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(precession of a body's axis of rotation)

A nutation is a movement of the direction of a body's axis of rotation, such that a point where the axis emerges from the body describes a circle as the axis rotates.

For Earth, the term precession is used for a long, regular cycle of Earth's axial tilt of about 25772 years whereas nutation is used for small irregularities in the precession (e.g., times when the change is preceding sightly faster or slower), and other smaller changes in the axial tilt, due to conditions on Earth moving portions of its mass, and due to gravitational interactions with other astronomical bodies. Earth's nutation is sufficiently significant (several arcseconds) that it must be taken into account in determining accurate and precise coordinates for a celestial object, and is especially need in astrometry and in the determination of orbits of solar system bodies.

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