function [ x, y ] = ellipse_xy( a, e )
% ELLIPSE_XY Generate the x, y coordinates of an ellipse with specified
% semi-major axis a and eccentricity e
%   The units of x and y are determined by a
%   The coordinate system is such that the center of the ellipse is at
%   (0,0), with the semi-major axis along a.
b = a * sqrt(1-e^2);

theta = 0 : 0.01 : 2 *pi;

r = a * (1-e^2)./(1+e.*cos(theta));

x = r .* cos(theta);
y = r .* sin(theta);

% This has the origin at one focus.  Shift the origin to the center of the
% ellipse
x = x + a*e;
end