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.