def pointing_vector(self, s, time):
        """
        s is the spin vector in roche coordinates
        time is the current time
        """
        t = time - self._t0
        longitude = self._longitude + self._dlongdt * t

        # define the basis vectors in the spin (primed) coordinates in terms of
        # the Roche coordinates.
        # ez' = s
        # ex' =  (ex - s(s.ex)) /|i - s(s.ex)|
        # ey' = s x ex'
        ex = np.array([1., 0., 0.])
        ezp = s
        exp = (ex - s*np.dot(s,ex))
        eyp = np.cross(s, exp)

        return np.sin(self._colat)*np.cos(longitude)*exp +\
                  np.sin(self._colat)*np.sin(longitude)*eyp +\
                  np.cos(self._colat)*ezp
s is the spin vector in roche coordinates
        time is the current time