Precession – As Earth rotates, it wobbles slightly upon its axis, like a slightly off-center spinning toy top. This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon that cause Earth to bulge at the equator, affecting its rotation.

What are the effects of precession?

Precession causes the stars to change their longitude slightly each year, so the sidereal year is longer than the tropical year. Using observations of the equinoxes and solstices, Hipparchus found that the length of the tropical year was 365+1/4−1/300 days, or 365.24667 days (Evans 1998, p. 209).

What causes the precession of the Earth?

Answer: The precession of the Earth's rotational axis comes about due to tidal torques caused mostly by the differential pull on the Earth by the Moon and the Sun. These tidal torques cause the Earth to become less than spherical (oblate).