Nutation

Nutation is a slight irregular motion (etymologically a "nodding") in the axis of rotation of a planet, due to the fact that the tidal forces which cause precession of the equinoxes vary over time so that the speed of precession is not constant.

In the case of Earth, the principal sources of tidal force are the Sun and Moon, which continually change location relative to each other and thus cause nutation in Earth's axis. Earth's nutation has a period of 18.6 years, the same as that of the precession of the Moon's orbital nodes. Nutation was discovered in 1728 by the English astronomer James Bradley, but was not explained until 20 years later.