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

30 THE ARCTIC HOME IN THE VEDAS. There is another consideration which we must take into account in estimating the severity of winter or the mildness of summer in any hemisphere. If the summer be defined to be the period of time required by the earth to travel from one equinoctial point Q' to another equinoctial point Q, this interval cannot always be constant for we have seen that the winter and summer points (P and A), and with them the equinoctial points (Q and Q') are not stationary, but revolve along the orbit once in 21,000 years. Had the orbit been a circle, the lines qq' and pa will have always divided it in equal parts. But the orbit being an ellipse these two sections are unequal. For instance, suppose that the winter occurs when the earth is at P, then the duration of the summer will be represented by Q'AQ, but when the winter occurs at A the summer time will be represented by QPQ', a segment of the ellipse necessarily smaller than Q'AQ. This inequality is due to the ellipticity of the orbit, and the more elongated or elliptic the orbit is, the greater will be the difference between the durations of summer and winter in a hemisphere. Now the ellipticity of the orbit is measured by the difference between the mean and the greatest distance of the earth from the sun, and is called in astronomy the eccentricity of the earth's orbit. This eccentricity of the earth's orbit is not a constant quantity, but varies, though slowly, in course of time, making the orbit more and more elliptical, until it reaches a maximum value, when it again begins to reduce until the original value is reached. The duration of summer and winter in a hemisphere, therefore, varies as the value of the eccentricity of the earth's orbit at that time; and it has been stated above that the difference between the duration of summer and winter also depends on the position of the equinoctial line, or of the points in the earth's orbit at which the winter and the summer in a hemisphere occur. As the joint result of