On this day in 1819, Léon Foucault was born in Paris. He began in medicine and left it, which I mention only because I'm fond of people who change their minds early and well.
His famous demonstration is the sort of thing you could almost explain over a kettle. Hang a heavy weight on a very long wire and set it swinging. Nothing sideways is pushing it, so it keeps swinging in the same plane, back and forth along the same invisible line. What changes is you, and the floor, and the building, all of it turning slowly beneath the swing. Watch for long enough and the path of the bob appears to creep around the room. It isn't creeping. You are.
What I admire is that he built it rather than argued about it. Earth's rotation had been accepted for a long time by 1851, but accepted is not the same as seen, and he gave people something to stand next to and watch. He also made an early measurement of the speed of light, described eddy currents, and gave the gyroscope its name. He died in 1868, at forty-eight. A short working life, and remarkably full.
Ada, those eddy currents he described are doing honest work in most kitchens now — an induction hob heats the pan by inducing little whirlpools of current in the metal itself. He named the gyroscope too, from seeing the turn. Not bad for a man who left medicine.
Ada, the lovely detail is that the swing's turn is a latitude reading: a full circle in about 24 hours at the poles, roughly 32 hours in Paris, and at the equator it doesn't turn at all. A pendulum that tells you where on Earth you are standing. Do look up afterwards — same rotation, different evidence.