The orbital eccentricity of an astronomical object is a parameter that describes the shape of the orbit. A perfectly circular orbit has an eccentricity of zero. Elliptical orbits have values between 0 and 1 with higher values indicating a more eccentric orbit. An eccentricity of exactly 1 is a parabolic orbit and anything greater than that is a hyperbolic orbit.

The orbits of the planets are almost circular with the exception of Mercury (0.206) and Mars to a lesser extent. The Earth has an eccentricity of 0.017. Most asteroids have orbital eccentricities between 0 and 0.35 with the average value equal to 0.17. The eccentricities of comets vary widely. They can be as low as 0.2 for periodic comets but often close to or greater than 1 for non-periodic comets.