The mean time taken by any object in the solar system to move between successive returns to the same position, relative to the sun as seen from the earth. Since a planet is best observed at opposition the synodic period of a planet, S, is easier to measure than its sidereal period, P. For inferior planets 1/S=1/P−1/E; for superior planets 1/S=1/E−1/P, where E is the sidereal period of the earth.