the remoteness of Saturn from the Sun its satellites play a somewhat important part in the Saturnian sky as reflectors of sun-light. Nevertheless the space occupied by all of them, taken together, is (as stated on a previous page) only about 6 times that covered by the Moon. Lockyer thus summarises the phases of Saturn's ring as seen by an observer placed on the planet itself:-" As the plane of the ring lies in the plane of the planet's equator, an observer at the equator will only see its thickness, and the ring therefore will put on the appearance of a band of light passing through the East and West points and the zenith. As the observer, however, increases his latitude either North or South, the surface of the ring-system will begin to be seen, and it will gradually widen, as in fact the observer will be able to look down upon it; but as it increases in width it will also increase its distance from the zenith, until in lat. 63° it is lost below the horizon, and between this latitude and the poles it is altogether invisible. Now the plane of the rings always remains parallel to itself, and twice in Saturn's year—that is, in two opposite points of the planet's orbit it passes through the Sun. It follows, therefore, that during onehalf of the revolution of the planet one surface of the rings is lit up, and during the remaining period the other surface. At night, therefore, in one case, the ring-system will be seen as an illuminated arch, with the shadow of the planet passing over it, like the hour-hand over a dial; and in the other, if it be not lit up by the light reflected from the planet, its position will only be indicated by the entire absence of stars. "But if the rings eclipse the stars at night, they can also eclipse the Sun by day. In latitude 40° we have morning and evening eclipses for more than a year, gradually extending until the Sun is eclipsed during the whole day-that is, when its apparent path lies entirely in the region covered by the ring; and these total eclipses continue for nearly 7 years: eclipses of one kind or another taking place for 8 years 292 days. This will give us an idea how largely the apparent phenomena of the t Elementary Lessons in Astronomy, p. 117. heavens, and the actual conditions as to climates and seasons, are influenced by the presence of the ring." The only physical fact which has been discovered in relation to the satellites of Saturn concerns Iapetus. Cassini lost that satellite soon after its discovery, but a larger telescope enabled him to find it again, and moreover to ascertain that it was subject to considerable variations of brilliancy. Sir W. Herschel, with a view of establishing this fact beyond doubt, paid much THE APPARENT ORBITS OF THE SEVEN INNER SATELLITES OF SATURN TO FACILITATE THEIR IDENTIFICATION (1888). The date of Titan's Eastern Elongation being known (= 0), it will on subsequent days be attention to Iapetus. He was able to confirm Cassini's opinion, and decided that it actually did experience a considerable loss of light when traversing the Eastern half of its orbit. He found that 7° past Opposition was the place of minimum light. The conclusions deducible from this are (as Cassini himself pointed out), that the satellite rotates once on its axis in the same time that it performs one revolution round its primary; and that there are portions of its surface which are almost entirely incapable of reflecting the rays of the Sun. 1 The mass of Saturn has been given at so 3355 by Laplace; at 1 by Bouvard; and at by Newton; at 50 by Bessel. Jacob thought from his own observations that the mass of the whole Saturnian system did not differ much from 373. The most recent value is A. Hall's, 375 Бу E "The most ancient observation of Saturn which has descended to us was made by the Chaldæans, probably at Babylon, in the year 519 of Nabonassar's period, on the 14th of the month Tybi, in the evening; when the planet was observed to be 2 digits below the star in the Southern wing of Virgo, known to us as > Virginis. The date given by Ptolemy, who reports this observation in his Almagest [lib. xi.], answers to B.C. 228, March 1".” An occultation of this planet by the Moon is recorded to have been observed by one Thius, at Athens, on Feb. 21, 503 A.D. Cassini observed in 1692 the occultation of a star by Saturn's satellite Titan. No other instance of this kind is on record. From Saturn the Sun appears only about 3' in diameter, and the greatest elongations of the planets are: Mercury, 2° 19′; Venus, 4° 21′; Earth, 6° 1′; Mars, 9° 11'; Jupiter, 33° 3′-so that a Saturnian, assuming his visual powers to resemble ours, can only see Jupiter, Uranus, and Neptune with the naked eye, and Mars perhaps with some optical aid. Saturn, on account of its slow dreary pace, was chosen by the alchemists as the symbol for lead. In computing the places of Saturn, the Tables of A. Bouvard, published in 1821, were long used, but new Tables by Le Verrier have superseded them. Tables of the satellites have still to be formed, and are a great desideratum. u Hind, Sol. Syst., p. 117. Ꭱ CHAPTER XIII. URANUS. H Circumstances connected with its discovery by Sir W. Herschel.