Earth Rotation Day

It’s Earth Rotation Day, in honor of Foucault, who did this:

On January 8, 1851, Foucault performed an experiment in the cellar of his home, in which he swung a five-kilogram weight attached to a two-meter-long pendulum. He put sand underneath it to mark the pendulum’s path, allowing him to see any changes in it. He observed a slight clockwise movement in the plane—the floor, and thus the earth, were slowly rotating; the pendulum kept its position. His experiment showed that the earth rotated on its axis. No longer was it just a hypothesis.