Léon Foucault

French physicist who, in 1851, gave the first simple, direct demonstration of the Earth's rotation - with a pendulum.

In February 1851 he publicly demonstrated a long pendulum at the Paris Observatory; weeks later he hung a 28 kg bob on a 67 m wire from the dome of the Panthéon. The plane of the swing slowly rotated relative to the floor - about 11.3°/hour at Paris's 48.9° N, a full circle in ~31.8 hours - exactly the rate the rotation predicts (15°/hour × sin(latitude)). No engines, no electronics: just inertia and a turning floor. In 1852 he followed it with the gyroscope. Foucault pendulums are now in science museums worldwide - and the same 15°/hour is what a $20,000 flat-earth gyroscope 'could not accept' in 2017.

Built on the fact that the Earth's rotation was already inferred (the stars' daily motion, Eratosthenes' geometry) but had never been directly shown. Enabled the Foucault-pendulum experiment card and the gyroscope line of instruments (Knodel's FOG, aircraft navigation) - the pendulum and the gyroscope are the same experiment with different hardware.