Chuck Coker

On March 31, 1851, at the request of Napoleon Bonaparte, Léon Foucault demonstrated his pendulum experiment at the Panthéon in Paris. For the exhibition, he used a 28-kilogram (61.7-pound) sphere hanging on a 67-meter (219.8-foot) wire.

Foucault, pronounced /fuːˈkoʊ/ in English, /fuˈko/ in French, performed his first pendulum experiment on January 6, 1851. The experiment was designed as a simple, easy-to-see proof of the rotation of the Earth. He built the first pendulum in the cellar of his house. It was made from a 5-kilogram (11.0-pound) weight hanging from a 2-meter (6.6-foot) wire. He observed a small movement of the oscillation plane of the pendulum, showing that the Earth was rotating under the swinging pendulum. In reality, the oscillation plane stayed stationary, relative to the far-off stars in universe, while the Earth rotated underneath it.

Though the Panthéon experiment was the most famous, the first public demonstration had already taken place in February at the Observatoire de Paris.

The angular speed of the Foucault pendulum, measured in clockwise degrees per sidereal day, varies with latitude.

The speed can be calculated with the formula

ω = 360 × sin(φ)
where ω is the angular speed and φ is the latitude in degrees north.

For example, at 30° North latitude, the pendulum rotates 180º per sidereal day:
φ = 30°
ω = 360 × sin(30)
ω = 360 × 0.5
ω = 180

At the equator, the pendulum does not rotate:
φ = 0°
ω = 360 × sin(0)
ω = 360 × 0
ω = 0

At the north pole, the pendulum rotates 360º clockwise per sidereal day:
φ = 90°
ω = 360 × sin(90)
ω = 360 × 1
ω = 360

At the south pole, the pendulum rotates 360º counter-clockwise per sidereal day:
φ = 90°
ω = 360 × sin(-90)
ω = 360 × -1
ω = -360