Earth's Rotation Day

On January 8, 1851, French Physicist Leon Foucault invented a pendulum device to prove the Earth’s rotation. It had long been believed that the Earth rotated, but Focault discovered that he could demonstrate this with a simple experiment using a tall pendulum with a heavy weight.

He constructed an early form of this device in his house’s cellar, made up of a 5-kilogram weight attached to a 2-meter cable. Underneath the pendulum, he placed sand to mark the path of the swinging. He observed a slight clockwise movement of the plane—which meant that even if the pendulum was swinging straight back and forth, the Earth underneath it moved slowly. Excited with his discovery, Focault sent invitations to Parisian scientists for a demonstration of his refined pendulum apparatus on February 3, 1861 at the Paris Observatory.

The best known version of the device, called Focault pendulum after its inventor, was comprised of a 67 m long wire and a 28 kg brass-coated lead bob as the weight. During Focault’s first public demonstration of the device in March 1861, it was suspended from the dome of the Pantheon in Paris and made to swing freely. The idea was to show that the plane (ground) in which the pendulum sways rotates with time. The pin placed at the bottom of the weight marked the track of the pendulum’s swinging against the plane, proving the Earth’s rotation. The experiment showed that the pendulum’s swing rotated clockwise at 11 degrees per hour and created a full circle in 32.7 hours.

Today, Focault pendulums can still be found in many museums and science centers around the world, swinging continuously to show and remind curious visitors around the world of the simple experiment that proved the Earth’s rotation.