This is variable to the extent of more than 31,000 miles, on account of the eccentricity of its orbit, and the eccentricity itself is variable, in consequence of the perturbing attractions of the earth and the sun, so that the distance of the moon from the earth is continually changing.

It may be as far away as 253,000 miles and as near as 221,600 miles.
Although the orbit of the moon is generally represented, for convenience, as an ellipse about the earth, it is, in reality, a varying curve, having the sun for its real focus, and always concave toward the latter.

This is a fact that can be more readily explained with the aid of a diagram.
[Illustration: THE MOON'S PATH WITH RESPECT TO THE SUN AND THE EARTH.] In the accompanying cut, when the earth is at _A_ the moon is between it and the sun, in the phase called new moon.

At this point the earth's orbit about the sun is more curved than the moon's, and the earth is moving relatively faster than the moon, so that when it arrives at _B_ it is ahead of the moon, and we see the latter to the right of the earth, in the phase called first quarter.

The earth being at this time ahead of the moon, the effect of its attraction, combined with that of the sun, tends to hasten the moon onward in its orbit about the sun, and the moon begins to travel more swiftly, until it overtakes the earth at _C_, and appears on the side opposite the sun, in the phase called full moon.