Léon Foucault uses a pendulum to demonstrate the rotation of the Earth. It is the first direct visual evidence not based on watching the stars circle in the sky.
Jean Bernard Léon Foucault was born in 1819. His mother wanted him to become a doctor, but he dropped out of medical school
when he made his first scientific discovery: He couldn’t stand the sight of blood.
Without formal scientific training, he worked as a lab assistant and continued tinkering. He used the new Daguerreotype photographic process
to take the first photograph of the sun. Together with Armand Fizeau, in 1850 he devised a way to use rotating mirrors to measure the speed of light. They observed that light travels more slowly in water than in air.
Scientists had been trying for two centuries to drop objects from towers and measure their drift as the planet spun beneath them. It didn't work: too quick, too crude, too many interfering factors.
Foucault had an insight. A pendulum hanging on a wire and swinging directly north and south would appear to the observer to slowly move its plane of oscillation as the Earth turned underneath it.
To grasp this, just picture a pendulum at the North Pole. It starts at zero degrees longitude and swings back and forth, as the Earth spins below it. For every hour it's going back and forth, the Earth will have moved 15 degrees of longitude eastward. The effect is less farther away from the poles, but it's still there.
After weeks of work
in the cellar of his home, Foucault hung a 5-kilogram (11-pound) pendulum from a 2-meter (6½-foot) cable in January 1851. He observed a small clockwise motion of the pendulum's apparent plane of oscillation. The pendulum was going straight back and forth, but the Earth moved for Foucault.
(Sources disagree on whether the crucial experiment took place on Feb. 6, Feb. 7 or Feb. 8. We've taken the middle course here.)
Foucault refined his apparatus and also derived his "sine law" showing the governing influence of latitude on how much a free-swinging pendulum would move. Specifically, the angular speed (in clockwise degrees per sidereal day) is 360 times the sine of the latitude. A Foucault pendulum will rotate through a full 360 degrees at the North Pole (the sine of 90 degrees is 1), but not at all at the equator (the sine of zero degrees is zero).
Foucault arranged a demonstration for the scientists of Paris on Feb. 3. He told them, "You are invited to see the Earth turn." And so they did, as they watched Foucault's pendulum
move on an 11-meter wire at the Paris observatory.
French President Louis Napoléon was a science buff, and he arranged for Foucault to give a public demonstration of his remarkable pendulum on March 31. Under the lofty roof of the Pantheon in Paris, Foucault hung a 62-pound brass sphere on a 220-foot cable. A pointer attached to the bottom of the sphere traced patterns in sand on a low wood platform.
The public was dazzled. President Napoleon soon became Emperor Napoleon III, and he gave Foucault the position of Physicist Attached to the Imperial Observatory. While there, Foucault's work on the centrifugal governor improved the precision of surveying instruments
Despite Foucault's imperial support, the university-trained scientists of Paris sniffed at him as an untrained upstart. They turned him down several times for membership in the French Academy of Sciences, before finally admitting him in 1865. Foucault died in 1868 at the age of 49.
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