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“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 y Virginis. The date given by Ptolemy, who reports this observation in his Almagest [lib. xi.), answers to B.C. 228, March 14."
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'-80 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the places of Saturn, the Tables of A. Bouvard, published in 1821, were long used, but new Tables by Le Verrier have superseded them.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.
Circumstances connected with its discorery by Sir W. Herschel.-Names proposed
for it.-Early obserrations.—Period, &c.-Physical appearance.- Belts risible in large telescopes.- Position of its aris.- Attended by 4 Satellites.—Table of them.— Miscellaneous information concerning them.-Mass of Uranus.—Tables of Uranus.
ON 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 26a. 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
• On this remark of Arago's Holden motion. Does any one suppose that . a says:-“This is an entire misconception, new and singular star' like this would since the new planet was detected by its have been once viewed and then for. physical appearance and not by its gotten?” (Life of W. Herschel, p. 49.)
labours of former astronomers shewed that Uranus had been observed and recorded as a fixed star on 20 previous occasions : namely, by Flamsteedin 1690, on Dec. 13; in 1712, on March 22; in 1715, on Feb. 21, 22, 27, and April 18 (all 0.s.); by Bradley in 1748, on Oct. 21; in 1750, on Sept. 13, and in 1753, on Dec. 3; by Mayer in 1756, on Sept. 25; and by Le Monnier no less than 12 times—in 1750, on Oct. 14 and Dec. 3; in 1764, on Jan. 15; in 1768, on Dec. 27 and 30; in 1769, on Jan. 15, 16, 20, 21, 22 and 23; and in 1771, on Dec. 18. Had Le Monnier been a man of order and method it can scarcely be doubted that he would have anticipated Sir W. Herschel. Arago recollected to have been shewn by Bouvard one of Le Monnier's observations of the planet written on a paper bag, which originally contained hair-powder purchased at a perfumer's !
It will readily be understood that these early observations have been of great service to computers, inasmuch as they have been enabled to determine the elements of the planet's orbit with greater accuracy than they could otherwise have done simply by the aid of modern observations.
Uranus revolves round the Sun in 30,686-7 days, or rather more than 84 of our years, at a mean distance of 1,781,944,000 miles. The eccentricity of its orbit, which amounts to o‘04667 (rather less than that of Jupiter), may cause this to extend to 1,865,107,000 miles, or to fall to 1,698,781,000 miles. The apparent diameter of Uranus varies but slightly, as seen from the Earth; and its mean value is about 3:4". (Seeliger, 3.82" : Millosevich, 3.96.") The real diameter is about 31,000 miles. Sir W. Herschel saw the planet's outline strongly elliptical in 1792 and 1794, after having noted it to be round in 1782. Mädler at Dorpat in 1842 and 1843 measured the ellipticity to be ito or ii. Arago however pointed out that a polar compression may exist but not always be visible, because a spheroid, when viewed in the direction of its axis, will necessarily present a truly
Le Verrier, in his investigation of the theory of Uranus, rejected Flamsteed's observation of Feb. 22, 1715, and
adopted another dated April 18, 1715. (Grant, Hist. Phys. A st., p. 165.)
circular outline, and this seems both the proper and a sufficient way of reconciling discordances on the subject which have been noted. Buffham on Jan. 25, 1870, thought that the ellipticity was “obvious d.” Safarik after many observations between 1877 and 1883 considered the ellipticity to be “striking” and therefore in fact “considerable o.” Prof. C. A. Young in 1883 measured the planet on several occasions and obtained an ellipticity of 4. He considers that there can be no “reasonable doubt that the planet's disc is considerably flattened, its equator lying sensibly in the same plane with the satellite-orbits?.” Schiaparelli too in 1884 obtained as he thought clear proofs of an ellipticity of is. But the measures of Seeliger at Munich and Millosevich at Rome in 1883 negative the idea.
It has been calculated that the amount of light received by Uranus from the Sun is equal to about the quantity which would be afforded by 300 Full Moons. The inhabitants of Uranus can see Saturn, and perhaps Jupiter, but none of the planets included within the orbit of the latter.
The physical appearance of Uranus may be disposed of in a few words. Its disc is commonly considered to be uniformly bright, bluish in tinge and without spots or belts. Yet both Lassell and Buffham have fancied they have seen traces of an equatorial belt and of inequalities of brilliancy on the planet's surface. Writing in 1883 Prof. C. A. Young says :" Whenever the seeing was good 2 belts were always faintly but unmistakeably visible on each side of the equator much like the belts of Saturn. On one or two occasions other belts were suspected near the poles 8." Schiaparelli too with an 8-inch refractor has detected faint spots and differences of colour on the disc of Uranus. The period of axial rotation is unknown, but analogy h leads us to suppose that it does not differ materially from that of Jupiter or Saturn. Buff ham has ventured on a conjecture that some indications of
d Month. Not., vol. xxxiii. p. 164. Jan. 1872.
• Ast. Nach., vol. cv. No. 2505. Ap. 14, 1883. Observatory, vol. vi. p. 183. June, 1883.
'Observatory, vol. vi. p. 331. Nov. 1883.
6 Observatory, vol. vi. p. 331. Nov. 1883.
h See p. 68, ante.