Glossary N

Nutation. This refers to an oscillation in the Earth's axis of rotation caused principally by the gravitational effects of the Moon upon it. This is separate from, and superimposed upon, precession. Nutation is a comparatively small but complex motion. Its largest component (9.2 seconds of arc in obliquity, equivalent to 284 metres at the equator) is caused by the regression of the nodes of the Moon's orbit and has a cycle length of 18.61 years. Other smaller components generally have a cycle length of under a year.
     Nutation was unknown to ancient or medieval astronomers and was discovered by James Bradley in 1728. However, taking account of the nutation is of some importance for predicting the positions of celestial objects in ancient times.