At this point the moon's orbit about the sun has a shorter radius
of curvature than the earth's. In traveling from _C_ to _D_ the moon
still moves more rapidly than the earth, and, having passed it, appears
at _D_ to the left of the earth, in the phase called third quarter. Now,
the earth being behind the moon, the effect of its attraction combined
with the sun's tends to retard the moon in its orbit about the sun,
with the result that the moon moves again less rapidly than the earth,
and the latter overtakes it, so that, upon reaching _E_, the two are
once more in the same relative positions that they occupied at _A_, and
it is again new moon. Thus it will be seen that, although the real orbit
of the moon has the sun for its center of revolution, nevertheless, in
consequence of the attraction of the earth, combined in varying
directions with that of the sun, the moon, once every month, makes a
complete circuit of our globe.
The above explanation should not be taken for a mathematical
demonstration of the moon's motion, but simply for a graphical
illustration of how the moon appears to revolve about the earth while
really obeying the sun's attraction as completely as the earth does.
There is no other planet that has a moon relatively as large as ours.
The moon's diameter is 2,163 miles. Its volume, compared with the
earth's, is in the ratio of 1 to 49, and its density is about six
tenths of the earth's.
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