The French scientist Jean-Bernard-Léon Foucault made his most famous pendulum when he suspended a 28-kg brass-coated lead bob with a 67-m-long wire from the dome of the Panthéon in Paris.

He set it in motion and to mark its progress he attached a stylus to the ball and placed a ring of damp sand on the floor below.

The audience was amazed to see the pendulum inexplicably appeared to rotate. It left a slightly different, but precise, trace in the sand with each swing.

Amazingly, they were witnessing that the seemingly motionless floor of the Panthéon was slowly moving. It was Foucault’s way of showing that the earth revolves on its axis.

At the latitude of Paris, the pendulum’s swing rotated clockwise 11° per hour, making a full circle in 32.7 hours. In the Southern Hemisphere it would rotate counterclockwise. If it was set up on the Equator, it wouldn’t revolve at all. In much more recent experiments with a pendulum at the South Pole, the period of rotation is 24 hours.

It is such a simple and elegant demonstration of a complex idea – precession as a form of parallel transport – that it fascinates children and adults.

Foucault pendulums are installed around the world, at universities, science museums and planetariums. The United Nations headquarters in New York City has one. The largest Foucault pendulum in the world, Principia, is housed at the Oregon Convention Center.

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