212 



PHYSIOGRAPHY 



Inclination of Axis and its Effects 



The sun's rays illuminate one-half of the earth all the time. 

 The border of the illuminated half is called the circle of illumination 

 (Fig. 206). All places on one side of the circle of illumination have 



Fig. 206. Diagram to illustrate the fact that half of the earth is lighted by 

 the sun at any one time. The parallel lines at the right show the direc- 

 tion of the sun's rays. The part of the earth not shaded is lighted by the 

 sun. The other half is in darkness. The line between the illuminated 

 half and the half which is not illuminated is the circle of illumination. 



day, while all places on the other side have night. If the axis 

 about which the earth rotates were perpendicular to the plane in 

 which the earth revolves about the sun, the circle of illumination 

 would always pass through the poles. Under these conditions, 

 half of the equator and half of every parallel of latitude would be 

 illuminated all the time, as in Fig. 206. If the half of each parallel 

 was always illuminated, the days and nights would be equal every- 

 where, for it takes just as long for a place at A (Fig. 206) to move 

 to B (six hours, half of a twelve-hour day) as for it to move from 

 B to A' (half of a twelve-hour night) . 



Since days and nights are not equal at all seasons in most parts 

 of the earth, it proves that the axis on which the earth rotates is 

 not perpendicular to the plane of its orbit. 



Again, if the earth rotated on an axis perpendicular to the 

 plane of its orbit, the sun would always be equally high at any 

 given place at the same hour of the day. But this is not the case. 

 In the United States, for example, the sun is much higher above 



