ANALOGY BETWEEN THE DRIFT OF PLANETARY VORTICES AND THE PRECESSION OFA SPINNING BODY

The physical origin of the well-known expression for the drift velocit y of a monopole vortex on a rotating planet is clarified. It is shown that this drift can be obtained from the equation of motion of a small , rotating disk on the surface of the planet. Because of the slightly oblate shape of a rotating planet, a net force is directed on the disk from the equator toward the closest pole. The drift along the latitud e circle caused by this force is completely analogous to the precessio n of a spinning body.