SUN AND CLIMATIC VARIATION 43 



There is consequently a slight rise in temperature to the 

 advantage of the former. The converse occurs every 10,500 

 years. 



The eccentricity of the earth's orbit is also subject to very 

 important variations : to-day it is -gV- But it can increase to 

 T j5 ; that is to say, the orbit may be extended by the earth 

 going further from the sun than it does to-day, and also by 

 coming nearer to it ; and this would lead, of necessity, to a 

 greater difference between the hot and the cold season than 

 that prevailing to-day, especially if the line of the solstice 

 coincided with the major axis of the orbit. Conversely, if the 

 eccentricity were zero, that is to say, if the terrestrial orbit 

 became circular, a possible contingency, and the earth remained 

 the same distance from the sun throughout the year, the 

 seasons would be less marked than they are to-day. Such 

 alternations must actually have occurred in the course of the 

 millions of years represented by geological periods. But we may 

 go further and say that in the present phase of our planetary 

 system, the angle formed by the plane of the equator and the 

 ecliptic varies only within narrow limits, from about 21 59' 

 to 34° 36'. These two planes cannot coincide, which would 

 eliminate the seasons. But other astronomical conditions 

 affect the position of the earth's axis ; every change in the shape 

 of the earth, which is not rigid, and every modification in the 

 distribution of the matter which composes its mass, may bring 

 about a displacement of the axis of rotation in relation to the 

 surface, or, to put it otherwise, a displacement of the line of 

 the Poles, and may, in consequence, fundamentally alter the 

 climates of different regions of the globe. It has even been 

 supposed that a greater accumulation of ice at one of the 

 Poles than at the other can produce such an effect. 



Neither has the sun itself remained unchanged. After the 

 earth was detached from it the sun continued to shrink both 

 as a result of cooling and because it had thrown off two other 

 planets, Venus and Mercury, which naturally Jed to a reduction 

 in its size. It would be sufficient for the sun to have an apparent 

 diameter of 47 to cause the lighted area of the earth greatly 

 to exceed that of the unlighted, 1 instead of their being equal 

 as is the case to-day. Under those conditions, however, the 

 long solar nights would disappear and there would be no more 



1 Vbis, 32. 



