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An orbital parameter describing the eccentricity of the orbit ellipse. Eccentricity e is the ratio of half the distance between the foci c to the semi-major axisa: e=c/a. For example, an orbit with e=0 is circular, e=1 is parabolic, and e between 0 and 1 is elliptic.

NOTE:
The list of defined terms to the left is by no means exhaustive with
respect to the general subject of solar system dynamics.
The terms listed are thought to be those more commonly used on our site
and thus appropriate for inclusion on this page.