The solar system: a descriptive treatise upon the sun, moon, and planets, including an account of all the recent discoveries

Front matter

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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 descriptive 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, therefore, 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 principles 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 certain points connected with their investigations than was to be found in printed authorities, I have to return my best thanks.

J. EUSSELL HIND.

Contents

CIlAl'TKR

I. THE SUN,

HI. VENUS,

Chapter II. The Inferior Planets

WITHIN the orbit of the Earth revolve the two planets Mercury 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 illuminated 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 passage 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 Mercury 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 beginning of the present century and paid much attention to the subject, considered he had decided evidence of the existence of high mountains on the surface of Mercury, and it was by examining 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. Herschel 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^ : consequently 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 cannot, 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 without once seeing Mercury. Even Copernicus, the celebrated reviver 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 illuminated, 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 increases, 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 diminishes 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 becomes 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 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 satellite, 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 movements 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 obtained, 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 passage 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 prediction 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 remarked 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 November, 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 parallaxes 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

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 November, 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 twentyfifth 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.

M. s.

19 20 12

4 46

18 43 44

3 30 42

6 38 29

7 47 2

12 37 37

5 18 18

14 44 56

5 8 40

6 27 4

5 15 12

MERGURY. 29

In describing their observations of the transits of Mercury, astronomers make use of the terms internal and external contacts. 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 degree of accuracy, and is therefore less important than the observation 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 external 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 appeared 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 ascribed partly to atmospheric circumstances ; but this cause alone is not sufficient to account for them completely, for different 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 Observatory, though others afforded no indications of it. Another 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 telescope by DoUond and another by Fraunhofer showed the spot in precisely the same manner, though various eye-pieces, magnifying 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 descended 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 joining 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 reports 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 agreement. 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 present 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 computation 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 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 publication. 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 calculations.

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 sensibly than that of Mercury, owing to the greater extent of variation 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* 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 rotation. 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 prismatic 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 reflector 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 margin 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 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, ascertained that the time of rotation is 23h. 21m. 7.98s., a result which has been .pretty generally received, though it may hereafter 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 rotation 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 indicated 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.

was of opinion that the time of revolution could not be so long as twenty-four days, though, as above stated, his own experience did not enable him to assign the precise period. The late Professor De Vico, in the admirable sky of Rome, frequently saw spots upon Venus ; but for a steady view of them it was necessary to wait for the very best atmospheric conditions, 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 inferior 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.

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 decision. 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 perceived 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 distant from one another. Kodkier, Horrebow, and others at Copenhagen, with a refracting telescope, and Montbarron at

Auxerre, with a Gregorian reflector, repeatedly saw the attendant 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 satellite 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 century can have been mistaken. In this state the question at present remains.