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Orbital eccentricity
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In astrodynamics, orbital eccentricity shows how much the shape of an object's orbit is different from a circle.
Eccentricity ([math]e\,\![/math]) is defined for all circular, elliptic, parabolic and hyperbolic orbits. It can take the following values:
 for circular orbits: [math]e\,\![/math] is equal to zero,
 for elliptic orbits: [math]e\,\![/math] is more than zero but less than 1,
 for parabolic trajectories: [math]e\,\![/math] is equal to 1,
 for hyperbolic trajectories: [math]e\,\![/math] is more than 1.
Finding eccentricity
Use this formula:
[math]e_{obj}=\frac {r_ar_p} {r_a+r_p}[/math], where e_{obj} is the eccentricity, r_{a} is the apoapsis (far point) of the object's orbit, and r_{p} is the periapsis (near point) of the object's orbit. The near and far points are the apsides.ru:Кеплеровы элементы орбиты#Эксцентриситет

