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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 elliptical 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

Here is a formula to find eccentricity:

[math]e_{obj}=\frac {r_a-r_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.