Nutation:

Nutation is, in astronomy, a small irregularity in the precession of the equinoxes. Precession is the slow, top-like wobbling of the spinning Earth, with a period of about 26,000 years. Nutation (Latin nutare, "to nod") superimposes a small oscillation, with a period of 18.6 years and an amplitude of 9.2 seconds of arc, upon this great slow movement.

The cause of nutation lies chiefly in the fact that the plane of the Moon's orbit around the Earth is tilted by about 5 from the plane of the Earth's orbit around the Sun. The Moon's orbital plane precesses around the Earth's in 18.6 years, and the effect of the Moon on the precession of the equinoxes varies with this same period. The British astronomer James Bradley announced his discovery of nutation in 1748.