def _geometrical_delay(self, alpha_x, alpha_y, delta_T):
        """
        geometrical delay (evaluated at z=0) of a light ray with an angle relative to the shortest path

        :param alpha_x: angle relative to a straight path
        :param alpha_y: angle relative to a straight path
        :param delta_T: transversal diameter distance between the start and end of the ray
        :return: geometrical delay in units of days
        """
        dt_days = (alpha_x**2 + alpha_y**2) / 2. * delta_T * const.Mpc / const.c / const.day_s * const.arcsec**2
        return dt_days