Angle variable
= Angle variable
{title2=$\theta_i\in\mathbb R/(2\pi\mathbb Z)$}
A periodic canonical coordinate paired with an <action variable>. In an integrable background <Hamiltonian> $H_0(\mathbf J)$, the angles advance uniformly at $\dot\theta_i=\partial H_0/\partial J_i$, while the <action variables> remain constant. The periodicity permits an integer-indexed <Fourier series> on each regular orbital torus.