nutasjon

Nutation is the small periodic oscillation in the orientation of Earth's rotation axis relative to the fixed stars, superimposed on the long-term precession of the equinoxes. In contrast to precession, which is a slow 26,000-year cycle, nutation produces shorter-term wiggles of the axis with amplitudes of only a few arcseconds.

Nutation arises primarily from gravitational torques exerted by the Moon and Sun on Earth's equatorial bulge. The dominant term is the lunisolar nutation with a period of about 18.6 years, tied to the regression of the Moon's nodes. The largest contributions are about 17.2 arcseconds in longitude (Δψ) and 9.2 arcseconds in obliquity (Δε), with many smaller terms also present.

Because the angular changes involved are very small, nutation is a subtle effect, but it is essential for precise celestial coordinates. It is included in modern precession–nutation models used to transform coordinates between celestial and terrestrial reference frames. Models such as the IAU 2000A (and later refinements) provide the mathematical expressions for the nutation in longitude and obliquity, Δψ and Δε, respectively. Observations from very-long-baseline interferometry (VLBI) and other astrometric data are used to refine these models.

Nutation affects ephemeris calculations, navigation, and the alignment of telescopes, and it is a standard topic in astronomy and geodesy for high-precision celestial mechanics.