-Names proposed for it.-Early observations.—Period, &c.—Physical appearance.—Belts visible in large telescopes.-Position of its axis.—Attended by 4 Satellites.—Table of them.-Miscellaneous information concerning them.-Mass of Uranus.-Tables of Uranus. N March 13, 1781, whilst engaged in examining some small stars in the vicinity of H Geminorum, Sir W. Herschel noticed one which specially attracted his attention: and desirous of knowing more about it, he applied to his telescope higher magnifying powers, which (in contrast to their effect on fixed stars) he found increased the apparent diameter of the object under view considerably; this circumstance clearly proving its non-stellar character. Careful observations of position shewing it to be in motion at the rate of 21" per hour, Herschel conjectured it to be a comet, and made an announcement to that effect to the Royal Society on April 26. Four days after its first discovery it was observed by Maskelyne, then Astronomer Royal, who seems to have suspected at the time its planetary character, and in the course of the following 2 or 3 months it received the attention of all the leading observers of Europe. So soon as sufficient observations were accumulated, attempts were made by various calculators to assign parabolic elements for the orbit of the new body; though but little success attended their efforts. It was found that although a parabola might be obtained which would represent with tolerable accuracy a limited number a Phil. Trans., vol. lxxi. p. 492. 1781. of observations, yet a larger range always revealed discrepancies which defied all endeavours to reconcile them with positions assigned on any parabolic hypothesis. The final determination was only arrived at step by step, and to Lexell must be ascribed the credit of first announcing, with any amount of authority, that the stranger revolved round the Sun in a nearly circular orbit, and that it was a planet and not a comet; though priority for this honour has been contested on behalf of Laplace. The question of a name for the new planet was the next subject of debate. Herschel himself, in compliment to his sovereign and patron King George III, proposed that it should be called the Georgium Sidus; Lalande or, as some say, Laplace suggested the personal name of Herschel; but neither of these gave satisfaction to the Continental astronomers, who all declared for a mythological name of some kind. Prosperin considered Neptune appropriate, on the ground that Saturn would then be found between his two sons Jupiter and Neptune. Lichtenberg advanced the claims of Astræa, the goddess of justice, who fled to the confines of the system. Poinsinet thought that as Saturn and Jupiter, the fathers of the gods, were commemorated astronomically, it would be unpolite longer to exclude the mother, Cybele. Ultimately, however, Bode's Uranus prevailed over all others. A symbol was manufactured out of the initial of Herschel's surname, though in Germany, at the instigation of Köhler, one not differing much from that of Mars was adopted. It soon became a matter of inquiry whether the new planet had ever been seen before, and here may be brought in a note of Arago's :-" If Herschel had directed his telescope to the constellation Gemini 11 days earlier (that is, on March 2 instead of March 13), the proper motion of Uranus would have escaped his observation, for on the 2nd the planet was in one of its stationary points. It will be seen by this remark on what may depend the greatest discoveries in astronomy b." A careful inspection of the b On this remark of Arago's Holden says: "This is an entire misconception, since the new planet was detected by its physical appearance and not by its motion. Does any one suppose that' a new and singular star' like this would have been once viewed and then forgotten?" (Life of W. Herschel, p. 49.) |