MOVEMENT OF THE EARTH 23 



This gives the direction of the rising sun, which we call 

 east, and of the setting, which we call west. A line which 

 runs at right angles to the one joining east and west, i.e. 

 one running parallel to the axis of the earth, is said to run 

 north and south. Thus the points of the compass, as well 

 as day and night, are determined for us by the earth's rota- 

 tion. The north star, which is so important to the sailor 

 in determining his direction, is simply a star which is 

 almost in line with the axis of the earth. It is the rota- 

 tion of the earth which gives us also our means of measur- 

 ing time. 



As was seen in the previous experiment, the direction of 

 the axis of a revolving globe has much to do with the light 

 which different parts of it will receive from a luminous 

 object. The hemisphere which is inclined toward the 

 luminous object will have a larger part oi; its surface 

 illuminated and therefore each place on it will be longer 

 in the light during a revolution than when the hemisphere 

 is inclined in the opposite direction. 



As the axis of the earth is inclined to a line drawn from 

 the earth to the sun, the light the earth receives is similar 

 to that received by the globe in the last part of the experi- 

 ment. Thus the days and nights vary in length during 

 the year, because in summer the northern hemisphere is 

 inclined toward the sun and in winter away from it. 



10. The Movement of the Earth around the Sun. The earth 

 not only turns on its axis every day, but it travels around 

 the sun, continually changing its position in relation to 

 the stars. It moves with the tremendous average velocity 

 of about 19 miles a second. It is this revolution around 

 the sun which gives us our measure of time which we call 

 a year. It takes 365 days and a fraction to complete this 

 revolution, so we consider 365 days to be a year, and add a 

 day practically every fourth year to make up the fraction. 



