When considered in regard to size, constitution, development, and their likeness to each other, the planets are sometimes distinguished as the terrestrial planets and the major planets. This need occasion no confusion with the general division of them into major and minor planets, because, as has been said, when simply “the planets” are mentioned, these seven large planets are always the ones that are meant, the others being usually called asteroids, or planetoids. The terrestrial planets are Mercury, Venus, Earth, and Mars. As the name implies, they are so called because they are in some respects similar to the earth. The major planets are Jupiter, Saturn, Uranus, and Neptune. They are all larger than the terrestrial planets, and, in addition, have some other characteristics in common which the planets of the other group do not have. The two classes represent different stages of evolution. The four planets forming the terrestrial group are sometimes called the inner planets, and the four major planets are then known as the outer planets. The point of division in mind then is the space between Mars and Jupiter. This is so vast in comparison with the spaces between the other planets from the sun out to Mars that it becomes a convenient dividing-line, particularly as the groups divided by it are in some respects essentially different from each other. Of the four planets which have an especial interest to us because of their being the ones most easily seen, two are terrestrial, or inner, planets, Mars and Venus, and two are major, or outer, planets, Jupiter and Saturn. The differences between the two classes are solely matters of constitution and situation, and have nothing to do with their appearance to us. Venus, the brightest of them all, belongs to one group; Jupiter, the second in brilliancy, belongs to the other. That there is at least one other planet beyond the present boundary of our system (which is the orbit of Neptune) seems to be quite probable. Some astronomers think there may be several others. There are certain perturbations, or irregularities, in the movements of Neptune which the influence of Uranus does not account for, and they seem to indicate that there is some disturbing body even beyond the orbit of that farthest known planet.