In astrodynamics, orbital eccentricity shows how much the shape of an object's orbit is different from a circle.
Eccentricity () is defined for all circular, elliptic, parabolic and hyperbolic orbits. It can take the following values:
 for circular orbits: is equal to zero,
 for elliptic orbits: is more than zero but less than 1,
 for parabolic trajectories: is equal to 1,
 for hyperbolic trajectories: is more than 1.
Finding eccentricity
Use this formula:
, 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.
Images for kids

Gravity Simulator plot of the changing orbital eccentricity of Mercury, Venus, Earth, and Mars over the next 50,000 years. The arrows indicate the different scales used. The 0 point on this plot is the year 2007.
ru:Кеплеровы элементы орбиты#Эксцентриситет
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Orbital eccentricity Facts for Kids. Kiddle Encyclopedia.