Ellipticity

Given a Spheroid with equatorial radius $a$ and polar radius $c$,

$$e \equiv \cases{ \sqrt{a^2-c^2\over a^2} & $a>c$\ {\rm\ (ob... ...qrt{c^2-a^2\over a^2}. & $a<c$\ {\rm\ (prolate\ spheroid)}\cr}$$

See also Flattening, Oblate Spheroid, Prolate Spheroid, Spheroid