The proof that Earth rotates was achieved before space travel with some brilliant physics.

What is the shape of our planet? A rotating spheroid, right? You know this because of astronomy, or maybe you live in the 21st century. A couple of centuries ago, it was still a mystery yet to be resolved. People for all purpose believed that the earth was flat and all celestial bodies moved over the Earth. This is obvious because you seem stationary, whereas the sun rises and set, the moon rises and set, and the stars seem to do as such. So how on Earth were people able to prove that the earth was a rotating spheroid. It got down to some brilliant physics — long before the first spacecraft left for space — by one medical school dropout.

Related media: How We Proved Earth Rotates Using a Giant Swinging Ball


We’re Turning Around

Jean Bernard Léon Foucalt

Jean Bernard Léon Foucault — commonly called Léon — was born in Paris, France, in 1819. As a child, he was an introvert and usually shy. He never did well in school, but her mother urged him on to become a doctor, so he enrolled as a medical student. While in medical school, he discovered that he had a discretion upon seeing blood, so he had to dropout.

Despite his poor academic background, he had the passion for science which led him to seek a job as a laboratory assistant. There he dabble in the invention of the Daguerre photographic process, and incidentally produced the world’s first photograph of the sun. He was also able to devise a method of measuring the speed of light, and later demonstrated that light travels faster in air than in water in 1850.

Astronomers, as at the time, knew that something was turning around, but they just weren’t sure of what it was. If you’ve ever gone star gazing, you’ll notice that the stars appear to be moving over like circles in the sky — if you’re patient enough. Before the 17th century, people believed that the stars were just holes in a giant sphere that moved over the Earth. Several astronomers throughout the course of history, including Galileo Galilei and Giordano Bruno, proposed the idea of a heliocentric universe. They were either jailed or executed — respectively — for their heretic ideas.

During the generation of Léon, most educated intellectuals agreed with Galileo and his idea that the Earth rotates. The only available evidence as at that time was astronomical observations. However, no one had been able to prove it with a demonstration yet; until one fateful night in the history of science, in January 1851, Léon had a stroke of genius. He realized that if a pendulum hanged up and was going to swing freely, it would show the effects of the Earth’s rotation by (wobbling) as it swings.


You Gotta Swing

This is Léon’s proof: Imagine you’re standing on the North Pole and you have a giant and tall pendulum that’s swinging from side to side, tracing its path with marks made on the ground. As the Earth rotates beneath, the pendulum would keep swinging in it’s original direction, irrespective of the ground beneath it. What this means is that, over the course of a day, the path of the pendulum would appear to be shifting bit by bit until it has marked a line at every degree of a 360-degree circle. The pendulum is just swinging side to side; its the Earth that’s moving beneath it. That proves that the Earth rotates.

Léon realized that the pendulum would only mark a 360 degree in a day at either of the poles; it would mark different angles at different latitudes; and it wouldn’t mark anything at all at the equator. Léon formularized the sine law to define how long it would take to mark a different angle at any given latitude. It is said that in Paris — where he successfully completed his experiment in solitude — the pendulum would mark a 270-degree angle in a day.


That’s The Swinging Truth

Foucault demonstrates that the earth rotates with a swinging pendulum

On February 2, 1851, Léon sent series of letters to all scientists in Paris to witness the proof of the Earth’s rotation. The next day, the scientists assembled together in the Meridian Room of Paris Observatory as they witnessed the successful demonstration of Foucault’s swinging pendulum for the first time. A month later, Léon demonstrated it again for the public in the Pantheon, where he hung a 28 kilogram (62 pound) brass bob from a 6 meter (220 feet) long wire, and let it swing through a thin layer of sand on the floor.

Few years later, Léon yet again invented another method of demonstrating the Earth’s rotation — the gyroscope — and came up with a device that swung his pendulum perpetually. He was even appointed to the position of Physicist Attached, by Napoleon III, to the Imperial Observatory. And after all this, the French Academy of Science denied all his applications. It took multiple petitions before he was finally elected in 1865. He passed on three years later.

Today, you can see full-scale Foucault pendulums in museums and universities all over the world. They remind us of how a passion for science can lead to one of science great discoveries.

Image: Shutterstock | A display of Foucault’s pendulum at a museum in Paris


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Written by: Nana Kwadwo, Tue, Jan 01, 2019.

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