.IS,r*fDES &; Co., NEW YORK. blnti'reil iLCi:ordiD^ to Act of Congreaa in the yeiir IS69, by A. S. BARNES & CO , ic the Clerk'B Office of the United Stiit«3 District Court for the Southern District of New 1 orb PREFACE. DuEiNG the past few years great advances have been made in astronomical science. A new hori- zontal parallax of the sun has been established. This has materially altered the estimated distances, etc., of the planets. The sun is much nearer us than we supposed, and light has lost a httle of its wonder- ful velocity. Much additional information has been obtained concerning Meteors and Shooting Stars. The investigations connected with Spectrum Analy- sis have been especially suggestive. Thus on every hand the facts of Astronomy have been accumulat- ing. As yet, however, they are scattered through many expensive foreign works, and are consequently beyond the reach of most of our schools. It has been the aim of the author to collect in this httle volume the most interesting features of these larger works. Believing that Natural Science is full of fas- cination, he has sought to weave the story of those far-distant worlds into a form that may attract the attention and kindle the enthusiasm of the pupil. The work is not written for the information of scien- tific men, but for the inspiration of youth. The pages therefore are not burdened with a multitude b PBEPAOE. of figures wliich no memory could possibly retain. Mathematical tables and data, Questions for Ke- view, and also a Guide to the Constellations, are given in the Appendix, where they may be useful for constant reference. The author would call particular attention to the method of classifying the measurements of Space, and the practical treatment of the subjects of Parallax, Harvest Moon, Eclipses, the Seasons, Phases of the Moon, Time, Nebular Hypothesis, &c. To teachers heretofore compelled to use a cum- bersome set of charts, it is hoped that the star maps here offered will present a welcome substitute. The geometrical figures showing the actual appearance of the constellations, wiU relieve the mind confused with the idea of numberless rivers, serpents, and classical heroes. The brightest stars only are given, since in practice it is found that pupils remember the general outlines alone. Finally, the author commits this little work to the hands of the young, to whose instruction he has consecrated the energies of his life, in the earnest hope that, loving Nature in all her varied phases, they may come to understand somewhat of the wis- dom, power, beneficence, and grandeur displayed in the Divine harmony of the Universe. "One God, one law, one element, And one far-off Divine event To which the whole creation moves." PEEFAOE. I The following works, among others, have been Ireely con- sulted in preparing this volume: The Heavens Guillemin. Astronomy Chambers. Introduction to Astronomy Hind. Solar System Hind. Popular Asti'onomy Aiiy. Popular Astronomy Arago. Astronomy Norton. Astronomy Eobinson. Asta'onomy .Loomis. Age of Fable Bulfinoh. Poetiy of Science Hunt. Outlines of Astronomy Herschel. Popular Asti'onomy Mitchell. Asti'onomy and Physics Whewell. Annual of Scientific Discovery Kneeland. The Chemical News. SUGGESTIONS TO TEACHERS. This -prork is designed to be recited in the topical metliod. On naming tlie title of a paragraph, the pupil should he able to draw on the blackboard the diagram, if any is given, and state the substance of what is contained in the book. It will be noticed that the order of topics, in ti'eatiug of the planets and also of the constellations, is uniform. If a portion of the class write theii- topics in full upon the blackboard, it will be found a, valuable exercise in spelling, punctuation, and composition. Every point which can be illustrated in the heavens should be shown to the class. No description or apparatus can equal the reality in the sky. After a constellation has been traced, the pupil should be practised in star-map drawing. Much profit- able instruction can be obtained in this way. For the pur- pose of more easily finding the heavenly bodies at any time, Whitall's Movable Plantsphekk is of great service. It may be procured of the publishere of this work. " Orreries are of little account." A tellurian is invaluable in explaining Precession of the Equinoxes, Eclipses, Phases of the Moon, etc. Messre. J. Nellegar & Co., Albany, N. Y., furnish a good instru- ment at a low price. The article on " Celestial Measm'ements," near the close of the work, should be constantly refeiTed to dur- ing the term. In the figures, the right-hand side represents the west and the left-hand the east. When it is important to obtain this idea con-ectly, the book should be held up toward the south- em sky. TABLE OF CONTENTS. CELESTIAL MAP. I. INTRODUCTION. PAG8 HISTORY OF ASTRONOMY i6 SPACE 35 THE THREE SYSTEMS OF CIRCLES ... 37 II. THE SOLAR SYSTEM . . 43 THE SUN 46 THE PLANETS 65 Vulcan 82 Mercury 83 Venus 89 The Earth 96 The Seasons no Precession and Nutation . . .120 Refraction, Aberration and Parallax . 130 The Moon 139 Eclipses 155 The Tides ... ... 165 Mars 168 The Minor Planets ... .172 Jupiter .... ... 175 Saturn 182 Uranus .189 Neptune 191 METEORS AND SHOOTING STARS . . 194 12 TABLE OF CONTENTS. PASB COMETS 206 ZODIACAL LIGHT 217 m. THE SIDEREAL SYSTEM . . 219 THE STARS 221 THE CONSTELLATIONS 234 Northern Circumpolar Constellations . 234 Equatorial Constellations .... 242 Southern Constellations .... 263 DOUBLE STARS, COLORED STARS, VARIABLE STARS, CLUSTERS, MAGELLANIC CLOUDS, NEBULiE, &C 265 THE MILKY WAY 280 THE NEBULAR HYPOTHESIS . . . .282 CELESTIAL CHEMISTRY.— Spectrum Analysis 284 TIME 288 CELESTIAL MEASUREMENTS .... 298 APPENDIX 311, Tables 312 Questions ■ . 315 Guide to the Constellations . . .331 Index 335 INTRODUCTION. Astronomy (astron, a star, and nomas, a law) treats of tlie Heavenly Bodies — tlie sun, moon, planets, stars, and, as our globe itself is a planet, of the earth, also. It is, above all others, a science that cultivates the powers of the imagination. Yet all its theories and distances are based upon the most rigorous mathematical demonstrations. Thus the study has at once the beauty of poetry and the ex- actness of Geometry. The Appearance of the Heavens to an Observer. — The great dome of the sky filled with glittering stars is one of the most sublime spectacles in nature. To enjoy this fully, a night must be chosen when the air is clear, and the moon is absent. We then gaze upon a deep blue, an immense expanse studded with stars of varied color and brilliancy. Some shiue with a vivid Hght, perpetually changiag and twinkling; others, more constant, beam tranquilly and softly upon us ; while many just tremble into our sight, like a wave that, struggling to reach some far-off land, dies as it touches the shore. In the presence of such weird and wondrous beauty, the 14 mTEODUcmoN. tenderest sentiments of tlie heart are aroused — a feeling of awe and reverence, of softened melan- choly mingled with a thought of God, comes over us, and awakens the better nature within us. Those far-off lights seem full of meaning to us, could we but read their holy message ; they become real and sentient, and, like the soft eyes in pictures, look lov- ingly and inquiringly upon us. We come into com- munion with another life, and the soul asserts its immortality more strongly than ever before. We are humbled as we gaze upon the infinity of worlds, and strive to comprehend their enormous distances, their magnificent retinue of suns. The powers of the mind are aroused, and eager questionings crowd upon us. What are those glittering fires? What their distances from us ? Are they worlds like our own ? Do living, thinking beings dwell upon them ? Are they carelessly Scattered through infinite space, or is there an order of the universe ? Can we ever hope to fathom those mysterious depths, or are they closed to us forever ? Many of these problems have been solved ; others yet await the astronomer whose keen eye shall be strong enough to read the myste- rious scroU of the heavens. Two hundred genera- tions of study have revealed to us such startling facts, that we wonder how man in his feebleness can grasp so much, see so far, and penetrate so deeply into the mysteries of the universe. Astron- omy has measured the distance of many of the stars, and of all the planets ; computed their weight and INTKODUCTION. 15 size, their days, years, and seasons, with many of their physical features ; made a map of the moon, in some respects more perfect than any map of the earth ; tracked the comets ia their immense sidereal journeys, marking their paths to a nicety of which wo can scarcely conceive, and at last it has analyzed the structure of the sun and far-o£f stars, announ- cing the Tery elements of which they are composed. Observing for several evenings those stars which shine with a clear distinct light, we notice that they change their position with respect to the others. They are therefore called "planets" (Hterally, waii- derers). Others remain immovable, and shine with a shifting, twinkhng light. They are termed the "fixed stars" although it is now known that they also are in motion — ^the most rapid of any known even to Astronomy — ^but through such immense or- bits that they seem to us stationary. Then, too, diagonally girdling the heavens, is a whitish, vapory belt — ^the Milky Way. This is composed of multi- tudes of millions of suns — of which our own sun itself is one — so far removed from us that their light mingles, and makes only a fleecy whiteness. This magnificent panorama of the heavens is before us, inviting our study, and waiting to make known to ua the grandest revelations of science. 1(3 INTEODUCTION. DESCRIPTIYE ASTRONOMY. HISTOET. AsTEONOMY is the most ancient of all sciences. The study of the stars is doubtless as old as man himself, and hence many of its discoveries date back of authentic records, amid the dim mysteries of tra- dition. In tracing its history, we shall speak only of those prominent facts which mil best enable us to understand its progress and glorious achievements. The Chinese. — This people boast much of their astronomical discoveries. Indeed their emperor claims a celestial ancestry, and styles himself " Son of the Sun." They possess an account of a conjunc- tion of four planets and the moon, which must have occurred a century before the Flood. They have also the first record of an ecHpse of the sun, which took place about two hundred and twenty years* after the Deluge. It is reported that one of their kings, two thousand years before Christ, put to death the principal officers of state because they had failed to calculate an approaching eclipse. * October 13, 3127 b. c. HISTORY. 17 The Chaldeans. — The Chaldean shepherds, watch- ing their flocks by night under the open sky, could not fail to become familiar -with many of the move- ments of the heavenly bodies. When Alexander took Babylon, two centuries before Christ, he found in that city a record of their observations reaching back about nineteen centuries, or nearly to the con- fusion of tongues at the Tower of Babel. The Chaldeans divided the day into twelve hours, in- vented the sun-dial, and also discovered the " Saros " or "Chaldean Period," which is the length of time in which the eclipses of the sun and moon repeat themselves ia the same order. The GEECiiNS. — In the seventh century b. c. Tholes, noted for his electrical discoveries, acquired much renown, and established the first school of Astronomy in Greece. He taught that the earth is round, and that the moon receives her light from the sun. He introduced the division of the earth's surface into zones, and the theory of the obliquity of the ecliptic. He also predicted an eclipse of the sun which is memorable in ancient history as having terminated a war between the Medes and Lydians. These nations were engaged in a fierce battle, but the awe produced by the darkening of the sun was so great, that both sides threw down their arms and made peace. Thales had two pupils, Anaximander and Anaxagoras. The first of these taught that the stars are suns, and that the planets are inhabited. He erected the first sun-dial, at Sparta. The second 18 INTEODUOTION. mamtained that there is but one God, that the sun is solid, and as large as the country of Greece, and attempted to explaia ecHpses and other celestial phenomena by natural causes. For his audacity and impiety, as his countryman considered it, he and his family were doomed to perpetual banish- ment. Pythagoras founded the second celebrated astro- nomical school, ab Crotona, at which were educated hundreds of enthusiastic pupils. He knew the causes of eclipses, and calculated them by means of the Saros. He was most emphatically a dreamer. He conceived a system of the universe, in many re- spects correct ; yet he advanced no proof, and made few converts to his views, and they were soon weU- nigh forgotten. He held that the sun is the centre of the solar system, and that the planets revolve about it in circular orbits ; that the earth revolves daily on its axis, and yearly around the sun ; that Venus is both morning and evening star ; that the planets are inhabited — and .he even attempted to calculate the size of some of the animals in the moon ; that the planets are placed at intervals cor- responding to the scale in music, and that they move in harmony, making the "music of the spheres," but that this celestial concert is heard only by the gods — ^the ears of man being too gross for such divine melody. Evdoxiis, who lived in the fourth century b. c, in- vented the theory of the Crystalline Spheres. He HISTOEY. 19 held that the heavenly bodies are set, like gems, in hollow, transparent, crystal globes, which are so pure that they do not obstruct our view, while they all revolve aroimd the earth. The planets are placed in one globe, but have a power of moving themselves, under the guidance — as Aristotle taught — of a tutelary genius, who resides in each, and rules over it as the mind rules ove^ the body. Hipparchus, who flourished in the second century B. c, has been called the " Newton of Antiquity." He was the most celebrated of the Greek astrono- mers. He calculated the length of the year to with- in six minutes, discovered the precession of the equi- noxes, and made the first catalogue of the stars — 1081 in number. The Egyptiaks. — ^Egypt, as well as Chaldea, was noted for its knowledge of the sciences long before they were cultivated in Greece. It was the practice of the Greek philosophers, before aspiring to the rank of teacher, to travel for years through these countries, and gather wisdom at its fountain-head. Pythagoras spent thirty years in this manner. Two hundred years after Pythagoras, the celebrated school of Alexandria was established. Here were concentrated in vast libraries and princely halls nearly all the wisdom and learning of the world. Here flourished all the sciences and arts, under the patronage of munificent kings. At this school Ptol- emy, a Grecian, wrote liis great work, the "Alma- gest," which for fourteen centuries was the text- 20 INTEODUCTION. book of astronomers. In this work was given what is known as the "Ptolemaic System." It was founded largely upon the materials gathered by prsTious astronomers, such as Hipparchus, whom we have already mentioned, and Eratosthenes, who computed the size of the earth by the means even now considered the best— the measurement of an arc of the meridian. Ptolemaic Theoky. — The movements of the planets were to the ancients extremely complex. Venus, for instance, was sometimes seen as " evening star" in the west, and then again as " morning star" in the east. Sometimes she seemed to be moving in the same direction as the sun, then going apparently behind the sun, appeared to pass on again in a course directly opposite. At one time she would recede from the sun more and more slowly and coyly, until she would appear to be entirely station- ary; then she would retrace her steps, and seem to meet the sun. AU these facts were attempted to be accounted for by an incongruous system of " cycles and epicycles," as it is called. The advo- cates of this theory assumed that every planet re- volves in a circle, and that the earth is the fixed centre around which the sun and the heavenly bodies move. They then conceived that a bar, or some- thing equivalent, is connected at one end with the earth ; that at some part of this bar the sun is at- tached ; while between that aud the earth, Venus is fastened — not to the bar directly, but to a sort of BISTORT. 21 crank ; and further on, Mercury is hitclied on in the same way. In the cut, let A be the earth, S the sun, ABDF the bar (real or imaginary), BG the short bar or crank to -which Venus is tied, D E another bar for Mercury, F G another bar, with still another short crank, at the end of which, H, Mars is attached. THE PTOLEMAIC THEORY. Thus they had a complete system. They did not exactly understand the nature of these bars— whether they were real or only imaginary — but they did comprehend their action, as they thought ; and so they supposed the bar revolved, carrying the sun and planets along in a large circle about the earth ; while all the short cranks kept flyhig around, thus sweeping each planet through a smaller circle. By this theory, we can see that the planets would sometimes go in front of the sun and sometimes behind; and their places were so accurately pre- dicted, that the error could not be detected by the rude instruments then in use. As soon as a new motion, of one of the heavenly bodies vr&s discov- ered, a new crank, and of course a new circle, was 22 INTEODUCTION. added to account for the fact. Thus the system became more and more complicated, until a com- bination of five cranks and circles was necessary to make the planet Mars keep pace with the Ptolemaic theory. No wonder that Alfonso, king of Castile, and a very celebrated patron of Astronomy, revolted at the cumbersome machinery, and cried out, " If I had been consulted at the creation, I could have done the thing better than that !" Astrology. — After the death of Ptolemy, Astron- omy ceased to be cultivated as a science. The Romans, engrossed with schemes of conquest, never produced a single great astronomer. Indeed, when Julius Caesar reformed the calendar, he obtained the assistance, not of a Eoman, but of Sosigenes an Alex- andrian. The Arabians studied the stars merely for purposes of soothsaying and prophecy. They pro- fessed to foretell the future by the appearance of the planets or stars. AH of the ancient astronomers shared more or less in this superstition. Tiberius, emperor of Rome, practised Astrology. Hippoc- rates himself, the " Father of Medicine" whoflouT' ished in the 4th century B. C, ranked it among the most important branches of knowledge for the phy- sician. Star-diviners were held in the greatest estimation. The system continued to increase in credit until the Middle Ages, when it was at its height of popularity. The issue of any important undertaking, or the fortunes of an individual, were foretold by the astrologer, who drew up a Horoscope, HISTOEY. 