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_.