Eccentricity (e)

Eccentricity describes the “roundness” of an orbit. It is a ratio between the semi-minor axis and semi-major axis of an orbit.


A “perfect” circular orbit has an eccentricity value of 0. The more eccentric the orbit (as e↑), the less circular the orbit.

An orbit is a trajectory with an e value less than 1 because an orbit is a closed path around a central body.

A trajectory with an eccentricity greater than or equal to 1 is an open trajectory, and the “orbiting” body will constantly move away from the central body.

For orbits or closed trajectories: 0 ≤ e < 1

  • Circle: e = 0
  • Ellipse: 0 < e < 1

For open trajectories: e ≥ 1

  • Parabola: e = 1
  • Hyperbola: e > 1