*This page refers to eccentricity 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:

## Calculation

**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]).

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 .

## See also

## External links

Last updated: 06-02-2005 05:02:39