mean anomaly

Shows constant areas being swept out per unit time   by an object in an elliptical orbit, and   by an imaginary object in a circular orbit with the same period. The angular sweep rate varies for the eliptic case. Also shows comparison of mean anomaly and true anomaly for two units of time. Note to avoid overlapping, the circular orbit has been magnified; in true scale the major axis diameter would be equal for ellipse and circle while the minor axis will be less for the ellipse sweeping out correspondingly less area per unit time (less angular momentum).

In celestial mechanics, the mean anomaly is the fraction of an elliptical orbit's period that has elapsed since the orbiting body passed periapsis, expressed as an angle which can be used in calculating the position of that body in the classical two-body problem. It is the angular distance from the pericenter which a fictitious body would have if it moved in a circular orbit, with constant speed, in the same orbital period as the actual body in its elliptical orbit.