n fact, the moon revolves
around the earth in twenty-seven days, and its rotation about its axis
is performed in twenty-seven days also. You may illustrate the
movement of the moon around the earth by walking around a table in a
room, keeping all the time your face turned towards the table; in such
a case as this you not only perform a motion of revolution, but you
also perform a rotation in an equal period. The proof that you do
rotate is to be found in the fact that during the movement your face
is being directed successively to all the points of the compass. There
is no more singular fact in the solar system than the constancy of the
moon's face to the earth. The periods of rotation and revolution are
both alike; if one of these periods exceeded the other by an amount so
small as the hundredth part of a second, the moon would in the lapse
of ages permit us to see that other side which is now so jealously
concealed. The marvellous coincidence between these two periods would
be absolutely inexplicable, unless we were able to assign it to some
physical cause. It must be remembered that in this matter the moon
occupies a unique position among the heavenly host. The sun revolves
around on its axis in a period of twenty-five or twenty-six days--thus
we see one side of the sun as frequently as we see the other. The side
of the sun which is turned towards us to-day is almost entirely
different from that we saw a fortnight ago. Nor is the period of the
sun's rotation to be identified with any other remarkable period in
our system. If it were equal to the length of the year, for instance,
or if it were equal to the period of any of the other planets, then it
could hardly be contended that the phenomenon as presented by the moon
was unique; but the sun's period is not simply related, or indeed
related at all, to any of the other periodic times in the system. Nor
do we find anything like the moon's constancy of face in the behaviour
of the other planets. Jupiter turns now one face to us and then
another. Nor is his rotation related to the sun or related to any
other body, as our moon's motion is related to us. It has indeed been
thought that in the movements of the satellites of Jupiter a somewhat
similar phenomenon may be observed to that in the motion of our own
satellite. If this be so, the causes whereby this phenomenon is
produced are doubtless identical in the two cases.

So remarkable a coincidence as that which the moon'