Foucault's Pendulum at the Panthéon Paris
How a 28-kilogram bob on a 67-metre wire proved the Earth turns — and how to watch it happen during your visit
In March 1851, the physicist Léon Foucault suspended a 28-kilogram brass-coated lead bob on a 67-metre wire from the dome of the Panthéon and invited Parisians to watch the Earth turn beneath it. The demonstration was the first laboratory experiment to make the rotation of the Earth directly visible to the eye, without astronomical instruments. The original bob is now at the Musée des Arts et Métiers; an exact-replica pendulum has hung in the Panthéon since 1995. This guide explains how the experiment works, why it works at the latitude of Paris, what to expect when you stand under it today, and where to position yourself to see the rotation effect inside a single visit.
The 1851 demonstration — apparatus and audience
Foucault's pendulum was not an abstract physics experiment. It was a public demonstration commissioned to make a previously theoretical effect — the rotation of the Earth — directly observable to anyone willing to stand and watch. After a smaller prototype at the Paris Observatory in February 1851, Foucault was invited by Prince Louis-Napoléon (then President of the Second Republic, soon to be Emperor Napoleon III) to install a full-scale version under the central dome of the Panthéon, where the 67-metre clearance from dome oculus to floor allowed an unusually long wire and therefore a slow, easily observed period of swing. The bob — a 28-kilogram brass-coated lead sphere — was released with a burnt-thread mechanism, in which a fine cord holding the bob to one side was burned through with a candle so that no lateral push could be imparted at the moment of release. A sand tray on the floor recorded the swing pattern as a series of fine ridges that visibly rotated over the course of the day, and an iron point fixed to the underside of the bob inscribed the rotation directly into the sand.
The Panthéon installation opened to the public on 26 March 1851 and was an immediate sensation. Newspapers reported steady queues of Parisians waiting their turn under the dome, and the demonstration was repeated by Foucault several times a day during the opening weeks. Visitors who came back to the same spot after a few hours could see, with their own eyes, that the plane of the swing had rotated relative to the sand tray and the building beneath it. The effect required no instrument other than patience — the rotation is about 11.3 degrees per hour at the latitude of Paris, perceptible within an hour and unmistakable within three. Foucault was awarded the Légion d'honneur the following year, the gyroscope (which he invented in 1852 to confirm the same effect by a different method) became a standard piece of physics-lab apparatus, and the phrase pendule de Foucault entered everyday French. The bob remained in the Panthéon until 1855, when the building was returned to religious use and the pendulum was transferred to the Conservatoire des Arts et Métiers, where the original still hangs today.
Why it works — and why latitude matters
The principle is simpler than the maths suggests. A free-swinging pendulum maintains the plane of its swing relative to the fixed stars, not relative to the ground beneath it. Because the Earth rotates once per sidereal day, the floor underneath the pendulum turns while the swing plane does not — and to an observer standing on the floor, it looks as though the swing plane itself is rotating. The apparent rotation rate is given by ω = 360° × sin(φ) per sidereal day, where φ is the latitude of the installation. At the latitude of Paris (48.86° N), sin(φ) is roughly 0.752, which gives an apparent rotation of about 271 degrees per day, or 11.3 degrees per hour, completing a full revolution in approximately 31 hours 50 minutes.
The effect is strongest at the poles, where a pendulum's swing plane appears to rotate a full 360 degrees in a single sidereal day (23 hours 56 minutes), and zero at the equator, where sin(φ) is zero and the pendulum's swing plane does not appear to rotate at all. A Foucault pendulum in Quito or Singapore would simply swing back and forth without any apparent precession, no matter how long you watched. The fact that Paris sits at roughly 49 degrees north was lucky for the demonstration — high enough latitude to give a visibly fast rotation within a single afternoon, low enough that the swing plane does not move so fast as to be confusing. A southern-hemisphere installation, by contrast, would rotate the opposite way: counter-clockwise rather than clockwise.
Removal, restoration, and the 1995 replica
The original pendulum did not stay in the Panthéon continuously. When Napoleon III returned the building to Catholic use in 1852 and re-designated it as a national basilica, the experiment was discontinued; the bob was formally moved to the Conservatoire des Arts et Métiers (now the Musée des Arts et Métiers) in 1855. The building swung back to secular use definitively in 1881, but the pendulum did not return — partly because the museum had taken curatorial responsibility for the original apparatus, partly because the demonstration was, by then, scientifically uncontroversial and no longer needed.
The decision to reinstate a working pendulum at the Panthéon was made in 1995, in advance of the broader CMN refurbishment of the monument, and an exact replica of Foucault's 1851 apparatus was hung from the dome at its original location. The replica uses the same wire length (67 metres) and a bob of equivalent mass, and is restarted each morning by Panthéon staff using the same burnt-thread release Foucault devised. The replica was briefly damaged in April 2010 when the suspension cable failed and the bob fell, denting the marble floor and the bob itself; the original 1851 bob at the Musée des Arts et Métiers is still on display there, now in a case adjacent to a working pendulum installed in the museum's nave.
