An ellipse is a curve that is the locus of all points in the plane the sum of whose distances r_{1} and r_{2} from two fixed points F_{1} and F_{2} (the foci) separated by a distance of 2c is a given positive constant 2a). This results in the two-center bipolar coordinate equation:
r_{1}+r_{2}=2a,
where a is the semimajor axis and the origin of the coordinate system is at one of the foci. The corresponding parameter b is known as the semiminor axis. |