code: def pointing_vector(self, s, 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
docs: s is the spin vector in roche coordinates
          time is the current time