IIH>IIIIIIIII!II ;!:{'! iliii M mmwM Ism ' ^ Jl hi- mm 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/cu31924031323714 POPULAR SCIENCE. THE SOLAR SYSTEM: A DESCRIPTIVE TREATISE UPON THE SUN, MOON, AND PLANETS, INCLUDING In Slttnunt nf oil iliB Ei«nt SistniiwiBB BY j/EUSSELL hind, FOREIGN SECRETARY OF THE ROYAL ASTRONOMICAL SOCIETY OF LONDON, CORRBSPONDINa MEMBER OP THE NATIONAL INSTITUTE OF FRANCE, AND OF THE PUILOMATIIIC SOCIETY OF PARIS, ETC., ETC., AND FORMERLY OF THE ROYAL OBSERVATORY, GREENWICH. NEW YORK: GEO. P. PUTNAM, 155 BEOADWAY. 1852. DR. MARSHALL HALL, FELLOW OP THE EOYAL COLLEGE OF PHTSIOliNS, AND OF THE EOTAL SOCIETY, AND OF THE HOTAL SOCIETY OF EDINBUKGH, FOKEIGN ASSOCIATE OF THE NATIONAL ACADEMY OF MEDICINE OF FEANCE, ETC., ETC., AND THE DISCOVEEEE OF THE SPINAL SYSTEM IN PHYSIOLOGY, ®:i)i0 Srcotise IS INSCEIEED, AS A SLIGHT TOKEN OF GEATITUDB AND ESTEEM, AND OF ADMIEATION OF HIS SCIENTIFIC LABOES, BY THE AUTHOR. PREFACE. THE present work differs in its arrangement and general con- tents from any exclusively astronomical treatise with which I am acquainted. I have had in view the production of a descrip- tive work, presenting the reader with the latest information on all points connected with the Solar System, yet written in a style as popular as the nature of the subject will admit. It will, there- fore, be understood that this little volume has no pretences to the character of an explanatory treatise on astronomy, but is rather addressed to that numerous class of readers whose time and inclination do not permit of any regular study of the princi- ples of the science, but are yet desirous of informing themselves as to the present state of our knowledge of the heavenly bodies, what has already been accomplished, and how much there yet remains to be done. I have thought it necessary, however, to introduce frequent explanatory remarks, for the more ready comprehension of those parts of the work, which, without such additions, might appear obscure or unintelligible. The present treatise is confined to the 8un^ Moon, and Planets; but, if life and health be spared me, I hope to carry out the same plan to Cometary and Meteoric Astronomy, and also to the Stars and Nebulai. The subjects are ah so widely different, that it is no disadvantage to treat of them in separate works. To M. Le Verrier, and to those English Astronomers who have kindly furnished me with more definite information on cer- tain points connected with their investigations than was to be found in printed authorities, I have to return my best thanks. J. EUSSELL HIND. Grove Road, St, John's Wood, London, December, 1851, CONTENTS. CIlAl'TKR I. THE SUN, II. TIIE INlTKiaOE PLANETS. — MEEOUKY, HI. VENUS, IV. THE EAETH, .... V. THE MOON, .... VI. ECLIPSES OP TIIE SUN AND MOON, VII. THE SUPEEIOE PLANETS. — ^MAES, VIII. THE MINOR OK TJLTEA-ZODIAOAL PLANETS, CEEES, 113; PALLAS, 115; JUNO, 117; vesta, 118 ASTE^A, 120; HEBE, 121; lEis, 122; ploea, 124; METIS, 125 ; iiYGEiA, 126 ; paetiienope, 127 ; vic- TOEIA, 127; EGEEIA, 128 ; IRENE, 129 ; EUNOMIA, 130. IX. JUPITER, X. SATUEN, XI. UEANUS, XII. NEPTUNE, FAUU 11 23 33 45 5G 85 107 112 132 144 165 175 THE SOLAR SYSTEM: OR THE SUN, MOON, AND PLANETS. CHAPTEE I. THE SUN. O THE Suu, as the great originator of light and heat, and the mighty centre of the system, first claims our attention. The distance of this splendid luminary from the earth, which is employed by astronomers as a common unit of meas- urement, has been ascertained with very great accuracy from the transit of Venus over the Sun's disc in lYGO. It will readily be imagined that an exact knowledge of this distance is of high importance in various astronomical investigations, and it has accordingly formed the subject of several elaborate inquiries. Professor Encke, of Berlin, has produced a masterly treatise on the results to be deduced from the transit of 1769 ; he con- cludes that at the mean distance of the earth from the Sun, the equatorial semi-diameter of our globe would subtend an angle of 8".5'7'76,* which is called the equatorial horizontal * Since the above was written, Mr. Adams has drawn my attention to a remark of Professor Encke's, in his Astronomical Jahrbuch for 1852, from which it appears, that in order to satisfy the observations of the transit of Venus by Pere Hell, in 1769, a small correction is 12 THE SOLAR SYSTEM. parallax of the Sun ; hence we infer by trigonometry, that this himinary is separated from us by 24,047 times the earth's equatorial radius, or, more exactly, 95,298,260 English miles. And this is the most probable value that we are able to derive from existing data, though it is possible future observations may furnish a result with greater pretensions to accuracy. It may be safely asserted, that we know the true distance of the earth from the Sun, within the 300th part of the whole ; a most sat- isfactory conclusion, considering the magnitude and importance of the question. Knowing the mean distance of the Earth from the Sun in semi-diameters of our globe, it is easy to determine the real diameter of the solar orb referred to the same unit of measure- ment. The best and latest observations prove that when the Sun is at his mean distance from us, the diameter subtends an angle of 32' 0"; and there appears to be little, if any, appre- ciable difference between the diameters measured in a vertical and horizontal direction. Hence we conclude that the true diameter of the Sun exceeds the equatorial radius of the earth 223.83 times, or measures 887,076 miles. This enormous globe has, therefoi'e, 1,401,910 times the volume of the earth, and the mass is found to be upwards of 355,000 times greater. The appearance of the Sun, with the aid of telescopes, may be briefly described. When we examine his disc through the intervention of a dart glass, we perceive upon it black spots, or maculce, surrounded by a lighter shade, or penumbra, which in most cases has a similar form to the inclosed spot, though necessary to the above value of the Equatorial Horizontal Parallax. It is, however, so small, that it has not been thought necessary, or even advisable, to recompute the various distances of planets from the Sun, &c., given in this work. Professor Encke's result for the Paral- lax is confirmed in a remarkable manner by the independent research- es of a Spanish Astronomer, Don Jose de Ferrer. — Author. THS SUN. 13 tbis does not invariably happen, several dark spots being occa- sionally included in a common penumbra. Generally they are confined to zones, extending 35° on each side of the Solar Equator, leaving an intermediate belt where they appear much more rarely. They have now and then been noticed in higher latitudes ; but these instances may be considered as forming exceptions to the general rule. The solar spots are not perma- nent, they change their form from day to day, or even from hour to hour, sometimes vanishing in an incredibly short space of time, while others make their appearance as suddenly. The dark, or central part of the spot, disappears first, and the penum- bra gradually closes in upon it. When a spot is observed for any length of time, it is found to change its apparent position on the Sun's surface, becoming visible at first upon the eastern side, and in somewhat less than a fortnight disappearing near the western limb, while after the lapse of another like period, if it remain as before, it will re-appear upon the eastern limb, and again traverse the disc. To account for these phenomena, it is necessary to admit that the Sun rotates upon his axis in a direction similar to that of the diurnal revolution of the earth, or from west to east. Tobias Mayer records the appearance of a black spot upon the Sun on March 15, 1758, the diameter of which was one twentieth of that of the Sun. Sir W. Herschel saw one on the 19th of April, 1779, sufficiently large and well-defined to be visible to the naked eye. More recently M. Schwabe, of Dessau, who has paid much attention to solar phenomena, has observed several spots without the aid of a telescope. One visible in June, 1843, measured 167" in breadth, and was seen with the naked eye for a whole week. As one second of arc on the Sun's surface includes a breadth of 460 miles, we infer that the spot viewed by M. Schwabe must have occupied a space 77,000 miles in diameter, or ten times greater than that 14 THE SOLAR SYSTEM. of tbe Earth. A group of spots with the penumbra surround- ing it, will frequently cover a much larger poi'tion of the Sun's disc. One noticed in April, 1845, measured 5' 20", and another on the 6th of December, of the same year, was nearly of equal length. A cluster of spots seen at the Cape of Good Hope by Sir John Herschel, at the end of March, 183Y, covered an area of nearly five square minutes, a space which the reader will duly appreciate on remembering that the diameter of the Sun is only thirty-two minutes. A minute in linear dimension on his disc being 27,500 English miles, and a square minute 756,000,000. Sir John Heischel observes, that we have an area of 3,780,000,000 miles included in one vast region of dis- turbance on this occasion. M. Schmidt, of Bonn, counted up- wards of two hundred single spots and points in one of these large groups visible on the 26th of April, 1846, and one hundred and eighty in another cluster in August of the preceding year. It has been found by continual observation of the spots that their number varies considerably in different years. It will sometimes happen on every clear day during a particular year, the Sun's disc always contains one or more of them, while, in another year, for weeks or even months together, no spots of any kind can be perceived. M. Schwabe, after twenty-five years close attention to the appearance of the Sun's surface, thinks he has discovered something like regularity in the preva- lence or otherwise of these phenomena, and is induced to sup- pose that the period of variation in the number is not far from ten years. It is not easy to imagine any adequate cause for this cyclical appearance of the spots ; but in the present state of astronomy it is unsafe to reject any of the indications of careful observation, simply because we cannot fully account for them.* We would particularly recommend the solar phenom- *M. Schwabe has furnished a table exhibiting the number of days in each year between 1826 and 1843 on which the sun was free from THE SUN. 15 ena to the attention of amateur astronomers, who with ordi- nary telescopes may do good service to the science by regularly watching and mapping down the spots day by da)' ; a work almost beyond the power of the professed observer, who has so many other claims upon his time and attention. Besides the dark spots already described, we remark upon the Sun's disc curved lines or streaks of light of a more lumi- nous character than the rest of the surface, which are generally found in the neighborhood of the black spots, or where they have previously existed ; not unfrequently the dark spots break out amongst them. These phenomena are termed faculce (lichtsireifen by the Germans), and are considered by Sir John Herschel as the ridges of immense waves in the luminous re- gions of the Sun's atmosphere, indicative of violent agitation spots, and the number of groups showed. This table is interesting in more than one point of view, and is here subjoined : — ■ 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 Groups of spots observed. 118 161 225 199 190 149 84 33 51 173 272 333 282 162 152 102 68 34 Days on which the Sun wjxs free from spots. 22 2 1 3 49 139 120 18 3 15 64 149 Number of ob- serving days. 277 273 282 244 217 239 270 267 273 244 200 168 202 205 263 283 307 324 16 THE SOLAR SYSTEM. in the neighborhood. Th&faculm are not so generally noticed as the spots, possibly because they require much better optical means to show them well. Yet M. Schmidt says in the year 1845 he never saw them at all, though during the early part of the year he used one of Fraunhofer's celebrated telescopes, of four feet focal length ; but on one day in 1844 they were unusually distinct and visible in considerable numbers. Care- ful examination, with proper optical aid, shows that the Sun's disc is covered with a fine mottled appearance, consisting of minute points, or, as Sir John Herschel terms them, " dark dots or pores,'' which are constantly undergoing some alteration. The appearance presented by this uniform mottling of the Sun's disc has been aptly compared by the same eminent astronomer to the " slow subsidence of some flocculent chemical precipitates in a transparent fluid, when viewed perpendicularly from above." The rotation of the Sun upon his axis was inferred, as al- ready remarked, from observations on the positions of the spots upon his disc on successive days. Astronomers have diflfered a good deal in the periods they assign to this rotation ; still it is certain that we have now approximated within a very few hours of the truth.* Perhaps the period assigned by M. Bianchini, from very careful measures, in the year ISlY, may be taken as * We subjoin the times of the Sun's rotation, according to the va- rious astronomers, from the age of Cassini to the present day : — Cassini I. by comparing his own observations with d. h. m. those of Scheiner, &c., 25 14 5 De La Hire 25 8 56 Lalande 25 10 Flauguergues, from observations in 1798, . 