Spherical Pendulum, Actions, and Spin Dedicated to John Ross on the occasion of his 70th birthday

The classical and quantum mechanics of a spherical pendulum are worked out, including the dynamics of a suspending frame with moment of inertia . The presence of two separatrices in the bifurcation diagram of the energy-momentum mapping has its mathematical expression in the hyperel-liptic nature of the problem. Nevertheless, numerical computation allows to obtain the action variable representation of energy surfaces, and to derive frequencies and winding ratios from there. The quantum mechanics is also best understood in terms of these actions. The limit ! 0 is of particular interest, both classically and quantum mechanically, as it generates two copies of the frameless standard spherical pendulum. This is suggested as a classical interpretation of spin.