sidereal period Orbital period of a planet or other celestial body with respect to a background star. It is the true orbital period. Sidereal time is local time reckoned according to the rotation of the Earth with respect to the stars. The sidereal day is 23 hours, 56 minutes and 4 seconds of mean solar time, nearly 4 minutes shorter than the mean solar day. The sidereal year is equal to 365.25636 mean solar days.

si·de·re·al day • n. Astron. the time between two consecutive transits of the First Point of Aries. It represents the time taken by the earth to rotate on its axis relative to the stars, and is almost four minutes shorter than the solar day because of the earth's orbital motion.

sidereal day The time it takes for the Earth to complete one rotation on its axis such that a particular point on the surface returns to its former position in relation to the position of the fixed stars. The sidereal day is 4.09 minutes shorter than the mean solar day, because the movement of the Earth in its solar orbit is imposed on its rotational motion.

year Length of time taken by the Earth to circle once round the Sun in its orbit. It is defined in various ways, such as the sidereal year, which is timed with reference to the fixed stars.