In 1851, French physicist Léon Foucault achieved something extraordinary with an experiment so simple that it needed no telescope, complex machinery or advanced mathematics. He gave people a way to watch the Earth rotate beneath their feet.His invention, now known as the Foucault pendulum, turned an invisible planetary motion into something people could see happening in front of them.When Foucault invited scientists to witness his demonstration at the Paris Observatory in February 1851, his invitation carried a remarkably simple message: “You are invited to see the Earth turn.”And that was essentially what the experiment allowed them to do.How a swinging pendulum revealed Earth's rotationA Foucault pendulum appears to slowly change the direction in which it swings. But the pendulum itself is not constantly turning.Instead, it continues swinging in almost the same plane relative to an inertial frame of reference while the Earth rotates beneath it.Because observers are standing on the rotating planet, the pendulum's path appears to gradually shift.That slow change provided direct physical evidence that Earth was rotating on its axis.Foucault first presented the experiment to scientists at the Paris Observatory on February 3, 1851. The demonstration attracted considerable attention and was soon recreated on a much larger scale at the Panthéon in Paris.The giant pendulum that made the invisible visibleThe Panthéon installation featured a brass sphere weighing about 28 kilograms, suspended from a wire roughly 67 metres long.Sand was spread across the floor beneath it so that the pendulum could leave a visible trace as it swung.At first, the movement appeared almost impossible to detect.But over time, the line traced by the pendulum gradually shifted.Nobody was pushing it. No mechanism was secretly changing its direction.The apparent change was caused by the rotation of the Earth beneath the swinging pendulum.That made the experiment particularly powerful. Instead of being told that Earth rotates, people could simply stand there and watch evidence of that rotation accumulate.The effect changes depending on where you areThe rate at which a Foucault pendulum appears to rotate depends on its latitude.At the North and South Poles, the plane of the pendulum's swing completes one apparent rotation in roughly a sidereal day.At the Equator, there is no apparent rotation of the swing plane.Between the poles and the Equator, the effect varies with latitude according to what became known as Foucault's sine law.The same basic experiment could therefore reveal something about Earth's position and rotation simply by changing where it was performed.The idea still fascinates people todayMore than 175 years after Foucault's demonstration, large pendulums continue to hang inside museums, universities and public buildings around the world.One well-known example is located at the United Nations Headquarters in New York. The pendulum was presented by the Government of the Netherlands in 1955 and hangs beneath the ceiling of the Visitor's Lobby.Its slow movement continues to give visitors a visual demonstration of something that is otherwise difficult to perceive: the ground beneath them is constantly rotating.A remarkably simple experiment with a huge ideaModern science has given humanity extraordinary ways to study Earth and the universe. We have satellites orbiting the planet, spacecraft travelling across the Solar System and instruments capable of detecting phenomena billions of light-years away.Yet the Foucault pendulum remains compelling because it requires almost none of that complexity.A long wire.A heavy weight.A swinging motion.And enough patience to watch.That is all it takes to turn Earth's rotation from an abstract scientific fact into something tangible.Foucault's experiment remains a reminder that some of the most memorable scientific demonstrations are not necessarily the most complicated ones. More than a century and a half later, a slowly swinging weight can still make people stop and realise that the entire world beneath them is moving.
