Eccentricity (e) is a number indicating the degree to which an orbit is non-circular, 0 meaning circular, between 0 and 1 being Elliptical, 1 being parabolic, and greater than 1 being hyperbolic.
The eccentricity of an elliptical orbit is:
ra - rp e = ——————— ra + rp
Earth's orbit has an eccentricity of 0.0167. The Moon's is 0.0549.
Laplace Radius (rL)
Semi-Major Axis (a)