Sidereal time is time based on Earth's rotation with respect to the stars. Local sidereal time is given by the right ascension of the observer's meridian. Greenwich sidereal time is the sidereal time on the prime meridian. A sidereal day is Earth's rotation period with respect to the stars (23 hr 56 min 4 sec); a sidereal month is the Moon's orbital period around Earth with respect to the stars (27.32166 days); a sidereal year is Earth's orbital period around the Sun with respect to the stars; sidereal period is the orbital period of a planet around the Sun, or of a moon around its primary, with respect to the stars.