# Orbital eccentricity

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_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.