25 1 2 Delambre, 25 17 Mossotti, 25 10 13 Taylor In 1835-6 25 14 Petersen, 25 4 30 Laugier 25 8 10 THE SUN. 17 one of the best results ; this gives 25d. "Zh. 48m. for one side- real revolution upon the axis, agreeing closely with the more recent calculation of M. Laugier. Besides the time of rota- tion, observations of the solar spots enable us to ascertain the position of the equator, and its nodes in reference to the ecliptic or the great circle of the heavens in which the plane of the earth's path lies. According to the eminent mathematician and astronomer, M. Delambre, the angle between the solar equator and the ecliptic is 7° 19', and the longitude of the node, or the point where the equator intersects the ecliptic is 80° 45'. Some later observations by Dr. Petersen at Altona, assign 6" 51' for the inclination, and 73° 29' for the position of the node. There ai'e difficulties in the way of an exact determination of these quantities, and not practical ones only, for some astrono- mers have strongly suspected that the spots really alter their position upon the Sun's disc, in which case the apparent diur- nal movement of the spots given by our observations will not be the real change due to axial rotation, but must be partly influenced by the proper motion of the spot itself. Hence probably arise the discordances which are apparent in the re- sults of different astronomers, and in the times of rotation de- duced by the same observer from observations of different spots. The Earth is in the line of nodes about the first weeks of June and December, and at these times the spots, in traversing the Sun's disc, appear to us to describe straight lines. As our globe recedes from the line of nodes, the apparent paths be- come more and more elliptical, until we have advanced through an arc of longitude of 90°, or arrived at our greatest heliocen- tric declination, when the ellipticity reaches its maximum, di- minishing again as we are carried forward to the other node. The paths of the solar spots consequently present the gi-eatest curvature about the commencement of March and the middle of September. 18 THE SOLAR SYSTEM. The discovery of the spots is usually dated about the begin- ning of the seventeenth century, or soon after telescopes came into use. It appears, from the papers of our countryman Har- riot, that he observed them on the 8th of December, 1610. Christopher Scheiner, Professor of Mathematics at Ingoldstadt, remarked them in March following, and published a volumi- nous work upon the subject, entitled Hosa TJrsina. The cele- brated Galileo noticed the spots about the same time, and, in a tract printed in 1613, he affirms that he had shown them to several persons at Rome in 1611, and had mentioned their ex- istence to other friends at Florence some months previous. John Fabricius observed them at Wittenburg about the same time as Scheiner, and gave an account of them in a small work published in June, 1611. All these discoveries were very prob- ably entirely independent of each other ; but it seems quite certain that the first notice of a solar spot is to be dated at a much earlier period. Adelmus, a Benedictine monk, in a life of Charlemagne, mentions a black spot observed upon the Sun in the year SOY, on the 16th of the calends of April or March iVth : this circumstance is recorded by many historians, includ- ing Bede, Polydorus Virgil, and Almoin, monk of St. Germain de Pres. Averroes, a Spanish Moor, is reported to have ob- served dark spots upon the Sun's disc about the middle of the twelfth century. It has been suggested that the otherwise mysterious diminutions of the Sun's light when there was no eclipse, mentioned more than once by historians,* may have been owing to a great accumulation of spots upon his disc ; but * A remarkable instance is recorded by Keppler, Astronomice pars optica, in the following words — " Refcrt Gemma Pater et Filius. anno 1547 ante conflictum Caroli V. cum Saxoniae Duce, solera per tres dies sen sanguine perfusum comparuisse ut etiam stellse plerseque in meri- die conspicerentur." The battle alluded to is that of Muhlberg, which was fought on the 24th of April, 1547. THE SUN. 19 it certainly appears questionable whether they could congi-egate in such numbers as to materially lessen the intensity of the so- lar rays. A great number of opinions have been advanced with regard to the nature of the solar spots. Scheiner at first considered them to be solid bodies revolving round the Sun, and very near his surface ; in this opinion he was followed by Malapert, who termed them Sidera Austriaca Periheliaca ; by John Tarde, who, in his turn, called them Borhoina Sidera, as having been discovered in the reign of Louis XIII. ; and by the capuchin Antonio de Rheita, who thought he had accounted for the /ac- ulce, or luminous spots, also, by supposing them to be owing to the intense light reflected from the revolving planets on the Sun's surface. Galileo differed entirely from Scheiner and his follower's, regarding the spots merely as clouds or exhalations from the Sun's surface, and urging as a fatal objection to Schei- ner's theory, that they are ever changing their form and gen- eral appearance, sometimes vanishing suddenly, and bursting forth again with equal rapidity in other places. The idea of their being solid bodies was therefore soon rejected. The opinion prevaihng among the best authorities of the present day is, that these spots are portions of the dark body or surface of the Sun, which are occasionally rendered visible from the temporary removal of the interposing luminous atmosphere, owing to local causes of disturbance, which, whatever be their true nature, appear to be predominant in the equatorial regions. Sir William Herechel has accounted for the penumbra, and general appearance of the spots, by supposing the existence of a transparent medium, which sustains the luminous atmosphere at a great altitude above the Sun's solid dark body, " carrying on its upper surface a cloudy stratum, which, being strongly illuminated from above, reflects a considerable portion of the light to our eyes, and forms a penumbra, while the solid body. 20 THE SOLAR SYSTEM. shaded by the clouds, reflects none." The disturbances which give rise to the visibility of the spots, Sir William thinks to be due to powerful upward currents of the atmosphere. Before closing our account of solar phenomena, we must not omit a brief notice of the zodiacal light. In these high lati- tudes, it is not usually visible except about the months of March iind April, in the evenings, after Sun-set, and September and October, in the mornings, before Sun-rise ; yet in some years it has exhibited itself in uncommon brilliancy as early as Janu- ary. In tropical climates, the zodiacal hght is far brighter, and more sharply defined than we ever see it in this country. Its appearance is that of a conical-shaped light, extending from the horizon nearly along the course of the ecliptic, the vertex at- taining distances of 70° or 80° from the Sun's place, or, as some observations would show, extending 100° from the same point. Hence it is evident its real extent must include the orbits both of Mercury and Venus, and possibly even that of the Earth. The visible length above the horizon, and the breadth of the light at its base, vary under different circum- stances, the latter from about 10° to 30°. The general opin- ion is, that the axis of the zodiacal light is in the plane of the Sun's equator. M. Houzeau has endeavored to show, by cal- culation of a considerable number of observations by Cassini and others, that the elements of the zodiacal light are materially different from those of the Suu's equator : he fixes the node of the light in 2° heliocentric longitude, subject to a probable error of 12° or 13^, and its inclination to the plane of the ecliptic 3°. 35', subject to an uncertainty of rather more than 2°. With these elements he finds his series of sixty observations rather better represented than if the elements of the Sun's equator were employed ; but the preference to be given to the former is by no means decided. The subject deserves further investi- gation, when a much larger number of observations are in our THE SUN. 21 possession than that employed by M. Houzeau. At present, we think the evidence against the supposed coincidence of the above elements by no means sufficient to outweigh the proba- bilities in its favor derived from other considerations. Sir John Herechel suggests that the zodiacal light may be " no other than the denser part of the resisting medium,'' which, as we are now aware, has disturbed the movements of one, at least, of the periodical comets, " loaded perhaps with the actual materials of the tails of millions of those bodies of which they have been stripped in their successive perihelion passages ;" and the same eminent astronomer shows that it cannot be, as some persons have supposed, an atmosphere of the Sun, in the common ac- ceptation of the term, for dynamical reasons. In connection with Sir John Herschel's idea relative to the nature of the light, it is perhaps worthy of mention, that during the visibility of the magnificent comet of March 1843, which exhibited a tail 50° long, and almost grazed the solar orb, the zodiacal light was unusually briUiant — so much so, in fact, that some confusion was caused' by the puMication of descriptions, of the latter phenomenon, which the observer appears to have mistaken for the cometary train. On no recent occasion has the Light shown itself so conspic- uously oi' for so long a period, as during the early part of the year 1850. From the middle of January to the latter end of March it was constantly visible on clear evenings, but was brightest early in February, when it decidedly excelled the most condensed part of the Via Lactea about the constellation Cygnus. Observers who paid particular attention to the posi- tion of the borders of the light among the stars on this occasion, from pretty distant stations in England, have suspected the ex- istence of a very sensible parallax, but it is hardly necessary to remark that the apparent variation in the position of the out- line, as assigned at two distant places on the same evening, 22 THE SOLAR SYSTEM. may be satisfactorily accounted for by the supposition of vary- ing atmospheric conditions. It is not possible to admit the re- ality of the parallax, if the luminosity observed in the western heavens in the early part of 1850, v^ere, as we are at present under the necessity of regarding it, an appearance of the zodi- acal light. The firet particular description of this phenomenon was given by Cassini the Elder in 1683, but it had been previously treated of by Descartes and Childrey, and it seems probable that it may have been remarked more than two thousand years CHAPTER II. THE INFERIOR PLANETS. WITHIN the orbit of the Earth revolve the two planets Mer- cury and Venus, recognized as such from the most remote antiquity. We know that these bodies move in smaller orbits than our globe ; first, because they never appear in the opposite part of the heavens to that which the Sun occupies, or, to use astronomical language, never come into opposition with that luminary : secondly, because under telescopic aid they present every variety of phase from the thin crescent to the fully illu- minated disc, which should occur if, receiving their light from the Sun, they were always situated within the Earth's orbit ; and, thirdly, because at certain times we actually observe them projected upon the disc of the Sun in their passage between that body and our globe, and have watched them in their pas- sage over him ; a phenomenon known as a transit. MERCnRY. S The first of the inferior planets is Mercury, who performs his revolution round the Sun in 87d. 23h. 15m. 43.9s. at a mean distance of 36,890,000 miles. When between the Earth and the Sun, or near the time of inferior conjunction, the disc of this planet, as viewed from our globe, subtends an angle of about twelve seconds of arc, but the diameter dwindles down as Mer- cury approaches the opposite part of the orbit, where the breadth would not exceed five seconds. 24 THE SOLAR SYSTIIM. The constant proximity of the planet to the solar rays, has greatly interfered with observations of its physical appearance. The German astronomer, Sohroter, who observed at the begin- ning of the present century and paid much attention to the sub- ject, considered he had decided evidence of the existence of high mountains on the surface of Mercury, and it was by exam- ining them at various times that he concluded the planet had a revolution upon its axis in 24h. 5m. 28s. ; but this inference may yet require very considerable modification. Sir W. Her- schel never remarked any spots upon the planet's surface, by which he could approximate to the time of rotation, nor are we aware that any astronomer since the time of Schroter has been able to add to our knowledge on these points. The eccentricity of the orbit of Mercury, or the deviation of his orbit from a circle, is much larger than in the case of any other of the old planets, and this circumstance, combined with the great inclination of his equator to the plane of his annual path, which Schroter thinks may amount to 70°, must produce a vast variety of seasons, with great extremes of heat and cold. At perihelion. Mercury is only 29,305,000 miles from the Sun's centre, while in the opposite part of the orbit, or in aphelion, he reaches to 44,474,000, making a variation of distance arising from the eccentricity of his annual track, of no less than 15,169,000 miles, which is nearly five times as great as in the case of the Earth. The elongation or angular distance of Mercury from the Sun, measured as an arc of longitude, is never so great as 30^ : con- sequently he cannot be seen except in strong twilight, either morning or evening, and under the most favorable circumstances does not appear conspicuous to the naked eye, but twinkles like a star of the third magnitude with a pale rosy light. We can- not, therefore, too highly appreciate the diligence and attention of the ancient astronomers, who were not only aware of the MERCURY. 