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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:[1]

  • for circular orbits: ,
  • for elliptic orbits: ,
  • for parabolic orbits: ,
  • for hyperbolic orbits: .


For elliptic orbits, eccentricity can be calculated from distance at periapsis and apoapsis:


  • is distance at periapsis (closest approach),
  • is distance at apoapsis (farthest approach).