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 .
The true anomaly is one of three angular parameters ("anomalies") that define a position along an orbit, the other two being the eccentric anomaly and the mean anomaly.
From state vectors
For elliptic orbits true anomaly can be calculated from orbital state vectors as:
- is orbital velocity vector of the orbiting body,
- is eccentricity vector,
- is orbital position vector (segment fp) of the orbiting body.
For circular orbits the true anomaly is undefined because circular orbits do not have a uniquely determined periapsis. Instead one uses the argument of latitude :
Circular orbit with zero inclination
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:
- is x-component of orbital position vector ,
- is x-component of orbital velocity vector .
From the eccentric anomaly
The relation between the true anomaly and the eccentric anomaly E is:
where is the polar argument of the vector (available in many programming languages as the library function atan2(y, x) in Fortran and MATLAB, or as ArcTan(x, y) in Wolfram Mathematica).
Radius from true anomaly
The radius (distance from the focus of attraction and the orbiting body) is related to the true anomaly by the formula
where a is the orbit's semi-major axis (segment cz).
- Murray, C. D. & Dermott, S. F. 1999, Solar System Dynamics, Cambridge University Press, Cambridge. ISBN 0-521-57597-4
- Plummer, H.C., 1960, An Introductory treatise on Dynamical Astronomy, Dover Publications, New York. Template:OCLC (Reprint of the 1918 Cambridge University Press edition.)