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.