Definitions

# Eccentric anomaly

The eccentric anomaly is the angle between the direction of periapsis and the current position of an object on its orbit, projected onto the ellipse's circumscribing circle perpendicularly to the major axis, measured at the centre of the ellipse. In the diagram below, it is E (the angle zcx).

## Calculation

In astrodynamics eccentric anomaly E can be calculated as follows:

$E=arccos \left\{\left\{1-left | mathbf\left\{r\right\} right | / a\right\} over e\right\}$

where:

• $mathbf\left\{r\right\},!$ is the orbiting body's position vector (segment sp),
• $a,!$ is the orbit's semi-major axis (segment cz), and
• $e,!$ is the orbit's eccentricity.

The relation between E and M, the mean anomaly, is:

$M = E - e , sin\left\{E\right\}.,!$

This equation can be solved iteratively, starting from $E_0 = M$ and using the relation $E_\left\{i+1\right\} = M + e,sin E_i$.

The equation can also be expanded in powers of $e$, as long as $e < 0.6627434$. The first few terms of the expansion are:

• $E_1 = M + e,sin M$
• $E_2 = M + e,sin M + frac\left\{1\right\}\left\{2\right\} e^2 sin 2M$
• $E_3 = M + e,sin M + frac\left\{1\right\}\left\{2\right\} e^2 sin 2M$

+ frac{1}{8} e^3 (3sin 3M - sin M). For references on details of this derivation, as well as other more efficient methods of solution, see Murray and Dermott (1999, p.35). For a derivation of the limiting value of $e$ see Plummer (1960, section 46).

The relation between E and ν, the true anomaly, is:

$cos\left\{nu\right\} = \left\{\left\{cos\left\{E\right\} - e\right\} over \left\{1 - e cdot cos\left\{E\right\}\right\}\right\}$

or equivalently

$tan\left\{nu over 2\right\} = sqrt tan\left\{E over 2\right\}.,$

The relations between the radius (position vector magnitude) and the anomalies are:

$r = a left \left(1 - e cdot cos\left\{E\right\} right \right),!$

and

$r = a\left\{1 - e^2 over 1 + e cdot cos\left\{nu\right\}\right\}.,!$