Building a Foucault Pendulum — Proving Earth Rotates with a Swinging Weight

On 3 February 1851, Léon Foucault hung a 28-kilogram brass bob from a 67-metre wire beneath the dome of the Panthéon in Paris and set it swinging. Over the course of hours, the plane of the pendulum's swing slowly rotated clockwise — not because any force was pushing it sideways, but because the Earth itself was rotating beneath it. The pendulum's plane of oscillation remains fixed in space (by Newton's first law), while the floor turns underneath. At the North Pole, the floor would complete a full 360-degree rotation in 24 hours; at lower latitudes, the rate is slower, proportional to the sine of the latitude. Foucault's demonstration was the first direct, visual proof that the Earth rotates — something long accepted theoretically but never before observed in a laboratory. The experiment caused a sensation: Napoleon III ordered a repeat at the Panthéon, and within months Foucault pendulums were swinging in public buildings across Europe. This blueprint builds a functional Foucault pendulum with a heavy bob on a long wire, a low-friction pivot, and a graduated reference ring, capable of demonstrating Earth's rotation over the course of several hours.