Page 239 - Lokmanya Tilak Samagra (khand 2)

26 SAMAGRA TILAK - 2 • THE ARCTIC HOME

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 7i 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 posi-
tion 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 eclip-
tic, 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 ecccenticity. Supposing, there-
fore, 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