THE EARTH AS A PART OF THE SOLAR SYSTEM 517 



earth and sun and bodies of the solar system seem large 

 as compared with the distances on the earth, yet they are 

 very small when compared with the distances between the 

 earth and the stars. 



Motion of earth. The earth is traveling around the sun 

 at a tremendous speed in its annual orbit. During the yearly 

 journey we are traveling through space at the rate of eighteen 

 miles every second. Every twenty-five minutes we travel 

 a distance equal to the circumference of the earth, every 

 four hours a distance equal to that to the moon, and every 

 two months a distance equal to that to the sun. 



At the same time we are moving in another direction, 

 through the daily rotation of the earth. People at the 

 equator travel at the rate of a thousand miles an hour. We 

 do not realize that we are moving in these two ways because 

 all the objects on the earth are moving with us and we are 

 all held on the earth by the action of gravity. 



Sun. The sun is the center of the solar system, around 

 which the planets revolve, and it is by far the largest body in 

 the system. Its diameter is almost nine hundred thousand 

 miles, about one hundred times that of the earth. If the 

 size of the earth were represented by a marble a half -inch 

 in diameter, it would take a sphere four feet through to 

 represent the sun. Or if a baseball were taken to represent 

 the earth, it would take a sphere about twenty-five feet in 

 diameter to represent the sun. If we could imagine the 

 earth to be placed at the center of the sun with the moon 

 at its average distance away, the sun would extend out to 

 the moon and almost as far beyond it. 



The distance from the earth to the sun varies from month 

 to month. Strange as it may seem, we are nearer the sun 

 in winter than in summer. The average distance is ninety- 

 three million miles. 



Sun's heat. The sun is the source of the heat and light 

 that make life possible on the earth. Without the sun's 