23 representing the position of the stars and planets at the beginning of the enterprise, or at the birth of the person. It was a complete and complicated system, and contained regular rules, which guided the interpretation, and which were so abstruse that they required years for their entire mastery. Venus foretold love ; Mars, war ; the Pleiades,* storms at sea. The ignorant were not alone the dupes of this visionary system. Lord Bacon be- lieved in it most firmly. As late even as the reign of Charles II., Lilly, a famous astrologer of that time, was called before a committee of the House of Commons to give his opinion on the probable issue of some enterprise then under consideration. How- ever foolish the system of Astrology itself may have been, it preserved the science of Astronomy during the Dark Ages, and prompted to accurate observa- tion and diligent study of the heavens. The Copeenican System. — About the middle of the sixteenth century, Copernicus, breaking away from the theory of Ptolemy, which was still taught in all the institutions of learning in Europe, revived the theory of Pythagoras. He saw how beautiful- ly simple is the idea of considering the sun the gi-and centre about which revolve the earth and all the planets. He noticed how constantly, when we are riding swiftly, we forget our own motion, and think that the trees and fences are gliding by us in * PlC'-ya-dEz. 24 INTRODUCTION. the contrary directioB. He applied this thought to the movements of the heavenly bodies, and main- tained that, instead of all the starry host revolving about the earth once in twenty-four hours, the earth simply turns on its own axis : that this produces the apparent daily revolution of the sim and stars ; while the yearly motion of the earth about the sun, transferred in the same manner to that body, would account for its various movements. Though Coper- nicus thus simplified so greatly the Ptolemaic the- ory, he yet found that the idea of circular orbits for the planets woiild not explain aU the phenomena ; he therefore still retained the " cycles and epicycles" that Alfonso had so heartily condemned. For forty years this illustrious astronomer carried on his ob- servations in the upper part of a humble, dilapi- dated farm-house, through the roof of which he had an unobstructed view of tjie sky. The work con- taining his theory was at last published just in time to be laid upon his death-bed. Tycho BbjVH', a celebrated Danish astronomer, next propounded a modification of the Copemican system. He rejected the idea of cycles and epi- cycles, but, influenced by certain passages of Scrip- ture, maintained, with Ptolemy, that the earth is the centre, and that all the heavenly bodies revolve about it daily in circular orbits. Brahe was a noble- man of wealth, and, in addition, received large sums from the Government. He erected a magnificent observatory, and made many beautiful and rare in- HISTORY. 25 struments. Clad in his robes of state, he watched the heavens with the intelligence of a philosopher and the splendor of a king. His indefatigable in- dustry and zeal resulted in the accumulation of a vast fund of astronomical knowledge, which, how- ever, he lacked the wit to apply to any further ad- vance in science. His pupil, Kepler, saw these facts, and in his fruitful mind they germinated into three great truths, called Kepler's laws. These constitute almost the sum of astronomical knowledge, and form one of the most precious conquests of the human mind. They are the three arches of the bridge over which Astronomy crossed the gulf between the Ptol- emaic and Copernican systems. Kbplee's Laws. — Kepler, taking the investigations of his master, Tycho Brahe, determiaed to find what is the exact shape of the orbits of the planets. He adopted the Copernican theory, that the sun is the centre of the system. At that time all be- lieved the orbits to be circular. Since, as they said, the circle is perfect, is the most beautiful figure in nature, has neither beginning nor ending, therefore it is the only form worthy of God, and He must have used it for the orbits of the worlds He has made. Imbued with this romantic view, Kepler commenced with a rigorous comparison of the places of the planet Mars, as observed by Brahe, with the places as stated by the best tables that could be computed on the circular theory. For a time they agreed, but in certain portions of the 2 26 rNTEODUCnON. orbit the observations of Brali^ ■would not fit the computed place by eight minutes of a degree. Be- lieying that so good an astronomer could not be mistaken as to the facts, Kepler exclaimed, " Out of these eight minutes we will construct a new theory that will explain the movements of all planets." He resumed his work, and for eight years continued to imagine every conceivable hypothesis, and then pa- tiently to test it — "hunt it down," as he called it. Each in turn proved false, until nineteen had been tried. He then determined to abandon the circle and adopt another form. The ellipse suggested itself to his mind. Let us see how this figure is made. Fig. 2. Attach a thread to two pins, as at F P in the figure; next move a pencil along with the thread, the latter being kept tightly stretched, and the point will mark a curve which is flattened in proportion HISTORY. 27 to the length of the string we use, — the longer the string, the nearer a circle will the figure become. This figure is the ellipse. The two poiats F F are called the /oct (singular, /ocms). We can now under- stand Kepler's attempt, and the glorious triumph which crowned his seventeen years of unflagging toil. First Law. — With this figure, he constructed an orbit, having the sun at the centre, and again fol- lowed the planet Mars in its course. But very soon there was as great discrepancy between the observed and computed places as before. Undismayed by this failure, Kepler assumed another hypothesis. He determined to place the sun at one of the foci of the ellipse, and once more "hunted down" the theory. For a whole year he traced the planet along the imagiaary orbit, and it did not diverge. The truth was discovered at last, and Kepler an- nounced his first great law — Planets eevolve m ellipses, with the sun at ONE FOCUS. Second Lata. — Kepler knew that the planets do not move with equal velocity in the different parts of their orbits. He next set about establish- ing some law by which this speed could be deter- mined, and the place of the planet computed. He drew an eUipse, and marked the various positions of the planet Mars once more. He soon found that when at its 'perihelion (point nearest the sun) it moves the fastest, but when at its aphelion (point furthest from the sun) it moves the slowest. Once 28 . INTEODUCTION. more he "hunted down" various hypotheses, until at last he discovered that "while in going from B to A the planet moves very slowly, and from D to Fig. 3. very rapidly; yet the space inclosed between the liaes SB and SA is equal to that inclosed between S D and S C. Hence the second law — A LINE CONNECTING THE CENTEE OE THE EARTH WITH THE CENTRE OE THE SUN, PASSES OVER EQUAL SPACES IN EQUAL TIMES. Third Law. — Kepler, not satisfied with the dis- covery of these laws, now determined to ascertain if there were not some relation existing between the times of the revolution of the planets about the sun and their distances from that body. With the same wonderful patience, he took the figures of Tycho Brahe, and began to compare them. He tried them in every imaginable relation. Next he took their squares, then he attempted their cubes, and lastly he combined the squares and the cubes. Here was the secret ; but he toiled around it, made a blunder, HISTOEY. 29 and waited for months, until, once more, his patience triumphed, and he reached the third law — The squabbs of the times of eevolution op the planets about the sun, are peopobtjonal to the cubes of theie mean distances from the sun.* In rapture over the discovery of these three laws, SO marked by that divine simplicity which pervades all the laws of nature, Kepler exclaimed, " Nothing holds me. The die is cast. The book is written, to be read now or by posterity, I care not which. It may well wait a century for a reader, since God has waited six thousand years for an observer."t Galileo. — Contemporary with Kepler was the great Florentine philosopher, Galileo. He discovered the laws of the pendulum and of falling bodies, as we have already learned in Natural Philosophy. He, however, was educated in and believed the Ptolemaic theory. A disciple of the Copernican theory hap- pening to come to Pisa, where Galileo was teaching * For example : The square of Jupiter's period is to the square of Mars' period, as the cube of Jupiter's distance is to the cube of Mars' distance ; or, representing the earth's time of revolu- tion by P, and her distance from the sun by p, then letting D and d represent the same in another planet, we have the proportion P^ D^ ; : ij' : fp. f Kepler, strangely enough, believed in the " Music of the Spheres." He made Saturn and Jupiter take the bass, Mars the tenor, Earth and Venus the counter, and Mercury the ti'eble. This shows what a streak of folly or superstition may run through the character of the noblest man. However, as John- son says, a mass of metal may be gold, though there be in it a little vein of tin. 30 INTEODTTCTION. as professor in the University, drew his attention to its simplicity and beauty. His clear discriminating mind perceived its perfection, and he henceforth advocated it with all the ardor of his tmconquerable zeal. Soon after he learned that one Jansen, a Dutch watchmaker, had invented a contrivance for making distant objects appear near. With his profound knowledge of optics and philosophical instruments, Galileo instantly caught the idea, and soon had a telescope completed that would magnify thirty times. It was a very simple affair — only a piece of lead pipe with glasses set at each end ; but it was the first telescope ever made, and destined to over- throw the old Ptolemaic theory, and revolutionize the whole science of Astronomy. Discoveries made with the telescope. — Galileo now examined the moon. He saw its mountains and val- leys, and watched the dense shadows sweep over its plains. On January 8, 1610, he turned the telescope toward Jupiter. Near it he saw three bright stars:, as he considered them, which were invisible to thta naked eye. The next night he noticed that those stars had changed their relative positions. Aston- ished and perplexed, he waited three days for a fair night in which to resume his observations. The fourth night was favorable, and he again founr' the three stars had shifted. Night after night he watched them, discovered a fourth star, and finallj- found that they were aU rapidly revolving around Jupiter, each in its elliptical orbit, with its own rate mSTOEY. 31 of motion, and ajl accompanying the planet in its journey around the sun. Here was a miniature Copernican system, hung up in the sky for all to see and examine for themselves. Reception of the discoveries. — Galileo met with the most bitter opposition. Many refused to look through the telescope lest they might become victims of the philosopher's magic. Some prated of the wickedness of digging out valleys in the fair face of the moon. Others doggedly clung to the theory they had held from their youth up. As a specimen of the arguments adduced against the new system, the following by Sizzi is a fair instance. " There are seven windows in the head, through which the air is admitted to the body, to enlighten, to warm, and to nourish it, — two nostrils, two eyes, two ears, and one mouth. So in the heavens there are two favorable stars, Jupiter and Venus ; two unpropitious. Mars and Saturn ; two luminaries, the Sun and Moon ; and Mercury alone, undecided and indifferent. From which, and from many other phenomena of Nature, such as the seven metals, etc., we gather that the number of planets is necessarily seven. Moreover, the satellites are in- visible to the naked eye, can exercise no influence over the earth, and would be useless, and therefore do not exist. Besides, the week is divided into seven days, which are named from the seven planets. Now, if we increase the number of planets, this whole system falls to the ground." Newton. — As we have seen, the truth of the Co- 32 INTEODUCTION. pernican system was fully established by the discov- eries of Galileo with his telescope. Philosophers gradually adopted this view, and the Ptolemaic theory became a relic of the past. In 1666, Newton, a young man of twenty-four years, was spending a season in the country, on account of the plague which prevailed at Cambridge, his place of resi- dence. One day, while sitting in a garden, an apple chanced to fall to the ground near him. Eeflecting upon the strange power that causes all bodies thus to descend to the earth, and remembering that this force continues, even when we ascend to the tops of high mountains, the thought occurred to his mind, " May not this same force extend to a great distance i)ut in space ? Does it not reach the moon ?" Laios of Motion. — To understand the jjhilosophy of the reasoning that now occupied the mind of Newton, let us apply the laws of motion as we have learned them in Philosophy. "When a body is once set in motion, it will continue to move forever in a straight line, unless another force is applied. As there is no friction in space, the planets do not lose any of their original velocity, but move now with the same speed which they received in the beginning from the Divine hand. But this would make them all pass through straight, and not circular orbits. What causes the curve? Obviously another force. For example : I throw a stone into the air. It moves not in a straight line, but in a curve, because the earth constantly bends it downward. HISTOEY. 33 Application. — Just so tlie moon is moving around the earth, not in a straight line, but in a curve. Can it not be that the earth bends it downward, just as it does the stone ? Newton knew that a stone falls toward the earth sixteen feet the first second. He imagined, after a careful study of Kepler's laws, that the attraction of the earth diminishes according to the square of the distance. He knew (according to fche measurement then received) that a body on the surface of the earth is four thousand miles from the centre. He applied this imaginary law. Sup- posw it is removed four thousand miles from the sur/iice of the earth, or eight thousand miles from the centre. Then, as it is twice as far from the centre, its weight wiU be diminished 2^, or 4 times. If it were placed 3, 4, 5, 10 times further away, its weight would then decrease 9, 16, 25, 100 times. If, then, the stone at the surface of the earth (four thousand miles from the centre) falls sixteen feet the first second, at eight thousand miles it would fall only four feet ; at 240,000 miles, or the distance of the moon, it would fall only about one-twentieth of an inch (exactly .053). Now the question arose, " How far does the moon fall toward the earth, i. e., bend from a straight line, every second ?" For sev- enteen years, with a patience rivalling Kepler's, this philosopher toiled over interminable columns of fig- ures to find how much the moon's path around the earth curves each second. He reached the result at last. It was nearly, but not quite exact. Disap- 2* 34 INTKODUCTION. pointed, lie laid aside his calculatioBS. liepeatedly he reviewed them, but could not find a mistake. At length, while in London, he learned of a new and more accurate measurement of the distance from the circumference to the centre of the earth. He has- tened home, inserted this new value in his calcula- tions, and soon found that the result would be cor- rect. Overpowered by the thought of the grand truth just before him, his hand faltered, and he called upon a friend to complete the computation. From the moon, Newton passed on to the other heavenly bodies, calculating and testing their orbits. At last he turned his attention to the sun, and, by reasoning equally conclusive, proved that the attrac- tion of that great central orb compels all the planets to revolve about it in elliptical orbits, and holds them with an irresistible power in their appointed paths. At last he announced this grand Law of Gravitation : EVEEY PAETICLE OF MATTER IN THE UNTVEESE AT- TRACTS EVERT OTHER PARTICLE OE MATTER WTTH A FORCE DIRECTLY PROPORTIONAL TO ITS QUANTITY OP MATTER, AND DECREASING AS THE SQUARE OF THE DIS- TA^'CE INCREASES. SPACE. 