Eccentric anomaly

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 angle measured at the center of the ellipse between the orbit's periapsis and the current position. The eccentric anomaly is one of three angular parameters that can be used to define a position along an orbit, the other two being the true anomaly and the mean anomaly.

Graphical representation

Consider the ellipse with equation given by:
where is the semi-major axis and is the semi-minor axis.
For a point on the ellipse,, representing the position of an orbiting body in an elliptical orbit, the eccentric anomaly is the angle in the figure. The eccentric anomaly ' 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 ', and opposite side that passes through the point. The eccentric anomaly is measured in the same direction as the true anomaly, shown in the figure as. The eccentric anomaly in terms of these coordinates is given by:
and
The second equation is established using the relationship
which implies that. The equation 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 as the distance from to the major axis, and its hypotenuse equal to the semi-minor axis of the ellipse.

Formulas

Radius and eccentric anomaly

The eccentricity is defined as:
From Pythagoras's theorem applied to the triangle with as hypotenuse:
Thus, the radius 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.

From the true anomaly

The true anomaly is the angle labeled in the figure, located at the focus of the ellipse. It is sometimes represented by or. The true anomaly and the eccentric anomaly are related as follows.
Using the formula for above, the sine and cosine of are found in terms of :
Hence,
where the correct quadrant for is given by the signs of numerator and denominator, so that can be most easily found using an atan2 function.
Angle is therefore the adjacent angle of a right triangle with hypotenuse adjacent side and opposite side
Also,
Substituting as found above into the expression for, the radial distance from the focal point to the point, can be found in terms of the true anomaly as well:
where
is called the semi-latus rectum in classical geometry.

From the mean anomaly

The eccentric anomaly is related to the mean anomaly by Kepler's equation:
This equation does not have a closed-form solution for ' given '. 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.