Mean Anomaly
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Mean Anomaly
Area swept out per unit time     by an object in an elliptical orbit, and     by an imaginary object in a circular orbit (with the same orbital period). Both sweep out equal areas in equal times, but the angular rate of sweep varies for the elliptical orbit and is constant for the circular orbit. Shown are mean anomaly and true anomaly for two units of time. (Note that for visual simplicity, a nonoverlapping circular orbit is diagrammed, thus this circular orbit with same orbital period is Not shown in true scale with this elliptical orbit. For scale to be true for orbits of equal period, diameter of circle must equal major axis of ellipse, thus these orbits must intersect at two points. Visually simplified, but Theoretically confusing.)

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.[1][2]

## Definition

Define T as the time required for a particular body to complete one orbit. In time T, the radius vector sweeps out 2? radians or 360°. The average rate of sweep, n, is then

${\displaystyle n={\frac {2\pi }{T}}={\frac {360^{\circ }}{T}},}$

which is called the mean angular motion of the body, with dimensions of radians per unit time or degrees per unit time.

Define τ as the time at which the body is at the pericenter. From the above definitions, a new quantity, M, the mean anomaly can be defined

${\displaystyle M=n(t-\tau ),}$

which gives an angular distance from the pericenter at arbitrary time t,[3] with dimensions of radians or degrees.

Because the rate of increase, n, is a constant average, the mean anomaly increases uniformly (linearly) from 0 to 2? radians or 0° to 360° during each orbit. It is equal to 0 when the body is at the pericenter, ? radians (180°) at the apocenter, and 2? radians (360°) after one complete revolution.[4] 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) n?t where ?t represents the time difference.

Mean anomaly does not measure an angle between any physical objects. It is simply a convenient uniform measure of how far around its orbit a body has progressed since pericenter. The mean anomaly is one of three angular parameters (known historically as "anomalies") that define a position along an orbit, the other two being the eccentric anomaly and the true anomaly.

## Formulae

The mean anomaly M can be computed from the eccentric anomaly E and the eccentricity e with Kepler's Equation:

${\displaystyle M=E-e\sin E.}$

Mean anomaly is also frequently seen as

${\displaystyle M=M_{0}+n\left(t-t_{0}\right),}$

where M0 is the mean anomaly at epoch and t0 is the epoch, a reference time to which the orbital elements are referred, which may or may not coincide with τ, the time of pericenter passage. The classical method of finding the position of an object in an elliptical orbit from a set of orbital elements is to calculate the mean anomaly by this equation, and then to solve Kepler's equation for the eccentric anomaly.

Define ? as the longitude of the pericenter, the angular distance of the pericenter from a reference direction. Define l as the mean longitude, the angular distance of the body from the same reference direction, assuming it moves with uniform angular motion as with the mean anomaly. Thus mean anomaly is also[5]

${\displaystyle M=l-\varpi .}$

Mean angular motion can also be expressed,

${\displaystyle n={\sqrt {\frac {\mu }{a^{3}}}},}$

where ? is a gravitational parameter which varies with the masses of the objects, and a is the semi-major axis of the orbit. Mean anomaly can then be expanded,

${\displaystyle M={\sqrt {\frac {\mu }{a^{3}}}}(t-\tau ),}$

and here mean anomaly represents uniform angular motion on a circle of radius a.[6]

Mean anomaly can be expressed as a series expansion of the eccentricity e and the true anomaly ?,[7]

${\displaystyle M=\nu -2e\sin \nu +\left({\frac {3}{4}}e^{2}+{\frac {1}{8}}e^{4}\right)\sin 2\nu -{\frac {1}{3}}e^{3}\sin 3\nu +{\frac {5}{32}}e^{4}\sin 4\nu +\cdots }$

A similar formula gives the true anomaly directly in terms of the mean anomaly:[8]

${\displaystyle \nu =M+\left(2e-{\frac {1}{4}}e^{3}\right)\sin M+{\frac {5}{4}}e^{2}\sin 2M+{\frac {13}{12}}e^{3}\sin 3M+\cdots }$

## References

1. ^ Montenbruck, Oliver (1989). Practical Ephemeris Calculations. Springer-Verlag. p. 44. ISBN 0-387-50704-3.
2. ^ Meeus, Jean (1991). Astronomical Algorithms. Willmann-Bell, Inc., Richmond, VA. p. 182. ISBN 0-943396-35-2.
3. ^ Smart, W. M. (1977). Textbook on Spherical Astronomy (sixth ed.). Cambridge University Press, Cambridge. p. 113. ISBN 0-521-29180-1.
4. ^ Meeus (1991), p. 183
5. ^ Smart (1977), p. 122
6. ^ Vallado, David A. (2001). Fundamentals of Astrodynamics and Applications (second ed.). El Segundo, CA: Microcosm Press. pp. 53-54. ISBN 1-881883-12-4.
7. ^ Smart, W. M. (1953). Celestial Mechanics. Longmans, Green and Co., London. p. 38.
8. ^ Roy, A.E. (1988). Orbital Motion (1 ed.). Bristol, UK; Philadelphia, PA: A. Hilger. ISBN 0852743602.