Any orbit in planetary dynamics can be assumed to be of conic cross-section shape. The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape. Eccentricity may be interpreted as a measure of how much this shape deviates from a circle.
Eccentricity () is strictly defined for all circular, elliptic, parabolic and hyperbolic orbits and may take following values:
- for circular orbits: ,
- for elliptic orbits: ,
- for parabolic orbits: ,
- for hyperbolic orbits: .