# eccentricity

Eccentricity is a parameter that describes the shape of an orbit, or any curve that is a conic section. The eccentricity of an ellipse
is the ratio of the distance between the foci and the major
axis. Equivalently, the eccentricity of an orbit is (*r _{a}* -

*r*) / (

_{p}*r*+

_{a}*r*) where

_{p}*r*is the apoapsis distance and

_{a}*r*is the periapsis distance. An almost circular orbit has an eccentricity close to zero. The more elongated the orbit, the higher the eccentricity, up to a maximum value, for a closed orbit, of 1. As a guide, the orbital eccentricity of most of the planets in the Solar System is less than 0.1 (0.017 in the case of Earth), although that of Mercury and Pluto exceeds 0.2. Eccentricity is one of the orbital elements.

_{p}