nutation, in astronomy, a small irregularity in the precession of the equinoxes. Precession is the slow, toplike wobbling of the spinning Earth, with a period of about 25,772 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 Earth is tilted by 5.15° from the plane of Earth’s orbit around the Sun. The Moon’s orbital plane precesses around 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.