text: A similar principle which conserves a quantity can be obtained for the rotating frame also, known as the Poincaré–Bjerknes theorem, named after Henri Poincaré and Vilhelm Bjerknes, who derived the invariant in 1893 and 1898. The theorem can be applied to a rotating frame which is rotating at a constant angular velocity given by the vector , for the modified circulation
  Here  is the position of the area of fluid. From Stokes' theorem, this is:
  The vorticity of a velocity field in fluid dynamics is defined by:
  Then: