The Arctic Home in the Vedas c5.16

c5.16

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 during winter, whatever the eccentricity of the earth may be. But though these figures are not affected by the eccentricity of the orbit, yet we have seen that the duration of the summer or winter does vary as the eccentricity. Supposing, therefore, that we have the longest winter in the northern hemisphere, we shall have to distribute 229 heat-units over 166 days of a short summer, and 136 heat-units over 199 days of a long winter of the same period. In other words, the difference between the daily average heat in summer and winter will, in such a case, be the greatest, producing shorter but warmer summers and longer and colder winters; and ice and snow accumulated in the long winter will not be melted or removed by the heat of the sun in the short summer, giving rise thereby to what is known as the Glacial period in the northern hemisphere. From what has been stated above, it may be seen that the southern hemisphere during this period will have long and cool summers and short and warm winters, a condition precisely reverse to that in the northern hemisphere. In short the Glacial and Inter-Glacial periods in the two hemispheres will alternate with each other every 10,500 years, if the eccentricity of the earth be sufficiently great to make a perceptibly large difference between the winters and the summers in each hemisphere.