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

THE GLACIAL PERIOD. ·31· these two variations, the difference between the durations of summer and winter would be the longest, when the eccentricity of the earth is at its maximum and according as the winter and summer occur at the points of perihelion or aphelion. It has been found that this difference is equal to 33 days at the highest, and that at the present day it is about 7½ days. Thus if the winter in the northern hemisphere occurs when the earth is at P in its orbit and the eccentricity is at its maximum, the winter will be shorter by 33 days than the summer of the time. But this position will be altered after 10,500 years when the winter, occurring at A, will, in its turn, be longer than the corresponding summer by the same length of time, viz., 33 days. Now, since the earth describes equal areas in equal times in its orbit, Herschel supposed that in spite of the difference between the duration of summer and winter noticed above, the whole earth received equal amount of heat while passing from one equinox to another, the "inequality in the intensities of solar radiation in the two intervals being precisely compensated by the opposite inequality in the duration of the intervals themselves." Accepting this statement Dr. Croll understated his case to a certain extent. But Sir Robert Ball, formerly the Astronomer Royal of Ireland, in his recent work On the Cause of an Ice Age has demonstrated, by mathematical calculation, that the above supposition is erroneous, and that the total amount of heat received from the sun by each hemisphere in summer and winter varies as the obliquity of the earth or the inclination of its axis to the ecliptic, but is practically independent of the eccentricity of the earth's orbit. Taking the total sun-heat received in a year by each hemisphere to be 365 units, or on an average one unit a day, and taking the obliquity to be 23° 27', Sir. Robert Ball has calculated that each hemisphere would receive 229 of these heat-units during summer and only 136