

Ch. xiil] astronomical causes affecting climate. 



209 



Sir John Herschel, in 



1832,"* entertained the question 



whether there are any astronomical causes which might offer 

 a possible explanation of the difference between the actual 

 temperature of the earth's surface and the climates which 

 appear formerly to have prevailed. ' Geometers/ he observed, 

 ' had demonstrated the absolute invariability of the earth's 

 mean distance from the sun, whence it would seem to follow 



that the mean 



annual supply of light and heat would be 



alike invariable. This, however, is not exactly true : the 

 total quantity of heat received in one revolution is inversely 

 proportional to the minor axis ; ' still, as the extreme amount 

 of difference in the quantity of heat annually received, owing 

 to such change in the minor axis, can never by possibility ex- 

 ceed the whole supply in a ratio of more than 1,003 to 1,000, 

 it may, he says, be neglected in our geological speculations. 



But there is another way in which changes in the excen- 

 tricity of the orbit affect climate. Climate depends not 

 merely on the absolute amount of heat, but on the manner in 

 which it is distributed through different parts of the year, 

 especially in the polar and circumpolar zones of the earth. 



mor 



circular, but only at a very slow and somewhat irregular 

 rate, and it will become in 23,980 years after a.d. 1800 nearly 



minimum 



as circular as it can ever be, or will approach a 

 excentricity, after which it will again increase at the same 

 slow rate. These perturbations are caused by the attraction 

 of the nearest and largest planets, Jupiter and Saturn playing 

 the principal part, and the other planets, especially Venus 



Mars 



a sensible influence. 



It has long 



been known that the deviation of the orbit from a circle 

 could never exceed certain limits, and these limits were very 

 nearly defined by Lagrange towards the end of the last 



i exact! v bv Tip v9rv\9v in 1839. The ex- 



mor 



treme range of excentricity as expressed by the difference 

 in distance of the earth from the sun in aphelion and peri- 



helion amounts in round numbers 



a little more than 



minimum 



ing half a million. In other words, recent observation having 



* Trans. Geol. Soc, 2nd series, vol. iii. 



