Foucault pendulum at the Panthéon in Paris (photo by “Arnaud 25,” public domain)
The apparent rotation of the Foucault pendulum demonstrates the rotation of the Earth, and acts as a crude clock. The (apparent) angular rotation speed of the plane of the pendulum is proportional to the sine of the latitude at which the pendulum is located. See this 1915 article for an analysis.
Animation (by “Nbrouard”) of a Foucault pendulum. The blue line shows the movement of the pendulum bob in a frame of reference rotating with the plane of the pendulum, while the green line shows the movement of the pendulum bob relative to the ground.
The Foucault pendulum also appears in a famous novel by Umberto Eco. “I knew the earth was rotating, and I with it, and Saint- Martin-des-Champs and all Paris with me,” Eco writes, “and that together we were rotating beneath the Pendulum, whose own plane never changed direction.”