in the heavens
called the Pole Star, and we know that at any particular place it is seen
constantly in the north at a definite height above the horizon, which is
the latitude of the place. When watched carefully with a telescope it is
found to be not absolutely stationary, but to describe a small circle in
the heavens day by day, or rather night by night. These simple facts are
bound up with the phenomenon of the earth's rotation in this way: the axis
about which it is rotating points to the centre of that little circle, and
any change in the position of the axis can therefore be determined by
observing these motions of the Pole Star. Such changes may be of two
kinds: firstly, we might find that the size of the circle increased or
diminished, and this would mean that the earth's axis was pointing farther
away from the Pole Star or nearer to it--pointing, that is to say, in a
different direction in space. This actually happens (as has been known for
some thousands of years) owing to the phenomenon called "precession"; the
circle described by our Pole Star is at present getting a little smaller,
but it will ultimately increase in size, and after thousands of years
become so large that the Pole Star will entirely lose its character as a
steady guide to the North.

[Sidenote: Change of latitude.]

[Sidenote: Twenty years ago disbelieved.]

Secondly (and this is what more immediately concerns us), the centre of
the circle may alter its position and be no longer at the same height
above the horizon of any given place. This would mean that the earth's
axis was shifting _in the earth itself_--that the North Pole which our
explorers go to seek is not remaining in the same place. That it does not
change appreciably in position we know from familiar experience; our
climates, for instance, would suffer considerably if there were any large
changes. But astronomers are concerned with minute changes which would not
have any appreciable effect on climate, and the question has long been
before them whether, putting aside large movements, there were any minute
variations in position of the North Pole. Twenty years ago the answer to
this question would have been given decidedly in the negative; it was
considered as certain that the North Pole did not move at all within the
limits of our most refined astronomical observations. Accepted theory
seemed to indicate that any movements must in any case recur after a
period of ten months, and