eccentricity

A single number, e, measuring how squashed or stretched a conic is.

The eccentricity $e$ of a conic measures its shape independent of its size. For an ellipse, $e=\frac{c}{a}$ with $0\le e<1$ : $e=0$ gives a circle, and $e$ approaching $1$ gives an increasingly elongated ellipse. For a hyperbola, $e=\frac{c}{a}$ with $e>1$ always, since $c^2=a^2+b^2$ forces $c$ to exceed $a$ .

Examples

Earth's orbit has $e\approx 0.017$ (nearly circular); Halley's comet has $e\approx 0.97$ .