The principal bodies of the solar system are the planets. These constitute eight large bodies named in their order from the sun, beginning with the nearest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The last four planets, Jupiter, Saturn, Uranus, and Neptune are much larger than the others, and are therefore known as the _major planets_ in order to distinguish them from Mercury, Venus, Earth, and Mars, which are called the _minor planets_. You can remember the order in which the last three planets come by their initial letter, S-aturn, U-ranus, and N-eptune, spelling the word SUN, around which they all revolve. It may be interesting to state here that the ancients knew of seven only of these planets. Since, as they asserted, there were only seven days in the week, and seven openings into the head; i. e., two for the eyes, two for the nostrils, two for the ears, and one for the mouth, it was natural that there should be but seven planets. During later years, however, an eighth planet was discovered and named Neptune. It would be interesting to explain to you how the position of this planet was reasoned out by mathematical calculations, that is, in other words, how, as a result of such calculations, an astronomer was told that if he would point his telescope to a certain part of the heavens he would discover a new planet. He did this and located the planet Neptune. However interesting this story may be it belongs properly to astronomy, and will be described in full in the Wonder Book of Astronomy. In the opinion of some astronomers it is quite probable that a ninth planet will be found far beyond the orbit of Neptune. There may also be some additional planets discovered between Mercury and the Sun. Besides the eight known planets there exist, somewhere between the orbits of Mars and Jupiter, many smaller planets called _asteroids_, or _minor planets_. A long time ago it was pointed out by Bode that a curious relation exists between the distances of the planets from the sun. This relation or law is generally known, after the name of the astronomer who first called attention to it, as _Bode's Law_. No reason has been discovered for this arrangement of the planets, so that Bode's Law may be regarded as empirical. It may, however, be mentioned here that the distances of all the planets from the sun agrees with the law very closely, with the single exception of Neptune, which is quite at variance with the law.