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A quantity defined for a Conic Section which can be given in terms of Semimajor and Semiminor Axes for an Ellipse. For an Ellipse with Semimajor Axis $a$ and Semiminor Axis $b$,

e\equiv\sqrt{1-{b^2\over a^2}}\,.

The eccentricity can be interpreted as the fraction of the distance to the semimajor axis at which the Focus lies,

e={c\over a},

where $c$ is the distance from the center of the Conic Section to the Focus. The table below gives the type of Conic Section corresponding to various ranges of eccentricity $e$.

$e$ Curve
$e=0$ Circle
$0<e<1$ Ellipse
$e = 1$ Parabola
$e > 1$ Hyperbola

See also Circle, Conic Section, Eccentric Anomaly, Ellipse, Flattening, Hyperbola, Oblateness, Parabola, Semimajor Axis, Semiminor Axis

© 1996-9 Eric W. Weisstein