The Earth Turns in Paris
In March 1851, the French physicist Léon Foucault invited the public to the Panthéon in Paris to "see the Earth turn". Hanging from the central dome was a 67-meter (220 ft) steel wire holding a 28-kilogram (62 lb) brass-coated lead bob. As the pendulum swung, a stylus on its tip traced a line in a ring of sand on the floor. Spectators watched as the plane of the pendulum's swing slowly, visibly rotated clockwise, providing the first large-scale, direct proof of the Earth's rotation.
The experiment is its simplicity. The pendulum's swing plane is fixed in inertial space due to the conservation of momentum. The apparent rotation is actually the floor of the Panthéon—and the rest of the planet—rotating beneath the swinging bob. The demonstration, commissioned by Louis-Napoléon Bonaparte, was a success, attracting large crowds and making a complex astronomical fact clear to the public. Foucault had conducted a smaller, private version in his cellar and at the Paris Observatory just weeks earlier, but the Panthéon's height allowed for a dramatically long wire, making the effect slow and clear.