n, therefore, always keeps to the same regions of
the sky as the sun. The slant of the earth's axis thus regularly
displaces the position of both the sun and the moon to the north and
south sides of the equator respectively in the manner we have been
describing. Were the earth, however, a perfect sphere, such change of
position would not produce any effect. We have shown, however, that the
earth is not a perfect sphere, but that it is bulged out all round the
equator. The result is that this bulged-out portion swings slowly under
the pulls of solar and lunar gravitation, in response to the
displacements of the sun and moon to the north and to the south of it.
This slow swing of the equatorial regions results, of course, in a
certain slow change of the direction of the earth's axis, so that the
north pole does not go on pointing continually to the same region of the
sky. The change in the direction of the axis is, however, so extremely
slight, that it shows up only after the lapse of ages. The north pole of
the heavens, that is, the region of the sky towards which the north pole
of the earth's axis points, displaces therefore extremely slowly,
tracing out a wide circle, and arriving back again to the same position
in the sky only after a period of about 25,000 years. At present the
north pole of the heavens is quite close to a bright star in the tail of
the constellation of the Little Bear, which is consequently known as the
Pole Star; but in early Greek times it was at least ten times as far
away from this star as it is now. After some 12,000 years the pole will
point to the constellation of Lyra, and Vega, the most brilliant star in
that constellation, will then be considered as the pole star. This slow
twisting of the earth's axis is technically known as _Precession_, or
the _Precession of the Equinoxes_ (see Plate XIX., p. 292).

The slow displacement of the celestial pole appears to have attracted
the attention of men in very early times, but it was not until the
second century B.C. that precession was established as a fact by the
celebrated Greek astronomer, Hipparchus. For the ancients this strange
cyclical movement had a mystic significance; and they looked towards the
end of the period as the end, so to speak, of a "dispensation," after
which the life of the universe would begin anew:--

"Magnus ab integro saeclorum nascitur ordo.
Jam redit et Virgo, redeunt Saturnia regna;
. . . . .