Seeing the rotation during a one-hour visit
The rotation is genuinely visible within a single visit, but only if you know where to look. The sand tray beneath the pendulum is set with small wooden pegs arranged in a ring around the swing path; the bob knocks over a peg roughly every fifteen to twenty minutes as the swing plane rotates and a fresh peg comes into the bob's path. If you arrive at 10:00 when the staff has just released the pendulum and the first peg is still standing, then return at 10:20, you will see that a different peg is in the line of swing and the first is on the ground. This is the easiest way to observe the rotation without staying for the full thirty-two-hour cycle.
Photographically, the best position is on the eastern side of the nave, looking west across the swing plane, with the dome and the 67-metre wire foreshortened above the bob. A medium telephoto (50–85 mm equivalent) captures the bob and the sand tray together; wider lenses lose the bob in the geometry of the dome. The nave is lit from the dome oculus, so the bob is brightest at midday in summer and softer in winter; the brass coating reflects warm light against the cool stone of the nave. Tripods are not permitted in normal opening hours, so a fast lens or a high ISO is required to freeze the bob's motion.
Foucault's pendulum at the Panthéon vs other installations worldwide
The Panthéon installation is the most historically significant Foucault pendulum but not the largest or the most accurate. Working Foucault pendulums hang in physics museums and science centres around the world — from the Smithsonian and the Griffith Observatory in the United States to the United Nations headquarters in New York, and from the Deutsches Museum in Munich to the National Museum of Scotland in Edinburgh. Each is calibrated to the local latitude, and each provides a slightly different visible rotation rate: the higher-latitude installations rotate faster and the equatorial ones barely rotate at all.
The Panthéon's particular value is contextual rather than technical. Seeing the pendulum in the building where Foucault first hung it, under the dome he chose for the 67-metre clearance, surrounded by the tombs of the philosophers and scientists who made the Enlightenment intellectually possible, frames the experiment as a piece of European cultural history rather than a science-museum exhibit. Visitors who care about the physics will probably get a more legible demonstration at the Musée des Arts et Métiers (where the original bob and a separate working pendulum sit side by side, with explanatory panels in several languages); visitors who care about the moment in 1851 when an apparatus made the rotation of the Earth visible to the public will want to stand in the Panthéon.
常见问题
What is Foucault's pendulum and what does it prove?
It is a free-swinging pendulum whose swing plane appears to rotate over the course of a day, demonstrating that the Earth rotates beneath it. Léon Foucault installed the first public version under the dome of the Panthéon in March 1851 — the first direct laboratory proof of the Earth's rotation.
How heavy is the bob and how long is the wire?
The bob is a 28-kilogram brass-coated lead sphere; the wire is 67 metres long, hung from the central oculus of the Panthéon dome. The replica installed in 1995 uses the same dimensions as Foucault's original 1851 apparatus.
How fast does the pendulum rotate at Paris's latitude?
Approximately 11.3 degrees per hour, completing a full apparent rotation in about 31 hours 50 minutes. The rate is calculated as 360° × sin(latitude) per sidereal day; at the latitude of Paris (48.86° N), this works out to roughly 271 degrees per day.
Is the original 1851 pendulum still at the Panthéon?
No. The original bob was moved to the Conservatoire des Arts et Métiers (now the Musée des Arts et Métiers) in 1855 and is still on display there. The pendulum currently swinging at the Panthéon is an exact replica installed in 1995.
How does the effect change with latitude?
The apparent rotation rate is proportional to the sine of the latitude. At the poles (90°), a pendulum completes a full rotation in one sidereal day (23 hours 56 minutes). At the equator (0°), the swing plane does not appear to rotate at all. Southern-hemisphere installations rotate counter-clockwise rather than clockwise.
Can I actually see the rotation during a visit?
Yes. Small wooden pegs are arranged in a ring around the swing path beneath the bob; the pendulum knocks one over every 15–20 minutes as the swing plane rotates. Returning to the nave after 20–30 minutes is the simplest way to confirm the rotation with your own eyes.
When is the pendulum running?
Throughout normal opening hours. Staff restart the pendulum each morning using a burnt-thread release mechanism that avoids imparting any lateral push. Occasional restoration work pauses it; the Centre des monuments nationaux publishes any extended closure on their website.
Did the pendulum ever fail?
Yes — the suspension cable failed in April 2010 and the replica bob fell, denting the marble floor and the bob itself. The pendulum was repaired and reinstalled. The original 1851 bob at the Musée des Arts et Métiers is undamaged.
Why did Foucault choose the Panthéon?
Because the 67-metre clearance from the dome oculus to the floor allowed an unusually long wire. A long wire gives a slow oscillation period, which makes the rotation effect easier to observe over short periods of time. The Panthéon dome was the tallest interior space readily available to Foucault in 1851.
Where is the best place to stand to photograph the pendulum?
The eastern side of the nave, looking west across the swing plane with the dome and wire foreshortened above. A 50–85 mm equivalent lens captures the bob and sand tray together. Tripods are not permitted during normal opening hours, so use a fast lens or higher ISO to freeze the bob's motion.