๐ชSidereal Period Calculator
Convert between a planet's synodic period and its true sidereal period.
Sidereal Day vs Solar Day
A sidereal day (23h 56m 4s, or 23.9345 hours) is how long Earth takes to rotate once relative to the distant stars. A solar day (24h) is slightly longer because Earth also moves along its orbit during that rotation, so it must turn a little extra to face the Sun again. The difference is about 3 minutes 56 seconds per day.
Similarly, a sidereal year (365.256 days) โ one full orbit relative to the stars โ differs slightly from a tropical year (365.242 days) โ the time between successive spring equinoxes โ due to the slow precession of Earth's axis.
Runs entirely in your browser. No data is sent to a server.
Free Sidereal Period Calculator
Why does Mars seem to loop backward in the sky every couple of years, even though it never actually reverses direction? Our free sidereal period calculator untangles this by converting between a planet's synodic period โ how long it takes to return to the same position relative to Earth and the Sun โ and its true sidereal period, measured against the fixed stars.
Choose whether the planet orbits inside or outside Earth's orbit, enter a known period, and get the other instantly, checked against real planetary data for Mercury, Venus, Mars, Jupiter, and Saturn. A built-in explainer also covers the related sidereal-day-versus-solar-day distinction for Earth, showing why a sidereal day is about four minutes shorter than 24 hours.
No sign-up, no downloads, no limits โ just clear orbital mechanics whenever you need it. See the About page or the FAQ for more.
About the Sidereal Period Calculator
Features
Inferior/superior toggle
Uses the correct formula for each planet type.
Real planet presets
Mercury, Venus, Mars, Jupiter, and Saturn synodic periods.
Day/year explainer
Covers sidereal vs solar day and sidereal vs tropical year.
Edge-case guard
Flags near-equal synodic/sidereal values instead of dividing by zero.
Frequently Asked Questions
What is a sidereal period?
The time a planet takes to complete one orbit relative to the distant "fixed" stars.
What is a synodic period?
The time between successive similar configurations of a planet as seen from Earth (e.g. opposition to opposition).
How does this calculator work?
Choose whether the planet is inferior (inside Earth's orbit) or superior (outside), enter one period, and it solves for the other.
Is it free?
Completely free, no sign-up, downloads, or limits.
Why are synodic and sidereal periods different?
Because Earth is also moving around the Sun, so relative alignments take longer (or shorter) to repeat than a planet's true orbital period.
What is Mars's sidereal period?
About 687 Earth days, though its synodic period (how often it's closest to Earth) is about 780 days.
What's the difference between a sidereal day and a solar day?
A sidereal day (~23h56m) is how long Earth takes to rotate relative to the stars; a solar day (24h) accounts for Earth's orbital motion too.
What happens if synodic and sidereal periods are nearly equal?
The calculator flags this rare edge case instead of dividing by a near-zero number.
Is my data stored?
No; everything runs in your browser and isn't stored or sent anywhere.
Does it work on mobile?
Yes, fully responsive on all screen sizes.