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Space Calc
Doing the math, so you don't have to.

Planet Calendar


Given the planetary parameters of a "normal" planetary system—i.e. a star much more massive than any planet, this calculator just reads off the number of days in a year, as well as how long each day is.

Note that many real-world planetary systems have two central stars; the coarse day-night cycle result will not change significantly by this unless the planet is very close (in which case you have dynamical instabilities anyway), albeit you will get a secondary weak-day-strong-day cycle which may be significant.


Note: negative values indicate retrograde rotation.

Star's Mass
Distance from Star to Planet
Planetary Rotation Period
Time to rotate once with respect to the distant stars (sidereal day length)
Year Length
planetary days
Day Length

Credits / See Also

See also a planet's orbital period.

This calculator was suggested by user Katharina.