Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/885

] But it was found to be moving in a nearly circular path, and was presently recognised as one of the principal planets of the solar system. Herschel gave it the name of Georgium Sidus, and foreign astronomers called it Herschel ; but the name Uranus, suggested by Bode, of Berlin, is now ranian universally adopted. In 1787 Herschel discovered that it tellites. vas attended by two satellites, and he subsequently sup posed that he had discovered four others ; but there is great reason to believe that he had mistaken small stars near the planet for satellites. Two other satellites inferior to the rest have been discovered by Mr V. Lassell, com pleting the series of four satellites whose elements are given at p. 783. Recently Prof. Holden of Washington, U.S., after careful study of the motions of all four satellites, has found reason for believing that one of those discovered by Sir V. Herschel (subsequently to satellites 3 and 4) is identical with satellite 2, while another seems not impro bably identical with the innermost satellite. The satellites of Uranus are distinguished from all others in the solar system by the great inclination of their orbits to the ecliptic, amounting to nearly 80, and by their retrograde motion. Hind gives 79 26 as the inclination of the orbits of Titania and Oberon (3 and 4), and for the longitudes of their rising nodes 165 25 and 1G5 28 respectively. Little of interest has rewarded the telescopic study of Uranus ; nor has the spectroscope given any trustworthy evidence, though Secchi, Huggins, and Vogel recognise peculiarities distinguishing the spectrum of Uranus from that due to reflected sun-light.

The discovery of the planet Neptune ranks amongst the most brilliant of the scientific feats of the present century. For many years the orbit of Uranus had been the occasion of great embarrassment to astronomers, from the impossibility of adequately reconciling the ancient and modern observations by any one set of elements, and from the rapid increase of the error from year to year. Bouvard early suggested that some planet exterior to Uranus caused these apparent irregularities. Amongst the astronomers who entered seriously upon the task of determining the position of such a disturbing planet two arrived at a suc cessful solution of the problem, namely, Mr Adams and M. Leverrier. Adams had, ever since the year 1841, determined on attempting the solution of the problem relating to the unknown disturbances of Uranus, and in 1843 he began his investigations. In September 1845 he communicated to Professor Challis the values of the elements of the orbit of the supposed disturbing planet, and in the following month he communicated to the astronomer royal the same results slightly corrected. These communications did not lead, however, to any steps being taken to secure by observation the discovery of the planet till the summer of 1846, after the publication of Leverrier s second memoir, in which the same position, within one degree, was assigned to the disturbing planet as that given in Adams paper. Leverrier undertook the task of revising the theory of Uranus at the instance of M. Arago. His first memoir on the subject was read before the French Academy in Novem ber 1845, and his second memoir in June 1846. A third memoir, entitled &quot; Sur la planete qui produit les anomalies observees dans h mouvement d Uranus Determination de sa masse, de son orbite, et de sa position aduelle,&quot; was read on August 31, 1846. A fourth memoir, containing the re maining part of M. Leverrier s investigations, was read after the discovery of Neptune on October 5, 1846. The discovery of the disturbing planet followed almost immediately after the publication of the third memoir of Leverrier. He wrote to his friend Dr Galle, of Berlin, requesting him to search for the planet with the large refracting telescope of the Berlin Observatory, at the posi tion which he indicated to him. This letter reached Berlin on September 23, and on the same evening Galle observed all the stars in the neighbourhood of the place indicated, and compared their places with those given in Bremiker s Berlin Star-Map. This map had not yet reached England, and on this circumstance probably depended the priority of discovery at Berlin. Galle very quickly found a star of about the eighth magnitude, nearly in the place pointed out, which did not exist in the map. Little doubt was entertained at the time that this was the planet, &quot;and the observations of the next two days confirmed the discovery. After Leverrier had announced the results of his labours, agreeing, as we have seen, with Adams s results, the resources of the Cambridge Observatory were brought into use for the discovery of the planet, and a systematic search was begun by Professor Challis with the great Northum berland telescope. The sweeps of the portion of the heavens in which it was supposed the planet would be found were begun on July 29, and the planet was actually observed on August 4, but without recognition. After the discovery of the planet at Berlin, it was found that the planet had also been observed on August 12. If, therefore, Bre miker s map had been in the hands of Professor Challis, or if he had found leisure for the mapping of his observa tions from night to night, Neptune would have been infal libly detected within a very few days of the commencement of the search, and the whole glory of the discovery would have belonged to the English geometer. An ancient observation in I.alando s catalogue was discovered almost simultaneously by Mr Walker, of the Washington Observatory, and by Dr Peters at Altona. Two observations, the one in October 1845, and another in September 1846, were also detected by Mr Hind in Lament s Zones ; and these (especially the former) contri buted greatly to the construction of an accurate orbit. An excellent orbit was computed by the joint labours of the American astronomers Peirce and Walker.

Almost immediately after the discovery of Neptune, it was found to be attended by one satellite. This discovery was made by Mr Lassell in October 1846. The orbit of the satellite is inclined to the ecliptic at an angle of about 29. By observations of this satellite at Cambridge (in America), and at Poulkowa, two separate values of the mass, namely, 1 ^ and j 4 ] 88 have been deduced ; but the object is so difficult to observe, that a considerable time must elapse before any very accurate determination can be obtained. The satellite is supposed to travel in a retro grade direction, like the satellites of Uranus; but this has not yet been demonstrated.

—Comets.

The comets form a class of bodies belonging to the solar system, distinguished from the planets by their physical appearance and the great eccentricities of their orbits.

While the orbits of all the planets are confined within a Orbit* ol narrow zone, or to planes not greatly inclined to the c met. ecliptic, those of the comets are inclined in all possible angles. The figures of the orbits, instead of being nearly circular, like those of the planets, often have the appearance of being almost rectilinear. Hevelius seems to have been the first who discovered, by means of a geometrical construc tion, that the orbits might be represented by parabolas ; and Halley first calculated their elements on this hypothesis. After it was known, however, that certain comets return to the sun in the same orbits, it became necessary to adopt the opinion tha^, in conformity with the laws of Kepler, the cometary orbits are ellipses has Ing the sun in 