The gravitational pull of
the Sun and the Moon tug on the Earth's equatorial bulge. As the
Sun and the Moon move along the zodiac, each spends half their
time south of the bulge. The gravitational pull of the Sun and
the Moon thus tries to straighten up the Earth (that is they try
to reduce the Earth's tilt). However, the Earth is spining and
this spin interacts with the gravitational forces to create rotation.
A good demonstration of this effect is a toy top. But wait there
is more! Since the Earth's axis or rotation precesses, the Earth's
equatorial plane does as well. The means that the location where
the ecliptic crosses the celestial ecliptic also changes. Recall
that this intersection is the point in space defines the equinoxes.
Thus the equinoxes also shift their position in the sky just like
the NCP! Today the vernal equinox is located in the constellation
Pisces (the fishes). Just two thousand years ago (a heartbeat
to the universe), it was in Aries (the Ram). In about 600 years
the vernal equinox will be in the constellation Aquarius (the
waterbearer). Notice that the South Celestial Pole also traces
out a circle in the sky similar to that of the North Celesital
pole. After a period of about 26,000 years the equinoxes and poles
lie once again at nearly the same points on the celestial sphere.This
precession causes the coordinates of stars to gradually shift
and over a few years add up to significant changes. To cope with
this problem astronomers keep track of the date (called the epoch)
for which a set of stellar coordinates are correct. This menas
that star charts and catalogs are updated periodically.