Proving Earth's Rotation: The Foucault Pendulum
Foucault's pendulum proves Earth's rotation: a long pendulum's swing plane stays fixed while the floor rotates beneath it. Requires a long rope (67m originally), heavy weight, and low-friction pivot.
The Foucault pendulum is the simplest demonstration that Earth rotates. It's essentially just a long rope with a heavy weight — but the engineering matters. How it works: Set a pendulum swinging in a fixed plane. On a non-rotating surface, it would keep swinging in exactly the same direction forever. On Earth, the floor rotates beneath the pendulum while the pendulum's swing plane stays fixed relative to the stars. Over hours, the swing direction appears to rotate. At the North Pole: the pendulum appears to complete a full 360° rotation in ~24 hours. At the equator: no apparent rotation (the math involves the sine of latitude). In Paris (where Foucault first demonstrated it in 1851): ~32 hours for a full rotation. Engineering requirements: - Very long rope (Foucault used 67 meters in the Panthéon) — shorter pendulums have too much energy loss to show the effect before stopping - Heavy, symmetrical weight (Foucault used 28 kg) to resist air resistance - Low-friction pivot point — any friction transfers Earth's rotation to the pendulum, spoiling the demonstration - Still air — drafts push the pendulum off course Other proofs of Earth's motion: stellar parallax (stars shift position over a year), Coriolis effect (deflects moving objects), aberration of starlight, and satellite orbits.