Foucault's PendulumA pendulum let loose swings back and forth in a straight line, full stop — yet watch the golden trail on the floor: the swing-plane itself slowly rotates. That's not the pendulum turning, it's the ground turning underneath it as Earth spins. The rate depends only on latitude, fastest at the poles, frozen at the equator. Sped up here about 20,000× — the real thing takes a full day to complete one turn at the pole.The precession rate Ω·sinφ is identical, term for term, to the holonomy picked up when a vector is parallel-transported around a latitude circle on a curved sphere — the enclosed solid angle. Foucault's 1851 Panthéon demo is a room-sized version of the exact geometric-phase mathematics Michael Berry connected to quantum mechanics over a century later. Nothing about the pendulum's local swing "knows" it is rotating; only the closed loop around the curved Earth leaves a net twist behind, purely from the shape of the path.