function dchi = tau1(pt1, chi, e)
% Schwarzchild space-like geodesic
% ODE for proper time for the geodesics of the first kind
% e is eccentricity
% M is mass of the black hole
% l (latus rectum) is some positive constant
%e = 5; M = 3/14; l = 4.5;
%mu = M/l;
dchi = 2 *(1+e*cos(chi))^2 *sqrt(1-cos(chi/2));