these measurements may be as much as near thirteen million miles, or
about one seventh of the mean width of the orbit.

The first and most evident effect arising from these changes of the
orbit comes from the difference in the amount of heat which the earth
may receive according as it is nearer or farther from the sun. As in
the case of other fires, the nearer a body is to it the larger the
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