In the interaction the Moon tries to force the two axes to coincide. This force causes the direction in which the rotation axis points to change slowly. The motion is regular and is known as precession; the path on the Celestial Sphere is roughly circular. The radius of the circle is large, ~ 23.5 degrees (because for the most part, the angle which the Earth's rotation axis makes with the vertical doesn't change.). The period of the motion is long, 25,800 years, due to the weakness of the effect.
- Because the
point where the direction in which the rotation axis points changes, this
affects the period on which the seasons vary. This is why the Tropical
Year is shorter than the Sidereal Year.
- Because
the point where the projection
of the rotation axis of the Earth strikes
the Celestial Sphere moves around (the stars are still not in
motion), the
North Celestial Pole also moves through the stars.
This produces changes in the coordinates of
stars (even though they are not moving).
Today, it is fortuitous that
the North star is Polaris.
At other times, other stars will be
the North star:
In 12,000 years or so, Vega in the constellation Lyra will be near the North Celestical Pole.