Action-angle coordinates for the pendulum problem

The action-angle coordinates for the planar pendulum problem are expressed in terms of the Jacobi elliptic functions and integrals. In particular, we show that the Jacobi zeta function generates the canonical transformation from the pendulum coordinates $\vartheta$ and $p \equiv \partial\vartheta/\partial t$ to the action-angle coordinates $(J,\zeta)$ for both the librating pendulum and the rotating pendulum.