On March 31, 1851, a crowd of curious Parisians gathered at the Pantheon to witness a historic scientific demonstration. In the centre of the building, directly beneath its towering dome, they found a deceptively simple piece of equipment: a 28-kilogram brass-coated lead sphere, suspended from the building’s dome by a 67-metre-long wire. Beneath this was placed a wooden platform covered in a thin layer of sand. Once the crowd had settled, the organizer of the event, 32-year-old amateur scientist Léon Foucault, stepped forward and held a candle to the string anchoring the metal sphere to the wall. A few seconds later the string burned through, releasing the sphere. As the crowd watched, the pendulum swung slowly across the hall, a spike at its base scribing a line in the sand with each pass. At first nothing changed, but as the minutes ticked past something extraordinary began to happen. Slowly, the line in the sand began to shift, creeping steadily clockwise around the wooden platform. Within an hour it had rotated more than 10 degrees, while by the next day it had completed a full circle, returning to its original starting point. The implications of this strange phenomenon were not lost on the astonished onlookers. With only the simplest of devices, Léon Foucault had conclusively demonstrated what many had long suspected but had been unable to prove: that the earth does indeed rotate around its axis. This is the story of one of the most elegant and powerful experiments in the history of science.

For much of human history, the Earth was assumed to lie at the centre of the cosmos, while the sun, moon, stars, and planets rotated around it – the so-called geocentric model of the universe. But in the 16th and 17th Centuries, thanks to the efforts of scientists like Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei, humanity gradually adopted the heliocentric model, wherein the earth and the planets orbit around the sun. A major factor behind this cosmic shift was the problem of retrograde motion – the curious phenomenon whereby planets appear to briefly halt and reverse before resuming their regular paths. In the Second Century C.E, Roman astronomer Claudius Ptolemaeus reconciled retrograde motion with geocentrism by proposing that the planets moved in epicycles – smaller orbits within their regular orbits. But while the Ptolemaic model worked well enough for practical purposes like predicting astronomical events or setting the calendar, the concept of epicycles was clumsy and could not be explained by the known laws of motion. The geocentric model, by contrast, was far more elegant, handily explaining retrograde motion as the result of planets overtaking each other’s orbits. It also meshed perfectly with the laws of gravitation and motion discovered by Sir Isaac Newton a century later.

But while these theories and observations clearly established that the earth orbits around the sun, they said nothing about whether the earth rotated about its own axis. Astronomers had long assumed that the earth does rotate due to the circular paths traced by the sun and stars across the sky, but none had been able to produce any other solid evidence of this fact.

The first solid clue that the earth does indeed rotate came about largely by accident. In 1573, English astronomer Thomas Digges predicted that, if the earth really did orbit around the sun, the position of the stars should appear to shift slightly throughout the year – a phenomenon known as stellar parallax. Over the next century, several astronomers – including Frenchman Jean Picard and Englishmen John Flamsteed and Robert Hooke – confirmed that this was indeed the case. Unfortunately, the degree and annual cycle of this shift did not match that which would be produced by parallax. This discrepancy puzzled astronomers until 1728, when James Bradley, England’s third Astronomer Royal, realized that this shift – which he dubbed stellar aberration – was caused not by the movement of the earth around the sun, but by the earth’s rotation around its axis. The simplest analogy for this phenomenon would be walking in the rain. If you are standing still and no wind or other such factors, the rain that hits you is only from the top. But as you walk or run, raindrops will strike you from the front; consequently, though the rain is falling straight downwards, from your frame of reference your motion makes it appear as though the rain is coming down at an angle. Similarly, the motion of the earth through space makes the light from distant stars – which travels in straight lines – appear to be arriving at a slight angle, causing the apparent position of the light source to shift.

