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# Orbital Period - as a function of central body's density

## Description

The orbital period is the time taken for a given object to make one complete orbit around another object.

When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.
When a very small body is in a circular orbit barely above the surface of a sphere of any radius and mean density ρ, the orbital period equation simplifies as shown.
As an alternative for using a very small number like G, the strength of universal gravity can be described using some reference material, like water: the orbital period for an orbit just above the surface of a spherical body of water is 3 hours and 18 minutes. Conversely, this can be used as a kind of “universal” unit of time if we have a unit of mass, a unit of length and a unit of density.

Related formulas

## Variables

 T orbital period (sec) π pi G Newtonian constant of gravitation ρ density (kg/m3)