PRELIMINARY OUTLINE. 3 



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 miUion miles, and this fixes its revolution 

 at 365 i 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 

 th^ir. 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 -^\. This eccen- 

 tricity 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 23 i°, holds an intermediate position, some of the planets 

 having axes more inclined, as Saturn, 26|°, 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. 

 Its satellite. — The earth is peculiar in having one unusually large 

 satellite, which has a mass -gV of its own. The great planets have 

 several satellites whose combined mass exceeds that of the moon, 

 and perhaps in some few cases the individual satellites may be larger 

 than the moon, but they do not sustain so large a ratio to their planets, 



