the earth's pole should make a certain composite movement due to the two
separate movements. This is really the case, but there is a point to be
very carefully attended to, which at first seems almost paradoxical. We
have shown how the potency of the moon as a precessional agent exceeds
that of the sun, and therefore it might be thought that the composite
movement of the earth's pole would conform more nearly to a rotation
around the pole of the plane of the moon's orbit than to a rotation
around the pole of the ecliptic; but this is not the case. The
precessional movement is represented by a revolution around the pole of
the ecliptic, as is shown in the figure. Here lies the germ of one of
those exquisite astronomical discoveries which delight us by
illustrating some of the most subtle phenomena of nature.

The plane in which the moon revolves does not occupy a constant
position. We are not here specially concerned with the causes of this
change in the plane of the moon's orbit, but the character of the
movement must be enunciated. The inclination of this plane to the
ecliptic is about 5 deg., and this inclination does not vary (except within
very narrow limits); but the line of intersection of the two planes does
vary, and, in fact, varies so quickly that it completes a revolution in
about 18-2/3 years. This movement of the plane of the moon's orbit
necessitates a corresponding change in the position of its pole. We thus
see that the pole of the moon's orbit must be actually revolving around
the pole of the ecliptic, always remaining at the same distance of 5 deg.,
and completing its revolution in 18-2/3 years. It will, therefore, be
obvious that there is a profound difference between the precessional
effect of the sun and of the moon in their action on the earth. The sun
invites the earth's pole to describe a circle around a fixed centre; the
moon invites the earth's pole to describe a circle around a centre which
is itself in constant motion. It fortunately happens that the
circumstances of the case are such as to reduce considerably the
complexity of the problem. The movement of the moon's plane, only
occupying about 18-2/3 years, is a very rapid motion compared with the
whole precessional movement, which occupies about 26,000 years. It
follows that by the time the earth's axis has completed one circuit of
its majestic cone, the pole of the moon's plane will have gone round
about 1,400 times. Now, as this pole reall