French physicist Léon Foucault (1819–1868) is best known for developing the Foucault pendulum, a device that demonstrated once and for all that the Earth rotates. But he was also a master inventor, and he contributed to many different branches of science. Here are 15 things that you might not have known about the man behind the pendulum.

1. FOUCAULT SHOWED LITTLE PROMISE AS A YOUNGSTER.

From the beginning, he seemed ill-suited to schooling and studying; his attention often wandered. A childhood friend would later recall: “Nothing about the boy announced that he would be illustrious some day; his health was delicate, his character mild, timid and not expansive. The frailty of his constitution and the slow way he worked made it impossible for him to study at college. He was only able to study successfully thanks to the help of dedicated tutors watched over by his mother.” 

2. HE ABANDONED THE STUDY OF MEDICINE BECAUSE HE COULDN'T STAND THE SIGHT OF BLOOD.

In fact, he’s said to have fainted on seeing blood for the first time. Not surprisingly, he dropped out of medical school. Fortunately, he had other talents, and his aptitude for mechanics and invention was soon recognized. With almost no formal training, he succeeded in building a boat, a mechanical telegraph, and a steam engine.

3. FOUCAULT MEASURED THE SPEED OF LIGHT—AND GOT A PRETTY ACCURATE RESULT.

The technique involved sending a beam of light to a rapidly rotating mirror, where it would be reflected at a stationary mirror, then back to the rotating mirror. By measuring the amount that the mirror rotated while the beam traveled between the mirrors, the speed could be calculated. (The method had been developed by his countryman François Arago; Foucault took over after Arago’s eyesight began to fail.) Foucault’s eventual result was within 1 percent of the modern figure (299,792,458 km/sec).

4. HE DID PIONEERING WORK IN PHOTOGRAPHY, TOO.

Foucault worked with physicist Armand Fizeau to improve on the photographic techniques developed earlier by Louis Daguerre. Combining his photographic and astronomical talents, Foucault obtained the first detailed photographs of the surface of the Sun.

5. HE FIGURED OUT HOW TO IMPROVE THE ACCURACY OF TELESCOPE MIRRORS.

Since the time of Newton, astronomers knew that when building a telescope, a concave mirror (spherical or, even better, parabolic) could be used as part of an optical system to gather more light. But how do you know if your mirror is the right shape? Foucault developed a simple technique, known as the knife-edge test (shown above). The relatively simple—and cheap—test is used by amateur telescope makers to this day.

6. HE WAS JUST AS GOOD WITH MICROSCOPES AS WITH TELESCOPES.

Together with his professor, physician Alfred Donné, Foucault was a pioneer in “photomicrography”—taking photographs through a microscope. (It required, among other things, a powerful electric light source to illuminate the objects being photographed.) In 1845 Foucault and Donné published the first medical textbook that made extensive use of photomicrographs.

7. HE WAS CHUMMY WITH NAPOLEON III.

Louis-Napoléon Bonaparte—a nephew of Napoleon I, who had served as France’s president—seized absolute power following a coup in 1851, calling himself Napoleon III. And, as it happened, he was an amateur scientist. He supported Foucault, creating a post specifically for him—the scientist’s title would be “Physicist Attached to the Imperial Observatory.” This was lucky for Foucault, who at the time had no reliable source of income, other than serving as an editor of a scientific journal.

8. HIS FAMOUS PENDULUM DEMONSTRATES THE EARTH'S MOTION, WHICH HAD TROUBLED SCIENTISTS EVEN BEFORE SCIENCE WAS A THING.

Ancient thinkers had wondered if the Earth rotated, but there were obvious objections. For example, a non-spinning object dropped from a tower lands near the base of the tower; if the Earth rotated, shouldn’t it be swept away some distance? The full solution to this conundrum would come only with the work of Galileo and, later, Newton, who developed the modern idea of inertia.

9. THE HAND-WRINGING CONTINUED THROUGH THE MIDDLE AGES AND INTO THE RENAISSANCE.

The 14th-century thinker Nicole Oresme declared that there was no way to be sure if the stars revolved around the Earth or if the stars stayed put and the Earth spun, but he concluded that a stationary Earth was the more probable situation. When Nicolaus Copernicus (1473–1543) wrote his groundbreaking book On the Revolutions of the Heavenly Spheres (1543), he took it as a given that the Earth rotated on its axis once per day—though there was still no proof.

10. THEY ALSO WONDERED WHY THE EARTH SPINS.

Active at the turn of the 17th century, the English scientist William Gilbert—who was also Queen Elizabeth I's physician—was a devout Copernican. But he still wondered why the Earth turned. He conjectured—mostly correctly—that the Earth was a giant magnet and wondered if that was somehow responsible for the Earth’s rotation. It turns out, it is not. (Gilbert thought that the Earth’s magnetic axis and spin axis were one and the same; we now know they’re “off,” currently by about 10 degrees.) Gilbert thought that the Earth had a “magnetic soul,” and that this caused the planet to rotate, while at the same time causing a compass needle to point north.

11. THAT'S WHY FOUCAULT'S PENDULUM WASN'T AN ENTIRELY NEW IDEA.

Two centuries before Foucault, Galileo had understood the physics of the simple pendulum, and a few decades later, the Dutch physicist Christiaan Huygens would develop the pendulum clock, based on Galileo’s research. But it was Foucault who had the idea to use a pendulum to show that the Earth rotates. As the pendulum swings, the weight moves back and forth in a constant vertical plane while the Earth rotates beneath it.

12. THE PENDULUM DEMONSTRATES THE EARTH'S ROTATION, BUT IT'S NOT A 24-HOUR AFFAIR.

The plane of the pendulum’s swing rotates very slowly, eventually coming back to its original orientation. For example, if you start the pendulum swinging perfectly north–south, it eventually comes back to that orientation. But the period for this movement—its rate of “precession,” as physicists call it—depends on the latitude of the apparatus. At the north or south pole, the period is approximately 24 hours; at the latitude of Paris (about 49 degrees north), the period of precession is just under 32 hours.

13. FOUCAULT PENDULUMS ARE NOW SET UP ALL OVER THE WORLD.

This simple demonstration of the Earth’s rotation, first performed in Paris in 1851, caught the public’s imagination, and “Foucault pendulums” were set up in major American and European cities. The largest Foucault pendulum in the world, named the Principia, is housed at the Oregon Convention Center in Portland. The pendulum bob measures three feet across, weighs 900 pounds, and hangs from a 70-foot cable; each swing carries it 15 feet, taking about 10 seconds for a complete swing.

14. THE MOST FAMOUS FOUCAULT PENDULUM WAS STILL FOR A WHILE, BUT IT'S SWINGING AGAIN.

Foucault’s most famous demonstration took place in the Pantheon, in central Paris. Various versions of the pendulum have been mesmerizing visitors, more or less continuously, since 1851. However, the pendulum was removed when repair work on the building began in 2014. It was back to swinging in 2015, several years ahead of schedule. The rest of the Pantheon is still being restored.

15. HIS NAME IS INSCRIBED ON THE EIFFEL TOWER.

Foucault is one of the 72 scientists, mathematicians, and engineers whose names are inscribed in 60-cm-high letters on the side of the Eiffel Tower.