UNSW Bragg Student Science Prize winner Jessica Kitchen shares a pendulum conundrum story.

Throughout history, there have been a multitude of scientific theories that have changed the world. Scientists living in the time period now known as the scientific revolution were responsible for many of these innovative ideas about biology, medicine, anatomy, physics and astronomy.

One of these unconventional theories was the idea that the earth rotated on its axis. Although this was theorised by several prominent scientists as far back as ancient Greece, it was still not fully accepted by the end of the scientific revolution. In 1851 a man called Jean Bernard Léon Foucault devised the world’s first simple experiment to prove the Earth really does rotate.

Before the experiment took place, many people didn’t believe the Earth could possibly rotate. In ancient Egypt, an astronomer known as Ptolemy argued that if the Earth rotated, we would surely notice its motion – in fact, the Earth would be devastated by gales. As the Earth’s rotation is very smooth and does not speed up or slow down, this is obviously not the case. Other astronomers believed in the geocentric (or Earth-centred) model of the universe, which taught that the Earth remained stationary while the Sun, Moon and stars spun around the Earth at the centre of an imaginary sphere.

These ancient theories, contested throughout history and disproven during the scientific revolution, were finally proven wrong using a simple but impressive tool – an over 60 m-tall pendulum. Jean Bernard Léon Foucault created an experiment in which a 28 kg lead-filled brass sphere is suspended from an over 60 m-long wire. The original pendulum, which was affixed to the ceiling of the Panthéon in Paris, was released in a launching ceremony and began to swing back and forth. A point, fastened to the underside of the sphere, traced out a path in the sand that was placed under the pendulum. As the hours passed, the path of the pendulum, instead of remaining as a straight line, took the form of a flower with many thin ‘petals’.

This occurs because the Earth turns underneath the pendulum. As the pendulum swings back and forth, the line along which it swings undergoes a full clockwise rotation, causing the pendulum to turn and the pattern to change. (It turns clockwise if viewed from the North Pole. From the South Pole, the line along which the pendulum rotates spins counterclockwise.) Of course, the pendulum was not really turning; the launching ceremony, involving a thread that held the pendulum in place and was then burnt through, was designed specifically to prevent any sideways movement. Mathematical equations were calculated to explain the results, including the effects of latitude and air resistance.

So why does the result of this experiment matter? Sure, it shows that the Earth rotates, but people had already been theorising that for thousands of years! It’s true that scientists through the ages have suspected that the Earth rotates, but until the Foucault Pendulum Experiment, there was no proof. Those who suspected the Earth was stationary had always been the better-represented side of the argument, as simple observation seems to prove this. But the Foucault pendulum had another important purpose besides validating the theory of the Earth’s rotation – it showed how fast the Earth rotated. Calculations involving how many times the pendulum swung in a single day and night (or full rotation of the Earth) and by how much the pendulum’s path deviated from a straight line were used to calculate that the Earth spins at about 1600 km per hour at the equator (it’s slower at higher latitudes because of the Earth’s spherical shape; at higher latitudes the land has less far to travel to spin around from one point to the next in a full rotation). This information would later become useful when sending spacecraft and especially satellites into orbit.

In conclusion, the Foucault Pendulum Experiment is certainly a mind-blowing experiment, if for no other reason than the scale of the pendulum involved. Today, Foucault pendulums are displayed in museums all over the world, and new technologies such as an electromagnetic drive that eliminates the effects of friction are used to keep the pendulum running smoothly. A Foucault pendulum has even been positioned at the South Pole to show the Earth’s rotation at high latitudes. The fact that the pendulum’s swing differs depending on how distant you are from the equator has also been used to create an equation that allows you to determine your latitude from the pendulum’s swing. One thing’s for certain – thanks to the Foucault pendulum, scientists will never look at the Earth the same way again.

– Jessica Kitchen


Directorate for Education and Human resources Programs n.d., Foucault’s Pendulum, American Association for the Advancement of Science, accessed 1 August 2015