Synodic Period Calculator
Calculate a planet's synodic period — the time between successive oppositions or conjunctions as seen from Earth.
Select any planet or enter a custom period.
The synodic period is the time it takes for a planet to return to the same apparent position relative to the Sun as seen from Earth (e.g., from one opposition to the next).
It differs from the sidereal period (the true orbital period relative to the stars) because Earth is also moving.
Formula for an outer planet (P > 1 year):
1/P_syn = 1/P_Earth - 1/P_planet = 1 - 1/P_planet
For an inner planet (P < 1 year):
1/P_syn = 1/P_planet - 1/P_Earth = 1/P_planet - 1
Where all periods are in Earth years.
Planetary synodic periods:
| Planet | Sidereal Period | Synodic Period |
|---|---|---|
| Mercury | 0.241 yr | 115.9 days |
| Venus | 0.615 yr | 583.9 days |
| Mars | 1.881 yr | 779.9 days (2.135 yr) |
| Jupiter | 11.86 yr | 398.9 days |
| Saturn | 29.46 yr | 378.1 days |
| Uranus | 84.01 yr | 369.7 days |
| Neptune | 164.8 yr | 367.5 days |
Note on outer planets: The further away an outer planet is, the closer its synodic period is to 1 Earth year. This is because very distant planets hardly move relative to the stars over one year, so Earth essentially “laps” them once per year.
Oppositions (outer planets) occur when Earth is between the Sun and the planet — best time to observe. Conjunctions occur when the planet is behind the Sun (superior) or between Earth and Sun (inferior — inner planets only).