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Encyclopedia > Orbital eccentricity

(This page refers to eccitricity in astrodynamics. For other uses, see the disambiguation page eccentricity.)

In astrodynamics, under standard assumptions any orbit must be of conic 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.

Under standard assumptions eccentricity () is strictly defined for all circular, elliptic, parabolic and hyperbolic orbits and may take following values:

Eccentricity of an orbit can be calculated from orbital state vectors as a magnitude of eccentricity vector:

where:

For elliptic orbits it can also be calculated from distance at periapsis and apoapsis:

where:

## Examples

For example, the eccentricity of the Earth's orbit today is 0.0167. Through time, the eccentricity of the Earth's orbit slowly changes from nearly 0 to almost 0.05 as a result of gravitational attractions between the planets (see graph [1] (http://www.museum.state.il.us/exhibits/ice_ages/eccentricity_graph.html)).

Other values: Pluto 0.2488 (largest value among the planets of the Solar System), Mercury 0.2056, Moon 0.0554. For the values for all planets in one table, see de:Planet (Tabelle).

Results from FactBites:

 Milankovitch cycles: Definition and Much More from Answers.com (2527 words) The eccentricity is a measure of the departure of this ellipse from circularity. As the eccentricity of the orbit evolves, the semi-major axis of the orbital ellipse remains unchanged. This orbital precession is in the opposite sense to the gyroscopic motion of the axis of rotation, shortening the period of the precession of the equinoxes with respect to the perihelion from 26,000 to 20,000 years.
 eccentricity is ruled out (5498 words) In the Milankovitch theory, the peak near 0.01 (100 kyr period) is attributed to eccentricity, the peak near 0.024 (41 kyr period) to obliquity, and the peak near 0.043 (23 kyr period) to precession. The disagreement between the spectra of the climate and the spectrum of the eccentricity is evident. In contrast, the bispectrum of the eccentricity, Fig.
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