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 tw 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.