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 the earth's revolution when it is nearest the
sun, and the slowness when it is farther away.

There is a further complication arising from that curious phenomenon
called the precession of the equinoxes, which has to be taken into
account before we can sufficiently comprehend the effect of the
varying eccentricity of the orbit on the earth's seasons. To
understand this feature of precession we should first note that it
means that each year the change from the winter to the summer--or, as
we phrase it, the passage of the equinoctial line--occurs a little
sooner than the year before. The cause of this is to be found in the
attraction which the heavenly bodies, practically altogether the moon,
exercises on the equatorial protuberance of the earth. We know that
the diameter of our sphere at the equator is, on the average,
something more than twenty-six miles greater than it is through the
poles. We know, furthermore, that the position of the moon in relation