Planet

Planet ("a wanderer") is the name given to each of the bodies in the solar system, these bodies all moving in elliptical orbits with the sun in One focus. They received their name, on account of their motion, to distinguish them from the fixed stars. Although their movements can be so simply explained, they appear to be extremely irregular, and were a source of great difficulty to the ancient astronomers, who invented many theories to explain them. The planets, in the order of their distances from the sun, are Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, the planetoids occurring between Mars and Jupiter. Mercury and Venus are known as inferior planets, the others - those which are farther from the sun than the Earth is - being called the superior planets. When a planet and the sun are on the same side of the Earth, the three being in the same straight line, the planet is said to be in conjunction - inferior or superior, according as the planet or sun is nearer the Earth. When planet and sun are on opposite sides of the Earth, the planet is in opposition. The time between two successive conjunctions or oppositions is called the synodic period of the planet, and this is useful in calculating the relative distance of Earth and planet from the sun, and in determining the planet's periodic time (q.v.). The orbits of all the planets (except the planetoid Pallas) are very nearly in the plane of the ecliptic, the intersections of the plane of the planet's orbit with the ecliptic being called nodes. When an inferior planet is near one of these nodes at inferior conjunction, it looks like a dark spot on the sun's surface. This and other appearances show that the inferior planets are not self-luminous, but receive their light from the sun. Viewed through a powerful telescope, they are seen to exhibit phases similar to those of the moon, being sometimes crescent-shaped (near inferior conjunction), sometimes halved, sometimes gibbous, and, at superior conjunction, exhibiting a full bright disc. The superior planets appear practically with a full bright disc, Mars alone being somewhat gibbous at part of his course. The planets always appear, when seen, to shine with a steady light' thus differing from the twinkling stars; this gives a means of roughly distinguishing them in the sky. Several of the planets are accompanied by satellites or moons, these bodies revolving round the primary in the same way as the moon circles around the earth. Since the orbits of the planets are in most cases not far removed from the plane of the ecliptic, they are to be seen in a comparatively narrow belt of the heavens. The motions of the planets are in accordance with Kepler's Laws (q.v.); hence, the interior planets take far less time to revolve round the sun than the exterior ones - the periodic time increasing the farther away the planet is from the sun. The motion of a planet is not strictly uniform; it moves faster when at perihelion than at aphelion, and those planets whose orbits are most eccentric (i.e. differ most from circles) show the greatest difference between their quickest and slowest rates.