152 A YEAR IN SCIENCE 



Seasons. Seasonal changes in temperature are the 

 result of the revolution of the earth about the sun. 

 The path of the earth about the sun is known as its 

 orbit. This orbit is an ellipse ; consequently the dis- 

 tance from the sun to the earth varies from time to 

 time. As the earth moves, its axis (a line drawn 

 through the center of the earth from pole to pole) is 

 not perpendicular to the plane of its orbit. It" is 

 inclined toward this plane at an angle of about 23Vi> 

 degrees. This position of the axis, together with the 

 motion of the earth, causes the changes in the length 

 of day and night and also the succession of seasons. 



One half of the earth is being illuminated by the 

 sun's rays all of the time. If the earth's axis were 

 perpendicular to the plane of its orbit, day and night 

 would ahvays be of equal length. Since the rays of 

 light w r ould then always fall at the same angle and 

 for the same length of time at a given place on the 

 earth, there would be no change of season. 



We all know, however, that in June the sun shines 



more directly on the earth in the temperate latitude, 







and also for several hours more than it does in Jan- 

 uary. As a result, the temperature of several months 

 which we call summer, is higher than that of winter. 

 In autumn and spring the angle of the rays and the 

 length of daytime are intermediate. 



By referring to Figure 57 you will notice that the 

 hemispheres are at one time inclined toward the sun 

 and at another away from it. For example, from 



