THE CAUSE OF THE SEASONS 



27 



object at one of the foci, and move another small object 

 around the ellipse. The two objects will be closest together 

 when the moving object reaches one of the two points where 



9I,5OO,OOQMILES 



SUMMER WINTER 



FIGURE 7. THE EARTH'S VARIATION OF DISTANCE FROM THE SUN 



the straight line cuts the curve, and farthest apart when it 

 reaches the other point of intersection. 



Now the sun is at one of the foci of the ellipse in which 

 the earth moves, and so the distance between the sun and 

 the earth varies during the year. This variation is about 

 three millions of miles, the average distance of the earth 

 from the sun being about 93,000,000 miles. Strange as it 

 may seem, we are nearest 

 the sun in January and 

 farthest away in July. 

 (Figure 7.) 



MAH.CH [QUINQX 



The Cause of the Sea- 

 sons. Since the earth 

 moves around the sun 

 with its' axis inclined 23^ 

 from the perpendicular to 

 the plane of its orbit, the 

 northern and the southern 

 hemisphere will at different times be inclined toward and away 

 from the sun. (Figure 8.) In July the earth is farthest away 

 from the sun, but the northern hemisphere is then pointed 

 toward the sun, and the rays of heat from the sun fall more 

 nearly vertically upon this hemisphere than during the rest 



SEPTEMBER CQUINOX 



FIGURE 8. THE PATH OF THE EARTH 

 AROUND THE SUN 



Showing roughly the four positions men- 

 tioned in the text. 



