Orbital eccentricity

Amount by which an orbit deviates from a perfect circle / From Wikipedia, the free encyclopedia

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In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit (or capture orbit), and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the Galaxy.

An elliptic, parabolic, and hyperbolic Kepler orbit:
  Elliptic (eccentricity = 0.7)
  Parabolic (eccentricity = 1)
  Hyperbolic orbit (eccentricity = 1.3)
Elliptic orbit by eccentricity
  0.0 ·   0.2 ·   0.4 ·   0.6 ·   0.8