nutation

Nutation is a small periodic motion of the Earth's axis of rotation relative to distant stars, occurring on top of the long-term precession of the equinoxes. While precession traces a slow conical movement of the axis over about 26,000 years, nutation adds shorter oscillations.

Causes: The gravitational torques exerted by the Moon and the Sun on Earth's equatorial bulge produce a changing orientation of the rotation axis. The Moon's orbital plane around Earth is inclined to Earth's equator and slowly regresses, causing a time-varying torque that induces nutation.

Characteristics: The dominant nutation component has an 18.6-year period, linked to the regression of the Moon's ascending node. Nutation is described by two angles: delta psi, the nutation in longitude, and delta epsilon, the nutation in obliquity. Typical amplitudes are about 17 arcseconds for delta psi and about 9 arcseconds for delta epsilon. In addition, there are many smaller terms from solar and lunar tides with shorter periods, including diurnal and semi-diurnal components.

Observational and practical aspects: Nutation affects the precise orientation of the Earth relative to celestial reference frames. Astronomical ephemerides, star catalogs, and spacecraft navigation apply nutation corrections to convert between celestial coordinates and terrestrial coordinates.

Terminology: The concept of nutation can apply to other rotating bodies, but the Earth's nutation is the best known example. Full nutation models include many terms beyond the dominant 18.6-year component.