Plan′ets. If one observes the sky night after night he finds that practically all the stars maintain their relative positions; but there are certain heavenly bodies, besides the sun and moon, which form a striking exception to this general rule. There were five of these bodies known to the ancients who called them planets, the Greek word for wanderers. To these five bodies had been given the names of Mercury, Venus, Mars, Jupiter and Saturn. Copernicus, by placing the sun at the center of the solar system, showed that Tellus, the earth, also belongs in this group. On March 13, 1781, Sir William Herschel discovered what he at first thought was a comet, but which within a year proved to be another planet, the one we now call Uranus, On September 23, 1846, an eighth planet, Neptune, was found by Galle at Berlin almost exactly at the point in the heavens where it had been predicted by Leverrier (q. v.) in France and by Adams in England, a discovery which is justly celebrated as the most brilliant achievement of modern astronomy. Besides these, nearly 500 smaller planets called asteroids were discovered during the 19th century. See Astronomy.
The following table, giving the distances and periods of the various planets, is taken from Young’s General Astronomy:
| Name | Distance from Sun | Siderial Period |
| Mercury | 0.387 | 88 days |
| Venus | 0.723 | 224.7 “ |
| Earth | 1.000 | 365¼ “ |
| Mars | 1.523 | 687 “ |
| Mean Asteroid | 2.650 | 3 to 8 years |
| Jupiter | 5.202 | 11.9 years |
| Saturn | 9.539 | 29.5 “ |
| Uranus | 19.183 | 84.0 “ |
| Neptune | 30.054 | 164.8 “ |
It will be observed that the distance from the sun is given in terms of the distance of the earth as a unit. The relative sizes of the planets and the sun are well-shown in the accompanying figure:
For the laws which describe the motion of the planets about the sun see Kepler and Newton.
Concerning the individual planets it may be noted that
1. Mercury, which is nearest the sun, has the least diameter, the least mass and the greatest density of all the planets. Its density may be remembered from the fact that it is almost exactly that of the metal mercury, being 12½ times that of water. The diameter of this planet is about 3,000 miles. No satellite has been discovered for Mercury.
2. Venus has a diameter of 7,700 miles and a density 0.86 that of Earth, so that in size, surface, gravity she is not very different from our own planet. No satellite is known.
3. Earth. See Earth.
4. Mars has a diameter of 4,200 miles, and a density which is 0.73 that of Earth, so that bodies at the surface of Mars weigh only 0.38 what they would at the surface of Earth. Two satellites of this planet were discovered by Professor Hall at Washington in August, 1877. These two moons, Deimos and Phobos, are exceedingly minute, being only 7 and 5 miles, respectively, in diameter. The surface of Mars is covered with interesting markings.
5. Asteroids. See Asteroids.
6. Jupiter, the largest of all planets, has a diameter of 86,500 miles; and, while its density is only one quarter that of Earth, its mass is 316 times as great, so that a body on the surface of Jupiter weighs more than twice as much as the same body at the surface of Earth. The surface of this planet exhibits some very characteristic markings, especially belts and spots, which lead to the opinion that Jupiter is a body of very high temperature compared with the other planets. He has five satellites, four discovered by Galileo and one by Barnard in 1892 at Lick Observatory.
7. Saturn, with its system of rings, is conceded to be one of the most superb objects in the heavens. Although the diameter of the planet is only 73,000 miles, the outer ring is no less than 168,000 miles across. Two rings had been known for a long while, but the third ring was discovered by Bond in 1850. Pierce and Maxwell have proved that these rings are made up of discrete particles and that the rings, therefore, are neither solid nor liquid. See Maxwell, James Clerk. Saturn has eight satellites discovered between 1665 and 1848.
8. Uranus, discovered by the older Herschel, has a diameter of 32,000 miles; but its density is only about one fifth that of Earth, so that, notwithstanding its enormous bulk, surface gravity there is only 0.90 that of Earth. It is accompanied by four satellites.
9. Neptune, the most distant member of the family has a diameter of 35,000 miles and a density one fifth that of Earth. At an average distance of 2,800 millions of miles from the sun, it is absolutely invisible at Earth, except by the aid of a telescope. It has one satellite, discovered almost immediately after the discovery of the planet itself. See Miss Clerke’s History of Astronomy and Chambers’ Descriptive and Practical Astronomy.