THE CAUSE OF THE SEASONS 29 



face. Figure 10 shows that the same number of rays fall 

 upon a much smaller surface when the direction of the sun 

 is vertical than wh,en it is nearly horizontal. In the 30- 

 degree arcs there are 2^-, 7, and 9^ ray spaces respectively. 

 The sun is here considered to be vertical at the equator, 

 as it is on March 21, and September 23. Thus on these 

 days, other conditions being the same, about one fourth 



FIGURE 10. HEATING EFFECTS OF SUN'S RAYS 



Heating effects depend upon the angle at which the sun's rays strike 

 the earth's surface. 



as much heat from the sun falls upon the 30 about the 

 pole as upon the 30 north of the equator. 



When the northern hemisphere is inclined toward the 

 sun, the rays of the sun cover the north pole continuously 

 for six months, so that at this point there is no night for all 

 that time. The days are longer and the nights shorter 

 throughout all the northern hemisphere. More heat is, 

 therefore, received in the northern hemisphere during these 

 six months, not only because the rays of the sun fall more 

 nearly vertically but also because the length of the day is 

 increased. 



