The wobbling motion of a spinning top or gyroscope in which the axis of rotation gradually sweeps out a conical shape. The spinning Earth undergoes a slow precession, due to the combined gravitational attractions of the Sun, Moon, and planets. The Earth's pole takes about 25,800 years to describe one complete circle on the celestial sphere; this circle has a radius of approximately 23.5, i.e. the inclination of the Earth's axis. The equinoxes make one circuit of the ecliptic in the same time. As a result of precession, the right ascension and declination of stars change with time, so the date or epoch for which these coordinates apply must always be stated. See also lunisolar precession; planetary precession; precession of the equinoxes.

"precession" A Dictionary of Astronomy. 1997.