But before we discuss this matter, it will be well to state what is
known beyond peradventure about the moon.
Its mean distance from the earth is usually called, for the sake of a
round number, 240,000 miles, but more accurately stated it is 238,840
miles. 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.
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