The moon keeps the same side forever turned toward the earth,
behaving, in this respect, as Mercury does with regard to the sun. The
consequence is that the lunar globe makes but one rotation on its axis
in the course of a month, or in the course of one revolution about the
earth. Some of the results of this practical identity of the periods of
rotation and revolution are illustrated in the diagram on page 250. The
moon really undergoes considerable libration, recalling the libration of
Mercury, which was explained in the chapter on that planet, and in
consequence we are able to see a little way round into the opposite
lunar hemisphere, now on this side and now on the other, but in the
diagram this libration has been neglected. If it had been represented we
should have found that, instead of only one half, about three fifths of
the total superficies of the moon are visible from the earth at one time
or another.
[Illustration: PHASES AND ROTATION OF THE MOON.]
Perhaps it should be remarked that in drawing the moon's orbit about the
earth as a center we offer no contradiction to what was shown earlier
in this chapter. The moon does travel around the earth, and its orbit
about our globe may, for our present purpose, be treated independently
of its motion about the sun. Let the central globe, then, represent the
earth, and let the sun be supposed to shine from the left-hand side of
the diagram. A little cross is erected at a fixed spot on the globe of
the moon.
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