In orbital mechanics, the **eccentric anomaly** is an angular parameter that defines the position of a body that is moving along an elliptic Kepler orbit. The eccentric anomaly is one of three angular parameters ("anomalies") that define a position along an orbit, the other two being the true anomaly and the mean anomaly.

Consider the ellipse with equation given by:

where *a* is the *semi-major* axis and *b* is the *semi-minor* axis.

For a point on the ellipse, *P* = *P*(*x*, *y*), representing the position of an orbiting body in an elliptical orbit, the eccentric anomaly is the angle *E* in the figure. The eccentric anomaly *E* is one of the angles of a right triangle with one vertex at the center of the ellipse, its adjacent side lying on the *major* axis, having hypotenuse *a* (equal to the *semi-major* axis of the ellipse), and opposite side (perpendicular to the *major* axis and touching the point *P′* on the auxiliary circle of radius *a*) that passes through the point *P*. The eccentric anomaly is measured in the same direction as the true anomaly, shown in the figure as . The eccentric anomaly *E* in terms of these coordinates is given by:^{[1]}

and

The second equation is established using the relationship

- ,

which implies that sin *E* = ±*y*/*b*. The equation sin *E* = −*y*/*b* is immediately able to be ruled out since it traverses the ellipse in the wrong direction. It can also be noted that the second equation can be viewed as coming from a similar triangle with its opposite side having the same length *y* as the distance from *P* to the *major* axis, and its hypotenuse *b* equal to the *semi-minor* axis of the ellipse.

The eccentricity *e* is defined as:

From Pythagoras's theorem applied to the triangle with *r* (a distance *FP*) as hypotenuse:

Thus, the radius (distance from the focus to point *P*) is related to the eccentric anomaly by the formula

With this result the eccentric anomaly can be determined from the true anomaly as shown next.

The *true anomaly* is the angle labeled in the figure, located at the focus of the ellipse. It is sometimes represented by f or v. The true anomaly and the eccentric anomaly are related as follows.^{[2]}

Using the formula for r above, the sine and cosine of E are found in terms of f :

Hence,

Angle E is therefore the adjacent angle of a right triangle with hypotenuse adjacent side and opposite side

Also,

Substituting cos E as found above into the expression for r, the radial distance from the focal point to the point P, can be found in terms of the true anomaly as well:^{[2]}

where

is called *"the semi-latus rectum"* in classical geometry.

The eccentric anomaly *E* is related to the mean anomaly *M* by Kepler's equation:^{[3]}

This equation does not have a closed-form solution for *E* given *M*. It is usually solved by numerical methods, e.g. the Newton–Raphson method. It may be expressed in a Fourier series as

where is the Bessel function of the first kind.