fair round the short
vertical rod as an axis. Then the apple will move with respect to the
orange as the moon would move with respect to the earth on the
suppositions just made. No one in this case would say that the apple was
turning round on its axis, since its motion would be one of rotation
round the upright axis through the orange. Therefore, say the opponents
of the moon's rotation, no one should say that the moon turns round on
her axis.

Of course, the answer would be obvious even if the moon's motions were
as supposed. The moon is not connected with the earth as the apple is
with the orange in the illustrative case. If the apple, without rigid
connection with the orange, were carried round the orange so as to move
precisely as if it were so connected, it would unquestionably have to
rotate on its axis, as any one will find who may try the experiment.
Thus for the straight rod thrust through the apple substitute a straight
horizontal bar carrying a small basin of water in which the apple
floats. Sway the bar steadily and slowly round, and it will be found (if
a mark is placed on the apple) that the apple no longer keeps the same
face towards the centre of motion; but that, to cause it to do so, a
slow motion of rotation must be communicated to the apple in the same
direction and at the same rate (neglecting the effects of the friction
of the water against the sides of the basin) as the bar is rotating. In
my 'Treatise on the Moon' I have described and pictured a simple
apparatus by which this experiment may easily be made.

But, of course, such experiments are not essential to the argument by
which the paradox is overthrown. This argument simply is, that the moon
as she travels on her orbit round the sun--the real centre of her
motion--turns every part of her equator in succession towards him once
in a lunar month. At the time of new moon the sun illuminates the face
of the moon turned from us; at the time of full moon he illuminates the
face which has been gradually brought round to him as the moon has
passed through her first two quarters. As she passes onwards to new
moon again, the face we see is gradually turned from him until he
shines full upon the other face. And so on during successive lunations.
This could not happen unless the moon rotated. Again, if we lived on the
moon we should find the heaven of the fixed stars turning round from
east to west once in rather more than twenty-seven days; and unless