130 COSMOS. 



As the altered position of the major axis is capable of ex- 

 erting onty a very slight influence upon the temperature of 

 the Earth, so likewise the limits of the probable changes in 

 the elliptical form of the Earth's orbit are, according to Arago 

 and Poisson,^ so narrow that these changes could only very 

 slightly modify the climates of the individual zones, and that 

 in very long periods. Although the analyses which determ- 

 ine these limits accurately is not yet quite completed, still so 

 much, at least, follows from it, that the eccentricity of the 

 Earth's orbit will never equal those of the orbits of Juno, 

 Pallas, and Victoria. 



10. Intensity of the Light of the Sun upon the Planets. 

 -^If the intensity of light upon the Earth is taken as =1, 

 it will be found to be upon the other planets as follows 



Mercury 6 "674 



Venus 1-911 



Mars 0-431 



Pallas . . 0-130 



Jupiter 0-036 



Saturn O'Oll 



Uranus 0-003 



Neptune ... 0-001 



In consequence of the very great eccentricity of their orb- 

 its, the intensity of light on the following planets varies in 



Mercury, in perihelion, 10'58 ; in aphelion, 4-59 ; 



Mars " 0-52 ; " " 0'36 ; 



Juno " " 0-25; " " 0-09; 



while the Earth, owing to the slight eccentricity of its orbits, 

 has in perihelion 1-034, and in aphelion 0-967. If the sun- 

 light upon Mercury is seven times more intense than upon the 

 Earth, it must also be 368 times more feeble upon Uranus. 

 The relations of heat have not been mentioned here, because 

 they are complicated phenomena, dependent upon the exist- 

 ence or non-existence, of an atmosphere surrounding the plan- 

 it conveys to the Earth are the same while in the one hemisphere or 

 the other, north or south." 



* Arago, op. cit., p. 300-204. " L'excentricite," says Poisson (op. 

 cit., p. 38 and 52), "ayant toujours etc et devant toujours demeurer 

 tres petite, 1'iiifluence des variations seculaires de la quantite de chaleur 

 solaire recjue par la Terre sur la temperature moyenne parait aussi de- 

 voir etre tres limitee. On ne saurait admettre que 1'excentricite de la 

 Terre, qui est actuellement environ un soixautieme, ait jamais ete ou 

 devienue jamais un quart, comme celle de Junon ou de Pallas." "As 

 the eccentricity always has been, and always will be, very small, the 

 influence of the secular variations of the quantity of solar heat received 

 by the Earth upon the mean temperature would appear also to be very 

 limited. It can not be admitted that the eccentricity of the Earth, 

 which is actually about ^L, has ever been, or ever will be , as that 

 of Juno or Pallas." 