35 SPACE. We now in imagination pass into space, wMoh stretches out in every direction without bounds or measures. We look up to the heavens and try to locate some object among the mazes of the stars. We are bewildered, and immediately feel the neces- sity of some system of measurement. Let us try to understand the one adopted by astronomers. The Celestial Sphere. — The blue arch of the sky, as it appears to be spread above us, is termed the Celestial Sphere. There are two points to be no- ticed here. First, that so far distant is this imagi- nary arch from us, that if any two parallel lines from different parts of the earth are drawn to this sphere, they will apparently intersect. Of course this can- not be the fact ; but the distance is so immense, that we are unable to distinguish the Httle difference of four or even eight thousand miles, and the two lines wiU seem to unite : so we must consider this great earth as a mere speck or point at the centre of the Celestial Sphere. Second, that we must even neg- lect the entire diameter of the earth's orbit, so that if we should draw two parallel lines, one from each end of the earth's orbit, to the sphere, although these liaes would be 183,000,000 mUes apart, yet they would be extended so far that we could not separate them, and they would appear to pierce the sphere at the same point ; which is to say, that at 36 INTKODUCTION. that enormous distance, 183,000,000 miles shrink to a point. Consequently, in all parts of the earth, and in every part of the earth's orbit, we see the fixed stars in the same place. This sphere of stars sur- rounds the earth on every side. In the daytime we cannot see the stars because of the superior light of the sun ; but with a telescope they can be traced, and a skilful astronomer will find a star as well at noon as at midnight. Indeedi when looking at the sky from the bottom of a deep well or lofty chimney, if a bright star happens to be directly overhead, it can be seen with the naked eye even at midday. In )his way it is said a celebrated optician was first led to think of there being stars by day as well as by night. One half of the sphere is constantly visible to us ; and so far distant are the stars, that we see just o,K much of the sphere as we would if the upper part of the earth were removed, and we were to etand four thousand miles further away, or at the very centre of the earth, where our view would be bounded by a great circle of the earth. On the con- cave surface of the celestial sphere there are imag- iued to be drawn three systems of circles : the Hoei- ZON, the Equinoctial, and the Ecliptic Systems. Each of these has (1) its Principal Circle, (2) its Subordinate Circles, (3) its Points, and (4) its Meas- urements. SPACE. 37 I. The Horizon System. (a) The Principal Circle is the Bational Horizon. This is the great circle that, passing througK the centre of the earth, separates the visible from the invisible heavens. The Sensible Horizon is the small circle where the earth and sky seem to meet ; it is parallel to the rational horizon, but distant from it the semi-diameter of the earth. No two places have the same sensible horizon : any two on opposite sides of the earth have the same rational horizon. {b) The Subordinate Circles. — These are the Prime Vertical circle and the ileridian. A vertical circle is one passing through the poles of the horizon (the zenith and nadir). The Prime Yertical is a vertical circle passing through the Bast and West points. The Meridian is a vertical circle passing through the North and South points. (c) Points. — These are the Zenith, the Nadir, the N., S., B., and W. points. The Zenith is the point directly overhead, and the Nadir the one directly underfoot. They are also the poles of the horizon — i. e., the points where the axis of the horizon pierces the celestial sphere. The N., S., E., and W. points are familiar to aU. (d) Measurements. — These are Azimuth, Ampli- tude, Altitude, and Zenith distance. Azimuth is the distance from the meridian, meas- ured Bast or West, on the horizon (to a vertical circle passing through the object). 38 INTBODUOTION. Amplitude (the complement of Azimuth) is the distance from the Prime Vei-tical, measured on the horizon, North or South. Altitude is the distance from the horizon, meas- ured on a vertical circle toward the zenith. Zenith distanjce (the complement of Altitude) is the distance from the zenith, measured on a vertical circle, toward the horizon. The Horizon System is the one commonly used ia observations with Mural Circles and Transit In- struments. n. The Equinoctial System. (a) The PRmcrPAL Circle is the Equinoctial. This is the Celestial Equator, or the earth's equator, ex- tended to the Celestial Sphere. (b) Subordinate Circles. — These are the Horn- Circles (Eight Ascension Meridians) and the Decli- nation Parallels. The Hour Circles are thus lo- cated. The Equinoctial is divided into 360°, equal to twenty-four hours of motion — thus making 15° equal to one hour of motion. Through these divi- sions run twenty-four meridians, each constituting an hour of motion (time) or 15° of space. The Hoar Circles may be conceived as meridians of ter- restrial longitude (15° apart) extended to the Celes- tial Sphere. (See Colures, p. 40.) The Declination Parallels are small circles par- allel to the Equinoctial ; or they may be conceived SPACE. 39 as the parallels of terrestrial latitude extended to the Celestial Sphere. (c) The Points are the Celestial Poles and the Equinoxes. The Celestial Poles are the points where the axis of the earth extended pierces the Celestial Sphere, and are the extremities of the celestial axis, just as the poles of the earth are the extremities of the earth's axis. The North Point is marked very nearly by the North Star, and every direction from that is reckoned South, and every direction toward that is reckoned North, however it may conflict with our ideas of the points of the compass. The Equinoxes are the points where the Equi- noctial and the Ecliptic (the sun's apparent path through the heavens) intersect. (d) The Measurements are Bight Ascension (K. A.), Declination, and Polar Distance. Bight Ascension is distance from the Vernal Equi- nox, measured on the equinoctial eastward. B. A. corresponds to terrestrial longitude, and may ex- tend to 360° East, instead of 180° as on the earth. E. A. is never measured westward. The starting point is the meridian passing through the vernal equinox; as the meridian passing through Green- wich is the point from which terrestrial longitude is measured. Declination is distance from the equinoctial, meas- ured on any vertical circle or meridian North or South. It corresponds to terrestrial latitude. Polar distance (the complement of Dechnation) is 40 INTEODUCTION. the distance from the Pole, measured on a vertical circle. The Equiaoctial System is largely used by modem astronomers, and accompanies the Equatorial Tele- scope, Sidereal Clock, and Chronographs of the best Observatories. in. The Ecliptic System, (a) The Peinoipal Circle is the Ecliptic. This is the earth's orbit about the sun, or the apparent path of the sun in the heavens. It is inclined to the equinoctial 23° 28', which measures the inclina- tion of the axis of the earth to its orbit, and is called the obliquity of the ecliptic. (b) The Secondaey Circles are Circles of Celestial Longitude, the Golures, and Parallels of Celestial Latitude. The Circles of Celestial Longitude are now less employed. They are measured on the Ecliptic, as circles of Right Ascension (K. A.) are now measured on the Equinoctial. The Colures are two principal meridians ; the Equinoctial Colure is the meridian passing through the equinoxes ; the Solstitial Colure is the meridian passing through the solstitial points. The Parallels of Celestial Latitude are now little used, but are small circles drawn parallel to the ecliptic, as parallels of declination are now drawn parallel to the equinoctial. SPACE. 41 (c) The Points are the Poles of the Ecliptic, the Equinoxes, and the Solstitial Points. The Poles of the Ecliptic are the points where the axis of the earth's orbit meets the Celestial Sphere. (Little used.) The Equinoxes are the points where the ecliptic intersects the equinoctial. The place where the sun crosses the equinoctial* in going North, which occurs about the 21st of March, is called the Vernal Equinox. The place where the sun crosses the equinoctial in going South, which occurs about the 21st of September, is called the Autumnal Equinox. The Solstices are the two points of the echptic most distant from the Equator ; or they may be con- sidered to mark the sun's furthest declination. North and South of the equinoctial. The Summer Sol- stice occurs about the 22d of June ; the Winter Sol- stice occurs about the 22d of December. (d) The Measueements are celestial longitude and latitude. Celestial longitude is distance from the Vernal Equi- nox measured on the ecliptic, eastward. Celestial latitude is distance /rom the ecliptic meas- ured on a Secondary circle. The Zodiac. A belt of the Celestial Sphere, 9° on each side of the ecliptic, is styled the Zodiac. This is of very * This is what is commonly called " crossing the line." 42 INTEODUCTION. high antiquity, having been in use among the an- cient Hindoos and Egyptians. The Zodiac is di- vided into twelve equal parts — of 30° each — called Signs, to each of which a fanciful name is given. The following are the names of the Signs of the Zodiac. Aries t Taurus » Gemini n Cancer © Leo SI Virgo -nji Libra ^^ Scorpio TH, Sagittarius t Capricomus v3 Aquarius ^ Pisces a "The first, t, indicates the horns of the Earn; the second, » , the head and horns of the Bull ; the barb attached to a sort of letter th, designates the Scorpion ; the arrow, i , sufficiently points to Sagit- tarius ; v3 is formed from the Greek letters rp, the two first letters of To'tyor, a goat. Finally, a bal- ance, the flowing of water, and two fishes, tied by a string, may be imagined in ^, x, and k, the signs of Libra, Aquarius, and Pisces." I^fe Solar mim. *> In them hath He set a tabernacle for the sun." Psalm xix 4. THE SOLAR SYSTEM. The Solar System is comprised within the limits of the Zodiac. It consists of — • 1. The Sun — the centre. 3. The major planets — Vulcan (undetermined), Mercury Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. 3 The minor planets, at present one hundred and one in numher. 4. The satellites or moons, eighteen in number (the pathw of some extend outside the Zodiac), -which revolve around the different planets. 5. Meteors and shooting-stars. 6. Nine comets whose orbits have been computed, and over two himdred of which little is known. 7. The Zodiacal Light. How WE ARE TO IMAGINE THE SOLAR SYSTEM TO OUR- SELVES. — We -are to thiak of it as suspended in space ; being held up, not by any visible object, but in accordance with the law of Universal Gravitation discovered by Newton, whereby each planet attracts every other planet and is in turn attracted by all. First, the Sun, a great central globe, so vast as to overcome the attraction of aU the planets, and compel them to circle around him ; next, the planets, each turning on its axis while it flies around the 46 THE SOLAB SYSTEM. sun in an elliptical orbit ; then, accompanying these, the satellites, each revolting about its own planet, while aU whirl in a dizzy waltz about the central orb ; next, the comets, rushing across the planetary orbits at irregular intervals of time and space ; and finally, shooting-stars and meteors darting hither and thither, interweaving all in apparently inextri- cable confusion. To make the picture more wonder- ful still, every member is flying with an inconceiv- able velocity, and yet with such accuracy that the solar system is the most perfect timepiece known. THE Sim. Sign, 0, a buckler with its boss. Distance. — The sun's average distance from the earth is about 91^ million miles. Since the orbit of the earth is elliptical, and the sun is situated at one of its foci, the earth is nearly 3,000,000 miles farther from the sun in apheUon than in perihelion. As we attempt to locate the heavenly bodieg in space, we are immediately startled by the enormous figures employed. The first number, 91,500,000 miles, is far beyond our grasp. Let us try to comprehend it. If there were air to convey a sound from the sun to the earth, and a noise could be made loud enough to pass that distance, it would require over fourteen years for it to come to us. Suppose a railroad THE STJN. 47 could be built to the sun. An express-train, travel- ling day and night, at the rate of thirty miles an hour, would require 341 years to reach its desti- nation. Ten generations would be born and would die ; the young men would become gray-haired, and their great-grandchildren would forget the story of the begitming of that wonderful journey, and could find it only in history, as we now read of Queen Elizabeth or of Shakspeare ; the eleventh generation would see the solar depot at the end of the route. Yet this enormous distance of 91,500,000 miles is used as the unit for expressing celestial distances — as the foot-rule for measuring space ; and astron- omers speak of so many times the sun's distance as we speak of so many feet or inches. The Light of the Sun. — This is equal to 5,563 wax-candles held at a distance of one foot from the eye. It would require 800,000 fuU-moons to pro- duce a day as brilliant as one of cloudless sunshine. The Heat op the Sun. — The amount of heat we receive annually is sufficient to melt a layer of ice thirty-eight yards in thickness, extending over the whole earth. Tet the sunbeam is only -gTrolTnnr V^^^ as intense as it is at the surface of the sun. More- over, the heat and light stream off into space equally in every direction. Of this vast flood only one twenty-three hundred millionth part reaches the earth. It is said that if the heat of the sun were produced by the burning of coal, it would require a layer ten feet in thickness, extending over the whole 48 THE SOLAK SYSTEM. sun, to feed the flame a single hour. "Were the sun a sohcl body of coal, it would burn up at this rate in forty-six centuries. Sir John Herschel says that if a soUd cyhnder of ice 45 miles in diameter and 200,000 miles long were plunged, end first, into the sun, it would melt in a second of time. Apparent size. — It appears to be about a half de- gree iu diameter, so that 360 disks Hke the sun, laid side by side, would make a half circle of the celestial sphere. It seems a little larger to us in winter than iu summer, as we are 3,000,000 miles nearer it. If we represent the luminous surface of the sun when at its average (mean) distance by 1000, the same sur- face will be represented to us when in aphelion (July) by 940, and when in perihehon (January) by 1072. Dimensions. — Its diameter is about 850,000 miles.* Let us try to understand this amount by comparison. A mountain upon the surface of the sun, to bear the same proportion to the globe itself as the Dha-_ walaghiri of the Himalayas does to the earth, would have to be about six hundred miles high. Again : Suppose the sun were hollow, and the earth, as in the cut (Fig. 4), placed at the centre, not only would there be room for the moon to revolve in its regular orbit within the shell, but that would stretch off in every direction 200,000 mUes beyond. Its volume is 1,245,000 times that of the earth — * Pythagoras, whose theory of the universe Tvas in so many respects very lilse the one we receive, believed tlie sun to be 44,000 miles from the earth, and V5 miles in diameter. THE SUN. 49 i. e., it would take 1,245,000 earths to make a globe the size of the sun. Its mass is 674 times that of all the rest of the solar system. Its tveight may be expressed in tons thus, 1 , 910 , 278 , 070 , 000 , 000 , 000 , QOO , 000 , 000, Fig. 4. a number which is meaningless to our imagination, but yet represents a force of attraction which holds our own earth and aU the planets steadily in theii- places ; while it fills the mind with an indescribable awe as we think of that Being who made the sun, and holds it in the very palm of his hand. 50 THE SOLAB SYSTEM. The density of the sim is only about one-fourth that of the earth, or 1.43 that of water, so that the weight of a body transferred from the earth to the sun would not be increased in proportion to the comparative size of the two. On account also of the vast size of the sim, its surface is so far from its centre that the attraction is largely diminished, since that decreases, we remember, as the square of the distance. However, a man weighing at the earth's equator 150 lbs., at the sun's equator would weigh about 4,080 lbs., — a force of attraction that would inevitably and instantly crush him. At the earth's equator a stone falls 16 feet the first second ; at the sun's equator it would fall 437 feet. y Telescopic Ajppeaeance of the Sxjn : Sun-spots. — We may easily examine the sun at early morning or late at evening with the naked eye, and even at mid- day by using a smoked glass. The disk will appear to us perfectly distinct and circular, and with no spot to dim its brightness. If we use, however, a telescope of moderate power, taking the precaution to properly shield the eye with a colored eye-piece, we shallfinditssurface sprinkled with irregular spots, somewhat as shown in the accompanying figure. Curious opinions concerning solar spots. — The nat- ural purity of the sun seems to have been formerly an article of faith among astronomers, and therefore on no account to be called in question. Scheiner, it is said, having reported to his superior that he had seen spots on the sirn's face, was abruptly dis- THE SUN. 51 missed with these remarks : " I have read Aristotle's writings from end to end many times, and I assure you I do not find anything in them similar to that which you mention. Go, my son, tranquiUize your- self ; be assured that what you take for spots are the faults of your glasses or your own eyes." Fig. 5 Snu IM TELESCOPE. Discovery of the solar spots. — They seem to have been noticed as early as 807 a. d., although the tel- escope was not invented until 1610, and Galileo dis- covered the solar spots in the follo-wnng year. We 52 THE SOLAE SYSTEM. read in the log-book of the good ship Eichard of Arundell, on a voyage, in 1590, to the coast of Guinea, that " on the 7, at the going downe of the sunne, we saw a great black spot in the smme ; and the 8 day, both at rising and setting, we saw the like, — ^which spot to me seeming was about the big- nesse of a shilling, being in 5 degrees of latitude, and still there came a great biUow out of the souther board." Nwmher avd location of spots. — Sometimes, but rarely, the disk is clear. During a period of ten years, observations were made on 1982 days, on 372 of which there were no spots seen. As many as two hundred spots have been noticed at one time. They are mostly foimd in two belts on each side of the equator, within not less than 8° nor more than 35° of latitude. They seem to herd together — the length of the straggling group being generally par- allel to the equator. The size of the spots. — It is not uncommon to find a spot with a surface larger than that of the earth. Schroter measured one more than 29,000 miles in diameter. Sir J. W. Herschel calculated that one which he saw was 50,000 miles in diameter. In 1843 one was seen which was 14,816 miles across, and was visible to the naked eye for an entire week. On the day of the ecHpse in 1858, a spot over 107,000 miles broad was distinctly seen, and attracted general attention in this country. Some who read this paragraph will doubtless recall its ap- THE SUN. 53 Fig. 6. pearance. In 1839, Captain Davis saw one which he computed was not less than 186,000 miles long, and had an area of twenty-five billion square miles. If these are deep openings in the luminous atmos- phere of the sun, what an abyss must that be at "the bottom of which our earth could lie like a botilder in the crater of a volcano !" The spots consist of distinct parts. — From the ac- companying representation it will be seen that the spots generally consist of one or more dark portions called the umbra, and around that a grayish portion styled the pe- numbra (pene, almost, and um- bra, black). — Sometimes, how- ever, umbrae ap- pear without a penumbra, and vice versa. The umbra itself has generally a dense black centre, oaUed the nucleus. Besides this, the umbra is sometimes divided by luminous bridges. The spots are in motion. — They change from day to day ; but they all have a common movement. About fourteen days are required for a spot to pass BUN SPOTS. 