298 Mr. Herschel on the Astronomical Causes 



great periods of time either to mitigate or to exag-gerate the difference of 

 winter and summer temperatures, so as to produce alternately in the same 

 latitude of either hemisphere a perpetual spring-, or the extreme vicissitudes 

 of a burning summer and a rigorous winter. 



To show this, let us take at once the extreme case of an orbit as excentric 

 as that of Juno or Pallas, in which the greatest and least distances of the 

 sun are to each other as 5 to 3, and consequently the radiations at those di- 

 stances as 25 to 9, or very nearly as 3 to 1. To conceive what would be the 

 extreme effects of this great variation of the heat received at different periods 

 of the year, let us first imagine in our latitude, the place of the perigee 

 of the sun to coincide with the summer solstice. In that case the difference 

 between the summer and winter temperature would be exaggerated in the 

 same degree as if three suns were placed side by side in the heavens in the 

 former season, and only one in the latter, which would produce a climale per- 

 fectly intolerable. On the other hand, were the perigee situated in the winter 

 solstice, our three suns would combine to warm us in the winter, and would 

 afford such an excess of winter radiation as would probably more than coun- 

 teract the eflect of short days and oblique sunshine, and throw the summer 

 season into the winter months. 



The actual diminution of the excentricity is so slow, that the transition from 

 a state of the orbit, such as we have assumed, to the present nearly circular 

 figure, would occupy upwards of 600,000 years, supposing it uniformly 

 changeable; — this of course would not be the case : when near the maximum, 

 however, it would vary slower still, so tliat at that point, it is evident, a period 

 of 10,000 years would elapse without any perceptible change in the state of 

 the data of the case we are considering. 



Now this, adopting the very ingenious idea of Mr. Lyell*, would suffice, by 

 reason of the combined effect of the precession of the equinoxes and the 

 motion of the apsides of the orbit itself, to transfer the perigee from the sum- 



* Principles of Geology, p. 110. — Mr. Lyell, however, in stating the actual excess of eight days 

 in the duration of the sun's presence in the northern hemisphere over that in the southern, as pro- 

 ductive of an excess of light and heat annually received by the one over the other hemisphere, ap- 

 pears to have misconceived the effect of elliptic motion in the passage here cited ; since it is demon, 

 strable that, whatever be the ellipticity of the earth's orbit, the two hemispheres must receive equal 

 absolute quantities of light and heat per annum, the proximity of the sun in perigee exactly com- 

 pensating the effect of its swifter motion. This follows from a very simple theorem, which may be 

 thus stated : "The amount of heat received by the earth from the sun while describing any part 

 of its orbit, is proportional to the angle described round the sun's centre." So that if the orbit 

 be divided into two portions by a line drawn in any direction through the sun's centre, the heats 

 received in describing the two unequal segments of the ellipse so produced will be equal. 



