The Foucault pendulum at the California Academy of Sciences in San Francisco
Image credit: BrokenSphere [CC BY-SA 3.0 or GFDL], from Wikimedia Commons

Yes, I’m aware: the Earth rotates all day, every day. But on this date in 1851, Léon Foucault proved it scientifically by setting his eponymous pendulum in motion in Paris. You can see replicas of this pendulum in science and natural history museums all over the world. They’re kind of boring to watch, as they very slowly change direction and periodically knock over little pegs. But that subtle change in direction would occur only if the Earth is rotating. This simple invention was and remains a brilliant demonstration of what to many of us is a self-evident truth.