Mean anomaly

In the study of orbital dynamics the mean anomaly of an orbiting body is the angle the body would have traveled about the center of the orbit's auxiliary circle. Unlike other measures of anomaly, the mean anomaly grows linearly with time. The mean anomaly is conceptually an orbital clock that reads from 0 to 360 degrees or from 0 to 2π radians, passing "midnight" (zero) and restarting at perigee when each new orbit begins.

Because of the linear growth with time, the mean anomaly makes calculating the time of flight between two points on the orbit very easy. The mean anomalies for the two points are calculated and their difference is found. Knowing this, the ratio of this difference relative to the entire $2pi$ encompassing one orbit is simply equal to ratio of the time of flight to the period of one whole orbit (i.e. $frac\{M\_2\; -\; M\_1\}\{2pi\}\; =\; frac\{t\}\{T\}$).

When measured in radians the mean anomaly has a value of $0$ at the initial crossing of the orbit's point of periapsis, and a multiple of $2pi$ at any later crossing of the point periapsis. In the diagram below, the mean anomaly of point $p$ on the orbit around $s$ is given by angle $M$ (the angle $angle\; zcy$).

The point y is defined such that the circular sector area z-c-y is equal to the elliptic sector area z-s-p, scaled up by the ratio of the major to minor axes of the ellipse. Or, in other words, the circular sector area z-c-y is equal to the area x-s-z.

Calculation

In astrodynamics mean anomaly $M,!$ can be calculated as follows:

$M\; =\; M\_0\; +\; n(t-t\_0),!$

where:

• $M\_0,!$ is the mean anomaly at time $t\_0,!$,
• $t\_0,!$ is the start time,
• $t,!$ is the time of interest, and
• $n,!$ is the mean motion.

Alternatively:

$M=E\; -\; e\; cdot\; sin\; E,!$

where:

• $E,!$ is orbit's eccentric anomaly,
• $e,!$ is orbit's eccentricity.

See also

• Kepler's laws of planetary motion
• Eccentric anomaly
• True anomaly