THE EARTH IN THE SOLAR SYSTEM 3 



from the sun is about 92.9 million miles, and its period of revolu- 

 tion, 365^ days, is longer than that of any other one of the inner 

 planets, and shorter than that of any one of the outer group. The 

 orbit of the earth, like the orbits of the other planets, is an ellipse. 

 The inclination of the earth's axis, nearly 23,^, is less than that of 

 the axis of some planets, and more than that of others. 



The earth is peculiar in having one unusually large satellite, 

 which has a mass l /Si of its own. The larger planets have several 

 satellites whose combined mass exceeds that of the moon, and a few 

 individual satellites may be larger than the moon; but no other is I /Si 

 of the size of the planet about which it revolves. The moon has 

 played an important part in the history of the earth, for it is the 

 chief cause of tides, and tides are efficient in the wear of the shores 

 of the oceans and in the distribution of marine sediments. Tides 

 probably have been important ever since the ocean came into 

 existence. 



The most important external relation of the earth is its depend- 

 ence on the sun. Its mass is less than Vjooooo 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 determined its history. 

 A little heat and light are received from other bodies, and an im- 

 portant source of energy is found in the interior of the earth itself; 

 yet all of these are so far subordinate to the great flood of energy 

 which comes from the sun, that they are quite insignificant. The 

 dependence of the earth on the sun has been intimate throughout 

 its past history, and its future is locked up with the destiny of that 

 great luminary. 



Meteorites. There are multitudes of small bodies, called mete- 

 orites, passing through space in varying directions and with varying 

 velocities. Great numbers of these reach the earth daily as "shoot- 

 ing stars." Some meteorites revolve about the sun like planets, but 

 some of them do not belong to the sun's family. Some consist 

 almost wholly of metal, chiefly iron alloyed with a little nickel; 

 some consist of metal and rock intimately mixed; and some consist 

 wholly of rock. Since meteorites are thought to throw some light 

 on the early history of the earth, they are of interest to the geologist. 

 The amount of material added to the earth by the infall of meteorites 

 is now slight compared with the whole body of the earth; but their 

 contributions in the past may have been greater. 



