Page 236 - Lokmanya Tilak Samagra (khand 2)

THE GLACIAL PERIOD 23

tion of the problem is sought for in astronomy rather than in
geography. Changes which seem to be so gigantic on the globe
are, it is said, but daily wrought by cosmical forces with which
we are familiar in astronomy, and one of the chief merits of
Croll's theory is supposed to consist in the fact that it satisfac-
torily accounts for a succession of Glacial and Inter-Glacial
epochs during the Pleistocene period. Dr. Croll in his Climate
and Time and Climate and Cosmology has tried to explain and
establish his theory by elaborate calculations, showing that the
changes in the values of the variable elements in the motion of
the earth round the sun can adequately account for the climatic
changes in _the Pleistocene period. We shall first briefly state
Dr. Croll's theory and then the opinion of experts as regards
its probability.
Let PQ' A Q represent the orbit of the earth round the sun.
This orbit is an ellipse, and the sun, instead of being in the centre
C, is in one of the focu S or s. Let the sun be at S. Then the
distance of the sun from the earth when the latter is at P would
be shortest, while, when the earth is at A it will be the longest.
These points P and A are respectively called perihelion and
aphelion. The seasons are caused, as stated above, by the axis
of the earth being inclined to the plane of its orbit. Thus when the
earth is at P and the axis turned away from the sun, it will produce
winter in the northern hemisphere; while when the earth is at A,
axis, retaining its direction, will be now turned towards the sun,
and there will be summer in the northern hemisphere. If the
axis of the earth had no motion of its own, the seasons will
always occur at the same
pOints in the orbit of the
earth, as, for instance, the
winter in the northern
~ _ c: hemisphere at P and the
summer at A. But this
axis describes a small
circle round the pole of
the ecliptic in a cycle of
25,868 years, giving rise
to what is called the precession of the equinoxes, and consequently
the inclination of the earth's axis to the plane of its orbit is not
always the same at any given point in its orbit during this period.