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(degree to which an orbit is non-circular)

Eccentricity (e) is a number indicating the degree to which an orbit is non-circular, 0 meaning circular, between 0 and 1 being Elliptical, 1 being parabolic, and greater than 1 being hyperbolic.

The eccentricity of an elliptical orbit is:

    ra - rp
e = ———————
    ra + rp


  • e is the eccentricity
  • ra is the radius at apoapsis (furthest distance)
  • rp is the radius at periapsis (closest distance)

Earth's orbit has an eccentricity of 0.0167. The Moon's is 0.0549. Halley's Comet's is 0.97.

Eccentricity can aid in the study of Atmospheres of Extra Solar Planets, i.e., bodies orbiting stars, because the changes in EM Radiation from the host star allow observation of the atmosphere at multiple Temperatures using Light Curve Spectroscopic observations.

(orbits,celestial mechanics,measure)

Referenced by:
DQ Tau
HD 80606 b
Hill Radius
Kozai Mechanism
Kuiper Belt (K Belt)
Late Heavy Bombardment (LHB)
Orbital Element
Planet Nine
Planet Formation
Laplace Radius (rL)
Semi-Major Axis (a)
Tidal Heating