=The earth as a planet.=—Though supremely important to us, the earth is but one of the minor planets attendant upon the sun, and is in no very special way distinguished as a planetary body. Of the eight planets, four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while three, Mars, Venus, and Mercury, are smaller. There are a host of asteroids, but all together they do not equal the mass of the smallest planet. The average mass of the eight planets is more than fifty times that of the earth, while the largest, Jupiter, is more than three hundred times as massive as the earth. The earth’s position in the group is in no sense distinguished. It is neither the outer nor the inner, nor even the middle planet. Even in the minor group to which it belongs, it is neither the outermost nor the innermost member, though in this group it is the largest. Its average distance from the sun is about 92.9 million miles, and this fixes its revolution at 365¼ days, for its period of revolution is directly dependent on its distance from the sun, and is necessarily longer than the revolutions of the inner planets and shorter than those of the outer planets. Its rotation in twenty-four hours is not far different from that of its neighbor Mars, but is much slower than the more distant and larger planets, Jupiter and Saturn, which rotate in about ten hours. Comparison cannot be made with the innermost and outermost planets, because their rotations are not yet satisfactorily determined. The plane of the earth’s revolution lies near the common plane of the whole system, but this is not peculiar, as all of the planets revolve in nearly the same plane. Only a few of the small asteroids depart notably from this common plane. This has an important bearing on theories of the origin of the system, since this close coincidence of the planes of the orbits is not consistent with any haphazard aggregation of the material. Of similar importance is the fact that all of the planets revolve in the same direction and in ellipses that do not depart widely from circles. The eccentricity of the earth’s orbit is only about ¹⁄₆₀. This eccentricity varies somewhat, due to the disturbing influences of the other planets, and this variation has been regarded by some geologists as an influential cause of climatic changes, but its adequacy to produce great effects has been doubted by others. The inclination of the earth’s axis, now about 23½°, holds an intermediate position, some of the planets having axes more inclined, as Saturn, 26⅚°, and others less inclined, as Jupiter, 3°. The inclination of the axis is subject to trivial variations at present, and in the long periods of the past has possibly changed more notably. This possible change has also been thought to be a cause of climatic variation, but its efficiency has not been demonstrated.