Synodic Period of Two Planets Calculator

About this calculator

The synodic period of two planets is the time it takes for them to return to the same relative configuration as seen from the Sun — for instance, the time between successive oppositions or conjunctions — and it's different from each planet's own orbital (sidereal) period because both bodies are moving. It's calculated from 1/S = |1/P1 − 1/P2|, where P1 and P2 are the two orbital periods in the same time unit; applying it to Earth (1.0 year) and Mars (1.88 years) gives 1/S = |1/1.0 − 1/1.88| ≈ 0.532, so S ≈ 1.88 years, matching the roughly 26-month gap between Mars oppositions.

Because the inner planet always “laps” the outer one, the synodic period is always longer than the shorter of the two orbital periods — Mercury and Venus complete many orbits between each conjunction with Earth. Amateur astronomers planning observation windows, planetarium educators, and mission planners use synodic periods to predict when a planet will next be at opposition (best viewing) or to time interplanetary launch windows, since spacecraft trajectories to another planet are timed around its synodic alignment with Earth.