PLANETS (Gr. planetes, "a wanderer") are those heavenly bodies (including the earth) which belong to our solar system, and revolve in elliptic orbits round the sun. They are often denominated primary planets, to distinguish them from their moons or satellites, which are called secondary planets. The name planet is of considerable antiq uity, and was applied to these of the sun to distinguish them from the myriads of luminous bodies which stud the sky, and which present to the naked eye no indication of change of place (see S•its). The planets at present known are, in the order of their distance from the sun, Mercury, Venus, the earth, Mars, the planetoids (q.v.), Jupiter, Saturn, Uranus, and Neptune. Six of tMse, Mercury, Venus, the earth (which was •not, however, then reckoned a planet), Mars, Jupiter, and Saturn, were known to the ancients; Uranus was discovered by sir William Herschel (q.v.) in 1781; and Neptune, after having its position and elements determined theoretically by Lever rier and Adams, was discovered by M. Challis, and afterwards by Dr. Galle, in 1846. The planetoids, of which more than 180 are now known, have all been discovered during the present century. Five of the planets, the earth, Jupiter, Saturn, Uranus, and Nep tune, are attended by one or more satellites; Uranus (generally), Neptune, almost the whole of the planetoids, and all the satellites except the moon, are invisible to the naked eye. The visible planets can be at once distinguished from the fixed stars by their.clear steady light, while the latter have a sparkling or twinkling appearance. The compara tive proximity of the planets may be proved by examining them through a telescope of moderate power, when they appear as round luminous disks, while the fixed stars exhibit no increase of magnitude. The planets, as observed from the earth, move some times from w. to e., sometimes from e. to w.. and for some time remain stationary at the point where progression ends and retrogression commences. This irregularity in their movements was very puzzling to the ancient astronomers, who invented various hypoth eses to account for it. See PTOLEMAIC SYSTEM and EPICYCLE. The system of i.3oper nleitS, by assuming the sun, and not the earth, as the center of the system, explained with admirable simplicity what seemed before a maze of confusion.
The planetary orbits differ considerably in their degrees of eccentricity, the planet oids, Mars, and Mercury being most, and the larger planets least eccentric. No two planets move exactly in the same plane, though, as a general rule, the planes of the larger planets most nearly coincide with that of the ecliptic. The latter are consequently always to be found within a small strip of the heavens extending on both sides of the ecliptic; while the others have a far wider range, Pallas, one of them, having the angu lar elevation of its orbit no less than 42" above the ecliptic. According to Kepler's laws (q.v.), the nearer a planet is to the sun the shorter is the time of its revolution. The arrangement of the planets in the solar system bears no known relation to their relative size or weight, for though Mercury, Venus, and the earth follow the same order in size and distance from the sun, yet Mars, which is further from the sun, is much. less than either the earth or Venus, and the planetoids. which are still further off, are the least of all. Jupiter, which is next in order, is by far the largest, being about 1-.1 times as large as all the others together; and as we proceed further outwards. the phypets become smaller and smaller, Saturn being less than Jupiter, Uranus than Saturn, and .Neptune, t ban With reference to their distance from the sun, as compared with that of the earth, the planets are divided into superior and inferior; Mercury and Venus are consequently the only "inferior" planets, all the others being "superior." The inferior planets mast always he on the same side of the earth as the sun is, and can never be above the horizon of any place (not in a very high latitude) at midnight; they are always invisible at their superior and inferior conjunctions, except when, at the latter, a transit (see SUN) takes place. The superior planets are likewise invisible at conjunction, but when in oppoqi tion they are seen with the greatest distinctness, being then due south at midnight. The time which elapses from one conjunction to its corresponding conjunction is ealled.the synodic, period of a planet, and in the case of the inferior planets must always be greater than the true period of revolution.