130 Mr. J. Croll on the Physical Cause of the 



According to the calculations of Leverrier, the superior limit 

 of the earth's excentricity is -07775, and the inferior limit 

 •003314. The excentricity is at present diminishing, and will 

 continue to do so during 23,980 years from the year 1800*. 



The change in the excentricity of the earth's orbit may affect 

 the climate in two different ways ; viz., it may affect the climate 

 by either increasing or diminishing the mean annual amount of 

 heat received from the sun, or it may affect it by either increas- 

 ing or diminishing the difference between summer and winter 

 temperature. 



Let us consider the former case first. The total quantity of 

 heat, received from the sun during one revolution is inversely 

 proportional to the minor axis. 



The difference of the minor axis of the orbit when at its 

 maximum and its minimum state of excentricity is as 997 to 

 1000. This small amount of difference cannot therefore sensibly 

 affect the climate. Hence we must seek for our cause in the 

 second case under consideration. 



When the excentricity is at a maximum, the distance of the 

 sun from the earth, when the latter is in the aphelion of its 

 orbit, is no less than 102,256,873 miles; and when in the 

 perihelion it is only 87,503,039 miles. The earth is therefore 

 14,753,834 miles further from the sun in the former position 

 than in the latter. The direct heat of the sun being inversely 

 as the square of the distance, it follows that the amount of heat 

 received by the earth when in these two positions will be as 

 19 to 26. According to the determinations of Hansen re- 

 garding the present excentricity of the earth's orbit, the earth 

 during winter, when nearest to the sun, is 93,286,707 miles 

 distant. Suppose now that, according to the precession of the 

 equinoxes, winter in our northern hemisphere should happen 

 when the earth is in the aphelion of its orbit, at the time when 

 the orbit is at its greatest excentricity ; the earth would then be 

 8,970,166 miles further from the sun in winter than at present. 

 The direct heat of the sun would therefore be one-fifth less 

 during that season than at present; and in summer one-fifth 

 more than at present. The difference between the heat of 

 summer and winter in this case would be two-fifths greater than 

 at present. This enormous difference would affect the climate to 

 a very great extent. But if winter under these circumstances 

 should happen when the earth is in the perihelion of its orbit, 

 the earth would then be 14,753,834 miles nearer the sun in 

 winter than in summer. In this case the difference between 

 winter and summer in the latitude of this country would be 

 almost annihilated. But as the winter in the one hemisphere 

 * Connaissance des Temps for 1843 (Additions). 



