How Léon Foucault taught us to watch the world spin

Posted on 18 Sep 2013


Leon Foucault Google DoodleLéon Foucault and his pendulum demonstrated the turning of the Earth. Google honored the French physicist with an interactive doodle.

It’s easy to forget that while you read this article, you and your computer screen are racing through space at a nearly unimaginable speed. The earth constantly rotates on its axis at 1,037 miles per hour, orbiting the sun at 67,000 m.p.h. And the solar system whips around the galaxy at more than 420,000 m.p.h

Two centuries ago, it was difficult for scientists to model intricate planetary orbits. Léon Foucault helped devise a method to make celestial orbits a bit easier to understand.

Wednesday marks the 194th anniversary of the French physicist’s birth. To celebrate Mr. Foucault and his breakthrough pendulum, let’s take a look at how he was able to model Earth’s rotation.

Jean Bernard Léon Foucault was born in Paris in 1819. While Foucault received a medical education, the profession did not quite suit him. The young doctor is said to have a distaste for bloody medical dissections. But Foucault was brilliant when it came to making models, tools, and devices.

And Foucault’s craftsmanship came in handy.

Foucault and a series of teachers, bosses, and partners tackled many scientific questions by building contraptions that could make hard-to-grasp phenomenas more tangible. Foucault was able to measure the speed of light. He improved the daguerreotype, an early form of photography. He found a way to prove that light is a wave, not a beam of particles. He named the gyroscope, a stabilizing tool found in everything from toys to the International Space Station.

In 1851, Foucault made one of his best-remembered experiments: the scientist devised the first model to demonstrate the rotation of the earth on its axis.

People had tried many different ways to explain Earth’s rotation before Foucault. One group had even launched cannon balls up into the air with the hopes that the world would spin enough that they could measure the deviation once the ball plummeted back to earth. Compared to that loud, inaccurate (and dangerous) plan, Foucault’s solution was remarkably elegant. He strung up a brass weight at the end of six-foot wire. The metal ball hung over a pile of damp sand, just close enough that the brass brushed against the sand as it swung slowly back and forth. At first, the pendulum simply carved a straight line in the sand. But over the course of several hours, the line turned into a bow-tie shape.

Newton’s laws of motion state that an object will not change direction unless another force hits it. This means that while Foucault’s pendulum kept swinging in the same direction, the earth (and the sand on the ground) turned underneath it. It’s as if you drew a line back and forth repeatedly on a piece of paper, but then slowly rotated the sheet as you kept drawing – eventually the lines would form a circle.

Foucault’s experiment became a sensation. The French government even ordered a large-scale version that would hang inside the Pantheon in Paris, with a 219-foot, 61-pound pendulum suspended from the building’s dome. Modern-day pendulums hang in the United Nations headquarters in New York, the California Academy of Sciences in San Francisco, the Boston Museum of Science, and many other locations.

Since Earth spins on an axis, each of these “Foucault pendulums” turns at a slightly different rate, demonstrating that different parts of the globe rotate at different speeds. At the North Pole, a Foucault pendulum would turn 15 degree per hour, making a full 360-degree circle each day, while in Paris, the pendulum would only turn about 11 degrees an hour, requiring 32.7 hours to make a complete round. On the equator, the pendulum would not appear to spin at all.

In recognition of his many achievements, Foucault is among the 72 French engineers, scientists, and mathematicians whose names are engraved on the Eiffel Tower.

Can you visualize why a Foucault pendulum at the earth’s equator will not rotate? More from Wikipedia:

At either the North Pole or South Pole, the plane of oscillation of a pendulum remains fixed relative to the distant masses of the universe while Earth rotates underneath it, taking one sidereal day to complete a rotation. So, relative to Earth, the plane of oscillation of a pendulum at the North Pole undergoes a full clockwise rotation during one day; a pendulum at the South Pole rotates counterclockwise.

When a Foucault pendulum is suspended at the equator, the plane of oscillation remains fixed relative to Earth. At other latitudes, the plane of oscillation precesses relative to Earth, but slower than at the pole; the angular speed, ω (measured in clockwise degrees per sidereal day), is proportional to the sine of the latitude, φ:

\omega=360\sin\varphi\ ^\circ/daywhere latitudes north and south of the equator are defined as positive and negative, respectively. For example, a Foucault pendulum at 30° south latitude, viewed from above by an earthbound observer, rotates counterclockwise 360° in two days.

In order to demonstrate the rotation of the Earth without the complication of the dependence on latitude, Foucault used a gyroscope in an 1852 experiment. The gyroscope’s spinning rotor tracks the stars directly. Its axis of rotation is observed to return to its original orientation with respect to the earth after one day whatever the latitude, not being subject to the unbalanced Coriolis forces acting on the pendulum as a result of its geometric asymmetry.

Foucault’s Pendulum at the Ranchi Science Centre.

A Foucault pendulum requires care to set up because imprecise construction can cause additional veering which masks the terrestrial effect. The initial launch of the pendulum is critical; the traditional way to do this is to use a flame to burn through a thread which temporarily holds the bob in its starting position, thus avoiding unwanted sideways motion. Air resistance damps the oscillation, so some Foucault pendulums in museums incorporate an electromagnetic or other drive to keep the bob swinging; others are restarted regularly, sometimes with a launching ceremony as an added attraction.

A pendulum day is the time needed for the plane of a freely suspended Foucault pendulum to complete an apparent rotation about the local vertical. This is one sidereal day divided by the sine of the latitude.

Posted in: Earth, History, Technology