25 existence of the planet, but approximated very closely to his period, and were able to explain the general nature of his path in the heavens. Nevertheless we read of astronomers, and by no means inattentive ones either, who have lived and died with- out once seeing Mercury. Even Copernicus, the celebrated re- viver of the true system of the Universe, was never favored with a view of the planet, a circumstance attributed by Gassendi to the vapors prevailing near the horizon on the banks of the Vistula. On examining Mercury with telescopes of adequate power in diflferent parts of his orbit, we notice phases similar to those presented by the Moon in the course of her revolution round the Earth, with which every one is familiar. At the greatest elongations eastward or westward we see only half the disc il- luminated, as in the case of our own satellite at first or last quarter. As he moves towards superior conjunction, his form becomes gibbous, and the breadth of the illuminated part in- creases, or the outline of the disc becomes more nearly circular the nearer he approaches that position. Owing to the intensity of the solar light we lose the planet for some little time previous and subsequent to the superior conjunction, but, on emergence from the Sun's rays, we find the form still gibbous, the gibbosity being now on the opposite side. The illuminated part dimin- ishes as the planet draws near its greatest elongation, about which time it is again seen as a half moon under telescopic aid ; and, as it advances towards inferior conjunction, the form be- comes more nearly that of a crescent, until it is lost for the second time in the Sun's refulgence, except at certain epochs of not very frequent occurrence when we see it as a black spot traversing his disc ; a phenomenon appropriately termed a Transit of Mercury. The real diameter of Mercury appears to be about 2950 miles ; this value being deduced from very accurate measures 2 . / 26 THE SOLAR SYSTEM. taken during the last few years. There is but little difference between the polar and equatorial diameters, the compression probably not exceeding 1-150. As far as we are aware, Mercury is not attended by a satel- lite, and the determination of his mass, therefore, becomes a very difficult and uncertain matter. But it fortunately happens that we have a curious method of approximating to this element, \iz., by the perturbations produced by the planet in the move- ments of a comet known as Encke's, which revolves round the Sun in little more than three years, and occasionally approaches very near Mercury about the times of perihelion passage. On this subject we shall have more to say when we come to treat of the comets. We shall here merely state the result thus ob- tained, which indicates that the mass of the Sun exceeds that of the planet 4,865,750 times, or the mass, as usually expressed, is 1-4,865,750. The density of Mercury under this new mass is 1.12, that of the Earth being put equal to unity. In order that a transit of this planet over the disc of the Sun may take place, it is necessary that tBe Earth should be in the line of nodes of Mercury at, or very near, the time of his pas- sage through them, this bringing the three bodies veiy nearly in the same line. The nodes are situate at present in 46.7° and 226.7° of heliocentric longitude, at which points the Earth an-ives about the 10th of November and the 7th of May, and in consequence of the very slow sidereal motion of the nodes (amounting to only 13' in one hundred years), the transits of Mercury must occur for a long time to come in one or other of these months, those at the Ascending Node taking place in November, and those at the Descending Node in May. The fii-st recorded phenomenon of this kind occurred on the 7th of November, 1631. In a dissertation published at Leipsic in 1629, Kepler notified to astronomers, that according to his calculations a transit must occur on this day, since at the time MERCURY. 21 of conjunction he had found the latitude of Mercury by his tables less than the Sun's semi-diameter. This interesting pre- diction was verified by Gassendi, at Paris. -He discovered the planet on the disc of the Sun shortly before nine o'clock in the morning. At fii-st he thought it a spot which had not been re- marked on the preceding day, but continuing his observations, its motion was soon detected, and he saw the planet leave the Sun's disc on the western limb about half-past ten, a.m. It was found that Kepler's tables represented the circumstances with far greater precision than even the author himself had hoped for. The second observation of a transit of Mercury was made by Jeremiah Shakevley, on the morning of the 3d of Novem- ber, 1651, at Surat, in the East Indies. It is said Shakerley was so desirous of witnessing the phenomenon that, having found by his calculations it would be invisible in England, he made the voyage to India for the purpose. The third recorded transit was observed at Dantzic by the celebrated astronomer, Hevelius, on the 3d of May, 1661. He saw the planet on the Sun's disc four hours and a half. The next transit took place on the 1th of November, 16'7'7, and was witnessed by our illustrious Halley, at St. Helena, and bj' M. Gallet, at Avignon. Halley thought the times of ingress and egress might be observed within a single second of time, and pointed out how the Sun's parallax might be ascertained from such observations, taken at places widely distant from one another, remarking, however, that the difference of the par- allaxes would not be large enough to give very certain results. Accordingly, the transits of Mercury have not been employed for the above purpose, but we shall presently have occasion to notice a similar phenomenon in the case of Venus, which is far better adapted to give us a correct value of the Solar Parallax. A transit of Mercury occurred on November 10, 1690, and 28 THE SOLAR SYSTEM. was observed in China by the Jesuit missionaries, at Erfurt by Godfrey Kirch, and at Nuremberg by Wurzelbaur. Another in 1697, on November 3, was witnessed by astronomers at Paris, and other places. One on the ninth of November, 1723, was watched at Paris, Genoa, Bologna and Padua, but the Jlrst complete European observation of a transit of Mercury bears date November 11, 1736, when nearly all the astronomers of the time observed the planet in its progress across the Sun. Since this epoch the phenomena have been pretty closely watched. The transit of 1802, November 9, was seen by the well-known Jerome de Lalande, who was the more interested in it, inasmuch as he remarks it was the last he could hope to witness. That of 1832, May 5, was visible in this coimtry, though a general prevalence of unfavorable weather occasioned much disappointment. The next occurred on the 7th of No- vember, 1835, but was not visible in these parts of the Earth. Another on 1845, May 8, was partially observed in this country, and also the last, on November 8, 1848, which is the twenty- fifth that has occurred since the phenomenon was first noted by Gassendi. The following table exhibits the circumstances under which the remaining transits of the present century will take place. The numbers have no pretensions to extreme accuracy, as the tables, both of the Sun and planet, have been considerably improved since the calculations were made by M. Lalande : — Greenwich Mean Duration of Transit. Least Distnnce of Year akd Day. Time of Mercury from the Conjunction. Sun's centre. H. M. s. H. M. 8. M. s. 1861, November 11, 19 20 12 4 46 10 52 N. 1868, November 4, 18 43 44 3 30 42 12 20 S. 1878, May 6 6 38 29 7 47 2 4 39 N. 1881, November 7, 12 37 37 5 18 18 3 67 S. 1891, May 9, ... . 14 44 56 5 8 40 12 21 N. 1894, November 10, 6 27 4 5 15 12 4 20 N. MERGURY. 29 In describing their observations of the transits of Mercury, astronomers make use of the terms internal and external con- tacts. At the ingress or entrance of the planet upon the Sun's disc, the external contact takes place when the limb of the planet fii-st makes a perceptible indentation on the limb of the Sun ; this phenomenon can never be observed with any great de- gree of accuracy, and is therefore less important than the ob- servation of internal contact, or the moment when the whole disc of the planet is fairly projected on the Sun's surface. When a fine thread of light is seen between the outer limb of the planet and the Sun's limb, the internal contact has passed. At the egress, or on the planet's leaving the solar disc, these contacts of course recur, but in reversed order. The moment of internal contact is indicated by the disappearance of the thread of light, and that of external contact by the absence of all appearance of indentation or distortion of the Sun's limb. Before leaving this subject, we may notice several curious phenomena which have been remarked by astronomere during their observations of the Transits of Mercury. At the first ex- ternal contact, something like a penumbra or light shade upon the Sun's disc has been remarked. As the planet advances towards the internal contact, a " black drop" or line has ap- peared to connect its limb with that of the Sun, or the contour of the planet has been seen distorted in such a manner as to give it a pear-shaped form just before the formation of the luminous thread. Such appearances are doubtless to be as- cribed partly to atmospheric circumstances ; but this cause alone is not sufficient to account for them completely, for dif- ferent telescopes at the same station have frequently given very difierent results, the distortion of the outer limb of Mercury being apparent in some instruments, while in others nothing of the kind has been remarked. It happened thus at the last transit in November 1848, when an elongation of the planet's 30 THE SOLAR STSTSM. limb was distinctly seen with one telescope at the Royal Ob- servatory, though others afforded no indications of it. An- other singular appearance which has been mentioned by several observers at different transits, is that of a luminous spot or '' globule" upon the disc of the planet when projected upon the Sun. It was remarked- by Wurzelbaur at Nuremberg, in November 1697 ; again at Utrecht, in May 1832, by Professor Moll, who says its periphery was not well defined, but seemed gradually to sink from a grayish white to the dark color of the planet's disc : it was always situated in the same position, or a little south — preceding the centre of Mercury. A teles- cope by DoUond and another by Fraunhofer showed the spot in precisely the same manner, though various eye-pieces, mag- nifying from 96 to 324 times, were employed. A similar roundish spot of a grayish tinge was noticed at the last transit in 1848, in England and America. Luminous rings round the disc of the planet have been repeatedly noticed, and on other occasions dark or nebulous rings have been remarked. In 1'799 and 1832, the ring had a darker tinge of a violet hue, the color being strongest near the planet. These phenomena may probably arise from a simple cause, though at present it is very imperfectly understood. An American observer of the transit in 1848, says the dusky ring only appeared when the Sun was covered by a thin haze ; yet it is not improbable that the planet's atmosphere may cause a similar nebulous-looking ring. The " black drop" already mentioned as having been observed at the ingress, is occasionally recorded at the egress, the Hmb of Mercury being drawn towai-d that of the Sun, so as to cause a distortion in the opposite direction to that which is observed to take place at the planet's entrance upon the Sun's disc. Such are the most remarkable circumstances connected with the appearance of Mercury during his transits. MERCURY. 