Astronomy education tool
Orbital periods: sidereal vs synodic
Compare true orbital periods with the repeat cycles we observe from Earth for planets, dwarf planets, and the Moon.
Sidereal vs synodic
Compare orbital periods from Earth
Pick a body to compare its true orbit with the repeat cycle you see from Earth.
Sidereal period
1.88 Earth years
Synodic period
Repeat geometry seen from Earth
Orbital speed
Average orbital speed
Vs Earth year
Sidereal period / 365.26 days
When Mars rises in the same spot at the same time each night, that's the synodic cycle: Earth has caught up to the same viewing geometry again.
| Body | Type | Sidereal | Synodic from Earth | Speed | Earth comparison |
|---|---|---|---|---|---|
| Mercury | planet | 87.97 days | 115.88 days | 47.36 km/s | 0.24x Earth |
| Venus | planet | 224.7 days | 583.92 days | 35.02 km/s | 0.62x Earth |
| Earth | planet | 365.26 days | Not defined | 29.78 km/s | 1.00x Earth |
| Mars | planet | 686.97 days | 779.94 days | 24.07 km/s | 1.88x Earth |
| Jupiter | planet | 4,332.82 days | 398.88 days | 13.06 km/s | 11.86x Earth |
| Saturn | planet | 10,755.7 days | 378.1 days | 9.64 km/s | 29.45x Earth |
| Uranus | planet | 30,687.15 days | 369.66 days | 6.81 km/s | 84.01x Earth |
| Neptune | planet | 60,190.03 days | 367.49 days | 5.43 km/s | 164.79x Earth |
| Ceres | dwarf planet | 1,683.8 days | 466.44 days | 17.88 km/s | 4.61x Earth |
| Pluto | dwarf planet | 90,560.02 days | 366.74 days | 4.74 km/s | 247.93x Earth |
| Eris | dwarf planet | 203,643.69 days | 365.91 days | 3.62 km/s | 557.53x Earth |
| Makemake | dwarf planet | 112,320.49 days | 366.45 days | 4.42 km/s | 307.51x Earth |
| Haumea | dwarf planet | 104,035.85 days | 366.54 days | 4.53 km/s | 284.83x Earth |
| Moon | moon | 27.32 days | 29.53 days | 1.02 km/s | 0.07x Earth |
Source trail: NASA/JPL Solar System Dynamics planetary parameters, JPL Horizons, and the JPL Small-Body Database. Synodic values are Earth-relative mean intervals rounded for education.
What sidereal and synodic periods mean
A sidereal period is the real orbital period measured against the background stars. Mars takes about 687 Earth days to complete one sidereal orbit around the Sun. The Moon takes about 27.32 days to circle Earth relative to the stars. This is the period you want when asking how long one physical orbit takes.
A synodic period is different: it is the time between repeat viewing geometries from Earth. The familiar Moon example is the month from one new moon to the next. During the Moon's 27.32-day sidereal orbit, Earth also moves around the Sun, so the Moon needs roughly two extra days to line up with the Sun again. That is why the lunar phase cycle is about 29.53 days, not 27.32 days.
Planetary synodic periods work the same way. Venus completes a sidereal orbit in about 224.70 days, but its Earth-viewed synodic cycle is about 583.92 days. Mars takes about 686.97 days to orbit the Sun, yet oppositions recur about every 779.94 days because Earth must catch up to Mars again. The table cites NASA/JPL Solar System Dynamics, JPL Horizons, the JPL Small-Body Database, and NASA fact sheets; values are rounded for classroom comparison.
Sources: JPL planetary parameters, JPL Horizons, JPL Small-Body Database.