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:

• for circular orbits: ,
• for elliptic orbits: ,
• for parabolic trajectories: ,
• for hyperbolic trajectories: .

## Calculation

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

• is eccentricity vector.

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

• is distance at periapsis,
• is distance at apoapsis.

## 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 ).

Other values: Pluto 0.2488 (largest value among the planets of the Solar System), Mercury 0.2056, Moon 0.0554.