PLANET, a heavenly body which, to old-world observers, seemed to wander about aimlessly in the sky, thus marked ly contrasting with the orderly move ments of the fixed stars. Subsequently it was discovered that the seemingly er ratic bodies were as regular in their movements as the others, revolving, like the earth, around the sun. Planets are primary or secondary, the former re volving around the sun, the latter around the primaries. The primary planets known to the ancients were five: Mer cury, Venus, Mars, Jupiter, and Saturn. Omitting asteroids, comets, and meteoric rings, eight are now known. Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Twenty-one sec ondary planets are known, the Moon, two satellites of Mars, five of Jupiter, eight of Saturn, four of Uranus, and one of Neptune. The planets Mercury and Venus, being nearer than the Earth to the Sun, are called inferior planets; the others, being more distant, are termed superior. Another classification
is sometimes adopted, that into intra and extra-asteroidal planets; that is, those nearer and those more remote from the sun than the asteroids. Under the first are included, Mercury, Venus, the Earth, and Mars, all of which are comparatively small, while the others, Jupiter, Saturn, Uranus, and Neptune, are the giants of the system. For in stance, the Earth is 7,918 miles in diame ter, and Mars 4,200, but Jupiter is 85, 000. The intra-asteroidal planets com plete the annual revolution in short periods, the Earth, for example, hi 365.26 days, while Neptune takes to do so 60,127 days, or about 165 years. The minor planets, planetoids, or asteroids are between Mars and Jupiter.