In mechanics, nutation is a bobbing motion that accompanies the precession of a spinning rigid body, such as a top. Astronomical nutation refers to irregularities in the precessional motion of rotating bodies. A well-studied example is the irregularities in the precessional motion of the equinoxes caused by the varying torque applied to the Earth by the Sun and Moon. Astronomical nutation should not be confused with nutation as defined in mechanics; the latter is present even if the source of the torques is unvarying.