31 The most ancient observation of this planet that has de- scended to us is dated in the year of Nabonassar 494, or 60 yeai-s after the death of Alexander the Great, on the morning of the 19th of the Egyptian month Thoth, answering to the 15th of November in the year 265 before the Christian era. The planet was observed to be distant from the right line join- ing the stars called (S and S in Scorpio, one diameter of the Moon ; and from the star |?, two diameters towards the north, and following it in Right Ascension. Claudius Ptolemy re- ports this and many similar observations extending to the year 134 of our era, in his great work known as the Almagest. We have also observations of the planet Mercury by the Chinese astronomers, as far back as the year a.d. 118. These observations consist, for the most part, of approximations of the planet to stars. M. Leverrier, the eminent French geometer, has tested many of these Chinese observations by the best modern tables of the movements of Mercury, and finds, in the greater number of cases, a very satisfactory agree- ment. Thus, on the 9th of June 118, the Chinese observed the planet near a cluster of stars in the constellation Cancer, usually termed Praesepe ; the calculation from modern theory shows that on the evening of the day mentioned. Mercury was less than one degree distant from the group of stars. Although the extreme accuracy of observations at the pres- ent day renders it unnecessaiy to use these ancient positions of the planets in the determination of their orbits, they are still useful as a check upon our theory and calculations, and possess, moreover, a very high degree of interest on account of their remote antiquity. The tables of Mercury at present employed in the com- putation of Ephemerides, or for predicting the place of the planet at any time, as viewed ii'om the Earth or Sun, are those of Baron Lindenau, published in 1813. Ti ese tables still 32 THE SOLAR SYSTEM. agree, within moderate limits, with the results of observation ; but M. Leverrier has greatly improved the theory of the planet within the last few years ; and it is undei-stood that tables based upon his more correct elements are in course of publica- tion. We have given the numbers assigned by this eminent geometer in the table of Planetary Elements, together with those which form the basis of the Baron Lindenau's calcula- tions. CHAPTER III. VENUS. ? THE second planet in oider of distance from the Sun is Venus, the most conspicuous of all the members of the planetary S3'stem, when she is favorably placed with respect to our globe. Her sidereal revolution round the Sun is performed in 224d. 16h. 49m. 8s., at a mean distance of 68,770,000 miles, or nearly double that of Mercury. The apparent diameter of Venus varies much more sensi- bly than that of Mercury, owing to the greater extent of varia- tion of distance from the Earth. At inferior conjunction, or near this point, her disc subtends an angle of about seventy seconds of arc, — while, at superior conjunction, it is less than ten seconds. The disc is never fully illuminated except at superior conjunction, when the planet is lost in the Sun's rays : but with a good telescope we may trace the variations in her form from the full gibbous to the narrow crescent, — changes which follow the same law as in the case of Mercury. The real diameter of the planet is not very accurately known ; the best observations assign about 7,900 miles, or about the same as the diameter of the Earth. We have at present no means of determining from actual observation the exact difference between the polar and equatorial diameters, but it is certainly very small. The disc of Venus under telescopic vision is far too bright and glaring to allow of our obtaining any very precise knowl- 2* 34 THE SOLAR SYSTEM. edge of the constitution of her surface. The elder Cassini watched the planet attentively about the year 1667, and on several occasions remarked ill-defined dusky spots, vphich he observed vfith the view of ascertaining the time of axial rota- tion. This he considered to be about 23h. 16m. An Italian astronomer, Bianchini, soon afterwards published some observations, from which he inferred that Venus occupied no less than twenty-four days in revolving upon her axis. So marked a disagreement in the conclusions of two observers, could not fail to attract the attention of Sir William Herschel, who carried on for many years a careful examination of the planet's surface, partly with the view of determining which of the periods was the correct one. He occasionally saw spots upon the disc of Venus, particularly in the summer of 1780, but the result of his observations would not give the time of rotation. " For," he observes, 'i the spots assumed often the appearance of optical deceptions, such as might arise from pris- matic affections, and I was always very unwilling to lay any stress upon the motion of spots, that either were extremely faint and changeable or whose situation could not be precisely ascertained." The great power and light of the forty-feet re- flector was found to be rather an inconvenience than otherwise in these observations ; in fact large telescopes of any kind have seldom been employed with advantage on the planet Venus. Sir William Herschel considered he had- decisive evidence of the existence of a dense atmosphere, to the effects of which he attributed the appearance of a luminous border or bright mar- gin on the fully illuminated limb of the planet, from which the light diminished pretty suddenly. The prolongation of the cusps of Venus beyond a semicircle was also thought to be owing to the refraction in the atmosphere. The terminations of the cusps were always observed to be sharply defined and perfectly free from irregularities, such as the appearance of VENUS. 35 mountains on the surface might occasion. Schroter, who paid much attention to his observations on this planet, assures us that mountains exist upon it of fifteen and even twenty miles altitude, or of far greater height than any upon the earth, and he remarked further that the greatest inequalities are in the Southern hemisphere.* The same astronomer, from closely watching the atmospheric spots and appearance of the horns, and from eight observations of a fixed point on the surface, as- certained that the time of rotation is 23h. 21m. 7.98s., a result which has been .pretty generally received, though it may here- after be somewhat modified. In confirmation of this period of revolution, it may be remarked that Cassini II. was able to show the fallacy of Bianchini's inference by comparing all his father's observations together, and he further proved that the particulars given by Bianchini could be represented by a rota- tion of little less than one of our days.f Sir William Herschel * For a month following the llth of December, 1789, Schroter noticed that the Southern horn appeared blunt with an enlightened mountain in the dark part of the disc, which was found to be 18,000 toises in height, or rather less than twenty-two English miles. The highest mountain was supposed by Schroter to be 18,900 toises in altitude. These numbers, however, must be received with caution, for it may be doubted whether the micrometers, &c., employed by the diligent and able observer at Lilienthal, were sufficiently delicate for measures of this nature. His measures of the diameters of some of the minor planets are well known to be greatly in excess of the values given by the improved instruments of the present day. f The same remark may probably apply to some observations on a spot upon the disc of Venus, by M. Flaguergues, at Viviers, between the 7th and 13th of July, 1796, which are said to favor Bianchini's conclusion with respect to the time of rotation. The observations in- dicated that the axis of Venus is inclined at an angle of 73° 32', to that of the elliptic (which agrees with Cassini's result), the North Pole being directed to 321° 20' longitude. These particulars were communicated by M. Flauguergues to the Academy of Sciences at Nismes, and are pretty satisfactory as regards the position of the axis. 36 THE SOLAR SYSTEM. was of opinion that the time of revolution could not be so long as twenty-four days, though, as above stated, his own ex- perience did not enable him to assign the precise period. The late Professor De Vico, in the admirable sky of Rome, fre- quently saw spots upon Venus ; but for a steady view of them it was necessary to wait for the very best atmospheric con- ditions, even under an Italian sky, as the author was assured by this diligent observer. Professor Madler has recently made a series of observations, from which he deduces the amount of horizontal refraction, and finds it one sixth greater than in our own atmosphere. Venus is a morning star from inferior to superior conjunction, and an evening star from superior to inferior conjunction. Her greatest elongation from the Sun, in longitude, is about 47° 15', hence she is never observable more than from three to four hours after sun-set or before sun-rise. Occasionally she attains so great a degree of brilliancy as to he distinguishable at noon- day in a favorable sky, without the assistance of a telescope.* This happens once in eight years, when the planet is at or near its gi-eatest north latitude, and about five weeks from the time of inferior conjunction. One fourth of the disc, or rather less, * Claudianus relates that in the fourth year of Honorius, Emperor of the West, or a.d, 398, a star was seen in the day-time as bright as Arcturus appears at night. Venus might be observed at noon-day about the end of January and beginning of February in this year. It was probably this planet that attracted attention in the day-time in 984, according to a Saxon Chronicle, and again on Easter Sunday, 1008, and at the end of the year 1014. Several writers mention the appearance of a star at the sixth hour of the day, or about noon, on Palm Sunday 1077 (April 9) ; Venus was then approaching her inferi- or conjunction, and might probably be the object here referred to. On the 29th of January, 1280, she was seen by day-light, and again i'or some days about the 27th of May, 1363, like a very small star. Very recently the curiosity of the Parisian public was excited by the Jlscovery of a star in the day-time, which proved to be Venus. VENUS. 37 is illuminated, and under these circumstances the planet has been observed to cast a very sensible shadow at night. The elongation from the Sun at the time of maximum brilliancy is rather less than 40°, and the diameter of the illuminated part about ten seconds of arc; the phase is therefore similar to that presented by our own sateUite about three days from the New Moon. Astronomere are by no means satisfied whether the planet Venus be attended by a satellite or not. Observations have been made which are strongly in favor of the existence of such an attendant, but many of the most diligent observers of past and present times have watched the planet under every variety of climate and atmospheric condition and with telescopes of all kinds, without once obtaining a glimpse of a satellite. It is a question of great interest, and must remain open for future de- cision. We shall here briefly recapitulate the evidence in favor of a satellite. The celebrated Cassini was the first astronomer who noticed any suspicious object near the planet. On the mornings of January 25th, 1672, and August 2'7th, 1686, he distinctly per- ceived a luminous body, presenting on the first occasion the same phase as Venus, and about one quarter of her diameter. The well known optician. Short, remarked an object with the same phase as the planet, and about ten minutes of space from it, on the morning of the 3d of November, 1V40. In the month of May, 1761, M. Montaigne, at Limoges, saw what he considered to be a satellite on four evenings. It was always one fourth of the diameter of Venus, with precisely the same form, and changed its position with respect to the planet. In March, 1V64, the supposed satellite was observed by several astronomers, and, what is most important, at places widely dis- tant from one another. Kodkier, Horrebow, and others at Copenhagen, with a refracting telescope, and Montbarron at 38 THE SOLAR SYSTEM. Auxerre, with a Gregorian reflector, repeatedly saw the attend- ant between the 3d and 29th of that month. Its diameter was estimated as before at one fourth of that of the planet* Since that time, so far as we are aware, no suspicion of a satel- lite has been entertained by any observer. It has been urged, if there really be one in existence, it should have been readily seen at those times when Venus, like Mercury, has traversed the Sun's disc ; yet only two of the very many who watched the transits of the last century profess to have seen any object resembling an attendant upon the planet. Sir W. Herschel perceived no traces of a satellite, neither did Schroter, though he was most assiduous in his observations of Venus. Still it is not easy to understand how all the observers of the last cen- tury can have been mistaken. In this state the question at present remains. There are several methods by which the mass of this planet may be ascertained. The effect of its attraction upon our globe causes the Sun's place to differ by a sensible quantity from what it should be, supposing this attraction was not in force, and the planet also exercises an appreciable influence on the precession of the equinoctial points. The most accurate inves- tigations show that the mass is 401,8-39 times less than that of * Professor Lambert collected the observations together, and suc- ceeded iu deducing from them a pretty consistent orbit. The period of revolution assigned was lid. 5h. 13m., and the mean distance of the satellite from Venus 64i semi-diameters of the Earth, or about 255,000 miles. The eccentricity given by Montaigne's observations appeared to be 0.195, and the position of the aphelion, \a 1700, in longitude 256° : the node at the same epoch was 233°, and the plane of the orbit made with that of the elliptic an angle of 64°. There is one fatal objection to this orbit, notwithstanding its apparent agree- ment with observation ; if it were correct, the mass of Venus would be ten times greater than the value found from theory by other methods, Lambert's calculations will be found in Bode's Jharbuch for 1777. the Sun ; it is, therefore, a little smaller than the mass of the Earth, though a nearer approach to it than obtains with any of the other planets. Transits of Venus over the Sun's disc take place under the same circumstances as those of Mercury, or when she has the same heliocentric longitude as the Earth, at or near the times of the nodes. The present position of the line of nodes is in longitude 75°,6 and 255°,6, and the secular sidereal motion of this line is 31', wherefore for a long time to come the transits of Venus must occur early in June or December, those