by a section through the centre, perpendicular to the polar axis (i.e. in the equatorial plane). But: conic sections may occur on surfaces of higher order, too (see image). Celestial mechanics In celestial mechanics, for bound orbits in a spherical potential, the definition above is informally generalized. When the apocenter distance is close to the pericenter distance, the orbit is said to have low eccentricity; when they are very different, the orbit is said be eccentric or having eccentricity near unity. This definition coincides with the mathematical definition of eccentricity for ellipses, in Keplerian, i.e., potentials. Analogous classifications A number of classifications in mathematics use derived terminology from the classification of conic sections by eccentricity: Classification of elements of SL2(R) as elliptic, parabolic, and hyperbolic – and similarly for classification of elements of PSL2(R), the real Möbius transformations. Classification of discrete distributions by variance-to-mean ratio; see cumulants of some discrete probability distributions for details. Classification of partial differential equations is by analogy with the conic sections classification; see elliptic, parabolic and hyperbolic partial differential equations. See also Kepler orbits Eccentricity vector Orbital eccentricity Roundness (object) Conic constant References External links MathWorld: Eccentricity Conic sections Analytic geometry