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.