PLANET

(Yunanca köken — orijinale bakınız.) The name given by the ancient Greeks to a few bright and conspicuous stars which are constantly changing their apparent situations in the celestial sphere, and thus appear to wander among the constellations. The modern discovery of satellites and periodic comets has rendered it nece ary to adopt a more precise definition, in order to individualize the cla of objects to which the term is applied; and accordingly modern astronomers understand by the term planet a body which, in a telescope of sufficient power, exhibits a round and well defined disk, and which revolves about the sun in an elliptic orbit, not differing very greatly from a circle. The number of planets, including the earth, at present known to belong to the solar system, is eleven. Five of them — namely. Mercury, Venus, Mars, Jupiter, and Saturn — have been known from the earliest ages of astronomy; Uranus was discovered by Sir William Herschel in 1781; Ceres by Piazzi, at Pale'rmo, on the first day of the present century; and the discovery was soon followed by that of Juno, Pallas, and Vesta. The last four are never visible to the naked eye; under favourable circumstances Uranus may be discerned without a telescope; and Mercury, though it appears as a large star, is seldom to be seen (in our climates at least), in consequence of its proximity to the sun. Of the apparent Motions of the Planets — On watching the motions of any of the conspicuous planets for a few days or weeks, their change of position among the fixed stars becomes sufficiently apparent, even without the aid of an instrument to measure their relative distances. Their paths deviate little from that followed by the sun in his apparent annual revolution through the heavens; but their motions are exceedingly irregular. Sometimes they advance rapidly, then relax in their speed, come to a stop, and then move for a while in an opposite direction. Through the most considerable part of their orbits they move like the sun from west to east, in opposition to the apparent diurnal motion; their course is then said to be direct. When it lies in the opposite direction, their motion is retrograde; and between each change from the one direction to the other, they remain for a few days stationary. On the whole, however,the direct motion prevails, and the planets make the entire circuit of the heavens. These phenomena, which are called the s<a^?ows and retrogradations of the planets, may be exhibited in the following manner: — Let E C represent the ecliptic developed on a plane surface: the path of a planet, found by laying down its observed positions with reference to the ecliptic from day to day, will present the appearance of the zigzag line P Q R S. From P to Q the motion is direct, but becomes slower as the planet approaches to Q. At Q it is stationary; from Q to R retrograde; at R again stationary; from R to S direct, and so on. Such is the general character of the apparent motion; but the arcs and times of retrogradation differ greatly in respect of the different planets. 30 2 [s. 944]