code: def spin_in_system(incl, long_an):
      
      #print "*** spin_in_system", incl, long_an, np.dot(Rz(long_an), np.dot(Rx(-incl), np.array([0,0,1])))
      # Rz(long_an) Rx(incl) [0, 0, 1]
      return np.dot(Rz(long_an), np.dot(Rx(-incl), np.array([0.,0.,1.])))
docs: Spin in the plane of sky of a star given its inclination and "long_an"
  
         incl - inclination of the star in the plane of sky
         long_an - longitude of ascending node (equator) of the star in the plane of sky
  
      Return:
          spin - in plane of sky