V. PRECESSION

Due to the interaction between the Earth and the Moon, the rotation axis of the Earth is not stationary. The interaction arises because the Moon is not in the equatorial plane of the Earth; there is a misalignment between the spin axis of the Earth and the orbital axis of the Moon



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 quite regular; the path on the Celestial Sphere is roughly circular. The radius of the circle is large, ~ 23.5 degrees (is this a surprise?). The period of the motion is long, 26,000 years, due to the weakness of the effect.

Since 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), this means that the North Celestial Pole moves around 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:

The precession of the Earth leads to the difference between the Tropical and Sidereal years. How?

The Tropical year is shorter than the Sidereal year. What does this say about the direction in which the rotation axis of the Earth precesses?