If for some reason you're not enticed by the lush national parks near Valdivia, Chile, there's a mesmerizing attraction located along the banks of the Calle-Calle River: the Pendulo de Foucault, or a Foucault pendulum.

While sipping a coffee and taking in a breeze, you can watch a 220-pound chrome-plated lead ball swing from a 42-foot-long steel wire housed in a glass and steel tower and topped with an antique lighthouse lamp. And why would you want to watch this really heavy ball swing around an image of a compass?

So you know that the Earth is still spinning.

## As the world turns

The first Foucault (pronounced "Foo-koh") pendulum was constructed in 1851 by French physicist Jean Foucault and was a way to demonstrate the rotation of the Earth. Of course, by 1851, it wasn't exactly controversial that the Earth was rotating. What Foucault did with his pendulum was prove it without using astronomy.

Foucault suspended his pendulum — composed of a 61-pound weight and 200-foot-long cable — in the Pantheon in Paris over a pit of wet sand and gave it a little push to get it swinging. The long cable and the heavy weight ensuree that the pendulum would keep swinging for at least an hour. At the bottom of the weight, a pin traced a line in the sand as the pendulum moved back and forth.

When Foucault set the pendulum swinging, its path was set. It'll slow down, eventually, sure, but it won't deviate from the path of its swing, barring some sort of interference. Over time, however, the angles of the line the pin drew changed. After an hour, for instance, the pin drew a line in the sand that intersected with the first line at an angle of around 11.25 degrees. The result was proof that an outside force was exerting an influence on the pendulum's path: the Earth's rotation.

Animation of a fictitious Foucault pendulum of 67 meters released at a distance of 3/4 its length from the east with a null speed. The rotation of the Earth is also exaggerated and corresponds to a rotation in 110 seconds. It corresponds to the view taken from the plane of oscillations; the terrestrial reference frame turns. But the shadow of a yellow stick is added at the center for ease of comprehension, rotates faster than the plane of oscillations. In this pseudo-plane, the trace is not linear but corresponds to an ellipse. In this illustration, the pendulum is launched at noon at equinox. Thus the sun set is exactly six hours after the launch. (Image: Nbrouard/Wikimedia Commons)

But why 11.25 degrees? The pendulum moved in accordance with Foucault sine law, which predicts the amount of distortion of the pendulum's path based on the pendulum's latitude.

"Since all points on Earth's surface rotate as a unit, it follows that those located on the wider portions of the planet — nearer to the equator — must cover more meters each second (i.e., go faster) to 'keep up' with the points tracing smaller circles each day at the extreme northern and southern latitudes," Alicia Ault writes in Smithsonian magazine. "Though they don't feel it, a person standing in Quito, Ecuador, is moving with appreciably higher velocity than one in Reykjavik, Iceland."

As a pendulum swings, it moves further and closer to the equator, and since the velocities at these points differ, the pendulum's path shifts. It also means that pendulums at different locations move differently. In the North Pole, for instance, a pendulum would complete a 360-degree clockwise path in 23 hours, 56 minutes and 4.1 seconds, aka, a sidereal day, which is the amount of time it takes the Earth to complete a rotation relative to its stars as opposed to a solar day; at the South Pole, it would do the same thing, but in a counterclockwise motion. And if you set up a pendulum at the equator, it won't seem to move at all.

Since Foucault's experiment, his pendulums have become mainstays of museums around the world. The one in Valdivia, for instance, while outside, is across the street from the Centro de Estudios Científicos. Many museums employ some sort of drive to keep the pendulum swinging — air resistance won't move the pendulum's path, but it will eventually bring it to a standstill — or they let the pendulum wind down on its own and begin each day with a swinging ceremony.

It's difficult to miss Valdivia's Foucault pendulum. (Photo: Marco Antonio Correa Flores/Wikimedia Commons)

And speaking of the Valdivia pendulum, it's the southern most pendulum in the world. (There was a pendulum in Antarctica, erected to see how a pendulum would operate there.) The Centro de Estudios Científicos' pendulum was constructed in January 2007, but the original version proved a little unstable. It was replaced and it remained inside the building until April 2011, when it was moved outside into its current spot along Science Coastal Road.

The pendulum makes a complete turn — in a counterclockwise direction — every 37.5 hours, making a 9.6-degree turn every hour. While that's a long time to sip on a to-go coffee from the nearby Cafe De Luis, it might be worth it just so you can watch the world turn.

Swing by the Foucault pendulum of Valdivia
The Foucault pendulum of Valdivia, a 63-foot-tall swinging ball in Chile, entrances all who walk by and assures us that the world is still spinning.