We come now to a brief consideration of certain peculiarities in the motions of the moon, and in the phenomena of day and night on its surface.  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.

At A the moon is between the earth and the sun, or in the phase of new moon.  The lunar hemisphere facing the earth is now buried in night, except so far as the light reflected from the earth illuminates it, and this illumination, it is interesting to remember, is about fourteen times as great—­reckoned by the relative areas of the reflecting surfaces—­as that which the full moon sends to the earth.  An inhabitant of the moon, standing beside the cross, sees the earth in the form of a huge full moon directly above his head, but, as far as the sun is concerned, it is midnight for him.

In the course of about seven days the moon travels to B.  In the meantime it has turned one quarter of the way around its axis, and the spot marked by the cross is still directly under the earth.  For the lunar inhabitant standing on that spot the sun is now on the point of rising, and he sees the earth no longer in the shape of a full moon, but in that of a half-moon.  The lunar globe itself appears, at the same time from the earth, as a half-moon, being in the position or phase that we call first quarter.

Seven more days elapse, and the moon arrives at C, opposite to the position of the sun, and with the earth between it and the solar orb.  It is now high noon for our lunarian standing beside the cross, while the earth over his head appears, if he sees it at all, only as a black disk close to the sun, or—­as would sometimes be the case—­covering the sun, and encircled with a beautiful ring of light produced by the refraction of its atmosphere. (Recall the similar phenomenon in the case of Venus.) The moon seen from the earth is now in the phase called full moon.