moving the axis
gradually to positions where it approaches the plane of the orbit, he
will note that each stage of the change widens the tropic belt.
Bringing the polar axis down to the plane of the orbit, one hemisphere
would receive unbroken sunshine, the other remaining in perpetual
darkness and cold. In this condition, in place of an equatorial line
we should have an equatorial point at the pole nearest the sun; thence
the temperatures would grade away to the present equator, beyond which
half the earth would be in more refrigerating condition than are the
poles at the present day. In considering the movements of our planet,
we shall see that no great changes in the position of the polar axis
can have taken place. On this account the suggested alterations of the
axis should not be taken as other than imaginary changes.

It is easy to see that with a circular orbit and with an inclined axis
winter and summer would normally come always at the same point in the
orbit, and that these seasons would be of perfectly even length. But,
as we have before noted, the earth's path around the sun is in its
form greatly affected by the attractions which are exercised by the
neighbouring planets, principally by those great spheres which lie in
the realm without its orbit, Jupiter and Saturn. When these attracting
bodies, as is the case from time to time, though at long intervals,
are brought together somewhere near to that part of the solar system
in which the earth is moving around the sun, they draw our planet
toward them, and so make its path very elliptical. When, however, they
are so distributed that their pulling actions neutralize each other,
the orbit returns more nearly to a circular form. The range in its
eccentricity which can be brought about by these alterations is very
great. When the path is most nearly circular, the difference in the
major and minor axis may amount to as little as about five hundred
thousand miles, or about one one hundred and eighty-sixth of its
average diameter. When the variation is greatest the difference in
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 th