Happy birthday, Jean Bernard Leon Foucault, and thanks for the pendulum.
The French physicist and inventor was born in Paris on this day in 1819.
It may be hard to fathom, but the idea of Earth rotating on its axis, first proposed in the 6th century, took many centuries to gain favor, and many more to be demonstrated. The Copernican theory of celestial motions was well accepted by science by the time Foucault was born. It elegantly explained the apparent "rise" and "set" of the sun, but it was difficult to "prove" by experiment.
Some folks tried to drop stones down a mineshaft, to see if they deviated. Others tried something similar with the trajectory of cannonballs. But the mine shaft was too short, compared with Earth's radius, and the time traveled by the cannonballs likewise was too short to measure any difference.
The son of a publisher, Foucault showed an early aptitude for all things mechanical, and a growing aversion to all things bloody. So he gave up a medical curriculum and opted for physics.
He built his first pendulum with six feet of wire, an 11-pound ball and a candle that "launched" the ball by burning through a string to which the ball was attached (to prevent any directional effect of pushing the bob).
His pendulum became a sensation, and he constructed several for public displays, the most famous of them at the Pantheon in Paris.
Foucault went on to invent a gyroscope, and he tinkered with light experiments to "prove" light was a wave. Max Planck, Albert Einstein, Neils Bohr and others later went on to demonstrate it is both a particle and a wave, but more on that for their birthdays.
The California Academy of Sciences museum in San Francisco has a massive Foucault pendulum that swings through an arc of about 220 degrees daily.
Why not 360 degrees, you ask? The pendulum's motion is dependent on the latitude of Earth. Foucault's likewise moved 270 degrees in 24 hours. A pendulum at the North Pole would spin the full 360 degrees.
You're probably already wondering why the pendulum doesn't just slow down and stop, eventually. It does. In the early Foucault experiments, this didn't matter so much, because it swung long enough to see the floor shift in relation to the arc of the pendulum. But as the pendulum became more of a sensation, people invented ways to overcome the resistance that slows the bob.
Designers these days use electromagnets near the fastening point for the cable, to overcome that force and keep things swinging. At the academy, this electromagnet turns on and off when the cable passes a beam of light.
[For the Record, 2:40 p.m., Sept 18: A previous version of this post did not include the full name of French physicist Jean Bernard Leon Foucault.]