An ellipse consists of points whose distances to two fixed foci have a constant sum. For semiaxes , the foci lie at distance from the center along the major axis.椭圆上任一点到两个固定焦点的距离之和保持不变。设半轴为 ，焦点位于长轴上，距中心 。
As the point travels around the ellipse, the green and gold focal segments exchange length. The stacked bar below keeps the same total length, .光点沿椭圆运动时，绿色与金色焦距线段此消彼长。下方叠加条的总长度始终为 。