Eccentricity

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