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