54 THE SOLiR SYSTEM. across the disk of the sun from the eastern side or liTnb to the western ; in fourteen days it reappears, changed in form perhaps, but generally recognizable. The spots change their rapidity and apparent form as they pass across the disk. — A spot is seen on the eastern limb ; day by day it progresses, with a grad- ually increasing rapidity, until it reaches the cen- Fis. T. CHANGE nr SPOTS AS THEY CEOS9 THE DISK. tre ; it now gradually loses its rapidity, and finally disappears on the western Hmb. The diagram il- lustrates the apparent change which takes place in the form. Suppose at first it is of an oval shape ; as it approaches the centre it apparently widens and becomes circular. Having passed that point, it becomes more and more oval until it disappears. This change in the spots proves the sun's rotofion on its axis. — ^These changes can be accounted for only on the supoosition that the sun revolves on its axis : indeed, thev are the precise effects which the THE SUN. 55 laws of perspective demand in that case. About twenty-seven days (27 d., 7 h.) elapse from the ap- pearance of a spot on the eastern limb before it reappears a second time. During this time the earth has gone forward in its orbit, so that the location of the observer is changed ; allowing foi this, the sun's time of rotation is about twenty- five days (25 d., 8 h., 10 m. : Langier.) SYNODIC AND BIDKBEAL BBVOLrTIOK. Synodic and sidereal revolution of tJie spats. — We can easily understand why we make an aUowanco for the motion of the earth in its orbit. Suppose a 56 THE SOIiAE SYSTEM. solar spot at a, on a line passing from the centre of the earth to the centre of the sun. For the spot to pass around the sun and come into that same posi- tion again, requires about twenty-seven days. But during this time, the earth has passed on from T to T'. The spot has not only travelled around to a again, but also beyond that to a', or the distance from a to a' more than an entire revolution. To do this requires, as we have said, about two days. A revolution from a around to a' again is called a synodic, and one from a aroimd to a again is called a sidereal revolution. TJie spots apparently do not always move in straight lines. — Sometimes their path curves toward KABCH. TUNS. SEPTBKBSB. Fig. 9. the north, and sometimes toward the south, as in the figure. This can be explained only on the sup- position that the sun's axis is inclined to the ecKptic (7° 15'). b TJie spots have a motion of their own. — Besides the motion already named as assigned to the sun's rota- tion, the spots seem to have a motion of their own, THE SUN. 57 and this fact is undoubtedly the cause of the va- riation in the estimates made of the time of the sun's revolution on its axis. A spot on the equator Pig. 10. CYCLONE. performs a synodic revolution in about twenty-five days, vrhUe one haK way to either pole requires twenty-eight days. One spot was noticed which had a motion three times greater than that of clouds driven along by the most violent hurricane. Again, immense cyclones occasionally pass over the surface with fearful rapidity, producing rotation and sudden changes in the spots. At other times, however, the spots seem " to set sail and move across the disk of the sun like gondolas over a silver sea." j^ The spots change their real form. — Spots break out and then disappear under the very eye of the astron- omer. Wollaston saw one that seemed to be shat- 58 THE SOLAR SYSTEM. tered like a fragment of ice when it is thrown on a frozen surface, breaking into pieces, and sliding off in every direction. Sometimes one divides itself into several nuclei, while again several nuclei com- bine into one. Occasionally a spot will remain for six or eight rotations, while often one will last only half an hour. In one case, Sir. W. Herschel relates that when examining a spot through his telescope, he turned away for a moment, and on looking back it was gone. The appearance of the spots is periodical. — It is a remarkable fact that- the interval between the great- est and least number of spots is about 11.11 years. These variations seem also to be connected with periodical variations in the aurora, and magnetic •-arth-currents which interfere with the telegraph. The regular increase and diminution in the spots was discovered by Schwabe of Prussia, who watched the sun so carefully that it is said, " for thirty years the sun never appeared above the horizon without being confronted by his imperturbable telescope." Besides this, it has now been found that the activity of the sun's spots goes through another regular period of about 56 years. Independently of this conclusion, it has also been discovered that the aurora has a similar period of 56 years. The spots are influenced hy the planets. — They ap- pear to be especially sensitive to the approach of Venus, on account of its nearness, and of Jupiter, because of its size. The area of the spots exposed THE SUN. 59 to view from the earth is uniformly greatest when Venus is on the opposite side of the sun from us, and least when on the same side. When both Venus and Jupiter are on the side of the sun op- posite to us, the spots are much larger than when Venus alone is in that position. In part explana- tion of this influence of the planets, we may suppose that they withdraw heat or modify reflection on the disk of the sun exposed to their action, and thus cause a condensation of gases. The spots do not influence the fruip'iilness of the sea- son. — Sir W. Herschel first advanced the idea that years of abundant spots would be years also of plen- tiful harvest. This is not now generally received. What two years could be more dissimilar than 1859 and 1860 ? Both abounded in solar spots, yet one was a fruitful year and the other almost one of famine in Europe. The spot's are cooler than the surrounding surface. — It seems that the breaking out of a spot sensibly diminishes the temperature of that portion of the sun's disk. The faculse, on the other hand, do no* increase the temperature. (Secchi.) The spots are depressions below the Ijiminous surfai . — This was thought probable before, but is concl- sively proved by the photographs of the sun, which have been taken in large numbers of late at Kew Observatory. Comparative brightness of spots and sun. — If we represent the ordinary brightness of the sun by 60 THE SOLAE SYSTEM. Pig. 11. 1,000, then that of the penumbra would be 469, and that of the nucleus 7. There may be much light and heat radiated by a spot, which seems totally black as compared with the sun : we remember that when we look through even a Drumrrumd light at the svm, it appears as a black spot on the disk of that luminary. Facidce, wiEow-leaf, and mottled appearanoe.. — Be- sides the variety of spots already de- scribed, there are other curious ap- pearances worthy of note. Bright ridges or streaks appear, which constitute the most brilliant por- tions of the sun. — These are called /a- cwte. They vary from barely discem- ^^^'^^ ible, softly-gleaming tracts 1,000 mUes long, to lofty, piled-up, mountain- ous regions 40,000 miles long and 4,000 broad. Out- side of the spots, the entire disk of the sun is covered with minute shady dots, giving it a mottled appear- ance not unlike that of the skin of an orange, though less coarse. Under a large telescope the surface seems to be entirely made up of luminous masses, imperfectly separated by dark dots called pores. These masses are THE SUN. 61 said by Mr. Nasmyth to have a "willow-leaf" shape ; many observers apply other descriptive terms, such as " rice grains," " untidy circular masses," " things twice as long as broad," " granules," etc. The ac- companying cut represents the wUlow-leafed struc- ture of the luminous surface, and also the "bridges" Fig 12 WILLOW LEAX". spanning the solar spot. Indeed, it is said that the spots themselves always have their origin in a "pore," which appears to slowly increase and as- sume the blackness of an umbra, after which the penumbra begins to appear. Physical Constitution of the Sun. — Of the oonsti- 62 TKE SOLAE SYSTEM. tution of tlie sun, and consequent cause of the solar spots, very little is definitely known. We shall notice the various theories now adopted by differeht astronomers. Wilson's Theoby. — This theory supposes that the sun is composed of a solid, dark globe, surrounded by three atmospheres. The first, nearest the black body of the sim, is a dense, cloudy covering, pos- sessing high reflecting power. The second is called the photosphere. It consists of an incandescent gas, and is the seat of the light and heat of the sun. The third, or outer one, is transparent, very like our atmosphere. According to this theory, the spots are to be explained in the following manner. They are simply openings in these atmospheres made by powerful upward currents. At the bottom of these chasms we see the dark sun as a nucleus at the centre, and around this the cloudy atmosphere — the penumbra. This explains a black spot with its penumbra. Sometimes the opening in the photo- sphere may be smaller than that in the inner or cloudy atmosphere ; in that case there will be a black spot without a penumbra. It will be natural to suppose that when the heated gas of the photo- sphere or second atmosphere is thus violently rent asunder by an eruption or current from below, luminous ridges will be formed on every side of the opening by the heaped-up gas. This will ac- count for the faculce surrounding the sun-spots. It will be natural, also, to suppose that sometimes THE SUN. 63 the cloudy atmosphere below will close up first over the dark surface of the sun, leaving only an open- ing through the photosphere, disclosiog at the bot- tom a grayish surface of penumbra. We can readUy WILSON 3 THEOBT. see, also, how, as the sun revolving on its axis brings a spot nearer and nearer to the centre, thus giving us a more direct view of the opening, we can see more and more of the dark body. Then as it passes by the centre the nucleus will disappear, until finally we can see only the side of the fissure, the 64 THE SOLAE SYSTEM. penumbra, which, in its turn, will pass from oui sight. The existence of an outer atmosphere will account for the fact that the sun's margin is not so bright as its centre. Klbohhoff's Theoby. — This view differs essentially from that of Wilson. It considers the sun as an intensely white-hot solid or fluid body surrounded by a dense atmosphere of flame, filled with sub- stances yolatilized by the vivid heat. Changes of temperature take place"; which^give rise to tornadoes and violent tempests. Descending currents pro- duce openings filled with clouds, which appear as black spots on the sun's disk. A cloud once formed becomes a screen to shield the upper regions from the direct heat of the body of the sun. Thus a lighter clotid is produced, which giyes the appear- ance of a penumbra around the spots. Spectrum, analysis.— The hypothesis just given of the constitution of the sun rests upon the discov- eries of the spectroscope. This' subject will be treated hereafter under the head of Celestial Chem- istry. Wilson's theory is time-honored, but cotnpli- cated ; Kirchhoff's is modem, and partakes of the simplicity of true science. The Heat of the Sun. — This subject is not under- stood. Many theories have been advanced, but none has been generally adopted. Some have supposed the heat is produced by condensation, whereby the size of the sun is being constantly de- creased. The dynamic theory accoimts for the heat THE PLANETS. 65 and the solar spots by assuming that there are vast numbers of meteors reTolving around the sun, and that these constantly rain down upon the surface of that luminary. Their motion being stopped and changed to heat, feeds this great central fire. Were Mercury to strike the sun in this way, it would generate sufficient heat to compensate the loss by radiation for seven years. Many suppose that the heat of the sun is gradually diminishing. Of this we may be assured, there is enough to support life on our globe for millions of years yet to come. THE PLANETS. We shall describe these in regular order, passing outward from the sun. In this journey we shall ex- amine each planet in turn, noticing its distance, size, length of its year, duration of day and night, temperature of the climate, the number of its moons, and many other interesting facts, showing how much we can know of its world-life in spite of its wonder- ful distance. We shall encounter the earth in our imaginary wanderings through space, and shall ex- plain many celestial phenomena already partially familiar to us. In all these worlds we shall find traces of the same Divine hand, moulding and directing in conformity to one universal plan. The laws of Hght and heat will be invariable. The law 66 THE SOLAE SYSTEM. of gravitation, wliich causes a stone to fall to the ground, will be found to apply equally to the most distant planets. Even the very elements of which they are composed will be familiar to us, so that a book of natural science published here would, in all its general features, answer for use in a school on Mars or Jupiter. Chaeactebistics common to the Planets. {Hind.) — 1. They move in the same invariable direction around the sun ; their course, as viewed from the north side of the ecliptic, being contrary to the motion of the hands of a watch. 2. They describe oval or elliptical paths round the Sim — not, however, differing greatly from circles. 3. Their orbits are more or less inclined to the ecliptic, and intersect it in two points, which are' the nodes — one half of the orbit lying north and the other south of the earth's path. 4. They are opaque bodies like the earth, and shine by reflecting the light they receive from the sun. 5. They revolve upon iheir axes in the same way as the earth. This we know by telescopic observa- tion to be the case with many planets, and by anal- ogy the rule may be extended to all. Hence they will have the alternation of day and night like the inhabitants of the earth ; but their days are of dif- ferent lengths from our own. 6. Agreeably to the principles of gravitation, their velocity is greatest at those parts of their orbit THE PLANETS. 67 which are nearest the sun, and least at the parts which are most distant from it ; ia' other words, they move quickest ia perihelion, and slowest in aphelion. Comparison of the two Gkotjps of the Major Planets. (C/jam&ers.)^Separating the major plan- ets into two groups, if we take Mercury, Venus, the Earth, and Mars as belonging to the interior, and Jupiter, Saturn, Uranus, and Neptune to the exterior group, we shall find that they differ ia the following respects : 1. The interior planets, with the exception of the earth, are not, so far as we know, attended by any satellite, while the exterior planets all have satel- lites. We can but consider this as one of the many instances to be met with, in the universe, of the beneficence of the Creator, and that the satel- lites of these remote planets are designed to com- pensate for the small amount of light their primaries receive from the sun, owing to their great distance from that luminary. 2. The average density of the first group consid- erably exceeds that of the second, the approximate ratio being 5 : 1. 3. The mean duration of the axial rotations, or mean length of the day of the interior planets, is much longer than that of the exterior ; the average in the former case being about twenty-four hours, but in the latter only about ten hours. The Properties of the Ellipse. — In the figure, S 68 THE SOLAB SYSTEM. and S' are the fod of the ellipse ; AG is the major axis ; BD, th6 minor or conjugate axis ; O, the centre : or, astronomically, OA is the semi-axis-major or mean distance, OB the semi-axis-minor: the ratio of OS to OA is the eccentricity ; the least distance, SA, is the perihelion distance ; the greatest distance, SO, the aphelion distance. AH EUJfSE. Chaeaoteeistics of planetaby debit. — It wiU not be difficult to follow in the mind the additional Fig. 15. PLANETABT ORBITS. characteristics of a planet's orbit. The orbit or ellipse just given is laid on a plane surface. Now, THE PLANETS. 