The time required for a celestial body in the solar system to complete one revolution with respect to the fixed stars (as observed from a fixed point outside the system). A planet's sidereal period can be calculated from its synodic period or the length of time during which a body in the solar system makes one orbit of the sun relative to the Earth.

The sidereal period of the Moon or an artificial satellite of the Earth is the time it takes to return to the same position against the background of stars.

1. Time that is measured by the stars rather than by the sun: Sidereal time is based on the sidereal day, which is the unit of measurement.

The sidereal time is a unit of time used in astronomy, equal to the period of time in which the Earth makes one rotation relative to the stars.
2. The time taken between successive meridian passages of the same star (23 hours, 56 minutes, 4.091 seconds): The sidereal day is about four minutes shorter than the twenty-four hours of the "solar day".