e
share of light and heat which it will receive. In an orbit made
elliptical by the planetary attraction the sun necessarily occupies
one of the foci of the ellipse. The result is, of course, that the
side of the earth which is toward the sun, while it is thus brought
the nearer to the luminary, receives more energy in the form of light
and heat than come to any part which is exposed when the spheres are
farther away from each other in the other part of the orbit.
Computations clearly show that the total amount of heat and the
attendant light which the earth receives in a year is not affected by
these changes in the form of its path. While it is true that it
receives heat more rapidly in the half of the ellipse which is nearest
the source of the inundation, it obtains less while it is farther
away, and these two variations just balance each other.

Although the alterations in the eccentricity of its orbit do not vary
the annual supply of heat which the earth receives, they are capable
of changing the character of the seasons, and this in the way which we
will now endeavour to set forth, though we must do it at the cost of
considerable attention on the part of the reader, for the facts are
somewhat complicated. In the first place, we must note that the
ellipticity of the earth's orbit is not developed on fixed lines, but
is endlessly varied, as we can readily imagine it would be for the
reason that its form depends upon the wandering of the outer planetary
spheres which pull the earth about. The longer axis of the ellipse is
itself in constant motion in the direction in which the earth travels.
This movement is slow, and at an irregular rate. It is easy to see
that the effect of this action, which is called the revolution of the
apsides, or, as the word means, the movement of the poles of the
ellipse, is to bring the earth, when a given hemisphere is turned
toward the sun, sometimes in the part of the orbit which is nearest
the source of light and heat, and sometimes farther away. It may thus
well come about that at one time the summer season of a hemisphere
arrives when it is nearest the sun, so that the season, though hot,
will be very short, while at another time the same season will arrive
when the earth is farthest from the sun, and receives much less heat,
which would tend to make a long and relatively cool summer. The reason
for the difference in length of the seasons is to be found in the
relative swiftness of t