Page 238 - Lokmanya Tilak Samagra (khand 2)

THE GLACIAL PERIOD 25

rise to a severe winter. Calculating 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.
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 21000
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 inequa-
lity 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 quanity 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 dura-
tion 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 join
result of these two variations, the difference between the durations

• See Herschel's Outlines of Astronomy, Ed. 1883, Arts. 368, 369.