69 mcline it slightly, as compared with some other fixed plane ring, as in the cut. The astronomical fixed plane is the ecliptic. Imagine a planet follow- ing the incliaed ellipse ; at some point it must rise above the level of the fixed plane : this point is called the ascending node, and the opposite point of intersection is termed the descending node. A line connecting the two nodes is called the lin£ of the nodes. The longitude of the node is its distance from the first point of Aries, measured on the ecliptic. Following this method, we can get a very correct idea of a planetary orbit in space. CoMPAEATr?E Size OF Plambts. {Chambers.) — The following scheme will assist in obtainuig a correct notion of the magnitude of the planetary system. Choose a level field or common ; on it place a globe two feet in diameter fpi the Sun : Vulcan wiU then be represented by a smaU pin's head, at a distance of about 27 feet froni^ the centre of the ideal sun ; Mercury by a musta,r^rseed, at a distance of 82 feet ; Venus by a pea, at a distance of 142 feet ; the Earth, also, by a pea, at a distance of 215 feet ; Mars by a small pepper-corn, at a distance of 327 feet ; the minor planets by grains of sand, at dis- tances varying from 500 to 600 feet. If space wiU permit, we may place a moderate-sized orange nearly one-quarter of a mile distant from the start- ing point to represent Jupiter ; a small orange two- fifths of a mile for Saturn ; a full-sized cheny three- quarters of a mile distant for Uranus ; and lastly, a 70 THE SOLAK SYSTEM. plum I5 miles off for Neptune, the most distant planei yet known. Extending this scheme, we should find that the aphelion distance of Encke's comet would Fig. 16. UOiirAKATIVE SIZE OF PLANETS. be at 880 feet; the aphelion distance of Donati's comet of 1858 at 6 miles ; and the nearest fixed star at 7,500 miles. THE PLANETS. 71 According to this scale, the daily motion of Vulcan in its orbit would be 4§ feet ; of Mercury, 3 feet ; of Venus, 2 feet ; of the Earth, 1| feet ; of Mars, 1^ feet; of Jupiter, 10 J inches; of Saturn, 7^ inches ; of Uranus, 5 inches ; and of NeJ)tune, 4 inches. This illustrates the fact that the orbital velocity of a planet decreases as its distance from the sun increases. Conjunctions of Planets. — The grouping together of two or more planets within a limited area of the heavens is a rare event. The earliest record we have is the one of Chinese origin, already mentioned on page 16, wherein it is stated that a conjunction of Mars, Jupiter, Saturn, and Mercury occurred in the Fig. n. VENUS AMD JUPITEB IN CONJUSOHON, JANUARY 30, 1S68. reign of the Emperor Chuenhio. Astronomers tell us that this actually took place Feb. 28, 2446 b. c, and that they were between 10° and 18° of Pisces. This was before the Deluge, so that the fact must 72 THE SOLAB SYSTEM. have been afterward calculated and chronicled in tlieir records. In 1859, Venus and Jupiter came so near each other that they appeared to the naked eye as one object. In 1725, Venus, Mercury, Jupiter, and Mars appeared in the same field of the telescope. Abe the planets inhabited? — This question is one which very naturally arises, when we think of the planets as worlds in so many respects similar to our own. We can give no satisfactory answer. Many think that the only object God can possibly have in making any world is to form an abode for man. Ova own earth was evidently fitted up, al- though perhaps not created, for this express pur- pose. Everywhere about us we find proofs of special forethought and adaptation. Coal and oil in the earth for fuel and light, forests for timber, metals in the mountains for machinery, rivers for navigation, and level plains for com. Our own bodies, thej air, light, and heat are aU fitted to each other with exquisite nicety. When we turn to the planets, we do.; not know but God has other races of intelligent beings who inhabit them, or even entirely different ends to attain. Of this, however, we are assTu:ed, that, if inhabited, the conditions on which life is supported vary much from those familiar to us. We shall notice these more especially as we speak of the different planets. We shall see (1) how they differ in light and heat, from seven times our usual temperature to less than -5-5^7 5 (2) in the in- tensity of the force of gravity, from 2^ times that of THE PLANETS. 73 the earth to less than -^^ ; (3) in the constitution of the planet itseli, from a density ^ heavier than that of the earth to one only that of cork. The tem- perature sweeps downward through a scale of over Fie. la 8TZE Uff SUN AS SEEN FROM THE PLANET^. 2,000 in passing from Mercury to Uranus. No hu- man being could reside on the former, while we 4 74 THE SOLAS SYSTEM. carmot conceiTe of any polar iahabitant who could endure the mtense cold of the latter. At the sun, one of our pounds would weigh 27 pounds ; on our moon the pound weight would become only about 2 ounces ; while on Vesta, one of the planetoids, a man could easily spring sixty feet in the air and sustain no shock. Yet while we speak of these pecuUarities, we do not know what modification of the atmosphere or physical features may exist even on Mercury to temper the heat, or on Uranus to reduce the cold. With, however, all these diversi- ties, we must admit the power of an aU-wise Creator to create beings adapted to the Hfe and the land, however different from our own. The Power that prepared a world for us, could as easily and perfectly prepare one for other races. May it not be that the same love of diversity, which wiU not make two leaves after the same pattern nor two pebbles of the same size, delights in worlds peopled by races as diverse ? While, then, we cannot affirm that the planets are inhabited, analogy would lead us to think that they are, and that the most distant star that shines in the arch of heaven is filled with living beings under the care and govern- ment of Him who enlivens the, densest forest with the hum of insects, and populates even a drop of water with its teeming millions of animal- cule. DrvTSiONS OF THE Plahets. — The planets are di- vided into two classes : (1) Iriferio^, or those whose THE PLANETS. 75 orbits are within that of the earth — viz., Mercury, Venus ; (2) Superior, or those whose orbits are be- yond that of the earth — Mars, Jupiter, Saturn, Uranus, Neptune. Motions of a Planet as seen ebom the Sun. — Could we stand at the sun and watch the movements of the planets, they would all be seen to be revolv- ing with different velocities in the order of the zodiacal signs. But to us, standing on one of the planets, itself in motion, the effect is changed. To an observer at the sun all the motions would be real, while to us many are only apparent. The position of a planet, as seen from the centre of the sun, is called its hdioceniric place ; as seen from the centre of the earth, its geocentric place. When Venus is at inferior conjunction, an observer at the sun would see it in the opposite part of the heavens from that in which it would appear to him if viewed from the earth. Motions of an Ineekiob Planet. — ^An inferior planet is never seen by us in the part of the sky opposite to the sun at the time of observation. It cannot recede from him more than 90°, or \ the circumference, since it moves in an orbit entirely enclosed by the orbit of the earth. Twice in every revolution it is in conjxmction ( $ ) vdth the sim, — an inferior conjwnction (A) when it comes between the earth and the sun, and a superior conjunction (B) when the sun lies between it and the earth. (See Fig. 19.) 76 THE SOLAB SYSTEM. When the planet attains its greatest distance east or west (as we see it) from the sun, it is said to be at its greatest ekmgation, or in quadrature (a). QUADBATmUB AlTD CONJITNCTION, When passing from B to A it is east of the sun, and is " evening star ;" while passing from A to B it is west of the sun, and is " morning star." An in- ferior planet is only visible when near quadrature, and never when in superior conjunction, as its Ught is then lost in the greater jjriUiancy of the sun. THE PLANETS. 77 When in inferior conjunction, it sometimes passes in front of the sun, and appears to us as a round black spot swiftly moving across his disk. This is called a transit. RSTKOSBADB MOTION. Betrograde moiian of an inferior planet. — Suppose the earth to be at A, and the planet at B. Now, while the earth is passing to F, the planet will pass to D — the arc AF being shorter than BD, because the nearer a planet is to the sun the greater its velocity. While the planet is at B, we locate it a C on the ecKptic, in Gemini ; but at D, it appears to us to be' at G, in Taurus. So that the planet has retrograded through an entire sign on the ecliptic, while its course all the while has been directly for- 78 THE SOLAB SYSTEM. ward in the order of the signs ; and to an observer at the sun, such would have been its motion. Phases of an inferior planet. — ^An inferior planet presents aU the phases of the moon. At superior conjunction, the whole illumined disk is turned to- ward us ; but the planet is lost in the sun's rays : therefore neither Mercury nor Yenus ever presents a fuU circidar appearance, like the full moon. A little before or after superior conjunction, an inferior Fig. 21. V ~c^. C O CI w PHASES 07 AN INrBBIOB PLANET. planet may be seen with a telescope ; but the whole of the light side is not turned toward us, and so the planet appears gibbous, like the moon between first quarter and full. In quadrature, the planet shows us only one-half its illumined disk ; this decreases, becoming more and more crescent toward inferior conjunction, at which time the unillumined side is toward us. Motions of a Supeeiob Planet. — The superior planet moves in an orbit which entirely surrounds THE PLANETS. 79 that of the earth. When the earth is at E (Fig. 22), the planet at L is said to be ia opposition to the sun. It is then at its greatest distance from him- — 180°. The planet is on the meridian at midnight while the sun is on the corresponding meridian on the opposite side of the earth ; or the planet may be rising when the sun is just setting. When the planet is at N, it is in conjunction, and being lost in the sun's rays is invisible to us. Retrograde motion of a superior planet. — Suppose the earth to be at E and the planet at L, and that we move on to G while the planet passes on to O — the distance EG being longer than LO (just the reverse of what takes place in the movements of the inferior planets) ; at E, we should locate the planet at P on the ecliptic, in the sign Cancer ; but at G, it would appear to us at Q, in the sign Gemini, having apparently retrograded on the ecliptic the distance PQ, while it was all the while moving on in the direct order of the signs. Now, suppose the earth moves on to I and the planet to U, we should then see it at the point W, further on in the ecliptic than Q, which indicates direct motion again, and at some point near Q the planet must have appeared without motion. After this, it will continue direct until the earth has completed a large portion of her orbit, as we shall easily see by imagining various positions of the earth and planet, and then drawing lines as we have just done, noticing whether they indicate direct or retrograde motion. The greater 80 THE SOLAK SYSTEM. the distance of a planet the less it will retrograde, as we shall perceive by drawing another orbit out- side the one represented in the cut, and making the same suppositions concerning it as those we have already explained. Fig. 23. BETROGBADE MOTION OF X STTFEBIOB PLAITET. A^ SiDEREAi AND SYNODIC Eevoltjtion. — The interval of time required by a planet to perform a revolution from one fixed star back to it again, is termed a sidereal revolution {sidus, a star). 1. The interval of time between two similar con- THE PLANETS. 81 junctions of an inferior planet with the earth and sun is termed a synodic revolution. Were the earth at rest, there would be no difference between a sidereal and a synodic revolution, and the planet would come into conjunction twice in each revolution. Since, however, the earth is in motion, it follows that after the planet has completed its sidereal revolution, it must then overtake the earth before they can both come again into the same position with regard to the sun. The faster a planet moves, the sooner it can do this. Mercury, travelling at the greater speed and on an inner orbit, accom- plishes it much quicker than Venus. The synodic period always exceeds the sidereal. 2. The interval between two successive conjunc- tions or oppositions of a superior planet is termed a synodic revolution. Since the earth moves so much faster than any superior planet, it foUows that after it has completed a sidereal revolution it must then overtake the planet before they can come again into the same position with regard to the sun. The slower the planet moves, the sooner it can do this. Uranus, making a sidereal revolution in eighty-four years, can be overtaken more quickly than Mars, which makes one in less than two years. It conse- quently requires over a second revolution to catch up with Mars, -^ of one to overtake Jupiter, and but little over ^k^ of one to come up with Uranus. In- deed, the earth repasses Neptune in two days after it has finished a sidereal revolution. 82 THE SOLAS SYSTEM. Planets as Evening and Mobning Stabs. — The in- ferior planets are evening stars from superior to inferior conjunction, and the superior planets from opposition to conjunction. During the other half of their revolutions they are morning stars. Mercury, evening star 2 months. Venus, Mars, Jupiter, Saturn, Uranus. ■ W5 .13 6 6 To avoid filling the text with a multiplicity of figures, many interesting items are condensed in tables at the close of the volume. VULCAN, Supposed Discoveev. — Le Verrier, having detected an error in the asstuned motion of Mercury, sug- gested, in the fall of 1859, that there may be an interior planet, which is the cause of this disturb- ance. On this being made pubHc, M. Lescarbault, a French physician, and an amateur astronomer, stated that on March 26 of that year he had seen a dark body pass across the sun's disk, and that this might have been the unknown planet. Le Verrier visited him, and found his instruments rough and home-made, but singularly accurate. His clock was a simple pendtdum, consisting of an ivory ball hang- MERCURY. 83 ing from a nail by a silk thread. His observations were on prescription paper, covered with grease and laudanum. His calculations were chalked on a board, which he planed off to make room for fresh ones. Le Verrier became satisfied that a new planet had been really discovered by this enthusiastic ob- server, and congratulated him upon his deserved success. On March 20, 1862, Mr. Lummis, of Man- chester, England, noticed a rapidly-moving, dark spot, apparently the transit of an inner planet. Many other instances are given of a somewhat sim- ilar character. As yet, however, the existence of the planet is not generally conceded. The name Vulcan and the sign of a hammer have been given to it. Its distance from the sun has been estimated at 13,000,000 miles, and its periodic time (its year) at 20 days. MEECUEY. The fleetest of the goda. Sign, s , his wand. Desobiption. — Mercury is nearest to the sun of any of the definitely known planets. When the sky is very clear, we may sometimes see it, just after the setting of the sun, as a bright sparkling star, near the western horizon. Its elevation increases evening by evening, but never exceeds 30°.* If we watch it closely, we shall find that it again ap- * This distance varies much, owing to the eccentricitj- of Mcr cuiy's orbit. 84 THE SOLAE SYSTEM. proaclies the sun and becomes lost in his rays Some days afterward, just before sunrise, we can see the same star in the east, rising higher each morn- ing, until its greatest elevation equals that which it before attained in the west. Thus the planet appears to slowly but steadily oscillate like a pendulum, to and fro from one side to the other of the sun. The ancients, deceived by this, failed to discover the iden- tity of the two stars, and called the morning star Apollo, the god of day, and the evening star Mer- cury, the god of thieves, who walk to and fro in the - night-time seeldug plunder. The Greeks gave to Mercury the additional name of " The Sparkling One." The astrologists looked upon it as the malig- nant planet. The chemists, because of its extreme swiftness, applied the name to quicksilver. The most ancient account that we have of this planet is given by Ptolemy, in his Almagest ; he states its location on the 15th of November, 265 b. c. The Chinese also state that on June 9, 118 a. d., it was near the Beehive, a cluster of stars in Cancer. Astronomers teU us that, according to the best calculations, it was at that date within less than 1° of that group. On account of the nearness of Mercury to the sun, it is difficult to be detected.* It is said that Coper- nicus, an old man of seventy, lamented in his last moments that, much as he had tried, he had never * An old English -writer by the name of Goad, in 1686, humor- ously termed this planet, " A squinting lacquey of the sun, whc seldom shows his head in these parts, as if he were in debt." MERCUKY. 