value of that distance is 239,000 miles. In rare circumstances
it may approach to a distance but little more than 221,000 miles, or
recede to a distance hardly less than 253,000 miles, but the ordinary
fluctuations do not exceed more than about 13,000 miles on either side
of its mean value.

From the moon's incessant changes we perceive that she is in constant
motion, and we now further see that whatever these movements may be, the
earth and the moon must at present remain at _nearly_ the same distance
apart. If we further add that the path pursued by the moon around the
heavens lies nearly in a plane, then we are forced to the conclusion
that our satellite must be revolving in a nearly circular path around
the earth at the centre. It can, indeed, be shown that the constant
distance of the two bodies involves as a necessary condition the
revolution of the moon around the earth. The attraction between the moon
and the earth tends to bring the two bodies together. The only way by
which such a catastrophe can be permanently avoided is by making the
satellite move as we actually find it to do. The attraction between the
earth and the moon still exists, but its effect is not then shown in
bringing the moon in towards the earth. The attraction has now to exert
its whole power in restraining the moon in its circular path; were the
attraction to cease, the moon would start off in a straight line, and
recede never to return.

[Illustration: Fig. 24.--The Moon's Path around the Sun.]

The fact of the moon's revolution around the earth is easily
demonstrated by observations of the stars. The rising and setting of our
satellite is, of course, due to the rotation of the earth, and this
apparent diurnal movement the moon possesses in common with the sun and
with the stars. It will, however, be noticed that the moon is
continually changing its place among the stars. Even in the course of a
single night the displacement will be conspicuous to a careful observer
without the aid of a telescope. The moon completes each revolution
around the earth in a period of 27.3 days.

[Illustration: Fig. 25.--The Phases of the Moon.]

In Fig. 24 we have a view of the relative positions of the earth, the
sun, and the moon, but it is to be observed that, for the convenience of
illustration, we have been obliged to represent the orbit of the moon on
a much larger scale than it ought to be in comparison with the distance
of the sun. That half of th