n their minds to account for this
"earthlight," or "earthshine," as it is also called. Posidonius (135-51
B.C.) tried to explain it by supposing that the moon was partially
transparent, and that some sunlight consequently filtered through from
the other side. It was not, however, until the fifteenth century that
the correct solution was arrived at.

[Illustration: One side of the moon only is ever presented to the
earth. This side is here indicated by the letters S.F.E. (side facing
earth).

By placing the above positions in a row, we can see at once that the
moon makes one complete rotation on her axis in exactly the same time as
she revolves around the earth.

FIG. 15.--The Rotation of the Moon on her Axis.]

Perhaps the most remarkable thing which one notices about the moon is
that she always turns the same side towards us, and so we never see her
other side. One might be led from this to jump to the conclusion that
she does not rotate upon an axis, as do the other bodies which we see;
but, paradoxical as it may appear, the fact that she turns one face
always towards the earth, is actually a proof that she _does_ rotate
upon an axis. The rotation, however, takes place with such slowness,
that she turns round but once during the time in which she revolves
around the earth (see Fig. 15). In order to understand the matter
clearly, let the reader place an object in the centre of a room and walk
around it once, _keeping his face turned towards it the whole time_,
While he is doing this, it is evident that he will face every one of the
four walls of the room in succession. Now in order to face each of the
four walls of a room in succession one would be obliged _to turn oneself
entirely round_. Therefore, during the act of walking round an object
with his face turned directly towards it, a person at the same time
turns his body once entirely round.

In the long, long past the moon must have turned round much faster than
this. Her rate of rotation has no doubt been slowed down by the action
of some force. It will be recollected how, in the course of the previous
chapter, we found that the tides were tending, though exceedingly
gradually, to slow down the rotation of the earth upon its axis. But, on
account of the earth's much greater mass, the force of gravitation
exercised by it upon the surface of the moon is, of course, much more
powerful than that which the moon exercises upon the surface of the
earth. The tendency