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.
A Rotational Rate Dependent on Latitude
The pendulum's plane of swing does not complete a full 360-degree rotation in 24 hours. The rate of rotation is dependent on latitude, following the formula: Rotation per day = 360° × sin(latitude). At the North or South Pole (90° latitude), a pendulum would appear to complete a rotation in one sidereal day (about 23 hours and 56 minutes). At the equator (0° latitude), the plane would not appear to rotate at all.
At the Panthéon's latitude of approximately 48° 51′ N, the pendulum's plane rotates clockwise about 11.3 degrees per hour. This corresponds to a full circle in about 31 hours and 50 minutes. The slow, steady change was unmistakable to any observer who watched for even a short period. The original 1851 pendulum was moved to the Conservatoire des Arts et Métiers in 1855. The pendulum swinging in the Panthéon today is an exact replica that was installed in 1995.