Preliminary note. Beginners find two ways of regarding an ellipse easy to understand. The first presents it as the curve traced out by a point P (Fig. 1) when r + r′, the sum of P’s distances from two fixed points F and F′, is a constant, 2a. From this definition come the equivalences AA′ = 2a, CA = CA′ = a, and CF = CF′ = ae, where e is a fraction less than unity C is the “centre” of the ellipse, F and F′ its “foci”, CA = a the “semi-axis major”, CB = b the “semi-axis minor”, and e the “eccentricity”