PRELIMINARY OUTLINE 3 



sun. Of the eight planets, four, Jupiter, Saturn, Uranus, and 

 Neptune, are much larger than the earth, while three, Mercury, 

 Venus, and Mars, 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 planets is more than fifty times that of the 

 earth, and Jupiter, the largest, has more than three hundred times 

 the mass of the earth. The earth's position is in no sense distin- 

 guished. It is neither the outer nor the inner, nor even the middle 

 planet. Even in the inner group of four 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 its period of revolution is 365 J^ days, a period 

 longer than that of any of the inner planets, and shorter than that 

 of any of the outer ones. Its period of rotation is not very different 

 from that of Mars, but is much shorter than that of the larger 

 planets whose periods of rotation are known. The plane of the 

 earth's revolution approximates the planes of revolution of all the 

 other planets. The orbit of the earth, like the orbits of the other 

 planets, is an ellipse, and its eccentricity is about B ^. The in- 

 clination of the earth's axis, nearly 23}^ (23 27'), is less than that 

 of some planets, and more than that of others. 



Its satellite. The earth is peculiar in having one unusually 

 large satellite, which has a mass ^\ of its own. The larger plan- 

 ets 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 no other is ^ T of the size of 

 the planet about which it revolves. There is little doubt that the 

 moon has played an important part in the history of the earth. It 

 is the chief cause of oceanic tides, and the tides are efficient in the 

 wear of the shores of the oceans and in the distribution of marine 

 sediments. Tides have probably been of importance in the earth's 

 history ever since the ocean came into existence. 



Dependence on the sun. By far the most important external 

 relation of the earth is its dependence on the sun, of which it is 

 a mere satellite. Its mass is less than s^V that of the sun, 

 upon which it depends for nearly all its heat and light, and, through 

 these, for nearly all of the activities that have given character to 