in the former month at the Ascending Node, and those in the latter month at the Descending Node. Owing, however, to the length of time required after a conjunction to bring the Earth and planet into the same heliocentric position again, the transits of Venus are of rare occurrence, taking place at intervals of about eight and one hundred and thirteen years. They are phenom- ena of the highest importance as enabling astronomers to de- termine the distance of the Earth from the Sun, with far greater accuracy than any other method will give it. To do this suc- cessfully, it is necessary to have observations taken at places differing widely in latitude, so that the displacement of Venus upon the Sun's disc, owing to the effect of parallax, may be as large as possible. Now, under the circumstances that the tran- sits of Venus at present occur, the distance of the planet from the Sun is to its distance from the earth as 13 to 29, or very nearly as 2-^ to 1. Supposing we had observations taken at each of the poles of the Earth, it would be found that the dis- placement of Venus on the Sun's disc would occupy a space 2-^ times as great as the Earth's diameter, viewed from that lumi- nary, or five times as large as the Sun's horizontal parallax. Hence we see why the transits of Venus are so much more important than those of Mercury in the determination of this element, for similar considerations applied to the latter planet 40 THE SOLAR SYSTEM. will show that the displacement upon the Sun's disc, instead of exceeding the horizontal parallax, will be half as small again, so that any error entailed in the observation will have an effect upon the final result equivalent to more than twice the amount of that error. In the case of Venus, however, any error of observation can only influence the deduced parallax by one fifth of its actual amount. The observation consists in ascei'taining the time of duration of a transit, or the interval elapsing between the ingress and egress of the planet upon the Sun's disc. The theory of Venus and the Sun's diameter being well-known, such observations readily give the parallax of the planet, and hence that of the Sun. Venus was first observed upon the Sun's disc in the year 1639. Jeremiah Horrox, of Hoole, near Liverpool, while em- ployed in calculating an ephemeris of the planet from Lans- berg's tables, found that the geocentric latitude of the planet at the moment of inferior conjunction on the 24th of November, would be less than the Sun's semi-diameter, and consequently that it must appear upon his disc. These tables, however, had so often deceived him, that he had ]'ecourse to the Tahulm Hudolpfdnce, then newly published by Kepler, and based upon the observations of Tycho Brahe, the most exact astronomer of his age. According to Kepler's numbers, he found the transit of the planet equally certain, and, applying some corrections of his own, he expected to find the planet in conjunction at 4 p.m., on the 24th of November, about ten minutes south of the Sun's centre. Having thus satisfied himself that Venus must really appear projected upon the solar orb, he gave notice to his friend William Crabtree, a zealous astronomer, desiring him to observe it. Fortunately the planet was seen upon the Sun's disc by both observers, though Crabtree, interrupted by a cloudy sky, caught only a single glimpse of it. To make sure of the mat- VENUS. . 41 ter, Horrox commenced his examination of the Sun on the 23d of November, and repeatedly watched it until one o'clock, p.m., on the 24th, when he was called away by business. On return- ing at a quarter past three o'clock, he readily discerned the planet which had just fully imraerged upon the solar disc ; in fact, at the first view, its outer limb coincided with that of the Sun. He continued his observations until a few minutes before sun-set. Horrox transmitted the image of the Sun through a telescope into a darkened room, a mode of observation which was attended with great advantage. Such are the circumstances nnder which the planet Venus was for the firet time beheld as a black spot upon the Sun's disc. No other transit of Venus occuiTed until the 5th of June, 1T61. Dr. Halley had pointed out, many years previous, that the parallax of the Sun could be determined within a small fraction of a second from observations of this phenomenon, and a high degree of interest was awakened as that of 1761 drew near. Observers proceeded from Europe to distant parts of the earth to secure data for ascertaining this important quantity with exactness, and astronomers were on the watch from Tobolsk in Siberia to the Cape of Good Hope. The results have been discussed by Professor Encke, of Berlin, in a special treatise on the subject ; but it is found that the individuakMlues for the Sun's parallax do not agree so well as might haW been antici- pated, and it is most fortunate that another transit, on the 3d of June, 1769, has afforded more consistent numbei-s. Very extensive preparations were made for observing the transit of 1769. An expedition was equipped, on a large scale, and despatched to Otaheite, under the command of Captain Cook, and at the expense of the British Government. Continen- tal powers likewise joined in the preparations, and astronomers of various nations were sent out to the most advantageous points 42 THE SOLAR SYSTEM. for observation. The ingress of the planet on the Sun's disc was seen at almost all the observatories of Europe ; the egress at St. Petersburg, Pekin, Orenburg, Jakutsk, Manilla, Batavia, &c., and the complete duration of the transit at Cape Wardhus, Kola and Cajeneburg in Lapland, at Otaheite, Fort Prince of Wales and St. Joseph in California. The resulting parallax is considered certain within a very small fraction of a second of space ; separate investigations by Professor Encke and M. de Ferrer having led to precisely the same value. No transit of Venus has taken place since the year 1769 ; the next will occur on the morning of the 8th of December, 1874, but will be invisible in this country, the conjunction hap- pening soon after four o'clock in the morning, and the egress of the planet nearly two hours before sun-rise. Another transit will take place on the 6th of December, 1882 ; the entrance of Venus on the Sun's disc will be observable in England, and her progress across it may be watched till sunset ; but the egress will not occur until eight o'clock in the evening. No transit will happen during the twentieth century. The next, on the morning of the 7th of June, 2004, will be visible under favor- able circumstances in these parts of the world.* Similar phenomena to those we have already noticed as at- tending the transits of Mercury, take place on an extended scale during thos^ of Venus. A kind of lucid wave gave the first in- * Transits of Venus occurred as follow, according to the calcula- tions of M. Delambre : — .D. 902, November 26, 9 A.M. A.D. 1275, May 25, 10 p.m. " 910, November 32, 9 P.M. " 1283, May 23, 3 p.m. " 1032, May 24, 7 P.M. " 1388, November 26, 7 a.m. " 1040, May 22, 11 A.M. " 1396, November 23, 7 p.m. " 1145, November 26, 8 a.m. " 1518, May 26, 2 a.m. " 1153, November 23, 8 P.M. " 1526, May 23, 6 p.m. The hour given being that of conjunction of the Sun and planet in Greenwich time. Venus. 43 timation of the planet's approach to the Sun in 1T69, according to an observer at Greenwich ; this was followed by an apparent " boiling'' of the solar limb at the same place, which continued visible for some seconds. When the planet had partially en- tered upon the disc, a distortion of its outline was remarked by several persons, the planet assuming an oval or elongated ap- pearance ; that part of it, which was still off the disc, seemed to be surrounded by a faint border of light. The " black drop," shortly before internal contact, is mentioned by numerous ob- servers in Europe and America, and the completion of the luminous thread denoting that the contact had passed was very generally described. Narrow circles of light were noticed round the planet during its progress across the Sun, and one observer speaks of an illumination of the disc, possibly similar to that re- corded occasionally in the transits of Mercury. Similar ap- pearances to those noticed at the ingress have been found to offer themselves when the planet is leaving the Sun ; but, of course, in reversed order. The tables of Venus in present use for predicting the place of the planet in the heavens, are those of the Baron Lindenau, published at Gotha, in 1810. Within the last ten jean the elements have been much improved by several Enghsh astrono- mers, with the aid of observations taken at our Royal Observa- tory. A most important addition has also been made to the theory of the planet by Mr. Airy, since the appearance of the above tables, consisting of a long inequality affecting the places of the earth and planet, as viewed from the Sun, to a very sen- sible amount. It arises from the near commensurability of the mean motions of the two bodies, thirteen times the period of Venus being nearly equal to eiffht times the period of the Earth. This inequaUty goes through all its changes of magni- tude in about 240 years, and was at a maximum about the commencement of the present century, when the heliocentric 44 THE SOLAR SYSTEM. place of the Earth was changed two seconds of space, and that of Venus about three seconds, by this cause. At present the tables of Baron Lindenau are quite exact enough for all prac- tical purposes ; but from what has been said, it will be evident that astronomers have the means of improving them very con- siderably. Claudius Ptolemy has preserved for us, in his " Almagest," many observations of Venus by himself and other astronomers before him, at Alexandria, in Egypt. The most ancient of these observations is dated in the four hundred and seventy-sixth year of Nabonassar's era, and thirteenth of the reign of Ptolemy Philadelphus, on the night of the lYth of the Egyptian month Messori, when Timooharis saw the planet eclipse a star at the extremity of the wing of Virgo. The date answers to b.c. 271, October 12 a.m. Similar occultations of stars and planets by Venus have been witnessed in modern times. Regulus, the bright star in Leo, was twice echpsed by her in the sixteenth century : on the 16th of September, 15*74, according to Msestlin, and again on the 25th of September, 1598, as Kepler relates in his "Astronomise Pars Optica." Mars was occulted by Venus on the '3d of Oc- tober, 1590, and Mercury suffered a similar eclipse on the lYth of May, 1737. CHAPTER IV. THE EAETH. ® ¥E have now to consider the Earth on which we dwell, in its astronomical relations, as one of the primary planets re- volving round the Sun, next in order, beyond the orbit of Venus. Astronomical geography teaches us that the Earth is not a perfect sphere, but is somewhat flattened at the poles ; the equatorial diameter, therefore, being the greatest, as we shall presently see to be the case with the superior planets. The form of the Earth is, consequently, an oblate spheroid. The elaborate calculations of Mr. Airy, and the late Professor Bessel, have furnished us with a very exact determination of the actual dimensions of our globe. According to the former astronomer, the equatorial diameter measures 41,84'7,4:26 English feet, and the polar diameter 41,707,620 feet. These measures, reduced into miles, give 7925.6 and 7899.2 respectively : the compres- sion at the poles therefore amounts to 26^ miles, or it is about 1-3 00th part of the whole diameter. The transits of Venus, as already remarked, have given us the value of the Sun's equatorial horizontal parallax with great exactness. This quantity is really the angular measure of the Earth's equatorial semi-diameter, at our average distance from the Sun. Wherefore, knowing the number of miles in the di- ameter of our globe, we can readily ascertain by trigonometry 46 THE SOLAR SYSTEM. the mean distance of the Earth from the Sun in miles, which is 95,298,260. That great circle of the heavens which the Sun appears to us to describe in the course of a year, owing to the annual rev- olution of the Earth round that body, is called the ecliptic, and the plane of the ecliptic, or of the Earth's orbit, Ls employed in nearly all astronomical calculations as a fundamental plane of reference. The equator of the heavens, which is a projection of the terrestrial equator to the sphere of the fixed stars, makes an angle with the ecliptic of about 23°.2'7', termed the obli- quity of the ecliptic. The two points where the celestial equa- tor is intei-sected by the Sun's apparent path are called the equinoxes ; and those where the Sun is 90° distant from the equinoxes, or at his greatest north and south declinations, are called the solstices. The spring equinox is the point from which astronomers reckon the right ascensions along the equator, and the longitudes on the ecliptic. It is owing to the inclination of the ecliptic to the equator that, in the course of our annual revolution round the Sun, we experience the vicissitudes of the seasons — spring, summer, au- tumn, and winter. At present this inclination amounts to about 23° 27' ; but it is subject to a very slow diminution, not exceeding 48" in 100 yeare. It will not always, however, be on the decrease, for before it can have altered li°, the cause which produces this diminution must act in a contrary direc- tion, and thus tend to increase the obliquity. Consequently, tlie change of obliquity is a phenomenon in which we are con- cerned only as astronomers, since it can never become sufficiently great to produce any sensible alteration of climate on the Earth's surface. A consideration of this remarkable astronomical fact cannot but remind us of the promise made to man after the deluge, that " while the earth remaineth, seed-time and harvest, and cold and heat, and summer and winter, and day and night THE EARTH. 