Foucault Pendulum
Watch a pendulum's swing plane slowly rotate, proving the Earth turns beneath it. Jean Bernard Léon Foucault first demonstrated this in 1851 at the Panthéon in Paris, providing one of the most elegant proofs that our planet rotates.
Pendulum Status
Notable Latitudes
How it works
A Foucault pendulum is a tall, heavy pendulum free to swing in any vertical plane. Once set in motion, the plane of its swing appears to rotate slowly over the course of hours. This rotation is not caused by any force on the pendulum itself — rather, it is the Earth that turns beneath it while the pendulum maintains its original plane of oscillation (a consequence of inertia and angular momentum conservation).
The apparent rotation is caused by the Coriolis effect, a fictitious force that arises in a rotating reference frame. At latitude φ, the component of Earth's angular velocity perpendicular to the ground is Ω sin(φ), where Ω is Earth's rotation rate (360°/day). This gives a precession period of T = 24 h / sin(φ). At the North Pole (φ = 90°), the pendulum completes one full apparent rotation every 24 hours. At the equator (φ = 0°), there is no precession at all.
Léon Foucault first publicly demonstrated this phenomenon on 3 February 1851 beneath the dome of the Panthéon in Paris. His pendulum was a 28 kg brass bob on a 67-metre wire. The slow, unmistakable rotation of the swing plane captivated the public and provided a direct, visceral proof that the Earth rotates — something that had been accepted theoretically since Copernicus but never so simply demonstrated.
The trace pattern you see in the simulation is a rosette: each swing of the pendulum is offset by a small angle from the previous one. Over time, the path fills out a flower-like pattern. The number of "petals" depends on the ratio of the pendulum's swing period to its precession period. At higher latitudes (faster precession), the rosette has fewer, wider petals.