The Foucault Pendulum
September 18th is the birthday of Jean-Bernard-Léon Foucault (1819), born in Paris. He was trained in medicine, but became interested in physics. He is best known for inventing the pendulum that bears his name. He assembled it in 1851, a 62-pound iron ball swinging from a wire 220 feet long. He suspended it inside the dome of the Panthéon in Paris. He used it to prove that the Earth rotates on its axis. Once the pendulum is set in motion, it always swings along the same axis, but its position changes relative to the position of the Earth. As the Earth rotates counterclockwise, the pendulum appears to move in a clockwise direction. His pendulum caused a sensation among scientists and laypeople alike, and soon cities throughout Europe and America had suspended their own versions. You can still see them today in many science museums; sometimes a ring of dominoes is set up around the perimeter of the circle so you can see them being knocked down as the world turns.
Click to check out this explanation of the Foucault Pendulum