4Y shall not cease." The perturbation of obliquity, consisting merely of an oscillatory motion of the plane of the ecliptic, which will not permit of its ever becoming very great or very small, is an astronomical discovery in perfect unison with the declaration made to Noah, and explains how effectually the Creator had ordained the means for carrying out his promise, though the way it was to be accomplished remained a hidden secret, until the gi-eat discoveries of modern science placed it within human comprehension. Anaximander, a disciple of Thales, who was born in the third year of the forty-second Olympiad, or B.C. 610, is reported by Pliny to have been the first of the ancients by whom refer- ence is made to the obliquity of the ecliptic. Diogenes Laerces tells us that he erected a gnomon at Lacedemon, with which his observations were made. Other authorities attribute the first notice of the obliquity to Pythagoras, born about seventy years after Anaximander, while Diodorus Siculus, and after him Plutarch and Stobseus, inform us that CEnopides of Ohio ob- tained the knowledge of this inclination of the equator and ecliptic fi-om the Egyptians. Laplace, however, makes use of observations for ascertaining this angle said to have been taken in China by Tcheou-kong 1100 years before the Christian era. We subjoin a table exhibiting the various determinations of the obliquity of the ecliptic from the earliest times to the present day, from which the reader will see that observation had pointed out its gradual diminution long before analysis was suflBciently advanced to indicate the cause. Most of the ancient observa- tions by the Greeks and Arabians were taken with gnomons or armillae : their plan was to ascertain the length of the shadow in relation to the height of the gnomon, on the days of the sol- stices, when the Sun attains his greatest declinations north and south. Hence his altitude above the horizon could be found, and the difference between the results on the two solstitial 48 THE SOLAR SYSTEM. epochs would give the distance between the tropics, half of which distance is the inclination of the ecliptic to the equator. The Chinese observations for taking the obliquity were taken with similar instruments, and are hei'e given as reduced by Laplace in his paper on this subject, Connaissance des Temps, 1811 : — TABLE EXHIBITING THE PRINCIPAL DETEP-MINATIONa OF THE OBLiaUITY OP THE ECLIPTIC, IN ANCIENT AND MODERN TIMES. Obliquity. B.C. 1100. 324. 230. 140. 50. A.D. 140. 173. 461. 629. 830. 879. 987. 995. 1080, 1279. 1303. 1430. 1460. 1587. 1660. 1690. 1750. 1769. 1800. 1825. 1840. Tcheou-kong Pytheas of Marseilles .... Eratosthenes of Gyrene, by observations with armil- l3e erected in a portico at Alexandria Hipparchus, the great astronomer . Lieou-hang Claudius Ptolemy, the Egyptian astronomer Chinese observations at Layang Tsou-chong at Nan-king .... Litchun-fouDg at Siganfou Alraamun, son of the famous Haiun al Raschid Albategnius at Aracte .... Ahoul Wefa at Bagdad .... Abul Rihau with a quadrant 25 feet radius Arzachel in Spain Cocheu-kong with a gnomon 40 feet height Prophatius, a Spanish Jew Ulugh Beigh at Sarmarcand Regiomontauus in his tables Tycho Biahe, the celebrated Danish astronomer Hevelius at Dantzic Flamsteed, the first astronomer at Greenwich Bradley, La Caille, &c Maskelyne at Greenwich .... Delambre and others .... Bessel at Konigsberg .... By observations at Greenwich, Edinburgh, Cam- bridge, and other places .... 23 54 2 23 49 20 23 51 15 23 51 15 23 45 39 23 51 15 23 41 33 23 38 52 23 40 4 23 33 52 23 35 23 35 23 35 23 34 23 82 12 23 32 23 31 48 23 30 23 31 30 23 29 30 23 28 56 23 28 19 23 28 10 23 27 57 23. 27 43,4 23 27 36,5 THE EASTS. 49 The phenomenon known as the precession of the equinoxes was discovered by the celebrated astronomer Hipparchus, of Nicea, in the second century before the Christian era. By comparing his own observations of the • longitudes of several principal fixed stars with those of Timocharis and Aristyllus, taken at Alexandria about 150 years previously, he found thpy differed constantly in one direction — the distances of the stars from the first point of Aries having increased apparently at the rate of about 1° in a century. The effect was thus discovered, but the cause remained unknown till it was explained by our illustrious Newton. It consists chiefly in the action of the Sun and Moon upon the protuberant matter at the Earth's Equator : a minute effect is due to the influence of the planet Venus. It is called the precession of the equinoxes because the equinoc- tial point is carried forward in reference to the circle of diurnal movement, arising from the Earth's rotation on her axis ; con- sequently, it retrogrades upon the ecliptic, and thereby causes an increase in the distance of all stars from the first point of Aries, measured upon that circle. The present rate of progres- sion is about 1° 23' 44" in 100 years, or 501" annually. Up- wards of 25,800 years will be required for a complete revolu- tion of the equinoctial points. We may conceive the phenomena of precession to arise from the revolution of the pole of the celestial equator, or that point of the heavens to which the Earth's axis is directed, round the pole of the ecliptic, in the period of 25,800 years, at a mu- tual inclination of 23^ 28', and shall thus obtain an insight into the nature of another important inequality, called the nutation of the Earth's axis, which exercises very appi'eciable influence upon the positions of the stars, as we see them from the Earth, and goes through all its variations in somewhat less than nine- teen years, or in the course of one revolution of the Moon's nodes. The same cause which operates in producing the precession of 3 50 THE SOLAR SYSTEM. the equinoxes gives rise also to the nutation, in virtue of which the Earth's axis, instead of being continually directed to the same point in the celestial sphere, describes a small ellipse on the surface of the heavens, having the ratio of the greater axis to the lesser as 37 to 27, the length of the major axis in arc of a great circle being 18".6. Dr. Bradley first discovered and explained the nutation of the Earth's axis, soon after he had been led by his ovfn obser- vations to infer the existence of the aberration of light. We have, therefore, to thank this eminent astronomer for two of the most important discoveries connected with the science. The Earth's orbit is not circular, but an ellipse of very mod- erate eccentricity, the perihelion point in 1800 answering to 279° 30' 8'' of heliocentric longitude. The line of apsides is subject to an annual direct change of 11.29", independent of the effect of precession, so that, allowing for the latter cause of disturbance, the annual movement of the apsides in reference to the variable equinox may be roughly taken at one minute of arc. One important consequence of this slow motion of the greater axis of the Earth's orbit is the variation in the length of the seasons in different centuries. In the time of Hippar- chus, the longitude of the Sun's perigee -was between the au- tumnal equinoctial point and the winter solstice, and the au- tumn was the shortest of the seasons ; spring was longer than summer, and winter longer than autumn. About the middle of the thirteenth century the perigee coincided with the winter solstice, whence spring was equal to summer, and autumn to winter. In the year 1850 we find the time elapsing between d. h. m. The spring equinox and summer solstice . 92 20 57 The summer solstice and autumnal equinox 93 14 The autumnal equinox and winter solstice . 89 17 88 The winter solstice and spring equinox . . 89 1 17 THE EARTH. 51 Hence the spring has become shorter than the summer, and the autumn longer than the winter. About four thousand years before the Christian era, or singularly enough, near the epoch of the creation of man, ac- cording to chronologists, the perigee coincided with the vernal equinox, and the winter and spring were equal, and shorter than the summer and autumn, which were also equal. In A.D. 6485, or thereabouts, the perigee will have completed half a revolution, and will then coincide with the autumnal equi- nox ; summer will be equal to autumn, and winter to spring, but the former seasons will be the shortest. All these changes, it is to be observed, are due in the first place to the eccen- tricity of the Earth's orbit, and to the progressive movement of the line of apsides. The eccentricity, as we have stated above, is not large ; at the commencement of the present cen- tury it amounted to 0.0167923, but is subject to a slow dimi- nution, not exceeding 0.000044 in one hundred years. Sup- posing this change permanent, the Earth's orbit must eventually become circular, but the theory of attraction enables us to prove that the diminution is not to continue beyond a certain time, and although we are not yet in a condition to assign def- inite limits to the oscillations, we know that after the lapse of many thousand years the eccentricity will be stationary for a time, and afterward increase; and, without some external cause of perturbation, these variations, within certain not very distant limits, must continue throughout all ages. Were the Earth's orbit circular, and the plane of the equa- tor coincident with that of the ecliptic, the Sun would appear to describe an equal arc of the heavens day after day, and con- sequently the interval elapsing between two successive passages over the meridian of any place on the Earth's surface would be sensibly the same, and the solar day would be something like an equable period of time. But, as we have seen, the 52 THE SOLAR -SYSTEM. Earth's path round the Sun is elliptical, and the apparent diur- nal velocity of the Sun varies with our distance from him. Moreover, the equator is inchned to the plane of her path at an angle of about 23° 28', and this again has a great influence on the length of the arcs of liight Ascension, passed over by the Sun on successive days. It follows, therefore, that the solar day, or the interval elapsing between two consecutive meridian transits of the Sun, is of variable length. To secure an equable measure of time, astronomers assume the revolu- tion of a mean Sun in the plane of the equator with the real Sun's mean diurnal motion in Right Ascension, and the time intervening between two successive transits of the mean Sun is called a mean solar day, which is the unit of time in common use at present. The difference between the Right Ascension of the mean and true Suns is termed the Equation of Time, and in order to i-egulate a clock by observations of the time of culmination of the Sun it is necessary to ftnow the amount of this equation for each day, and we, accordingly, have it tabulated in astronom- ical ephemerides and almanacs. The equation is at a maxi- mum about the 10th of February, when an additional correc- tion of about 14m. 32s. is requii-ed to reduce apparent solar time to mean solar time, or, in other words, the mean Sun is on the meridian 14m. 32s. before the true Sun. On the loth of April there is no equation, the real and imaginary Suns being on the meridian at the same moment In the middle of May the equation again reaches a maximum at 3m. 54s., and disappears on the 15th of June. Another maximum occurs about the SVth of July, when a correction of 6m. lis. is required to be added to apparent solar time. On the 1st of September it again vanishes, but increases from that time until the beginning of November, when the equation amounts to 16m. l'7s., subtractive from apparent time, and again becomes THE EARTH. 