Precession-nutation

Precession-nutation is the displacement over time of the plane of the equator and the plane of the ecliptic relative to an inertial reference system, due to the gravitational action of the Moon, the Sun and the planets. The mathematical representation of this displacement contains secular terms, periodic series and Poisson series. By convention, precession is the name given to the secular terms and nutation is all the periodic and Poisson series.

Source: www.imcce.fr