• The amount by which an ellipse, such as an orbit, differs from an exact circle. Eccentricity is measured as the difference between the large and small radii (the semi-major and semi-minor axes) divided by the semi-major axis. Some Earth satellites are placed in virtually circular orbits, and the orbital eccentricities of most major solar system objects are less than 0.1, but Nereid, a satellite of Neptune, has an orbit with an eccentricity of 0.75.

Not what you were looking for?