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.

Another lapse of seven days, and the moon is at _D_, in the phase called
third quarter, while the earth, viewed from the cross on the moon, which
is still pointed directly at it, appears again in the shape of a huge
half-moon.

During the next seven days the moon returns to its origi