# orbital period

The orbital period is the time taken for a body to go once around a closed orbit. The orbital period of a planet is its "year".

For a body in an elliptical orbit of semimajor
axis *a* about a much more massive body of mass *M* the orbital
period *P* is given by

*P* = (2π*a*^{3/2}) / √(*gM*).

This is the mathematical formulation of the third of Kepler's laws of planetary motion.