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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 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. 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.
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forces to
create rotation. A good demonstration of this effect is a toy top.