In February 1851, visitors to the Panthéon in Paris were greeted by a notice inviting them to come and watch the Earth turn. Inside, a brass sphere weighing twenty eight kilograms hung from the centre of the dome on a wire sixty seven metres long, swinging slowly back and forth across a bed of sand.
Over the course of an hour, the line traced by the pointer in the sand rotated by about eleven degrees. Nobody had touched the pendulum. The floor had moved.
What Was Missing
By 1851 nobody with a scientific education doubted that the Earth rotates. What had never existed was a direct demonstration that an ordinary person could witness.
The evidence up to that point was indirect and required trusting somebody else’s measurements. James Bradley had discovered stellar aberration in 1729, a tiny annual shift in star positions caused by the Earth’s motion through space. Friedrich Bessel had measured stellar parallax in 1838. Both are real proofs and both involve angles of a fraction of an arcsecond, visible only through a good telescope to a trained observer.
This gap had consequences beyond physics. When Galileo was tried in 1633, one of his difficulties was precisely that no conclusive physical demonstration of the Earth’s motion existed. His own attempt, an argument from the tides, was wrong. Two hundred and eighteen years later, Foucault supplied what Galileo had needed, and did it with a weight on a string.
The Principle
A pendulum swings in a plane. Once it is set going, there is no horizontal force acting on it other than gravity and the tension in the wire, and neither of those can push it sideways out of that plane.
The plane of oscillation therefore stays fixed relative to space rather than relative to the building. If the ground underneath is rotating, the swing appears, to an observer standing on that ground, to rotate the other way.
The rate depends on latitude, and the relationship is straightforward. At the North or South Pole, the pendulum’s apparent rotation completes a full circle in exactly one sidereal day, because the ground is turning underneath it once per day. At the equator, the plane does not rotate at all. In between, the period is one day divided by the sine of the latitude.
Paris sits at 48 degrees 52 minutes north, which gives a rotation of about 11 degrees per hour and a full circuit in roughly 32 hours. That is fast enough to see within a few minutes of watching and slow enough to be unmistakably steady.
Building It
Foucault first tried the experiment in the cellar of his house in January 1851 with a two metre pendulum, and observed the effect. He repeated it at the Paris Observatory with an eleven metre wire, and then, with the support of Napoleon III, moved to the Panthéon.
The scale is not showmanship. A long pendulum has a long period and swings slowly, which reduces the disturbance from air resistance and from imperfections in the suspension. A heavy bob keeps going for hours. Both matter because the effect being measured is small and any asymmetry in the setup will produce a spurious rotation that swamps it.
The starting procedure is the delicate part. Releasing the bob by hand imparts a sideways component that turns the swing into a slow ellipse, which precesses on its own and ruins the result. Foucault’s solution was to draw the bob to one side with a cord, tie it, wait for all motion to die away, and then burn the cord through. The pendulum starts from rest with no lateral push.
A pointer on the bob traced a line in sand spread on the floor, so that the accumulating shift was visible as a fan of marks rather than something the audience had to take on trust.
Why the Public Demonstration Mattered
The Panthéon installation was a scientific result presented as a public spectacle, and it worked as both. Crowds came. Versions were built in observatories, universities and museums across Europe and America within months, and they remain one of the most common large exhibits in science museums.
The reason it is compelling is that the audience is inside the experiment. There is no instrument to interpret and no data to reduce. A heavy object swings in a straight line, the floor turns underneath it, and everyone in the room is turning with the floor at about a thousand kilometres per hour without feeling anything at all.
The Gyroscope
The following year, in 1852, Foucault built a device that demonstrates the same principle differently: a heavy disc mounted in gimbals so that it can spin freely in any orientation.
A spinning disc maintains the direction of its axis in space. Mounted so that friction is minimal, it appears to turn slowly relative to the room, for exactly the same reason the pendulum’s plane appears to turn.
Foucault named it the gyroscope, from the Greek for viewing the rotation, since that was its purpose. The practical consequences were enormous. A gyroscope holding a fixed direction independent of magnetic fields became the basis of the gyrocompass, and later of the inertial guidance systems that steer ships, aircraft, missiles and the spacecraft that navigated to the Moon.
Foucault’s Other Work
He began as a medical student and abandoned it, reportedly because he could not tolerate the sight of blood. He moved into experimental physics through photography, working with Hippolyte Fizeau on daguerreotypes, and had no formal physics training or university appointment for most of his career.
His experimental range was remarkable.
In 1850 he settled a two hundred year old argument about the nature of light. The particle theory requires light to travel faster in water than in air, while the wave theory requires the opposite. Using a rapidly rotating mirror, Foucault measured the speed in both media and found that light moves more slowly in water. This is the experiment that finished the corpuscular theory and confirmed the wave account, closing out the debate between Huygens and Newton.
He refined the rotating mirror method further and in 1862 produced a value for the speed of light of about 298,000 kilometres per second, within a fraction of a percent of the modern figure and a considerable improvement on earlier astronomical estimates.
He discovered the circulating currents induced in a conductor moving through a magnetic field, still called eddy currents or, in French, courants de Foucault. He invented the knife edge test for checking the figure of a telescope mirror, which amateur telescope makers use to this day, and introduced the silvered glass mirror that replaced the tarnish prone speculum metal used by Herschel and his contemporaries.
He died in 1868, aged forty eight, probably of multiple sclerosis.
The Physics Underneath
The pendulum’s behaviour is a consequence of the fact that Newton’s laws hold in an inertial frame and the surface of a rotating planet is not one. Working in the rotating frame requires introducing fictitious forces, of which the Coriolis force is the relevant one here, and the pendulum’s precession is exactly what that force predicts.
The same effect governs the rotation of weather systems, the deflection of long range artillery, and the circulation of ocean currents. The mathematics of motion in a rotating frame follows directly from the mechanics set out in Newton’s Principia, though Newton himself treated rotation in the context of an absolute space whose existence he defended with a different thought experiment involving a spinning bucket of water.
There is a subtlety worth noting. The pendulum’s plane stays fixed relative to the distant stars, not relative to any local structure. Why the local inertial frame should be tied to the average distribution of matter in the universe is a question Ernst Mach raised in the 1880s, and Mach’s principle was one of the ideas that pushed Einstein towards general relativity.
The Original
The pendulum was removed from the Panthéon after 1851 and reinstalled permanently in 1995. In 2010 the original wire snapped and the historic bob was damaged. The exhibit continues with a replacement.
The idea has needed no updating. It is one of the very few fundamental experiments in physics that requires no specialised equipment, produces a result visible to the naked eye, and can be assembled by anyone with a high ceiling and a heavy weight.
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