When e = 0, Eq. (1.1) becomes the well-known equation of the oscillations of a mathematical pendulum.
Its constant solutions q = 0 (mod 2~) and q = x (mod 2n) correspond to the stable lower position
of equilibrium and the unstable upper position of equilibrium. Those differing from constant solutions
correspond either to oscillations of the pendulum with arbitrary amplitude, or to rotations with arbitrary
mean angular velocity or to asymptotic motions.