# Dictionary:Fig E-12

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 ${\displaystyle =\varepsilon =c/a=(2f-f^{2})^{1/2}=[1-(1/E)^{2}]^{1/2}}$; ellipticity ${\displaystyle =E=a/b=1/(1-f)=(1-\varepsilon ^{2})^{-1/2}}$; flattening ${\displaystyle =f=(a-b)/a=1-1/E}$; and ${\displaystyle \theta }$ = eccentric anomaly when satellite is at S. The polar equation of an ellipse with one focus at the origin is ${\displaystyle \rho =\varepsilon h/(1-\varepsilon \cos \phi )}$; where h = distance from the focus to a directrix line.