Mars comes closest to Earth roughly every 26 months and the Moon’s phases repeat every 29.5 days – both are synodic periods, the time between successive alignments as seen from a moving observer.
How to use the Synodic Period Calculator
- Planet
- Planet sidereal period
- Reference body sidereal period
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose a planet (or enter its sidereal period) and the reference body’s sidereal period (Earth: 365.256 days).
Inputs explained
| Field | What to enter |
|---|---|
| Planet | Choose one: Mercury, Venus, Mars, Jupiter … |
| Planet sidereal period | Units: days, years |
| Reference body sidereal period | Units: days, years |
Example values
How the main result changes when Planet sidereal period changes and everything else stays at the example values above:
| Planet sidereal period (days) | Synodic period (days) |
|---|---|
| 343.49 | 779.9343 |
| 515.24 | 779.9343 |
| 686.98 | 779.9343 |
| 858.73 | 779.9343 |
| 1,030.47 | 779.9343 |
| 1,373.96 | 779.9343 |
Formula
1/S = |1/P₁ − 1/P₂|
Worked example
Inputs — Planet: Mars; Reference body sidereal period: 365.256 days.
Results — Synodic period = 779.93425 days.
Key terms
- Synodic period
- Time between successive identical alignments.
Frequently asked questions
Why is the lunar month longer than the Moon's orbit?
Earth moves around the Sun, so the Moon needs ~2 extra days to line up again.
How often is Mars at opposition?
About every 780 days.
What is the sidereal period?
The orbital period relative to the distant stars.
Why is Venus's synodic period so long?
Its period is close to Earth's, so it takes 584 days to lap us.