308 



MAN AND THE GLACIAL PERIOD. 



eccentricity of the earth's orbit when combined with the 

 precession of the equinoxes — another calculable astro- 

 nomical cause. 



It is well known that the earth's orbit is elliptical ; that 

 is, it is longer in one direction than in the other, so that 

 the sun is one side of the centre. During the winter of 

 the northern hemisphere the earth is now about three 



Fig. 99.— Diagram showing effect of precession : A. condition of things now; 

 B, as it will be 10,500 years hence. The eccentricity is of course greatly ex- 

 aggerated. 



million miles nearer the sun than in the summer; but 

 the summer makes up for this distance by being about 

 seven days longer than the winter. Through the preces- 

 sion of the equinoxes this state of things will be reversed 

 in ten thousand five hundred years ; at which time we 

 shall be nearer the sun during our northern summer, and 

 farther away in winter, our winter then being also longer 

 than our summer. Besides, through the unequal at- 

 traction of the planets the eccentricity of the earth's 

 orbit periodically increases and diminishes, so that there 

 have been periods when the earth was ten million five 



