270 



EFFECTS OF EXTREME EXCENTRICITY 



[Ch. XIII. 



determined the mean distance of the earth from the sun 

 to be 91,400,000 miles, the excentricity in the one case 



to -jig., and in the other, or 



3 6 



amounts 



of the whole. 



when it is least, to 



Whatever be the ellipticity of the earth's orbit, says Sir 

 J. Herschel, the two hemispheres must receive equal absolute 



oxim 



i.e., an 



sun in perigee, or its distance in apogee, exactly compensating 

 the effect of its swifter or slower motion.* But the same 

 writer, in 1858, alluding to some speculations of Reynauld, 

 speaks of the marked effects on climate which great vari- 

 ations in excentricity might produce, causing the charac- 

 ters of the seasons in the two hemispheres to be strongly 

 contrasted. ' In the northern (assuming the position of the 

 line of the apsides to be as now) we should have a short but 

 very mild winter, with a long but very cool summer 

 approach to perpetual spring ; while the southern hemisphere 

 would be inconvenienced, and might be rendered uninhabit- 

 able by the fierce extremes caused by concentrating half the 

 annual supply of heat into a summer of very short duration, 

 and spreading the other half over 

 sharpened to an intolerable intensity of frost when at its 

 climax, by the much greater remoteness of the sun;'f and 

 he goes on to observe, that, in consequence of the precession 

 of the equinoxes, combined with the secular movement of 

 the aphelion, the state of the northern and southern hemi- 

 spheres here alluded to, would in the course of about 11,000 

 years be reversed, and such alternations of climate must 

 in the immense periods of the past which the geologist con- 

 templates, have happened not once only, but thousands of 

 times ; and < it is not impossible/ he adds, ' that some of the 

 indications of widely different climates informer times may be 

 referable, in part at least, to this cause/ 



a long dreary winter, 



# 



This follows, observes Herschel, be divided into two portions by a line 



drawn in any direction through the sun's 



from a very simple theorem, which may 

 be thus stated : 



' The amount of heat centre, the heat received in describing 



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 



the two unequal segments of the ellipse 

 so produced will be equal. Geol. Trans, 

 vol. iii. part ii. p. 298 ; second series, 

 f Herschel' s Astronomy, Art. 368 c. 



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