FIG. E-12. Ellipse terminology. An ellipse is the locus of points for which the sum of the distances from the two foci is constant. A satellite follows an elliptical path about a body at one focus. If a = semimajor axis, b = semiminor axis, eccentricity ; ellipticity ; flattening ; and = eccentric anomaly when satellite is at S. The polar equation of an ellipse with one focus at the origin is ; where h = distance from the focus to a directrix line.