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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 ν or θ, or the Latin letter f, and is usually restricted to the range 0–360° (0–2π rad).
The true anomaly f 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 ν can be calculated from orbital state vectors as:
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:
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:
The relation between the true anomaly ν and the eccentric anomaly is:
or using the sine and tangent:
Alternatively, a form of this equation was derived by  that avoids numerical issues when the arguments are near , as the two tangents become infinite. Additionally, since and are always in the same quadrant, there will not be any sign problems.
The true anomaly can be calculated directly from the mean anomaly via a Fourier expansion:
with Bessel functions and parameter .
Omitting all terms of order or higher (indicated by ), it can be written as
Note that for reasons of accuracy this approximation is usually limited to orbits where the eccentricity is small.
The expression is known as the equation of the center, where more details about the expansion are given.
The radius (distance between 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.