sidereal period, in astronomy, length of time a body takes to complete an orbit relative to the fixed stars. See sidereal time.

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

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