In astronomy, the term period usually refers to how long an object takes to complete one cycle of revolution. In particular the **orbital period**of a star or planet is the time it takes to return to the same place in the orbit. The**spin period**of a star is the time it takes to rotate on its axis.Alternately, a wave period is the time taken for one complete cycle of the wave to pass a reference point.

The period,

of an orbit, rotation, or wave, is just the inverse of the frequency,**P**:**f**The orbital period,

of two bodies of mass**P**and**M**that are gravitationally bound and separated by a distance**m**is:**R**Here

is Newton's gravitational constant.**G**