Synodic Period Calculator: Time Between Repeats of the Same Alignment of Two Orbiting Bodies
How often two planets line up again.
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How this calculator works
Enter the orbital period of the first body.
Enter the orbital period of the second body.
The calculator gives the synodic period in days and years.
Formulas used
- \frac{1}{S} = \left|\frac{1}{T_1} - \frac{1}{T_2}\right|
- S = \frac{T_1T_2}{|T_1 - T_2|}
Worked example
With the default numbers in the calculator above:
- Synodic period: 779.934 days
- Synodic period in years: 2.1353 years
- Turning rate of the faster body relative to the slower one: 0.46158 degrees per day
Change the numbers in the calculator to see the full working, every step, for your own case.
Common questions
What is a synodic period?
The time between two successive times when two bodies are in the same position relative to each other, such as from one opposition of a planet to the next.
How often is Mars at opposition?
About every 780 days, which is roughly 26 months.
How is it different from the orbital period?
The orbital period is measured against the distant stars. The synodic period is measured against another orbiting body, such as the Earth.
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Any astronomy text (synodic and sidereal periods).
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