Four decades earlier, Sir Isaac Newton proposed yet another empirical test for the rotation of the earth. In his groundbreaking 1687 work Philosophiae Naturalis Principia Mathematica, Newton predicted that the centrifugal forces generated by the rotation of the earth would cause the planet to bulge around its equator. If this was true, then the force of gravity would be slightly stronger at the equator than at the poles – a phenomenon that could be measured, for example, by timing the swing of a pendulum. Indeed, such an experiment had already been carried out in 1673 by the French astronomer Jean Richer, who found that a seconds pendulum – that is, a pendulum constructed to make a complete swing every two seconds – was 2.8 millimetres shorter in French Guiana, South America, than in Paris. Later, in 1736, a pair of French expeditions were sent to measure the length of one degree of meridian arc near the pole and near the equator. They found that the earth is indeed flattened at the poles and bulged at the equator, confirming Newton’s prediction and further supporting the notion that the earth spins about its axis.

Yet another of Newton’s predictions was that a dropped object would fall slightly eastwards from its release point due to the earth rotating beneath it – a phenomenon now known as the Coriolis Effect. This effect is responsible for large weather systems like hurricanes and cyclones rotating clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere and must be accounted for when firing artillery shells over long distances. Contrary to popular belief, however, it is not responsible for making toilets flush in different directions in different hemispheres. A hurricane might be hundreds of kilometers in diameter and last for many days; your sink or toilet is very small in comparison and the time the Coriolis Effect has to influence the draining water is very small as well. In fact, when we are talking draining sinks, toilets, and bathtubs, the size and time scale is so small that the Coriolis Effect force is practically non-existent in terms of affecting the water in any way, especially when compared to the other forces in play here like the shape of the sink, the way the jets are pointed in the toilet, and things of this nature. In this case, the Coriolis Effect has about the same effect on the swirling water in your toilet as a butterfly’s wings flapping have on a Tornado.

In any event, based on Newton’s prediction, in 1679 fellow English scientist Robert Hooke attempted to measure the eastward deflection of a weight dropped from a height of 8.2 metres. However, this was too short a distance to obtain an easily measurable result, and it was not until 100 years later that a trio of scientists – Giovanni Guglielmini, Johan Benzenberg, and Ferdinand Reich – confirmed the existence of the Coriolis Effect by dropping weights from towers 150 metres in height.

But while these experiments were enough to convince the scientific community, their results were too minuscule and esoteric to be appreciated by the general public, who continued to harbour lingering doubts about the earth’s rotation. Enter Léon Foucault and his famous pendulum.

Jean Bernard Léon Foucault was born in Paris on September 18, 1819. His father, a publisher, died when Foucault was only nine while his mother was often in poor health, so he received most of his schooling at home. As a child he showed a remarkable aptitude for mechanics, constructing such devices as a working telegraph and steam engine. After completing a Bachelor of Arts, Foucault began studying to become a doctor. However, he could not stand the sight of blood and soon dropped out of medical school, instead becoming the laboratory assistant of bacteriologist Alfred Donné. Three years later, Foucault partnered with the physicist Hippolyte Fizeau, with whom he conducted various experiments on the nature and properties of light. He also became fascinated with Louis Daguerre’s newly-invented Daguerrotype photographic process and developed numerous improvements to it. By 1850, however, Foucault and Fizeau had fallen out and parted ways. Working independently, in 1851 the pair conducted experiments in which they measured the speed of light to within 5% of its currently accepted value. Foucault also determined that light travels more slowly through water than air, helping to disprove Sir Isaac Newton’s “corpuscular” or particle theory of light and sway the scientific establishment towards the wave theory of light – but that is a subject for another video.

One night in early January 1851, Foucault had what would turn out to be his most famous insight: that a large swinging pendulum could be used to demonstrate the rotation of the earth. The principle of the Foucault Pendulum is identical to that of the Coriolis Effect: like a flying artillery shell or falling weight, a swinging pendulum travels in a straight line or fixed plane independent of the earth’s rotation – in other words, it occupies an inertial frame. So long as no outside force acts upon it, the pendulum will continue to swing along this plane, meaning that as the earth spins beneath it, this plane will appear to rotate relative to its starting position – clockwise in the northern hemisphere and counter-clockwise in the south. The speed of this rotation depends on the pendulum’s latitude. At the poles, it will complete one rotation every 24 hours, while at the equator it will not rotate at all. At any other latitude, the rate of rotation is proportional to the sine of the angle of latitude. For example, in Paris, which lies at 48.85º North, a Foucault Pendulum will complete one rotation every 31.85 hours. As the suspension point of the pendulum is fixed to the earth, the plane of oscillation does not remain truly fixed but rather rotates at a rate of 180 degrees per day, returning to its original position every two days.

