sidereal year, time required for the earth to complete an orbit of the sun relative to the stars. The sidereal year is 365 days, 6 hr, 9 min, 9.5 sec of mean solar time (see solar time). It is 20 min longer than the tropical year because of the precession of the equinoxes; for this reason, the sidereal year does not stay in step with the seasons.