Editing Sidereal time (section)

==Effects of precession==

Earth's rotation is not a simple rotation around an axis that remains always parallel to itself. Earth's rotational axis itself rotates about a second axis, [[orthogonality|orthogonal]] to the plane of Earth's orbit, taking about 25,800&nbsp;years to perform a complete rotation. This phenomenon is termed the [[Axial precession|precession of the equinoxes]]. Because of this precession, the stars appear to move around Earth in a manner more complicated than a simple constant rotation.

For this reason, to simplify the description of Earth's orientation in astronomy and [[geodesy]], it was conventional to chart the positions of the stars in the sky according to [[right ascension]] and [[declination]], which are based on a frame of reference that follows Earth's precession, and to keep track of Earth's rotation, through sidereal time, relative to this frame as well. (The conventional reference frame, for purposes of star catalogues, was replaced in 1998 with the [[International Celestial Reference Frame]], which is fixed with respect to extra-galactic radio sources. Because of the great distances, these sources have no appreciable [[proper motion]].{{Sfn|Urban|Seidelmann|2013|page=105}}) In this frame of reference, Earth's rotation is close to constant, but the stars appear to rotate slowly with a period of about 25,800 years. It is also in this frame of reference that the [[tropical year]] (or solar year), the year related to Earth's seasons, represents one orbit of Earth around the Sun. The precise definition of a sidereal day is the time taken for one rotation of Earth in this precessing frame of reference.