In celestial mechanics, true anomaly is an angular parameter that defines the position of a body moving along a Keplerian orbit. It is the angle between the direction of periapsis and the current position of the body, as seen from the main focus of the ellipse (the point around which the object orbits).
The true anomaly is usually denoted by the Greek letters or, or the Latin letter, and is usually restricted to the range 0–360° (0–2π rad).
The true anomaly is one of three angular parameters (anomalies) that defines a position along an orbit, the other two being the eccentric anomaly and the mean anomaly.
For elliptic orbits, the true anomaly can be calculated from orbital state vectors as:
\nu=\arccos{{e ⋅ r
(if then replace by)
where:
For circular orbits the true anomaly is undefined, because circular orbits do not have a uniquely determined periapsis. Instead the argument of latitude u is used:
u=\arccos{{n ⋅ r
(if then replace)
where:
For circular orbits with zero inclination the argument of latitude is also undefined, because there is no uniquely determined line of nodes. One uses the true longitude instead:
l=\arccos{rx\over{\left|r\right|
(if then replace by)
where:
E
\cos{\nu}={{\cos{E}-e}\over{1-e\cos{E}}}
or using the sine[1] and tangent:
\begin{align} \sin{\nu}&={{\sqrt{1-e2}\sin{E}}\over{1-e\cos{E}}}\\[4pt] \tan{\nu}={{\sin{\nu}}\over{\cos{\nu}}}&={{\sqrt{1-e2}\sin{E}}\over{\cos{E}-e}} \end{align}
or equivalently:
\tan{\nu\over2}=\sqrt{{{1+e}\over{1-e}}}\tan{E\over2}
so
\nu=2\operatorname{arctan}\left(\sqrt{{{1+e}\over{1-e}}}\tan{E\over2}\right)
Alternatively, a form of this equation was derived by [2] that avoids numerical issues when the arguments are near
\pm\pi
| E | |
| 2 |
| \nu | |
| 2 |
| \tan{ | 1 |
| 2 |
(\nu-E)}=
| \beta\sin{E | |
\beta=
| e | |
| 1+\sqrt{1-e2 |
so
\nu=E+2\operatorname{arctan}\left(
| \beta\sin{E | |
M
\nu=M+2
| infty | |
| \sum | |
| k=1 |
| 1 | |
| k |
\left[
| infty | |
| \sum | |
| n=-infty |
| |k+n| | |
| J | |
| n(-ke)\beta |
\right]\sin{kM}
Jn
\beta=
| 1-\sqrt{1-e2 | |
Omitting all terms of order
e4
\operatorname{l{O}}\left(e4\right)
\nu=M+\left(2e-
| 1 | |
| 4 |
e3\right)\sin{M}+
| 5 | |
| 4 |
e2\sin{2M}+
| 13 | |
| 12 |
e3\sin{3M}+\operatorname{l{O}}\left(e4\right).
Note that for reasons of accuracy this approximation is usually limited to orbits where the eccentricity
e
The expression
\nu-M
The radius (distance between the focus of attraction and the orbiting body) is related to the true anomaly by the formula
r(t)=a{1-e2\over1+e\cos\nu(t)}
where a is the orbit's semi-major axis.