Examples
The eccentricity of the Earth's orbit is currently about 0.0167; the Earth's orbit is nearly circular. Over hundreds of thousands of years, the eccentricity of the Earth's orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets (see graph).
Mercury has the largest orbital eccentricity of the planets of the Solar System (e=0.2056). Before 2006, when Pluto was still considered a planet, it was considered the Solar System's planet with the largest orbital eccentricity (e=0.248). The Moon's value is 0.0549. For the values for all planets in one table, see List of gravitationally rounded objects of the Solar System.
Most of the Solar System's asteroids have orbital eccentricities between 0 and 0.35 with an average value of 0.17. Their comparatively high eccentricities are probably due to the influence of Jupiter and to past collisions.
The eccentricity of comets is most often close to 1. Periodic comets have highly eccentric elliptical orbits, with eccentricities just below 1; Halley's Comet's elliptical orbit, for example, has a value of 0.967. Non-periodic comets follow near-parabolic orbits and thus have eccentricities very close to 1. Examples include Comet Hale–Bopp with a value of 0.995 and comet C/2006 P1 (McNaught) with a value of 1.000019. As Hale–Bopp's value is less than 1, its orbit is elliptical and so the comet will in fact return. Comet McNaught has a hyperbolic orbit while within the influence of the planets, but is still bound to the Sun with an orbital period of about 105 years. As of a 2010 Epoch, Comet C/1980 E1 has the largest eccentricity of any known hyperbolic comet with an eccentricity of 1.057, and will leave the Solar System indefinitely.
Neptune's largest moon Triton has an eccentricity of 1.6 × 10-5, the smallest eccentricity of any known body in the Solar System; its orbit is as close to a perfect circle as can be currently measured.
Read more about this topic: Orbital Eccentricity
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—André Breton (1896–1966)
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