  function [xM,yM] = Mars(t)
% Mars is located at (xM,yM) at time t.
  PM = 206.669; AM = 249.209; rhoM = 686.971;
  theta = 2*pi*t/rhoM;
  xM = (PM-AM)/2 + ((PM+AM)/2)*cos(theta);
  yM = sqrt(PM*AM)*sin(theta);