Of a planetary orbit, the angle measured from the perihelion position, to the center of the circumscribing auxiliary circle, to the projected position of the planet on the circle.
The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a the semi-major axis. If e = 0, the orbit is a circle. If e < 1, the orbit is
an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The eccentricity is one of the six orbital elements that define a Keplerian orbit.