338 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



production of a large number of spots diminished the radiant heat and 

 occasioned great cold. But this supposition, nevertheless, remains in the 

 doubtful region of the merely probable, and can by no means be consid- 

 ered a settled fact. Secondly, it is quite otherwise as to the relative 

 position of the earth to the sun ; for a change in this position can be 

 submitted to exact calculation, and the influence readily determined it 

 would exercise upon the fluid envelope of the globe, which, by its mo 

 bility, is the true regulator of climates. 



The distance of the earth from the sun changes by virtue of the ec- 

 centricity of the terrestrial orbit, which has not been the same through- 

 out all periods, for it is diminishing now and will continue to diminish 

 for twenty-three thousand eight hundred and eighty years, commencing 

 from 1900 to increase again. Its smallest limit, according to Leverrier, 

 is 0.003314, and its greatest, 0.07775, a difference sufficient to impress, 

 by the alteration in the intensity of the sun's heat, a distinct character 

 upon the organisms of the corresponding epochs.* When the eccen- 

 tricity is very great, the sun is sensibly nearer to the earth at the peri- 

 helion. The temperature will then increase in inverse proportion to the 

 square of the distance, while, for the same reason, the heat will diminish 

 at the aphelion. This change will also affect the quantity of heat which 

 the earth annually receives from the sun. If, in fact, the .great axis re- 

 mains invariable during all variation of eccentricity, the temperature 

 must diminish, by virtue of the law that the total amount of heat received 

 from the sun during a revolution of the earth is inversely proportional 

 to the small axis, for the small axis diminishes when the eccentricity 

 increases, and vice versa. But as the difference of the small axis of the 

 terrestrial orbit between its two extremes is only 0.003 (as 997 is to 1,000), 

 this small quantity cannot sensibly affect climates, and it would not be 

 of any great consequence if it were omitted altogether. 



The value we place upon eccentricity is otherwise very important f 

 for when it is at the maximum, the distance which separates, at the 

 aphelion, the earth from the sun is 32,706,262 leagues (20 for a degree), 

 and when at the perihelion its distance is only 28,176,478 leagues. The 

 difference, 4,529,784 leagues (15,000,000 miles), shows that the earth is 

 by that amount nearer to the sun at the perihelion than at the aphelion. 

 The direct heat of the sun being inversely proportional to the square of 

 the distance, it follows that the quantity of heat received by the earth 

 in the two positions will be as 19 to 26. 



If we now suppose that, according to the precession of the equinoxes, 

 the winter of the boreal hemisphere coincides with the aphelion of the 

 terrestrial orbit to the period when the eccentricity is greatest, the 

 earth would then be 2,886,599 leagues (10,000,000 miles) farther off than 

 it is now. In consequence, the direct heat of the sun would be dimin- 

 ished a fifth of its present intensity, and the difference between the 

 temperature of the two extreme seasons would increase double that 



"' H. G. Bronii, Handbuch tiner Geschkhte dir Xatiir,t. i,p. 427, c. 