However, in order to achieve the desired results, a Foucault Pendulum must be very carefully constructed. The anchor point for the suspension cable must be built with a universal joint so that no direction of swing is preferred, while the cable itself must be as rigid, homogeneous, and free of imperfections as possible to prevent unwanted harmonic vibrations. As uneven air resistance can also deflect the pendulum, the bob must be made aerodynamic, symmetric, and as massive as possible. Finally, any extraneous forces during the pendulum’s launch can severely affect the swing – hence why Foucault Pendulums are usually launched by burning a string. Many current examples in science museums are also equipped with an electromagnetic system to counter the effects of air resistance and keep the pendulum swinging indefinitely.

On January 3, 1851, Foucault tried out his idea using a small pendulum suspended in the basement of his house. Having confirmed the basic principle, he sent out a letter to a group of scientists and dignitaries – including Emperor Napoleon III – stating:

“You are invited to see the earth turn.”

The first public demonstration of the Foucault Pendulum took place on February 3, 1851 in the Meridian Room of the Paris Observatory. But while the invited scientists were astonished by the dramatic experiment, due to Foucault’s lack of formal education – and not a bit of professional jealousy – the scientific establishment was slow to take him seriously. However, Foucault’s legacy was secured a month later when he conducted his famous public demonstration at the Pantheon, quickly propelling him to the heights of celebrity and causing tourists to flock to the exhibit in droves. Elegant and dramatic, the pendulum quickly dispelled any lingering doubts the public might have had about the rotation of the planet beneath their feet. After Foucault published his results later that year, the scientific community grudgingly accepted him into their ranks. In 1855 he was awarded the Copley Medal – the highest honour of the Royal Society of London – for his work on the relation between mechanical energy, heat, and magnetism, while that same year he was made Physicist of the Imperial Observatory at Paris – a position created specifically for him. Finally, in 1865, he was elected to the French Academy of Sciences.

Though best remembered for his pendulum, Foucault would go on to make many other important contributions to the fields of physics and technology. For instance, he discovered the eddy currents generated in a moving piece of copper by a magnet – a phenomenon now used to slow down high-speed trains; created a mechanism that made electric arc lighting practical; developed and named the gyroscope and used it to once again demonstrate the rotation of the earth; invented a new type of steam engine governor; and developed numerous improvements to telescope lenses and mirrors. But as they say: the candle that burns twice as bright burns half as long, and Léon Foucault died in Paris of multiple sclerosis on February 11, 1868. He was only 49.

Meanwhile, Foucault’s most famous experiment has taken on a life of its own. In 1855, the bob used in the original 1851 demonstration was moved to the Conservatoire des Arts et Métiers, while in 1902, another pendulum was temporarily installed at the Pantheon to mark the 50th anniversary of Foucault’s experiment. In 1995, the original pendulum was returned to the Pantheon while the now Musée des Arts et Métiers underwent renovations, being returned in 2000. On April 6, 2010, however, the suspension cable snapped, causing irreparable damage to the historic bob and the museum’s marble floor. The bob was thus retired and displayed in a separate glass case, while a replica pendulum was installed in its place. Another replica has also been operating at the Pantheon since 1995, while dozens of Foucault Pendulums have been installed in science museums around the world. Along with electromagnetic drives to keep them swinging, these pendulums often have other features like a circle of hinged pins that the bob gradually knocks over as it turns. But ultimately all work on the same elegant principle – demonstrating that sometimes the most powerful experiments are the simplest.

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