precessionnutation

Precessionnutation refers to the combined motion of the orientation of the Earth's rotation axis with respect to the distant stars. In astronomy, this motion is typically described as two components: precession and nutation.

Precession is a slow, wide-angle conical motion of the Earth's axis caused mainly by gravitational torques exerted by the Sun and Moon on the Earth's equatorial bulge. It causes the vernal equinox to drift along the ecliptic at a rate of about one degree every 72 years, completing a full cycle in roughly 26,000 years. This long-term change redefines the reference direction for celestial coordinates over centuries.

Nutation consists of shorter-period oscillations riding on top of the precession. These periodic terms arise from the varying gravitational influence of the Moon’s orbit and the Sun. The largest nutation term has an approximate 18.6-year period linked to the regression of the Moon’s ascending node, with numerous additional terms spanning months to years. Nutation produces small, periodic variations in right ascension and declination.

In practice, precessionnutation is modeled as a rotation of the celestial reference frame that is split into a precession part and a nutation part, and then combined to transform coordinates between epochs or between inertial frames such as the International Celestial Reference Frame and observed sky coordinates. Modern models use extensive series of periodic terms for nutation and time-dependent precession formulas, encoded in the IAU conventions and IERS standards. These models underpin precise astrometry, spacecraft navigation, and star catalogs.

See also: celestial mechanics, IAU resolutions, IERS conventions, equator and ecliptic.