In celestial mechanics, the mean anomaly is a parameter relating position and time for a body moving in a Kepler orbit. It is based on the fact that equal areas are swept at the focus in equal intervals of time.

The mean anomaly increases uniformly from 0 to Failed to parse (Missing texvc executable; please see math/README to configure.): 2\pi

The mean anomaly is usually denoted by the letter Failed to parse (Missing texvc executable; please see math/README to configure.): M , and is given by the formula:

where n is the mean motion, a is the length of the orbit's semi-major axis, Failed to parse (Missing texvc executable; please see math/README to configure.): M_\star

The mean anomaly is the time since the last periapsis multiplied by the mean motion, and the mean motion is Failed to parse (Missing texvc executable; please see math/README to configure.): 2\pi

The mean anomaly is one of three angular parameters ("anomalies") that define a position along an orbit; the other two being the eccentric anomaly and the true anomaly. If the mean anomaly is known at any given instant, it can be calculated at any later (or prior) instant by simply adding (or subtracting) Failed to parse (Missing texvc executable; please see math/README to configure.): \sqrt{\frac{ G( M_\star \! + \!m ) } {a^3}} \,\delta t