Definition
Eccentricity e is one of the six Keplerian elements. It equals the ratio of the distance between foci to the major axis. e = 0 gives a circle; 0 < e < 1 an ellipse; e = 1 a parabola (escape); e > 1 a hyperbola.
Solar system values
Most planets have low eccentricity (0.01–0.09). Mercury (0.21) and Mars (0.09) are exceptions among the eight planets. Comets can have e very close to 1.