85 been able to see it. In our latitude and climate, we can generally easily detect it if we watch for it at the time of its greatest elongation or quadrature, as given in the almanac. Motion in Space. — It revolves about the sun at a mean distance of 35,000,000 miles. Its orbit is the most eccentric (flattened) of any among the eight principal planets, so that although when in peri- helion it approaches to within 28,000,000 miles, in aphehon it speeds away 15,000,000 miles farther, or io the distance of 43,000,000 miles. Being so near the sun, its motion in its orbit is correspondingly rapid — viz., 30 miles per second. At this rate of speed, we could cross the Atlantic Ocean in two minutes. The Mercurial year comprises only about 88 days, or nearly three of our months. Mercury revolves upon its axis in about the same time as the earth, so that the length of the Mercurial day is nearly the same as that of the terrestrial one. Though Mercury thus completes a sidereal revolu- tion around the sun in 88 days, yet to pass from one inferior or superior conjunction to the same again (a synodic revolution) requires 116 days. The reason of this is, as already explained, that when Mercury comes around to the same spot in its orbit again, the earth has gone forward, and it requires 28 days for the planet to overtake us. Distance fkom the. Earth. — This varies stiU more than its sun distance. At inferior conjunction it is between the earth and the sun, and its mean dis- 86 THE SOLAS SYSTEM. tance from us is tlie difference between the distance of the earth and the planet from the sun : at supe- rior conjunction it is the sum of these distances. Its apparent diameter in these different positions varies in the same proportion as the distances, or as three to one. The greatest and least distances vary ac- cording as either planet may happen to be in aphe- lion or perihelion. If at inferior conjunction Mer- ctixy is in aphehon and the earth ia perihelion, its distance from us is only 90,000,000 - 43,000,000 = 47,000,000 miles. If at superior conjunction Mer- cury is in aphelion and the earth in aphelion also, its distance from us is 93,000,000 -|- 43,000,000 = 186,000,000 miles. Dimensions. — Mercury is about 3,000 miles in di- ameter. Its volume is about ^ that of the earth — !. e., it would require twenty globes as large as Mer- cury to make one the size of the earth, or 25,000,000 to equal the sun. Tet as it is j denser than the earth, its weight is nearly ^ that of the earth, and a stone let drop upon its surface would fall T^ feet the first second. Its specific gravity is about that of tin. A pound weight removed to Mercury would weigh only about seven ounces. Seasons. — As Mercury's axis is much inclined from a perpendicular, its seasons are pecuhar. There are no distinct frigid zones ; but large re- gions near the poles have six weeks continuous day and torrid heat, alternating with a night of equal length and arctic cold. The sun shines perpendic- MERCURY. 87 iilarly upon the torrid zone only at the equinoxes, while he sinks far toward the southern horizon at one solstice, and as far toward the northern hori- zon at the other. The equatorial regions, there- fore, modify their temperature during each rev- Fig.8? ORBIT AND SEASONS OP MEKOUET. olution from torrid to temperate, and the tropical heat is experienced alternately toward the north and south of what we call the temperate zones. There is no marked distinction of zones as with us, but each zone changes its character twice during the Mercurial year, or eight times during the terrestrial one. An inhabitant of Mercury 88 THE SOLAE SYSTEM. must be accustomed to the most sudden and "vio- lent vicissitudes of temperature. At one time the sun not only tlius pours down its vertical rays, and in a iew weeks after sinks far down toward the horizon, but, on account of Mercury's elliptical orbit, when in perihelion the planet approaches so near the sun that the heat and light are ten times as great as that we receive, while in aphelion it recedes so as to reduce the amount to four and a haK times (the average, however, is seven times), — a temperature sufficient to turn water to steam, and even to melt many of the metals. This entire round of transitions is swept through four times during one terrestrial year. The relative length of the days and nights is much more variable than with us. The sun, apparently seven times as large as it seems to us, must be a magnifi- cent spectacle, and illumine every object with insuf- ferable briUiancA'. The evening sky is, however, lighted by no moon. Telescopic Featuees. — ^Under the telescope. Mer- cury presents aU the phases of the moon, from a slender crescent to gibbous, when its light is lost in that of the sun. These phases prove that Mer- ciur^ is spherical, and shines by the light reflected from the sim. When in quadrature, it can some- times be detected with a telescope in daylight. Being an inferior planet, we can never see it when full, and hence the brightest, nor when nearest the earth, as then its dark side is turned toward us. Owing to the dazzling light, and the vapors almost VENUS. 89 always hanging around our horizon, this planet has not received much attention of late ; the cuts here given, and the remarks concerning its physical fea- tures, are based upon the observations of the older astronomers. It is thought by some to have a dense atmosphere loaded with clouds, which would materially diminish the intensity of the sun, and perhaps make Mercury quite habitable. Sir W. Herschel, however, emphatically denies this, and asserts that the atmosphere is too insignificant to be detected. There are some dark bands about its equator. It has lofty mountains, which intercept the light of the sun, and deep valleys plunged in shade. One mountain has been ascertained to be about ten miles in height, which is ^^ of the di- ameter of the planet. The height of the Dhawa- laghiri of the Himalayas is less than 29,000 feet, or y^Vir part of the earth's diameter. VENUS. The Qaeen of Beauty. Sign ? , a looking-glass. Description. — Venus, the next in order to Mer- cury, is the most brilliant of all the planets. When visible before sunrise, she was called by the ancients Phosphorus, Lucifer, or the Morning Star, and when she shone in the evening after sunset, Hesperus, Ves- per, or the Evening Star. She presents the same appearances as Mercury. Owing, however, to the greater diameter of her orbit, her apparent oscillations 90 THE SOLAE SYSTEM. are nearly 48° east and west of tlie sun,* or about 18° more tKan those of Mercury. She is therefore seen much earlier in the morning and much later at night. She is " morning star" from inferior to supe- rior conjunction, and " eTening star" from superior to inferior conjunction. She is the most brilliant about five weeks before and after inferior conjunc- tion, at which time the planet is bright enough to cast a shadow at night. If, in addition, at this time of greatest brilliancy, Venus is at or near her high- est north latitude, she may be seen with the naked eye in full dayhght.t This occurs once in eight years, in which interval the earth and planet return to the same situation in their orbits ; eight complete revolutions of the earth about the sun occupying nearly the same time as thirteen of Venus. This happened last in February, 1862. A less degree of brilhancy is attained once in twenty-nine months, under somewhat the same circumstances. Motion m Space. — Unlike Mercury, Venus has an orbit the most circular of any of the principal * This distance varies but little, owing to the slight eccentricity of Venus's orbit. t Arago relates that Bonaparte, upon repaiiing to the Luxem- boui'g, when the Directoiy was about to give him a fete, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. Upon inquiry, he learned that these curious persons were obseiv- ing with astonishment a star which they supposed to be that of the Conqueror of Italy. The emperor himself was not indifferent when his piercing eye caught the clear lusti'e of Venus smiling upon him at midday. VENUS. 91 planets. Her mean distance from the smi is about 66,000,000 miles, which varies at aphelion and peri- helion within the limits of a half million miles against 15,000,000 miles in the case of the former planet. She makes a complete revolution around the sun in about 225 days, at the mean rate of 22 miles per second ; hence her year is equal to about seven and one haK of our months. This is a sidereal revolu- tion, as it wotild appear to an observer at the sun, but a synodic revolution is 584 days. Mercury, we remember, catches up with the earth in 28 days after it reaches the point where it left the earth at the last inferior conjimetion. But it takes Venus nearly two and a haM revolutions to overtake the earth and come into the same conjunction again. This grows out of the fact that Yenus has a longer orbit to travel through, and moves only about one-fifth faster than the earth, while Mercury travels nearly twice as fast. The planet revolves upon its axis in about 24 hours ; so the day does not differ in length essen- tially from ours. Distance fkom the Earth. — The distance of Ve- nus from the earth, hke that of Mercury, when in inferior conjunction, is the difference between the distances* of these two planets from the sun, and when in superior conjunction the sum of these dis- tances. * Lot the pupil calculate the distances of the earth and Venua fi'om each other, when in perihelion and aphelion, as in the case of Mercury, (See tables in Appendix.) 92 THE SOLAR SYSTEM. The figure represents its apparent dimensions at the extreme, mean, and least distances from us. The variation is nearly as the numbers 10, 18, and 65. It would be natural to think that the planet is the brightest when the nearest, and thus the largest, EXTKEME, MEAN, AND LEAST APPARENT SIZE OP TENUS. but we should remember that then the bright side is toward the sun, and the unillumined side toward us. Indeed, at the period of greatest brilliancy of which we have spoken, only about one-fourth of the light is visible. At this time, however, many observers have noticed the entire contour of the planet to be of a dull gray hue, as seen in the cut. Dimensions. — Venus is about 7,500 miles in diame- ter. The volume of the planet is about four-fifths that of the earth, while the density is about the same. A stone let fall upon its surface would fall 14 feet in VENUS. 93 the first second : a pound weight removed to its equator would weigh about five-sixths of a pound. From this we see that the force of gravity does not decrease exactly in proportion to the size of the planet, any more than it increases with the mass of the sun. The reason of this is, that the body is brought nearer the mass of the small planet, and so feels its attraction more fully than when far out upon the extreme circumference of a large body, — the attraction increasing as the square of the dis- tance from the particles decreases. Seasons. — ^As the axis of Yenus is very much in- clined from a perpendicular, its seasons are similar to those of Mercury. The torrid and temperate Fig. 25. VENUS AT ITS SOLSTICE. zones overlap each other ; the polar regions having alternately at one solstice a torrid temperature, and at the other a prolonged arctic cold. The inequality 94 THE SOIAB SYSTEM. of the mghts is very marked. The heat and light are double that of the earth, while the circular form of its orbit gives nearly an equal length to its four seasons. If the inclination of its axis is 75°, as some as- tronomers hold, its tropics must be 75° from the equator, and its polar circles 75° from the poles. The torrid zone is, therefore, 150° in width. The torrid and frigid zones interlap through a space of 60°, midway between the equator and poles. Telescopic Featuees. — ^Venus, being an interior planet, presents, like Mercury, all the phases of the moon. This fact was discovered by Galileo, and was among the first achievements of his telescopic obser- vations. It had been argued against the Copemi- can system that, if true, Venus should wax and wane Hke the moon. Indeed, Copernicus himseK boldly declared that if means of seeing the planets more distinctly were ever invented, Venus would be found to present such phases. Galileo, with his telescope, proved this fact, and, by overthrowing that objec- tion, again vindicated the Copemican theory. This planet is not sensihlj flattened at the poles. It is thought to have a dense, cloudy atmosphere. This was established by the fact that at the transit of Venus over the sun in 1761 and 1769, a faint ring of light was observed to surround the black disk of the planet. The evidence of an atmosphere, as well as of mountains, rests very much upon the . peculiar appearance attending its crescent shape. VENUS. 95 (1.) The luminous part does not end abruptly ; on the contrary, its light diminishes gradually, which diminution may be entirely explained by the twi- light on the planet. The existence of an atmosphere Pig. 36. CKEBCENT AND SPOTS OP VENUS. which diffuses the rays of light into regions where the sun has already set, has hence been inferred. Thus, on Venus, the eyenings, like ours, are lighted by twilight, and the mornings by dawn. (2.) The edge of the enlightened portion of the planet is un- even and irregular. This appearance is doubtless the effect of shadows cast by mountains. Spots have been noticed on its disk which are consider-ed to be traceable to clouds. Indeed, Herschel thinks that we never see the real body of the planet, but only its atmosphere loaded with vapors, which may mitigate the glare of the intense sunshine. Satellites. — Venus is not known to have any DQOOn. 96 THE SOLAK SYSTEM. THE EAETH. Sign, ©, a circle witli Equator and Meridian. The Earth is the next planet we meet in passing outward from the sim. To the beginner, it seems strange enough to class our world among the heav- enly bodies. They are brilliant, while it is dark and opaque ; they appear Hght and airy, while it is solid and firm ; we see in it no motion, while they are constantly changing their position ; they seem mere poiats in the sky, while it is vast and extended. Yet at the very beginning we are to consider the earth as a planet shining brightly in the heavens, and appearing to other worlds as a star does to us : we are to learn that it is in motion, flying through its orbit with inconceivable velocity ; that it is not fixed, but hanging in space, held by an iuvisible power of gravitation which it cannot evade ; that it is small and insignificant beside the mighty globes that so gently shine upon us in the far-o£f sky; that our earth is only one atom in a universe of worlds, all fixm and solid, and equally well fitted to be the abode of life. ,-i-<^-— — Dimensions. — The earth is not " round like a ball," but flattened at the poles. Its form is that of an oblate spheroid. Its polar diameter is about 7,899 miles, and its equatorial about 7,925^. The com- pression is, therefore, about 26^ miles. (See table THE EABTH. 97 in Appendix.) K we represent the earth by a globe one yard in diameter, the polar diameter would be one-tenth of an inch too long. It has been recently Kg. 37. THE EAKTH IN SPACE. shown that the eqnatbf itself is not a perfect circle, but is somewhat flattened, since the diameter which 98 THE SOLAB SYSTEM. pierces the meridian 14° east of Greenwich is two miles longer than the one at right angles to it. The circumference of the earth is about 25,000 miles. Its density is about 5^ times that of water. Its weight is 6,069,000,000,000,000,000,000 tons. The inequalities of its surface, arising from build- ings, valleys, mountains, etc., have been likened to the roughness on the rind of an orange. This is not an exaggeration. On a globe sixteen inches in diameter, the land, to be in proportion, should be represented by the thinnest writing-paper, the hills by small grains of sand, and elevated ranges by thick drawing-paper. To represent the deepest wells or mines, a scratch might be made that would be invisible except with a glass. The water in the ocean could be shown by a brush dipped in color and Kghtly drawn over the bed of the sea. The Eotundity op the Earth. — This is shown in various ways, among which are the following: (1) By the fact that vessels have sailed around the eartii ;* * It is curious, in connection with this well-known feet, to re- (CaH the arguments urged by the Spanish philosophers against ,the ilieasoning of Columbus, when he assm'sd them that he jC0\dd »m.Ye at Asia just as certaiiily by sailing west as ,easu "JJow," they asked, "can the earth be round? If jit were, tbeji mx the opposite side the rain would fall upward, ftrees would, giiow with their branches down, and everything iwould be topgy-ftniwy. Every object on its surface would cer- tainly fall off; ^ijd Jf a ship by suling west should get around THE EAKTH. 99 (2) when a ship is coming into port we see the masts first ; (3) the shadow of the earth on the moon is circular; (4) the polar star seems higher in the heavens as we pass north ; (5) the horizon expands as we ascend an eminence* If we climb to the top of a hill, we can see further than when on the plain at its foot. Our eyesight is not improved ; it is only because ordinarily the curvature of the earth shuts off the view of distant objects, but when we ascend to a higher point, we can see farther over the side of the earth. The curvature is eight inches per mile, 2' X S""- = 32 inches for two miles, 3=" x 8"- for three nules, etc. An object of these respective heights would be just hidden at these distances. Appaeent AMD BEAii MoTiON. — ^In endeavoring to understand the various appearances of the heavenly bodies, it is well to remember how in daily life we transfer motion. On the cars, when in rapid move- ment, the fences and trees seem to glide by us, there, it would never be able to climb up the side of the earth and get back again. How can a ship sail up hill ?" * The history of aeronautic adventure affords a curious illustra- tion of this same principle. The late Mr. Sadler, the celebrated aeronaut, ascended on one occasion in a balloon from Dublin, and was wafted across the Irish Channel, when, on his approach to the Welsh coast, the balloon descended nearly to the surface of the sea. By this time the s\m was -set, and the shades of even- mg began to close in. He threw out nearly all his ballast, and suddenly sprang upward to a great height, and by so doing brought his horizon to dip below the sun, producing the whole phenomenon of a western sunrise. Subsequently descending in Wales, he, of course, witnessed a second sunset on the same evening. 