24 OUR OWN WORLD 



Of course it makes no difference to the eye whether a 

 light is brought toward the observer or the observer goes 

 toward the light. We are turned into and out of the 

 sunlight by the rotation of the earth. We speak of the 

 sun as rising high in the sky, but what really happens is 

 that we are turned so that the center of the earth, our 

 heads, and the sun come nearer and nearer toward a straight 

 line. 



When we say down we mean toward the center of the 

 earth, and when we say up we mean in the opposite direc- 

 tion. These are the only two directions that we could be 

 easily sure of, if it were not for the rotation of the earth. 

 This rotation 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. 



The rotation of ; the earth gives us also our means of measur- 

 ing time. 



Days and Nights of Varying Length. Experiment 4. (A) In 

 a darkened room place a globe a short distance from a small but strong 

 light. Rotate the globe with its axis at right angles to the line 

 which joins the centers of the globe and light. (Figure 5, A.) 

 How much of the globe is illuminated by the light ? Is the same 

 part of the globe illuminated all the time ? Does any place receive 

 light for a longer time during a rotation than any other place? 

 Remove the globe to the opposite side of the light without chang- 

 ing the direction of its axis. When rotated, is there any change 

 in the globe's illumination? 



