Meaning

• The orbital period of the Earth; a measure of the time it takes for the Sun to return to the same position with respect to the stars of the celestial sphere. A sidereal year is about 20.4 minutes longer than the tropical year (which is what is kept stable by the Gregorian calendar) due to precession of the equinoxes.

• The time it takes for any astronomical object in orbit around a central star to orbit that star, with respect to distant stars.