254 DYNAMICAL GEOLOGY. 



2. Changes in the time of the perihelion and aphelion. — The earth is now in 

 aphelion during the northern summer and southern winter. With aphelion 

 in winter, the winters are colder and the summers are warmer than with 

 perihelion in winter. The position of the major axis of the earth's orbit 

 (the extremities of which are the aphelion and perihelion points) is chang- 

 ing, and a complete revolution takes 110,000 years ; and since this change is 

 in the opposite direction from that of the precession of the equinoxes, above 

 stated, the cycle of the seasons is shortened from 25,868 years to about 

 21,000 years ; for, supposing the perihelion and either equinox to coincide, 

 and then the precession to move in its direction and the perihelion in the 

 opposite, at their respective rates, they would again be in conjunction, in 

 consequence of these rates, in 21,000 years. Hence, every 10,500 years, 

 the seasons become reversed, that is, the months of winter become the 

 summer months. Another consequence of this aphelion cycle is, that 

 the winter and summer intervals between the equinoxes vary in relative 

 lengths, the aphelion side being the longer. At the present time the aphel- 

 ion comes in summer, and the summer interval is therefore seven days longer 

 than the winter interval. 



3. Changes in the eccentricity of the earth's orMt. — The earth's ellip- 

 tical orbit varies slowly in eccentricity, — that is, in the length of its major 

 axis, — making the aphelion distance greater and the perihelion less, but not 

 varying the mean distance or the amount of heat received annually by the 

 earth from the sun. Maxima in the eccentricity occur once in 100,000 to 

 200,000 years. One maximum was passed, according to calculated results, 

 about 110,000 years since; another (higher), about 210,000 years since; the 

 next anterior (like the latter in height), about 750,000 years; and a maxi- 

 mum of extreme eccentricity, 850,000 years since. (Stockwell, 1868.) 



With the sun's mean distance 92,400,000 miles, the present aphelion dis- 

 tance is about 93,950,000 miles, and the perihelion, 90,850,000 miles, and 

 the eccentricity, 0*0168. But at the time of extreme maximum eccentricity 

 (= 0-075 nearly), referred to above, the aphelion distance would be about 

 99,300,000 miles, and the perihelion 85,500,000, making the sun 13|- millions 

 of miles nearer the earth in summer than in winter. 



Owing to the increasing eccentricity there is an increasing difference in 

 the length of the winter interval as compared with the summer interval; 

 and at an extreme maximum it is 33 days longer than the summer interval. 

 As the amount of heat which the earth receives varies inversely as the 

 square of the sun's distance, increasing eccentricity diminishes the amount 

 on the aphelion side and increases it on the other ; and if aphelion comes in 

 winter, the winter cold is greatly augmented, besides continuing longer. 

 The summers, on the contrary, would be proportionally hotter, but, in the 

 same proportion, shorter. With aphelion in summer, the winters would be 

 relatively mild and the summers cool. Herschel first drew attention to this 

 effect of extreme eccentricity (1858), and Croll used^the facts in his Cli- 

 mate and Time (1875) to account for the occurrence of glacial periods in 



