o that the time will at length
come when the pole will have departed a long way from the present Pole
Star. This movement is incessant. It can be easily detected and measured
by the instruments in our observatories, and astronomers are familiar
with the fact that in all their calculations it is necessary to hold
special account of this movement of the pole. It produces an apparent
change in the position of a star, which is known by the term
"precession."

[Illustration: Fig. 100.]

The movement of the pole is very clearly shown in the accompanying
figure (Fig. 100), for which I am indebted to the kindness of the late
Professor C. Piazzi Smyth. The circle shows the track along which the
pole moves among the stars.

The centre of the circle in the constellation of Draco is the pole of
the ecliptic. A complete journey of the pole occupies the considerable
period of about 25,867 years. The drawing shows the position of the
pole at the several dates from 4000 B.C. to 2000 A.D. A glance at this
map brings prominently before us how casual is the proximity of the pole
to the Pole Star. At present, indeed, the distance of the two is
actually lessening, but afterwards the distance will increase until,
when half of the revolution has been accomplished, the pole will be at a
distance of twice the radius of the circle from the Pole Star. It will
then happen that the pole will be near the bright star Vega or a
Lyrae, so that our successors some 12,000 years hence may make use of
Vega for many of the purposes for which the Pole Star is at present
employed! Looking back into past ages, we see that some 2,000 or 3,000
years B.C. the star a Draconis was suitably placed to serve as the
Pole Star, when b and d of the Great Bear served as pointers.
It need hardly be added, that since the birth of accurate astronomy the
course of the pole has only been observed over a very small part of the
mighty circle. We are not, however, entitled to doubt that the motion of
the pole will continue to pursue the same path. This will be made
abundantly clear when we proceed to render an explanation of this very
interesting phenomenon.

The north pole of the heavens is the point of the celestial sphere
towards which the northern end of the axis about which the earth rotates
is directed. It therefore follows that this axis must be constantly
changing its position. The character of the movement of the earth, so
far as its rotation is concerned, may be il