Eccentricity

Definition

Any orbit in planetary dynamics can be assumed to be of conic cross-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.

Eccentricity ( $e$ ) is strictly defined for all circular, elliptic, parabolic and hyperbolic orbits and may take following values:

for circular orbits: $e = 0$ ,
for elliptic orbits: $0 < e < 1$ ,
for parabolic orbits: $e = 1$ ,
for hyperbolic orbits: $e > 1$ .

==Calculation==^[1]

For elliptic orbits, eccentricity can be calculated from distance at periapsis and apoapsis:

e = \frac{d_{a} - d_{p}}{d_{a} + d_{p}}

= 1 - \frac{2}{(d_{a} / d_{p}) + 1}

where:

$d_{p}$ is distance at periapsis (closest approach),
$d_{a}$ is distance at apoapsis (farthest approach).

References

↑ Wikipedia article on eccentricity.

[1] Wikipedia article on eccentricity.

[1]