274 



CLIMATE OF THE EAETH AFFECTED BY 



[Ch. XIII. 



periods, and recent observation seems to favour the reality of 



such events. 

 In orcter ' 



extent to which I conceive that 



astronomical conditions may sometimes exaggerate the in- 

 tensity of cold, I shall begin by speaking of the movement 

 called the precession of the equinoxes, which is well known 

 to be due to the attraction of the sun and moon on the pro- 

 tuberant matter at the earth's equator (for if the earth were 

 a perfect sphere there would be no precession) ; the moon 

 exerting the greatest disturbing force, notwithstanding its 



mass 



of its greater proximity. 



By 



virtue of this movement, the different seasons of the northern 



made 



with all the points through which the earth passes in its 

 orbit, and consequently at all distances from the sun, con- 

 sistently with a given state of excentricity. 



Winter 



example, spring, autumn, and summer 



them 



•om 



intermed 



disturbing 



to the sun, as well as at every one of the 

 tances. This great cycle of change would be gone through 

 in 25,868 years were it not shortened by being combined 

 with another movement called the revolution of the apsides. 

 This last consists of a gradual change in the direction of the 

 major axis of the earth's orbit due to the same 

 forces which cause the ellipticity of the orbit to vary, namely, 

 the attraction of the larger and nearer planets. The result 

 of the combination of these two causes of perturbation is, 

 that in 21,000 years, the seasons will have made a complete 

 revolution so as again to coincide with the same point in 

 the orbit as at present, and in half that period, or about 

 1 0,500 years, the present astronomical state of things will be 

 reversed, so that winter will occur in our hemisphere in 

 aphelion, and at the south pole in perihelion. 



If the orbit were circular and our planet always equidistant 

 from the sun, the four seasons, so long as geographical cir- 

 cumstances remained constant, would always hold the same 

 relation to one another in respect to temperature in every 



phase of precession. 



ty 



be 



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