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Inside St. Isaac’s Cathedral, the Foucault pendulum demonstrates Earth’s rotation through precise harmonic motion. A heavy brass bob, suspended from a long wire, swings in a constant plane while the floor beneath slowly shifts due to planetary rotation. The pendulum’s period and amplitude are governed by classical mechanics, with minimal damping to preserve motion. Each pass knocks over calibrated markers, providing measurable evidence of angular displacement over time. The setup forms a clear, real‑world illustration of inertial frames and Coriolis effects. In this sacred space, physics turns into a visible, continuous experiment.