100 THE SOLAE SYSTEM. while we sit still and watch them pass. On a bridge, when we are at rest, we follow the undula- tions of the waves, until at last we come to think that they are stationary and we are sweeping down stream. "In the cabin of a large vessel going smoothly before the wind on still water, or drawn along a canal, not the smallest indication acquaints us with the ' way it is making.' We read, sit, walk, as if we were on land. If we throw a ball into the air, it falls back into our hand ; if we drop it, it ahghts at our feet. Insects buzz around us as in the free air, and smoke ascends in tiie same manner as it would do in an apartment on shore. If, indeed, we come on deck, the case is in some respects different ; the air, not being carried along with us, drifts away smoke and other light bodies such as feathers cast upon it, apparently in the opposite direction to that of the ship's progress; but in reality they remain at rest, and we leave them behind in the air."* - ' DlUENAL EeVOLUTION OF THE EaBTH ABOUND ITS Axis. — The earth, in constantly turning from west * " And what is the earth itself hut the good ship we are sailing in through the universe, hound round the sun ; and as we sit here in one of the ' herth^' we are unconscious of there heiug any 'way' at all upon the vessel. On deck, too, out in the open air, it's all the same as long as we keep our eyes on the ship ; but immediately we look over the sides— and the horizon is hut the 'gunwale' of our vessel — we see the blue tide of the great ocean around us go drifting by the ship, and sparkling with its mLlUon stars as the waters of the sea itself sparkle at night be- tween the tropics." THE EAETH. 101 to east, elevates our horizon above the stars on the west, and depresses it below the stars on the east. As the horizon appears to us to be sta- tionary, we assign the motion to the stars, think- ing those on the west which it passes over and hides to have sunk below it or set, and imagining those on the east it has dropped below to have moved above it or risen. So, also, the horizon is depressed below the sun, and we call it sunrise; it is elevated above the sun, and we call it sunset. We thus see that the diurnal movement of the sun by day and stars by night is a mere optical delu- sion — that here as elsewhere we simply transfer motion. This seems easy enough for us to under- stand, because the explanation makes it so simple ; but it was the " stone of stumbling" to ancient as- tronomers for two thousand years. Copernicus him- self, it is said, first thought of the true solution while riding on a vessel and noticing how he insensibly transferred the movement of the ship to the objects on the shore. How much grander the beautiful simplicity of this theory than the cumbersome com- plexity of the old Ptolemaic belief ! Diurnal motion of the Sun. — The explanation just given illustrates the apparent motion of the sun, and the cause of day and night. Suppose S to be the sun. E, the earth, turning upon its axis EF from west to east, has half its surface only illu- minated at one time by the sun. To a person at D, the sun is in the horizon and day commences, 102 THE SOLAB SYSTEM. the luminarj appearing to rise higher and higher in the heavens with a westerly motion, as the ob- server is carried forward easterly by the earth's diurnal rotation to A, where he has the sun in his Fig. 38. DAILT HOTION OP THIS BTJH. meridian, and it is consequently noon. The sun then begins to decline in the sky until the specta- tor arrives at B, where it sets, or is again in the horizon on the west side, and night begins. He moves on to C, which marks his position at midnight, the sun being then on the meridian of places on the opposite part of the earth, and he is then brought round again to D, the point of sunrise, when another day commences. (Hind.) , (s-*'^'^ The unequal rede of diurnal motion,. — ^Different points upon the surface of the earth revolve with different velocities. At the two poles the speed of rotation is nothing, while at the equator it is great- est, or over 1,000 miles per hour. At Quito, the circle of latitude is much longer than one at the mouth of the St. Lawrence, and the velocities vary ia the same proportion. The former place moves THE EABTH. 103 at the rate of about 1,038 miles per hour ; the lat- ter, 450 miles. In our latitude (41°) the speed is about 780 miles per hour. We do not perceiye this wonderful velocity with which we are flying through the air, because the air moves with US.* Yet were the earth suddenly to stop its rotation, the terrible shock would, without doubt, destroy the entire race of man, and we, with houses, trees, rocks, and even the oceans, in one confused mass, would be hurled headlong into space. On the other hand, were the rate of rotation to increase, the length of the day would be proportionately short- ened, and the weight of all bodies decreased by the centrifugal force thus produced. Indeed, if the rotary movement should become swift enough to * An ingenious inventor once suggested that we should utilize the earth's rotation, as the most simple and economical, as well as rapid mode of locomotion that could be conceived. This was to be accomplished by rising in a balloon to a height inacces- sible to aerial cuiTents. The balloon, remaining immovable in this calm region, would simply await the moment when the earth, rotating underneath, would present the place of destination to the eyes of travellers, who would then descend. A well- regulated watch and an exact knowledge of longitudes would thus render travelling possible from east to west, aU voyages north or south naturally being interdicted. This suggestion has only one fault; it supposes that the atmospheric strata do not revolve with the earth. Upon that hypothesis, since we rotate in our latitude with the velocity of 333 yards in a second, there would result a wind in the contrary direction ten times more violent than the most terrible hurricane. Is not the absence of / such a state of things a convincing prooif of the participation of the atmospheric envelope in the general movement? (Guillemin.) - 104 THE SOLAB SYSTEM. reduce the day to 84 minutes, or about yV ^^^ pres- ent length, the force of gravity would be entirely overcome, and all bodies *ould be without weight ; and if the speed were still farther increased, all loose bodies would fly off from the earth like water from a grindstone when swiftly turned, while we should be compelled to constantly " hold on" to avoid sharing the same fate. But against such a catastrophe we are assured by the immutability of God's laws. " He is the same yesterday, to-day, and forever." The earth has not varied in its revo- lution xIjt of a second in 2,000 years. Unequcd diurncH orlits of the stars. — ^Let O repre- sent our position on the earth's surface, E Z B our meridian ; E I B K our horizon ; P and P' the north THE EABTH. 105 and south poley, Z the zenith, Z' the nadir; and GICK the celestial equator. Now PB, it will be seen, is the elevation of the north pole above the horizon, or the latitude of the place. Suppose we should see a star at A, on the meridian below the pole. The earth revolves in the direction GIC ; the star win therefore move along A L to Z, when it is on the meridian above the pole. It continues its course along the dotted line around to A again, when it is on the meridian below the pole, having made a complete circuit around the pole, but not having descended below our horizon. A star rising at B would Just touch the horizon ; one at I would move on the celestial equator, and would be above the horizon as long time as it is below — twelve hours in each case ; a star rising at M, would just come above the horizon and set again at N. Unequal diurnab velocities of the stars. — The stars appear to us to be set in a concave shell which ro- tates daily about the earth. As different parts of the earth really revolve with varying velocities, so the stars appear to revolve at different rates of speed. Those near the pole, having a small orbit, revolve very slowly, while those near the celestial equator move at the greatest speed. Appearance of the, stars at different places on the earth. — ^Were we placed at the north pole, Polaris would be directly overhead, and the stars would seem to pass around us in circles parallel to the horizon, and increasing in diameter from the upper 106 THE SOLAE SYSTEM. to lower ones. Were we placed at the equator, the pole-star would be at the horizon, and the stars would move in circles exactly perpendicular to the horizon, and decreasing in diameter, north and south from those in the zenith, while we could see one half of the path of each star. Were we placed in the southern hemisphere, the ciremnpolar stars would rotate about the south pole, and the others in cir- cles resembling those in our sky, only the points of direction would be reversed to correspond with the pole. Were we placed at the south pole, the ap- pearance would be the same as at the north pole, except that there is no star to mark the direction of the earth's axis. Motion of the Earth in Space about the Sun.— The earth revolves in an elliptical path about the sun at a mean distance of 91^ million of miles. This path is called the ecliptic ; its eccentricity is about 3,000,000 miles; — this changes slightly, not more than TTnr*(TTnr P^'^ century, so that in time the orbit woiold become circular, were it not that after the lapse of some thousands of years, the eccentricity will begin to increase again, and vrill thus vary through all ages within definite, although yet un- determined limits. Its entii-e circumference is near- ly 600,000,000 miles, and the earth pursues this wonderful journey at the rate of 18 miles per second. This revolution of the earth about the sun gives rise to various phenomena, of which we shall now proceed to ^eak. THE EARTH. 107 1. Change in the appearance of the heavens in differ- ent months. — This is the natural result of the revolu- tion of the earth about the sun. In Fig. 30, suppose Hg.BO. H ♦ * * * * i * * APFEABANCB OT THE HEATENS m SUTEBENT SEASOHli A B C D to be the orbit of the earth, and E F G H the sphere of the fixed stars, surrounding the sun in every direction. When our globe is at A, the stars about E are on the meridian at midnight. Being seen from the earth in the opposite quarter 108 THE SOLAB SYSTEM. to the sun, they are most favorably placed for obser- vation. The stars at G, on the contrary, ■will be invisible, for the sun intervenes between them and the earth : they are on the meridian of tiie spectator about the same time as the sun, and are always hidden in his rays. In three months the earth has passed over one-fourth of her orbit, and has arrived at B. Stars about F now appear on the meridian at midnight, while those at E, which previously occupied their places, have descended toward the west and are becoming lost in the sun's refulgence, while those about G are just coming into sight in the east. In three months more the earth is situated at 0, and stars about G shine in the midnight sky, those at F having, in their turn, vanished in the west. Stars at E are on the meridian at noon, and consequently hidden in daylight; and those about H are just escaping from the sun's rays, and commencing their appearance in the east. One revolution of the earth brings the same stars again on the meridian at midnight. Thus it is that the earth's motion round the sun as a centre explains the varied aspect of the heavens in the summer and winter skies. (Hind.) 2. Yearly path of the sun through the heavens. — ^We have spoken of the diurnal motion of the sun. We now speak of its second apparent motion — ^its yearly path among the stars.* If we look at the accom- * This yearly movement of the sun among the fixed stars is aot as appai'ent to us as his daily motion, because his superior THE EABTH. 109 panying plate (Fig. 31), we can see how tKe motion of the earth in its orbit is also transferred to the sun, and causes him to appear to us to travel in a fixed path through the heavens. When the earth la in any part of the ecliptic, the sun seems to us to be in the point directly opposite. For example, when the earth is in Libra (===)* — autumnal equinox — tliC sun is in Aries (v) — vernal equinox ; when the sun enters the next sign, Taurus (8), the earth in fact has passed on to Scorpio (m). Thus as the- earth moves through her orbit, the sun seems to pass through the same path along the opposite side of the ecliptic, making the entire circuit of the heavens in the year, and returning at the end of that time to the same place among the stars. If the earth could leave a shining line as it passes through its orbit about the sun, we should see the sun apparently moving along this same line on the opposite side of the circle. We therefore define the edvptic as the real orbit of the earth about the sun, or the ajpparent path of the sun through the heavens. The ecliptic crosses the celes- tial' equator at two points. These are called the equinoxes, light blots out the stars. But if we notice a star at the western hoiizon just at sunset, we can tell what constellation the sun is tlien in: now wait two or three nights, and weshallflnd that star is set, and another has taken its place. Thus we can trace the sun through the year in his path among the fixed stars. * When we say " the earth is in Libra," we mean that a spec- tator placed at the sun would see the earth in that part of the heavens which is occupied by the sign Libra. 110 THE SOLAB SYSTEM. 3. An 'apparent movemerd of the sun, north and south. — Having now spoken of the apparent diurnal KaA annual motions of the sim, there yet remains a third motion, which has doubtless oftentimes at- tracted our attention. In summer, at midday, the sun is high in the heavens ; in the winter, quite low, near the southern horizon. In summer he is a long time above the horizon ; in the winter, a short time. In summer he rises and sets north of the east and west points ; in winter, south of the east and west points. This subject is so intimately connected with the next, that we shall understand it best when taken in connection with it. 4. Change of the Seasons. — ^Vaeiation in Length OP Day and Night.— By closely studying the accom- panying illustration and imagining the various posi- tions of the earth in its orbit, let us try to under- stand the several points. I. Ohliquity of the ediptic. — The axis of the earth is inclined 23^° from a perpendicular to its orbit. This angle is called the obUquity of the ecliptic. H. JParaUdisTn of the. axis. — ^In all parts of the orbit, the axis of the earth is parallel to itself and constantly points toward the North Star.* This is only an instance of what is very familiar to us all. Nature reveals to us nothing more permanent than the axis of rotation in anything that is rapidly turned. It is its rotation which keeps a boy's hoop * There is a slight variation from this, which we shall soon notice. THE ORBIT OF TEE EARTH. 112 THE SOLAU SYSTEM. from falling. For the same reason a quoit retains its direction when whirled, and it wiU keep in the same plane at whatever angle it may be thrown. A man slating a roof wishes to throw a slate to the ground ; he simply whirls it, and as it descends it will strike on the edge without breaking. As long as a top spins there is no danger of its falling, since its tendency to preserve parallel its axis of rotation is greater than the attraction of the earth. This wonderful law would lead us to think that the axis of the earth always points in the same direction, even if we did not know it from direct observation. m. The rays of the sun strike the various por- tions of the earth, when in any position, at different angles. — ^Example. When the earth is in Libra, and also when in Aries, the rays strike vertically at the equator, and more and more obliquely in the northern and southern hemispheres, as the distance from the equator increases, until at the poles they strike almost horizontally. This variation in the direction of the rays produces a corresponding variation in the intensity of the sun's heat and light at dif- ferent .places, and accounts for the difference between the torrid and polar regions. IV. As the earth changes its position the angle at which the rays strike any portion is varied. — Ex- ample. Take the earth as it enters Capricornus (mj) and the sun in Cancer (as) He is now over- head, 23^° north of the equator. His rays strike THE EAETH. 113 less obliquely in tlie northern hemisphere than when the earth was in Libra. Let six months elapse : the earth is now in Cancer and the sun in Capricomus; and he is overhead, 23^° sotdJi of the equator. His rays strike less obliquely in the southern hemisphere than before, but in the northern hemisphere more obliquely. These six months have changed the direction of the sun's rays on every part of the earth's surface. This accounts for the dif- ference in temperature between summer and winter. V. The Equinoxes. — ^At the equinoxes one half of each hemisphere is illuminated: hence the name Equinox (cequus, equal, and nox, night). At these points of the orbit the days and nights are equal over the entire earth,* each being twelve hours in length. VI. Northern and southern hemispheres unequally Uluminated. — While one half of the earth is con- stantly illuminated, at aU points in the orbit except the equinoxes the proportion of the northern or southern hemisphere which is in daylight or dark- ness varies. "When more than half of a hemi- sphere is in the light, its days are longer than the nights, and vice versa. "VH. The seasons and the comparative length of days and nights in the South Temperate Zone, ai any s-pedfied time, are the reverse of those in the North Temperate Zone, except at the Eqvmoxes, where they are cdike. * Except a small space at each pole. 114 THE SOLAE SYSTEM. "Vm. The earth at the Swmmer Solstice. — ^Wlien the earth is at the summer solstice, about the 21st of June, the sun is overhead 23^° north of the equator, and if its vertical rays could leave a gold- en line on the surface of the earth as it revolves, they would mark the Tropic of Cancer. The sun is at its furthest northern declination, ascends the high- est it is ever seen above our horizon, and rises and sets 23^° north of the east and west points. It seems now to stand still in its northern and southern course, and hence the name Solstice {sd, the sun, sto, to stand). The days iu the north temperate zone are longer than the nights. It is oiu: summer, and the 21st of June is the longest day of the year. In the south temperate zone it is winter, and the shortest day of the year. The circle that sepa- rates day from night extends 23|° beyond the north pole, and if the sun's rays could in like manner leave a golden line on that day, they would trace on the earth the Arctic Circle. It is the noon of the long six months polar day. The reverse is true at the Antarctic Circle, and it is there the midnight of the long six months polar night. IX. The earth at the Autumnal Equinox. — ^The earth crosses the aphehon point the 8th of July, when it is at its farthest distance from the sun, which is then said to be in apogee. The sun each day rising and setting a trifle further toward the south, passes through a lower circuit in the heavens We reach the autumnal equinox the 22d of Sep- THE EAETH. 