466 



THE ICE AGE IN NORTH AMERICA 



The seventh theory mentioned had in Mr. Croll and in Mr. 

 James Geikie most able and convincing advocates, and for 

 many years was generally accepted as sufficient and satisfac- 

 tory. Briefly stated, the theory is this: 



As is well known, the earth's orbit is not a circle, but an 

 ellipse, whose longer diameter at the present time exceeds 

 its shorter by about 3,000,000 miles. The sun is not in the 

 center, but is. in one of the foci of the ellipse, which at the 

 present is about 1,500,000 miles outside of the center. As 

 matters are now situated, therefore, the earth on the 21st of 

 June (when it is said to be in aphelion) is 3,000,000 miles 

 farther from the sun than it is on the 21st of December 

 (when it is said to be in perihelion) ; that is, during the pres- 

 ent winters of the northern hemisphere we are 3,000,000 

 miles nearer the sun than we are during the summers. 



of the 



Fig. 125.— Exaggerated view of the earth's orbit, showing the effects of precession o 

 equinoxes. A, condition of things now ; B. as it will be 10.500 years from now. 



But, if a line be drawn through the earth's orbit joining 

 the equinoxes — that is, the points passed through on the 20th 

 of March and the 22d of September — we shall find that the 

 winter is shorter than the summer. The period from the 

 20th of March to the 22d of September, which constitutes 

 the summer, is one hundred and eighty-six days, while that 



