Planet

(plan'ět.) [FromGr. planētēs-pla nao, I wander.] A celestial body revolving round the sun as a centre, and continually changing its position with respect to the fixed stars; whence the name. The planets are distinguished into primary and secondary. The primary planets are those which revolve round the sun as a centre; and the secondary, more usually called satellites or moons, those which revolve about a primary P. as a centre, and constantly attend it round the sun. The primary planets are Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the smaller planets, or asteroids, which have been discovered between the orbits of Mars and Jupiter. Saturn, Jupiter, Mars, and Uranus, being without the earth's orbit, are sometimes called the superior planets; Venus and Mercury,being within the earth's orbit, the inferior planets. These bodies are opaque, and receive their light from the sun; and they are distinguished from the fixed stars, not only by their motion or revolution, but by their not twinkling.-Motion of the planets. The primary planets all revolve about the sun, and are accelerated in their motions as they approach to him, but retarded as they recede from him, in such a manner that a ray, drawn from any one of them to the sun, always describes equal spaces or areas in equal times: from which it follows, that the power which bends their course into a curve line must be directed to the sun. This power is no other than that of gravitation, which increases according as the square of a P.'s distance from the sun decreases, and vice vers a. The universality of this law still further appears by comparing the motions of the different planets; for the power which acts on a planet near the sun is manifestly greater than that which acts on aP. more remote, both because it moves with greater velocity and because it moves in a le orbit, which has more curvature, and separates further from its tangent in arcs of the same length than in a greater orbit. To convey some idea of the space occupied by the planetary system if, indeed, the idea of space so vast be capable of comprehension sufficiently clear to have its due effect on the mind-it must be observed that the sun, which occupies so small a portion of that space, is 1,400,000 times larger than the earth. Sometimes the motion of the planets is the same as the apparent motion of the sun, from east to west; their course is then said to be direct; when they move in the opposite direction, it is retrograde; between each change, they remain for a few days stationary. The ancient astronomers found it extremely difficult to explain these phenomena, as they supposed the planets to revolve about the earth; but they are easily understood, on the supposition that they revolve about the sun; in which case they must nece arily present these appearances to a spectator on the eartir. Mercury and Venus, and Mars to a certain extent, exhibit phases, like the moon, and for a similar reason; and all the planets would do so, but for their distance from the sun being very great, compared with the distance of the (-ta'bl.) earth from (Land that Surv.) luminary. An instrument Plane Table, by means of which a plan is draught ed on the spot, without protraction or measurement of angles. [s. 714]