वेदों की प्राचीनता एवं काल-निर्णय / ओरायन (लोकमान्य बाल गंगाधर तिलक - खगोलीय वैदिक शोध)

THE GLACIAL PERIOD. ·29
cycle above mentioned, the winter in the northern hemisphere will
once occur at A, and the distance of the earth from the sun will
then be the longest. Now there is a vast difference between a
winter occurring when the earth is at P and a winter occurring
when it is at A. In the first case, the point P being nearest to the
sun, the severity of the winter will be greatly modified by the near-
ness of the sun. But at A the sun is farthest removed from the
earth, and the winter, when the earth is at A, will be naturally very
severe; and during the cycle the winter must once occur at A.
The length of the cycle is 25,868 years, and ordinarily speaking
half of this period must elapse before the occurrence of winter is
transferred from the earth's position at P to its position at A.
But it is found that the points P and A have a small motion of
their own in the direction opposite to that in which the line of
equinoxes QQ' or the winter point p moves along the orbit. The
above cycle of 25,868 years is, therefore, reduced to 20,984, or, in
round number, 21,000 years. Thus if the winter in one hemisphere
occurs when the earth is at P, the point nearest to the sun in the
orbit, it will occur in the same hemisphere at A after a lapse of
10,500 years. It may be here mentioned that in about 1,250
A. D., the winter in the northern hemisphere occurred when the
earth in its orbit was at P, and that in about 11,750 A. D. the
earth will be again at A, that is, at its longest distance from
the sun at the winter time, giving rise to a severe winter. Calculat-
ing backwards it may be seen that the last severe winter at A
must have occurred in the year 9,250 B. C.* It need not be
mentioned that the winter in one hemisphere corresponds with
the summer in the other, and that what is said about winter in
the northern hemisphere applies mutatis mutandis to seasonal
changes in the southern hemisphere.

• See Herschel's Outlines of Astronomy, Ed. 1883, Arts. 368, 369.