222 M. Arago on the Thermometrkal State 



The apparent orbit of the sun, that is to say, the real orbit 

 of the earth, is truly an ellipse very little different from a circle. 



In this ellipse the great axis perpetually maintains the same 

 length. The eccentricity, on the other hand, varies. 



The immutability of the great axis of a planet implies, ac- 

 cording to one of Kepler's laws, the immutability of the period 

 of the revolution of this planet around the sun. Thus, whatso- 

 ever may be the changes in the eccentricity of the earth's ellipse, 

 the length of the year will continue always the same. 



According to this conclusion, the problem before us will come 

 to this: Will the earth, considered as a whole, receive the same 

 quantity of heat from the sun, whether it move round this lumi- 

 nary in 365:J^ days in a perfect circle, or in an ellipse more or 

 less elongated ; always, however, having a greater axis equal to 

 the diameter of the circle ? 



We may suspect that the answer to this question will be ne- 

 gative ; that is to say, that the total quantity of heat received by 

 our globe, will increase with the eccentricity of the ellipse, if in 

 thought we will suddenly carry this eccentricity to the extreme 

 limit ; — if we will make the orbit so narrow, that the two sides 

 will almost graze the surface of the sun ; and thus make the 

 earth touch that luminary twice a year. In fact, an exact 

 calculation supplies the measure of the increase for every case ; 

 it teaches us, that the earth should annually receive from the 

 sun, total quantities of heat, inversely proportional to the lesser 

 axes of the elliptical orbits (with an invariable greater axis) in 

 which we successively circulate. 



At present the eccentricity of the terrestrial orbit diminishes ; 

 consequently the smaller axis increases ; and hence the heat 

 which we are receiving every succeeding year from the sun 

 should be becoming feebler. But, in truth,* this is nothing 

 more than a mere abstraction : the variation of eccentricity go- 

 ing on so slowly, that it would require more than 10,000 years 

 ere it would occasion a change that could be measured by the 

 thermometer, in the temperature of the earth. When we as- 

 cend only to the remotest periods of history, the influence of 

 this cause may then be wholly neglected. 



Herschel, who has recently been occupying himself with this 

 problem, in the hope of discovering the explanation of several 



