PLANETS. Those solid spherical bodies which belong to our solar system, and revolve in elliptic orbits round the sun. The name planet is of considerable antiquity, and was applied to these dependents of the sun to dis tinguish them from the other luminous bodies which stud the sky and present to the naked' eye no indication of change of relative place (see STARS), while the planets seem to wander about among them. The planets at present known are, in the order of their distance from the sun, Mer cury, Venus. the earth, Mars, the planetoids, Jupiter, Saturn, Uranus, and Neptime. Six of these, Mercury, Venus, the earth (which was not, however, then reckoned a planet). Mars, Ju piter, 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 Leverrier and Adams, was discovered by Galle in 184G. The planetoids, of which over 500 are now known, have all been discovered since Janu ary 1, 1801. Six of the planets, the earth, Mars, Jupiter, Saturn. Uranus, and Neptune. are at tended by one or more satellites: Uranus (gen erally), Neptune. almost all the planetoids, and all the satellites except our 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 com parative proximity of the planets may be proved by examining them through a telescope of moder ate power, when they appear as round luminous disks, while the fixed stars show as mere points of light even in the most powerful telescopes. The planets as observed from the earth move sometimes from west to east, sometimes from east to west, and at times remain stationary at the point where the apparent motion changes from the one direction to the other. This irregularity in their movements was very puzzling to the ancient astronomers, who invented various hy potheses to account for it. (See Prot.EmAtc SYSTEM; EPICYCLE.) The system of Copernicus, by assuming the sun, and not the earth, as the centre of the system, explained with admirable simplicity what seemed before a maze of con fusion, by showing that the planetary apparent motions are merely a result of real motions be longing to the planets in general and to the earth carrying the observer.
The planetary orbits differ considerably in their degrees of eccentricity, the planetoids, 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 he found within a small strip of the heavens extending on both sides of the ecliptic (q.v.) : while the others have a far
wider range. Pallas, one of the planetoids, has an angular elevation of orbit equal to 34° 41' above the ecliptic. According to Kepler's laws (see ASTRONOMY) . the nearer a planet is to the sun the shorter is the time of 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 farther from the sun, is much smaller than either the earth or Venus; and the planetoids. which are still farther off, are the smallest of all. Jupiter, which is next in order, is by far the largest, being about one and one-half times as large as all the others together; and as we proceed farther out ward the planets become smaller and smaller.
With reference to their distance from the sun, as with that of the earth, the planets are divided into superior and inferior: Mercury and are eonsequently the only inferior planets, all the others being superior. The in ferior planets must always be 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, ex cept when, at the latter, a transit (see TRANSIT OF VENUS) takes place. The superior planets are likewise invisible at conjunction, but when in opposition they are seen with the greatest dis tinctness, being then due south at midnight.
A trans-Neptunian planet, owing to disturb ances produced on the outer planets. is suspected to revolve outside the orbit of Neptune. Even before the discovery of Neptune it was suggested that the Uranian disturbances were not due to a single exterior planet alone. The matter was in vestigated by Todd (q.v.) in 1877. Ile computed the probable elements and searched the heavens on "30 clear, moonless nights, between November 3, 1877, and March 5, 1S78." Every effort to find the hypothetical planet has, however, so far proved fruitless. Consult Todd's account in the .t/act-icon Journal of Science (September, 1880).
In astronomical tables, almanacs, etc., the planets are for convenience denoted by symbols instead of their names, as follows: Mercury, ; Venus, r‘ ; the earth, 6 ; Mars, ; the plane toids, in the order of their discovery, 0, 0, C), etc.; Jupiter, if ; Saturn, ; Uranus, lti ; Neptune, I t ; the sun, 0 ; the puma], Q .
For a table of the periods, distances, size, density, etc., of the planets, see SOLAR SYSTEM. Sec also the several planets under their respective names. For the hypothetical planet Vulcan, see VULCAN; for the elements necessary to determine the orbit of a planet, see ORBIT. For theories regarding the origin of the system, see COSMOG ONY; NERUL.E.