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.