THE EARTH 



19 



DAY 



to be standing still while the objects along the track 

 whirl by. 



The succession of day and night, which is caused by 

 the rotation of the earth, has given man and many ani- 

 mals the habit of 

 working in the 

 light and resting 

 in the darkness. 

 Rotation is also 

 one of the factors 

 along with others 

 in producing tides, 

 influencing certain 

 belts of winds and 

 calms, affecting 

 the direction of 

 ocean currents, 

 and causing the 

 flattening of the 

 earth at the 

 poles. 



Directions. We say down when we mean toward 

 the center of the earth and up for the opposite direction. 

 In addition to these the rotation of the earth suggests 

 a natural system of directions by which the relative 

 positions of different places may be indicated. Toward 

 the North Pole on the surface of the earth is north; 

 toward the South Pole is south. The direction in which 

 the sun " rises" we call east; that in which it "sets "'is 

 west. However, the true east and west line, at any place, 

 is always at right angles to the north and south line. 



The Celestial Meridian. An imaginary line called 

 the celestial meridian is also an important factor in the 



FIG. 16. The Succession of Day and Night 

 Caused by the Rotation of the Earth. 