115 tember. The sun being now on the equinoctial, if its vertical rays could leave a line of golden light, they would mark on the earth the circle of the equator. It is autumn in the north temperate zone and spring in the south temperate zone. The days and nights are equal over the whole earth, the sun rising at 6 A. M. and setting at 6 p. M., exactly in the east and west where the equinoctial intersects the horizon. X. The earth at the Winter Solstice. — The sun after passing the equinoctial — "crossing the line," as it is called — sinks lower toward the southern ho- rizon each day. We reach the winter solstice the 21st of December. The sun is now directly overhead 23^° south of the equator, and if its rays could leave a line of golden light they would mark on the earth's surface the Tropic of Capricorn. It is at its furthest southern declination, and rises and sets 23J° south of the east and west points. It is our winter, and the 21st of December is the short- est day of the year. In the south temperate zone it is summer, and the longest day of the year. The circle that separates day from night extends 23^° beyond the south pole, and if the sun's rays in like maimer could leave a line of golden light they would mark the Antarctic Circle. It is there the noon of the long six months polar day. At the Arctic Circle the reverse is true ; the rays fall 28J° short of the north pole, and it is there the midnight of the long six months polar night. Here 116 THE SOLAE SYSTEM. again the sun appears to us to stand still a day or two before retracing its course, and it is there- fore called the Winter Solstice. XI. The earth at the Vernal Equinox. — The earth reaches its ferihelion about the 31st of December. It is then nearest the sun, which is therefore said to be in perigee. The sun rises and sets each day further and further north, and climbs up higher in the heavens at midday. . Our days gradually increase in length, and our nights shorten in the same proportion. On the 21st of March* the sun reaches the equinoctial, at the vernal equinox. He is overhead at the eqftator, and thte days and nights are again equal. It is our spring, but in the south temperate zone it is autumn. XH. The yearly path finished. — The earth moves on in its orbit through the spring and cummer months. The sun continues its northerly course, ascending each day higher in the heavens, and its rays becoming less and less oblique. On the 21st of June it again reaches its furthest northern decli- nation, and the earth is at the summer solstice. We have thus traced the yearly path, and noticed the course of the changing seasons, with the length of the days and nights. The same series has been repeated through all the ages of the past, and will be till time shall be no more. - ^ Xm. Distance of the earth from the sun varies. — * The precise time of the equinoxes and solstices varies each year, but within a small limit THE EARTH. 117 W« notice, from what we have just seen, that we are nearer the sun by 3,000,000 miles in winter than in summer. The obliqueness with which the rays strike the north temperate zone at that time pre- vents our receiving any special benefit from this favorable position of the earth. XIV. Southern summer. — The inhabitants of the south temperate zone have their summer while the earth is in perihelion, and the sun's rays are about ^ warmer than when in aphelion, our summer-time. This will perhaps partly account for the extreme heat of their season. Herschel tells us that he has found the. temperature of the surface soil of South Africa 159° F. Captain Sturt, in speaking of the extreme heat of Australia, says that matches accidentally dropped on the groimd were immediately ignited. The southern winters, for a similar reason, are colder ; and this makes the average yearly tempera- ture about the same as ours. XV. Extremes of heat and, cold not at the solstices. — We notice that we do not have our greatest heat at the time of the summer solstice, nor our greatest cold at the winter solstice. After the 21st of Jime, the earth, already warmed by the genial spring days, continues to receive more heat from the sun by day than it radiates by night : thus its tempera- ture still increases. On the other hand, after the 21st of December the earth continues to become colder, because it loses more heat during the night than it receives during the day. 118 THE SOLiE SYSTEM. XVI. Bwnvme.r longer than vnnter. — As the sun is not in the centre of the earth's orbit, but at one of its foci, that portion of the orbit which the earth passes through in going from the vernal to the autumnal equinox comprises more than one-haK the entire ecliptic. On this account the summer is longer than the wiuter. The difference is stiU fur- ther enhanced by the variation in the earth's ve- locity at aphelion and perihelion. The annexed table gives the mean duration of the seasons : SeaBons. Days. Seasons. Bays. Spring 92.9 Autumn 89.7 Summer 93,6 Winter 89.0 The difference of time in the earth's stay in the two portions of the ecliptic, as will be seen from the above, is 7.8 days. XVn. Varying vdocity of the earth. — ^We can see, by looking at the plate, that the velocity of the earth must vary in different portions of its orbit. When passing from the vernal equinox "to aphelion, the attraction of the sun tends to check its speed ; from that point to the autumnal equinox, the at- traction is partly in the direction of its motion, and so increases its velocity. The same principle applies when going to and from perihelion. XVm. Curious appearance of the sun at the north pole. — "To a person standing at the north pole, the sun appears to sweep horizontally around the sky every twenty-four hours, without any perceptible THE EABTH. 119 variation in its distance from the horizon. It is, however, slowly rising, until, on the 21st of June, it is twenty-three degrees and twenty-eight minutes above the horizon, a little more than one-fourth of the distance to the zenith. This is the highest point it «ver reaches. From this altitude it slowly de- scends, its track being represented by a spiral or screw with a very fine thread ; and in the course of three months it worms its way down to the horizon, which it reaches on the 22d of September. On this day it slowly sweeps aroimd the sky, with its face half hidden below the icy sea. It still continues to descend, and after it has entirely disappeared it is still so near the horizon that it carries a bright twilight around the heavens in its daily circuit. As the sun sinks lower and lower, this twihght grows gradually fainter, till it fades away. December 21st, the sun is 23° 28' below the horizon, and this is the midnight of the dark polar winter. From this date the sun begins to ascend, and after a time it is her- alded by a faint dawn, which circles slowly around the horizon, completing its circuit every twenty-four hours. This dawn grows gradually brighter, and on the 22d of March the peaks of ice are gilded with the first level rays of the six months day. The bringer of this long day continues to wind his spiral way upward, tiU he reaches his highest place on the 21st of June, and his annual course is completed." XIX. Besvlts, if the axis of the earth were perpen- dicular to, the edipUc.—Th.e sun would then always 120 THE SOLAB SYSTEM. appear to move through the equinoctial. He would rise and set every day at the same points on the horizon, and pass through the same circle in the heo.vens, while the days and nights would be equal the year round. There would be near the equator a fierce torrid heat, while north and south the climate would melt away into temperate spring, and, lastly, into the rigors of a perpetual winter. XX. BesuUs, if the equator of the earth were perpen- dicidar to the ecliptic. — ^Were this the case, to a spec- tator at the equator, as the earth leaves the vernal equinox, the sun would each day pass through a smaller circle, until at the summer solstice he would reach the north pole, when he would halt for a time and then slowly return in an inverse manner. In our own latitude, the sun would make his diurnal revolutions in the way we have just de- scribed, his rays shining past the north pole fur- ther and further, until we were included in the region of perpetual day, when he woidd seem to wind in a spiral course up to the north star, and then return in a descending curve to the equator. Pbecession of the Bquinoxes. — ^We have spoken of the equinoxes as if they were stationary in the orbit of the earth. Over two thousand years ago, Hipparchus found that they were falling back along the ecliptic. Modem astronomers fix the rate at about 50" of space annually. If we mark either point in the ecliptic at which the days and nights are equal over the earth, which is where the plane of the earth's THE EABTH. 121 equator passes exactly through the centre of the sun, we shall find the earth the next year comes back to that position 50" (20 m. 20 s, of time) earlier. This remarkable effect is called the Precession of the Equinoxes, because the position of the equinoxes in any year precedes that which they occupied the year before. Since the circle of the ecliptic is divided into 360°, it follows that the time occupied by the equinoctial points in making a complete revolution at the rate of 50.2" per year is 25,816 years. Results of the Precession of the Equinoxes. — In Fig. 31, we see that the line of the equinoxes is not at right angles to the ecliptic. In order that the plane of the terrestrial equator should pass through the sun's centre 50" earlier, it is necessary that the plane itself should slightly change its place. The axis of the earth is always perpendicular to this plane, hence it foUows that the axis is not rigorously parallel to itself. It varies in direction, so that the north pole describes a minute circle in the starry vault twice 23° 28' in diameter. To illustrate this, in the cut we suppose that after a series of years the position of the earth's equator has changed from efh {a g'Kl. The inclination of the axis of the earth, C P, to C Q, the pole of the ecliptic, remains unchanged ; but as it must turn with the equator, its position is moved from CP to CP', and it passes slowly aroimd through a portion of a circle whose centre is C Q. The direc- tion of this motion is the same as that of the hands of a watch, or just the reverse of that of the revolution 122 THE SOLAE SYSTEM. of the earth itself. The position of the north pole in the heavens is therefore gradually but almost insen- sibly changing. It is now distant from the north polar star about 1^-°. It will continue to approach CHANGE OF EARTH'S EQUATOR AND AXIS. it until they are not more than half a degree apart- In 12,000 years Lyra will be our polar star : 4,500 years ago the polar star was the bright star in the constellation Draco. As the right ascension of the stars is reckoned eastward from the vernal equinox along the equinoctial, the precession of the equinoxes increases the K. A. of the stars 50" per year. On this account, star maps must be accompanied by the date of their calculations, that they may be corrected to correspond with this annual variation. The con- stellations are fixed in the heavens, while the signs of THE EARTH. 123 the zodiac are not ; they are simply abstract divisions of the ecliptic which move with the equinox. When named, the sun was in both the sign and constellation Aries, at the time of the vernal equinox ; but since then the equinoxes have retrograded nearly a whole sign, so that now while the sun is in the sign Aries on the ecliptic, it corresponds to the constellation Pisces ia the heavens. Pisces is therefore the first constellation in the zodiac. (See Fig. 72.) Causes of the Precession of the Equinoxes. — Before commencing the explanation of this phenomenon, it is necessary to impress upon the mind a few facts. 1. The earth is not a perfect sphere, but is swollen at the equator. It is like a perfect sphere covered with padding, which iacreases constantly in thick- ness from the poles to the equator ; this gives it a turnip-like shape. 2. The attraction of the sun is Fig. 33. J INFLUENCE OP THE SUN ON A MOUNTAIN NEAK THE EQUATOR. greater the nearer a body is to it. 3. The attraction is not for the earth as a mass, but for each particle separately. In the figure, the position of the earth 124 THE SOLAE SYSTEM. at the time of the winter solstice is represented. P is the north pole, a b the ecliptic, C the centre of the earth, C Q a line perpendicular to the echp- tic, so that the angle QCP equals the obliquity of the ecliptic. In this position the equatorial pad- ding we have spoken of — the ring of matter about the equator — ^is turned, not exactly toward the sun, but is elevated above it. Now the attraction of the sun pulls the part D more strongly than the centre; the tendency of this is to bring D down to a. In the same way the attraction for C is greater than for I, so it tends to draw C away from I, and as at the same time D tends toward a, to puU I up toward b. The tendency of this, one would think, would be to change the inclination of the axis C P toward C Q, and make it more nearly perpendic- ular to the ecliptic. This would be the result if the earth were not revolving upon its axis. Let us con- sider the case of a mountain near the equator. This, if the sun did not act upon it, would pass through the curve HDE in the course of a semi-revolution of the earth. It is nearer the sun than the centre C is ; the attraction therefore tends to pull the mountain downward and tilt the earth over, as we have just described; so the mountain will pass fiirough the curve H/gr, and instead of crossing the ecliptic at E it will cross at g' a little sooner than it otherwise would. The same influence, though in a less degree, obtains on the opposite side of the earth. Tlie mountain passes around the earth in a curve nearer THE EAETH. 125 to h, and crosses the ecliptic a little earlier. The same reasoning will apply to each mountain and to all the protuberant mass near the equatorial regions. The final effect is to turn sHghtly the earth's equator so that it intersects the ecliptic sooner than it would were it not for this attraction. At the summer sol- stice the same tilting motion is produced. At the equinoxes the earth's equator passes directly through the centre of the sun, and therefore there is no ten- dency to change of position. As the axis C P must move with the equator, it slowly revolves, keeping its inclination unchanged, around C Q, the pole of the ecliptic, describing, in about 26,000 years, a minute circle twice 23° 28' in diameter. Precession illustrated in the spinning of a top. — This motion of the earth's axis is most singularly illus- trated in the spinning of a top, and the more remarkably because there the forces are of an opposite character to those which act on the earth, and so produce an opposite effect. We ■ have seen that if the earth had no rotation, the sun's attraction on the " padding" at the equator would bring C P nearer to C Q, but that in consequence of this rotation the effect really produced is that CP, the earth's axis, SPINNING OF A TOP. 126 THE SOLAB SYSTEM. slowly revolves around Q, the pole of the heavens, in a tlirection opposite to that of rotation. In Fig. 34, let C P be the axis of a spinning top, and C Q the. vertical Mne. The direct tendency of the earth's attraction is to bring C P further from C Q (or to make the top fall), and if the top were not spinning this would be the result; but in consequence of the rotary motion the incKnation does not sensibly alter (untU the spinning is retarded by friction), and so C P slowly revolves around C Q in the same direction as that of rotation. Nutation, (nutatio, a nodding). — We have noticed the sun as producing precession ; the moon has, however, treble its influence ; for although the moon's mass is not ^t.t^tt.tst part that of the sun, yet she is 400 times nearer and her attraction is cor- respondingly greater. The moon's orbit does not lie parallel to the ecliptic, but is inclined to it. Now the sun attracts the moon, and disturbs it as he would the path of the mountain we have just sup- posed, and the effect is the same — ^viz., the intersec- tions of the moon's orbit with the ecliptic travel backward, completing a revolution ia about 18 years. During half of this time the moon's orbit is inclined to the ecliptic in the same way as the earth's equator ; during the other half it is inclined in the opposite way. In the former state, the moon's attractive tendency to tilt the earth is very B ii; 11, and the precession is slow ; in the latter, the tendency is great, and precession goes on rapidly. THE EAETH. 127 The consequence of this is, that the pole of the earth is irregularly shifted, so ^^ gg that it travels in a slightly ./^'^^^^^''^^V^^ curved line, giving it a kind of F'^^ \) " wabbling" or " nodding" mo- (T %^ tion, as shown — though greatly q A exaggerated — ia Fig. 35. The ^ J) obliquity of the ecliptic, which ^, _