53 zero about the 25tli of December; thus there are four maxima in each year. The sidereal day is the time intervening between two con- secutive passages of the same star over a meridian ; it is con- sequently the length of the Earth's diurnal rotation upon her axis, and expressed in mean solar time is 23h. 60m. 4.9s. The sidereal day is subject to no sensible variation. It is in conse- quence of the acceleration of the sidereal upon the mean solar day that the aspect of the heavens is vai-ied at different times of the year, those stars vphich at one time appeared on the meridian at midnight gradually gaining upon it, until they are lost in the western heavens at sunset, to mate th^ir reappear- ance in the east after the lapse of a few months. The interval of time in which the Sun appears to us to complete an entire circuit of the heavens, in reference to the fixed stars, is called by astronomers a sidereal year, and con- sists of 365d. 6h. 9m. 10.7s. In this period, however, the equinox will have retrograded 50|", and the Sun will reach the same point of longitude from which he started in a shorter time, than would be required to elapse from the moment of his leaving a fixed star until he again returns to it. The revolu- tion in respect to the equinox is thus shorter than the revolulion in respect to the stai-s, by the interval occupied by the Earth in passing over an arc of 50-}-" upon her orbit, or by Oh. 20m. 22.9s., and we obtain 365d. 5h. 48m. 47. Ss., for the length of the revolution in reference to the equinoctial point, or, as it is termed, the tropical year. We have seen that the perihelion point of the Earth's orbit has an annual motion amongst the stars of about eleven sec- onds, by which quantity the longitude exceeds that of the pre- ceding year. If the Sun start from the place of the perihelion he will require a longer interval of time than the sidereal year to reach it again, and the excess will be equal to the time 54 THS SOLAR SYSTEM. necessary for the Earth to describe 11.29s. of her orbit, or 4m. 35.0s., which gives us 365d. 6h. 13m. 45.Ys. for the dura- tion of what is called the anomalistic year. This period is occasionally used in astronomical investigations, but mankind generally are more concerned in the tropical year, on which the return of the season depends. This year is subject at pres- ent to a slow diminution, amounting to little more than half a second in one hundred years, yet, like all variations of the kind by which the Earth's orbit is affected, the diminution is not to continue forever. The ancient Egyptian year consisted of 365 days, as we learn from Herodotus. The Thebans, or inhabitants of Upper Egypt, are said to have perceived the necessity of an addition of six hours to this period, in order to make it agree with the annual course of the Sun. In the time of Deraocritus, about 450 years before Christ, the year was supposed to consist of 365^ days. Eudoxus, who flourished soon afterwards, con- sidered it somewhat longer, while (Enopides, of Chius, men- tioned by Diodorus, made it 365d. 8h. 48m. The Calippic period of 76 years, commencing at the death of Darius, B.C. 329, consisted of 27,759 days, giving 365i days for the length of the year. The great astronomer, Hipparchus, found by his own ob- servations that the year of Calippus was somewhat too long, and, accordingly, diminished it by the three-hundredth part of a day, or 4m. 48s., whence he fixed the length of the tropical year at 365d. 5h. 55m. 12s., differing little more than six minutes from the best modern determination. Nearly three hundred years after the time of Hipparchus, Ptolemy appears to have investigated the length of a year, but concluded by adopting the duration assigned by the Greek astronomer, and employs it in the Solar Tables found in his Almagest. The Arabian Prince Albategnius, who lived at the latter THE EARTH. 55 ond of the ninth century, and observed at Aracte, in Chaldea, perceived the want of some correction to the Ptolemaic or Hipparchian year, and by compaiison of his own observations with those at Alexandria, inferred that the length of the tropi- cal year was 365d. 5h. 46m. 24s., as reported in his work De Scientia Stellarum. In the Alphonsine Tables, compiled about 1252, by Alphonsus X., King of Castile, with the as- sistance of the best astronomers of his age, we find the length of the year 365d, 5h. 49m. 16s., a very near approximation to the truth. Since this epoch, the duration of the tropical year has formed the frequent subject of investigation. The following table exhibits the principal determinations up to the present time : — Nicolas Copernicus, in 1543 365 Tycho Brahe, in 1602 Kepler, in the Tabula Rudolphince . . . Cassini, in 1743, by comparison of his own observations of Equinoxes with those of previous observers . .... Flamsteed, our first Astronomer Koyal . Halley, in his Astronomical Tables . . Lacaille, in his Tables 365 Bessel, in 1830, gives for 1800 . . . d. h. m. s. 365 5 49 6 365 5 48 45i 365 5 48 57.6 365 5 48 52.4 365 5 48 57.5 365 6 48 548 365 5 48 49 365 6 48 47.8 CHAPTER V. THE MOON, d THE Moon, the constant attendant of the Earth in her an- nual course round the Sun, is by far the nearest to us of all the heavenly bodies, being situated at an average mean dis- tance of only 238,650 miles. To her we ai-e indebted, not merely for illumining by her presence our dark winter nights, but likewise for that more important phenomenon, the tides of the ocean, in the production of which she has the greatest in- fluence, and it is not unlikely that this extraordinary luminary exercises an effect upon the Earth in other ways, of which we are not at present fully cognizant. The interval of time occupied by the Moon in performing one sidereal revolution round the earth,* or the time which elapses between her leaving a fixed star until she again returns to it, is found by the latest and most accurate investigation, to be 2'7d. Ih. 4.3ra. 11.461s., whence we find the mean tropical * Strictly speaking the centre of the Earth is not the point round which the Moon revolves ; but both bodies have a revolution round their common centre of gravity. This point is situated at an average distance of 2,690 miles from the centre of our globe, or about 1,270 miles beneath the surface, and subject to a variation of rather more than 165 miles, one way or the other, in consequence of the variable distance between the Earth and the Moon. The perturbation in the place of the Earth, or rather its reaction on the place of the Sun, owing to this motion round the centre of gravity, may affect the Sun's longitude more than five seconds of arc, and his latitude about 0.7". THE MOON. 57 revolution 27cl. Yh. 43m. 4.614s., since the equinox will have receded in a sidereal period S'VSS", a space which the Moon would require 6'847" to traverse with her average mean motion. The phenomena, termed the phases of the Moon, do not recur in the space of a sidereal revolution, for it is evident that the real motion of the Earth in that interval, giving rise to an apparent motion of the Sun, in the direction in which the Moon revolves, must cause the period between two conjunctions or oppositions to be longer than either the sidereal or tropical rev- olutions, the Moon having to traverse an arc equal to the angu- lar movement of. the Earth in the sidereal period, before she is again in a line (to speak roughly) with the Earth and Sun. The lunar month, or as it is usually called by astronomers, the synodical revolution of the Moon, is consequently longer than the sidereal period, and exceeds it by 2d. 5h. Ora. 51'41s., which is the time required by that body to describe, with her mean angular velocity of 13*1764° per day, the arc traversed by the Sun since the previous conjunction. Hence we find the duration of the synodical period to be 29d. 12h. 44m. 2'87s. The lunar phases depend on the position of the Moon with respect to the Sun, or what amounts to the same thing, her dis- tance from conjunction, which is termed in astronomical lan- guage, the age of the Moon. Being an opaque spherical globe, reflecting the Sun's light, she can only appear fully illuminated when opposite that luminary, and in all other positions her illu- minated disc appears less than a circle. Soon after conjunction ,with the Sun, she maybe seen as a very narrow crescent, a little above the western horizon at sun-set, for being then between the Earth and Sun, her illuminated surface is in a great measure turned from us. As she advances in her orbit, the dark part gradually diminishes until the Moon is 90° from conjunction, which is called the first quarter, and then the bright and unil- 3* 58 THE SOLAR SYSTEM. luroinated parts are equal. After this point, the illuminated surface increases till the Moon is in opposition, when it is said to he full, and presents to us her whole enlightened disc. The bright part then begins to diminish and again occupies one half of her surface when the Moon is 90^ from the conjunction, at the last quarter, becoming narrower as she approaches it, till a thin crescent above the eastern horizon shortly before sun-rise is all that remains. These phases are consequently recurrent after the interval of a synodical revolution, and depend upon the position of the visible, in reference to the enlightened, hemi- sphere. The eccentricity of the lunar orbit is considerable, and from this cause, the distance between the Earth and the Moon, at the perigee, may be no more than 225,560 miles, while at apogee it may increase to 251, '700 miles, the ellipticity of the orbit therefore producing a variation in the length of the radius vec- tor, or true distance from the Earth, of rather more than 26,000 miles. The mean inclination of the orbit to the ecliptic is, ac- cording to recent determination, 5° 8' 55'46", but this is sub- ject to a variation, one way and the other, of rather less than 23' : the latest tables of the Moon giving the greatest inclina- tion 5° 20' 6", and the least, 4° 57' 22". The line of nodes of the lunar orbit revolves round the ecliptic in 18yrs. 218d. 21h. 22m. 46s., in a retrograde direction, which is at the rate of 1^° in each sidereal peiiod, or somewhat more than 3' daily. This retrogression of the nodes is caused by the action of the Sun, which modifies the central gravity of the Moon towards the Earth. It is not, however, an equable motion throughout the whole of the Moon's revolution ; the node, generally speak- ing, is stationary when she is in quadrature, or in the echptic ; in all other parts of the orbit it has a retrograde motion, which is greater the nearer the Moon is to the syzigies, or the greater the distance from the ecliptic. The preponderating effect at the THE MOON. 59 end of each synodic period is, however, retrocessive, and gives rise to the revolution of the line of nodes in between eighteen and nineteen years. Hence it is necessary to distinguish be- tween the mean place of the node which supposes a regular movement of 3' 10" daily, contrary to the order of signs and its true place at any time. The backward movement of the line of nodes was discovered by the ancients, but first explained by Newton. The line of apsides or major axis of the lunar orbit has, from a similar cause, a direct motion on the ecliptic and accomplishes a whole revolution in 8yrs. 310d. 13h. 48m. 53s., so that in 4yrs. 155d. the perigee arrives where the apogee was before. This motion of the line of apsides, like the movement of the nodes, is not regular and equable throughout the whole of a lunar month, for when the Moon is in syzigies the line of apsides advances in the order of signs, but is retrograde in quadratures. But the preponderating effect in several revolutions tends to advance the apsides, and hence arises their direct revolution in between eight and nine yeare. The apparent diameter of the Moon, at her mean distance from the Earth, is 31' 19'8", but the eccentricity of her orbit causes a variation of about 4' 44", the maximum being 33' 32'', and the minimum 28' 48". According to Professor Madler, the real diameter of the Moon is 2159'6 English miles ; the recent measures of Dr. Wichmann make it 2162 miles, agreeing so nearly with the former value, that we may conclude we know the dimensions of the lunar orb very exactly. Taking the diameter at 2160 m.iles, which cannot be far from the truth, the circumference will be 6786 miles, and the bulk of the Moon will be to that of the Earth as 1 to 49-J-. Dr. Wichmann could not detect a sensible diflference between the equatorial and polar diameters. Some of the best determinations of the mass of the Moon 60 THE SOLAR SYSTEM. depend upon the amount of lunar nutation, which, as we have seen, is a periodical fluctuation in the position of the Earth's axis, arising from the variable direction of the line of nodes of the Moon's orbit. The Baron Lindenau, from his researches on nutation, concluded the mass to be l-SY'Y of that of the Earth ; Professor Henderson has more recently given 1-78-9. There are various other methods of ascertaining the mass of our satel- lite, as, for instance, the observation of the change in the Sun's longitude due to her attraction on the Earth. Mr. Airy's method of observing the positions of Venus near her inferior conjunction, already mentioned, which is similar in principle to the last, and the investigations of her effect upon the tides.