The Stalled Pendulum

The Foucault pendulum is the most elegant proof that Earth rotates. Set a pendulum swinging, wait, and the plane of oscillation precesses at a rate determined by latitude. At the poles, it completes a full rotation in 24 hours. At the equator, it doesn’t precess at all. The rate is sin(latitude) × 360°/day. Clean, universal, independent of the pendulum’s construction.

Salva and Salva (arXiv:2502.12230) show that real Foucault pendulums aren’t this simple. Support anisotropy — the fact that the cable attachment point isn’t perfectly symmetric — introduces Airy precession, which competes with the Coriolis precession from Earth’s rotation. The interaction between the two is sensitive to pendulum dimensions, support geometry, and initial conditions.

The most striking finding: at certain amplitudes, precession ceases entirely while the pendulum continues to oscillate. The Coriolis effect and the Airy effect cancel. The plane of swing freezes despite Earth continuing to rotate underneath it. Below this critical amplitude, precession resumes. There is an upper amplitude threshold above which the pendulum cannot demonstrate Earth’s rotation at all.

The structural insight: the Foucault pendulum’s pedagogical simplicity is an idealization. The “proof” that Earth rotates assumes the support is isotropic. In any real pendulum, the support has preferred directions, and these preferred directions create a competing precession that can overwhelm the Coriolis signal. The most carefully built demonstration pendulum in the world still has support anisotropy, and at the wrong amplitude, it shows nothing.

The demonstration works. But not at every amplitude. And the amplitude at which it fails is a property of the support, not of the Earth.