The Foucault pendulum was the first demonstration of the Earth’s rotation that did not rely on astronomical observation.
This was first demonstrated in 1851 by French physicist Jean Bernard Léon Foucault. Foucault noticed that if a pendulum is turned, it tries to keep swinging in its original direction. From this, he theorized that a pendulum could be used to observe the rotation of the Earth. Foucault went on to demonstrate the Earth’s rotation publicly at the Paris Observatory and then at the Pantheon in Paris.
But how does a pendulum demonstrate the Earth’s rotation?
As you watch the pendulum for just a short time, it will appear to be moving clockwise across the mosaic map on the floor.
But, since there is no outside force making the pendulum rotate as it swings, and no other outside force interrupting the swing, it must be the floor itself that is rotating, while the path of the pendulum’s swing remains constant.
The apparent rotation of a Foucault pendulum is affected by two kinds of motion: twisting and travelling. Twisting is circular motion directly on an axis. Travelling is circular motion around an axis.
To help understand this, consider two Foucault pendulums, one placed directly on the north pole and the other placed directly on the equator. At the north pole, the pendulum’s swing rotates in a complete circle every 24 hours – it twists but does not travel any distance. At the equator, the pendulum travels a wide path around the Earth’s axis but does not even begin to make a circle – it travels but it does not twist at all.
A Foucault pendulum placed in between the pole and the equator is affected by both twisting and travelling. The closer it is to the pole, the faster it circles. The library’s pendulum, located at 38 degrees, six minutes, does a complete circle about every 38 hours. Because air resistance would stop the pendulum’s motion, magnets at the top gently pull the pendulum to keep it moving, and the pendulum is suspended in a way that allows it to swing freely, regardless of the building’s movement.
For a more in-depth explanation of how a pendulum demonstrates the rotation of the earth, read About Foucault Pendulums  at the California Academy of Sciences.
The apparent rotation of the library pendulum is tracked by sensors in the mosaic on the floor, which light up as the plumb bob passes over them. The slow progress of the lights and the pendulum create a truly mesmerizing show.