* Koughly speaking, we shall be near the truth if we assume the mass of the Moon to be l-80th of that of the Earth. The most casual observer of the Moon can hardly fail to have remarked, that she always presents very nearly the same face towards us, and a little reflection will convince him that the cause must lie in the near equality of her periods of axial rotation and sidereal revolution round the Earth. Were these periods exactly equal, we should have the same hemisphere turned towards us without the slightest variation ; but the or- bital period is subject to small irregularities, while the time ■ of axial rotation remains constant, and for this reason a phenome- non termed the libration takes place, whereby we occasionally see a little more of one edge of the Moon than usual, either on the eastern or western sides of her equatorial regions. Galileo was the firet astronomer who remarked this periodical variation in the visible surface of the Moon, and the circumstance reflects no httle credit on his zeal and attention, for his instrumental means are well known to have been very small, notwithstanding the numerous discoveries we owe to him. Generally speaking, * By this method Laplace concluded the mass of the Moon to be l-73d of that of the Earth. THE MOON. 61 the Moon revolves on her axis in the period of one mean side- real revolution. The axis of the Moon is not quite, though very nearly, per- pendicular to the plane of her orbit, which allows of our seeing a little more of the polar regions at certain times than at others, a phenomenon called the libration in latitude. The angle be- tween the lunar equator and the plane of her path round the Earth is, according to Nicollet, 1° 28' 47", or, according to the more recent and elaborate researches of Dr. Wichmann, 1° 32' 9". By the parallactic libration we understand the difference in the position of a spot as viewed from th^ Centre and surface of the Earth. Professor Bessel and Dr. Wichmann have made some re- searches respecting the existence of a physical libration, or a real variation in the time of axial rotation of the Moon, such as would give rise to an apparent change in the position of the spots, periodical or otherwise ; but there does not at present appear to be sufficient reason for suspecting any inequality of this nature. The full Moon which falls nearest to the Autumnal Equinox has long received the name of the Harvest Moon, from the fact that the difference between the hours of her rising on two suc- cessive evenings is then at a minimum, and the long duration of moonlight, thus afforded soon after sun-set, is most advanta- geous to the farmer at this critical season. This near coinci- dence in the times of several successive risings takes place every lunar month, when the Moon is in the signs Pisces and Aries, but it has only been remarked when she is at full in these signs, and this can only happen in August or September. The least possible difference between two successive risings in this latitude is about seventeen minutes. When the Moon is in Libra, and at the same time near the descending node of her orbit upon 62 THE SOLAR SYSTEM. the ecliptic, the difference between the times of rising on two evenings is the greatest possible, and in London will amount to about Ih. 16m. The theory of the lunar motions is, perhaps, the most diffi- cult with which the astronomer has to deal, and it has accord- ingly occupied the attention of the most eminent mathema- ticians, from the time of Sir Isaac Newton to the pi-esent day. To bring it to the exact and elaborate form in which it now is, has required the utmost efforts of the observer as well as the physical astronomer, for it is a curious fact, that some of the most important of the smaller inequalities, as they are termed, of the Moon's mean mgtion, have been first detected by actual observation, and subsequently reconciled with the theory of gravitation as expounded by Newton. The larger inequahties are of such magnitude, that they were discovered with the rude instruments employed by Hipparchus in the second century be- fore the Christian era. It would lead us beyond the limits and plan of the present work, were we to enter into any explanatory account of the lunar perturbations generally, which, after all that can be said respecting them, are not easily intelligible without a knowledge of the mathematical processes by which they have been detected and reduced to calculation. We shall, therefore, content ourselves with a brief notice of the most im- portant inequalities affecting the mean longitude of our satellite. The most considerable is that termed the Evecti.on, the dis- covery of which we owe to the famous astronomer Hipparchus, in the second century before the Christian era. It depends upon the angular distance of the Moon from the Sun, and the mean anomaly of the former, and goes through its variations in a period of about 31d. 19h. 30m. At its maximum it may influence the Moon's longitude one way or the other about 1° 20'. It diminishes the equation of the centre in syzigies, and increases it in quadratures. THE MOON. 63 Another large inequality is called the Variation, the dis- covery of which has usually been attributed to Tycho Brahe, though M. Sedillot and otbei-s have claimed the merit of its first recognition for the Arabian astronomer, Aboul Wefa, who lived in the ninth century. It depends solely on the angular distance of the Moon from the Sun. and its peiiod is half a synodic revolution, or about 14d. 18h. The effect of this ine- quality is greatest in the octants, and disappears in syzigies and quadratures, the longitude of the Moon being altered thereby rather more than half a degree when the equation is at a max- imum. The Variaticm was the first lunar inequality explained by Sir Isaac Newton upon the theory of gravitation. The parallactic inequality arises from the sensible difference in the Sun's disturbing foi'ce, when the Moon is travelling over that semi-circumference of her orbit lying near the Sun, and when she is in the further semicircle. Small as is the change of distance at these times, the perturbation depending upon it is sufficiently large to produce an inequality, which at its max- imum may alter the Moon's longitude about two minutes of arc. The period during which it passes through all its varia- tions is one synodical revolution, or 29d. 12h. 44m. The Annual Equation is an inequahty caused by a varia- tion in the angular motion of the Moon, which becomes slower as the Earth and Moon are approaching the Sun, and acceler- ates as they recede from him. The motion of our satellite is slower than the mean motion during the time the Earth is moving from perihelion to aphelion, and more rapid as she passes from aphelion to periheUon, or in the present position of the line of apsides, the Moon moves slower between the end of December and June, than between the end of June and the end of December. The period of this inequahty is the anom- alistic solar year, and its maximum effect upon the Moon's lon- gitude amounts to 11' 10". 64 THE SOLAR SYSTEM. The Secular Acceleration of the Moon's mean motion is caused by a change in the eccentricity of the Earth's orbit, which has been slowly diminishing for many ages. It was dis- covered by Dr. Halley from a comparison of the periodic time of the Moon, deduced from recent observations, with that indi- cated by the Chaldean observations of eclipses at Babylon in the years "719 and 720 before the Christian era, and the Ara- bian observations in the eighth and ninth centuries, the result of which showed that the periodic time is now sensibly shorter than at the epoch of Chaldean eclipses. The cause of this diminution was not understood until the celebrated Laplace showed that it was similar in its origin to the much larger and more rapid fluctuation of period, which takes place according to the position of the Sun with respect to the line of apsides of the lunar orbit. The mean motion of the Moon is increased by this inequality, at the rate of rather more than ten seconds in one hundred years. As the diminution of eccentricity of the Earth's orbit is not a permanent change, though extending over a period whose duration is hardly yet calculable, so the acceleration of the Moon's mean motion is a cyclical inequality, and a time will arrive when it must altogether cease. After this remarkable equation had been detected by Dr. Halley, great doubts existed in some minds as to the possibility of ex- plaining it on the theory of gravitation. The elucidation which the subject has received at the hands of Laplace is, therefore, the more remarkable, and affords one of many instances where suspicions of the failure of Newton's law have ultimately tended only to its more striking confirmation. The Tables at present in general use for predicting the positions, eclipses, and other phenomena of our satellite, are those calculated by the French astronomer Burckhardt, and published at Paris in 1 8 1 2. They are founded principally upon the observations taken at our National Observatory at Green- THJS MOON. 65 wich, an establishment which was instituted with an especial view to the improvement of the lunar theory, and thereb)', of the art of navigation. Burckhardt's tables are used in the preparation of the Nautical Ephemerides annually issued by the governments of Great Britain, France, and Prussia. The late Baron Damoiseau was the author of two sets of lunar tables, which are based upon elements not very different from those employed by Burckhardt. The first set, according to the centesimal division of the circle, appeared in 1824, and the second, agrec^ably to the old or sexagesimal division, in 1828. Since the investigation of the elements of the existing tables, the lunar theory has been greatly improved by the laborious researches of M. Plana of Turin and Professor Hansen of See- berg, and several inequalities of long period have been discov- ered, which almost entirely reconcile the diiferences between the observed and tabular places of the Moon. But the most important work undertaken for perfecting our knowledge of her movements, and one which is hardly equalled for magnitude and intricacy in the history of astronomy, is the reduction of all the observations of the Moon taken at the Royal Observa- tory, Greenwich, between the years 1750 and 1830, which has been brought to a completion within the last few years, under the superintendence of the Astronomer Royal. There are about 8000 observations in all, and, for the attainment of the object in view, it was necessary to reduce the whole again, with the best modern elements, to compute the tabular places in dupli- cate, the tables themselves having been modified and extended so as to accord as nearly as possible with M. Plana's theory, and finally to determine the pricipal elements of the Moon's motion, from the whole mass of observations. Few persons who have not had an opportunity of viewing the manuscripts themselves can form any adequate idea of the enormous labor attending this valuable work. Twelve computers on an average were 66 THE SOLAR SYSTEM. engaged eight hours a-day for several years, the reductions having been commenced in earnest in August 1841, and the last sheets of the second large volume containing the results having passed through the press in the spring of 1848. Pro- fessor Hansen has undertaken a complete revision of the lunar theory, having at his command, where necessary, the Green- wich reductions of the 8000 observations, and the British Gov- ernment has lately provided funds to aid this distinguished mathematician in his important inquiry.* The Astronomer Royal communicated to the Royal Astronomical Society several years since, the most prominent conclusions at which he had himself arrived after discussing the Greenwich observations. The naked eye readily discerns that the disc of the full Moon is not uniformly bright : light and dark regions alternate upon it, giving the idea of continents and seas analogous to those on our own globe. In fact, the earlier selenographists considered the dull grayish spots to be water, and termed them the lunar seas, bays, and lakes. They are so called to the present day, though we have strong evidence to show that if water exist at all on the Moon, it must be in very small quantity. On apply- ing the telescope, with suitable magnifying power's, we perceive on every part of the surface, even in the midst of the so-called oceans and seas, annular spots evidently of a volcanic charac- ter, with extensive chains of mountains and steep isolated rocks, forming altogether a very rugged and desolate appearance. The * As one of the early fruits of this investigation, we may mention the discovery of two inequalities in the motion of our satellite, result- ing from the attraction of Venus, exercised directly in one instance and indirectly in the other. Great importance is attached to these discoveries, because, when their influence is taken into account, the positions of the Moon, calculated from theory, are almost precisely identical with those given by observations, which renders it certain that our knowledge of the movements of our nearest celestial neigh- bor is very nearly perfect. THE MOON. 67 craters are exceedingly numerous ; in some places they are thickly crowded together, small volcanoes having formed on the sides of the larger ones, — in other regions they are com- paratively isolated. Their dimensions are far greater than those of the largest volcanoes on the Earth, the breadth of the chasm occasionally exceeding one hundred miles, while the sides of the mountain attain a very considerable elevation. The best time for viewing a crater is when it is just clear of the dark part of the Moon, or when the Sun is just above its horizon : we can then trace the shadows thrown by the sides of the mountain upon its interior and exterior surface, and, by meas- uring the lengths of these shadows, we may approximate to its true altitude. Some of the steep isolated rocks throw their shadows for many miles across the plains surrounding them. The positions of the lunar spots upon the surface are usually given in selenograpMc longitudes and latitudes. In the large work of Professor Madler are found the results of a great num- ber of observations for fixing the exact places of the principal mountains and craters, first in longitude and latitude, and after- wards in the form of co-ordinates, to facilitate the construction of the lunar chart. The first quadrant contains west longitude and north latitude ; the second, east longitude and north lati- tude ; the third, east longitude and south latitude ; and the fourth, west longitude and south latitude. To distinguish the various spots from each other, two nomenclatures have been adopted by Hevelius and Riccioli re- spectively. The former made use chiefly of the names of places upon the Earth, the latter introduced the names of celebrities of all ages in science and literature ; and this method is the one adopted by Professor Madler, the greatest selenographer of the present day, and, in fact, universally followed by astrono- mei-s. Amongst the English names thus appropriated are those of Newton, Flamsteed, Bradley, Maskelyne, Airy, Dol- 08 THE SOLAR SYSTEM. lond, Cook, Herscliel, Eamsden, Sabine, WoUaston,