26 THE POLAR GLACIERS. 



Just so the sun lays hold of the protuberant rim of the great 

 terrestrial top, and immediately it begins to oscillate in the long 

 secular period of 25,868 years ; while the polar axis, extended to 

 the heavens, describes in the same length of time a small circle 

 of 23^ radius among the northern or southern stars. This is 

 the motion which causes what is called the precession of the equi- 

 noxes. The plane of the earth's equator crosses the plane of its 

 orbit ; and when the earth is at the points of junction, the days 

 and nights are equal the world over. These two points therefore 

 are the equinoxes ; and the earth passes through them about the 

 21st days of March and September. Owing to the rolling motion 

 of the equator above described, these points, always in the line 

 of intersection of the two planes, pass successively through the 

 twelve signs or constellations of the zodiac, making slowly the 

 entire circuit of the heavens. The vernal equinox, which now 

 points to, or is on a line between, the sun and the constellation of 

 the Fish, after about 26,000 years will have traveled the great 

 circle of the heavens and come back again to point to the same 

 cluster of stars, which is now overhead at midnight on the 21st 

 of March. 



But the time of this revolution, so far as it affects the climate 

 of the earth, is modified by the following circumstance : The 

 ellipse or oblong circle in which the earth revolves about the sun, 

 is itself all the time slowly revolving. The long diameter of it, 

 the major axis, makes a complete revolution in the heavens 

 once in 110,000 years. Now as this revolution is forward, or in 

 the the same direction among the constellations that the sun 

 appears to move, while that of the equinoxes is retrograde, it 

 follows that the extremities of the major axis, which are the 

 perigee and the apogee, advance to meet the equinoctial points ; 

 so that the revolutions, or rather the conjunctions, of the equi- 

 noxes, which have to do with terrestrial climate, are accomplished 

 in the shorter period of 21,000 years. 



Now all this astronomy amounts to simply this ; that in the 

 year of our Lord 1248 the earth was at its nearest approach to 

 the sun on the 21st day of December, our winter solstice ; and 

 that in 10,500 years from that time the same thing will happen 



