Eccentricity is a parameter that describes the shape of an orbit, or any curve that is a conic section. The eccentricity of an ellipse is the ratio of the distance between the foci and the major axis. Equivalently, the eccentricity of an orbit is (ra - rp) / (ra + rp) where ra is the apoapsis distance and rp is the periapsis distance. An almost circular orbit has an eccentricity close to zero. The more elongated the orbit, the higher the eccentricity, up to a maximum value, for a closed orbit, of 1. As a guide, the orbital eccentricity of most of the planets in the Solar System is less than 0.1 (0.017 in the case of Earth), although that of Mercury and Pluto exceeds 0.2. Eccentricity is one of the orbital elements.