Today's Google doodle celebrates the 194th anniversary of Léon Foucault's birth. Foucault was a French physicist who did work on such areas as magnetism and momentum, but his two most notable accomplishments were an early measure of the speed of light, and his pendulum demonstrating the rotation of the Earth. Imagine a pendulum hanging over a turntable. If you start the pendulum swinging, it will go back and forth in a straight line. If you start the turntable spinning, an ant on the turntable would see the pendulum doing loops from his perspective. If you attached traced out the route of the swinging pendulum on the spinning turntable, you'd get a design something like the Spirograph art I remember from when I was a kid. Foucalt found that with freely swinging pendulum with a long enough arm, you could actually see the effect of the spinning planet. This effect is strongest at the north and south pole, and disappears at the equator - so you can use the magnitude of the effect to determine your latitude.