Synodic Period Calculator

About this calculator

This calculator finds the synodic period — the time between successive alignments (e.g. conjunctions or oppositions) of two orbiting bodies as seen from a common reference point — using 1/S = |1/T₁ − 1/T₂|, where T₁ and T₂ are the bodies' sidereal (true orbital) periods. The synodic period differs from the sidereal period because both bodies are moving.

Astronomers and astronomy students use it to predict when planets will appear closest in the sky (like Mars oppositions, roughly every 26 months) or to determine phenomena like the lunar month's 29.5-day cycle relative to Earth's motion around the Sun. Enter the two orbital periods to find how often the bodies realign.