Geography of Foucault Pendulums

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At the California Academy of Sciences a special pendulum is on display called the Foucault Pendulum. This particular pendulum was used by French philosopher Leon Foucault to prove that the Earth rotates on its axis.

In 1851 Leon Foucault revealed his pendulum in the Paris Observatory; another was located in the massive dome of the Paris Pantheon, hanging from a 219-foot wire. The Foucault Pendulum in the California Academy of Sciences is a replica of Foucault’s famous creation, although it hangs from a 30-foot cable in one of the Academy’s buildings.


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The pendulum works to prove the rotation of the Earth on its axis using inertia and gravity by a simple experiment can be done nearly anywhere. Pins are set up 6 degrees apart in a circle around the pendulum. As the pendulum is set in motion it swings in an arc back and forth; as the Earth rotates it knocks the circled pins down.

Foucault's pendulum at the Academy of Sciences in San Francisco, California. Photo: Fovea Centralis
Foucault’s pendulum at the Academy of Sciences in San Francisco, California. Photo: Fovea Centralis

Location can affect the movement of a pendulum, much like location can influence the spiral of a hurricane or the direction water swirls down a drain. At the North Pole, a pendulum in motion will knock down a full 360 degree arc of pins in 24 hours, while the same pendulum would only knock down 220 degrees of pins in California.

The swing of the pendulum varies depending on its location on Earth.
The swing of the pendulum varies depending on its location on Earth.

At Earth’s poles the rotation of the Earth directly influences the way a pendulum swings. Anywhere else in the world, the Coriolis Effect comes into play. The Coriolis Effect makes objects that are rotating, like water down a drain or a storm, look like they are rotating left in the Southern Hemisphere and rotating right in the Northern Hemisphere. This force is stronger and weaker at different latitudes based on how close to the equator or the poles an object is.

For a pendulum like Foucault’s, the closer it is to the equator the more the Coriolis Effect is weakened. At the equator a pendulum wouldn’t knock over any of the pins circling it, but at the poles an entire 360 rotation would be completed in a 24 hour period of time. Pendulums around the world behave in the same way and yet can bring very different information about how the Earth’s rotation is affected by location to light.

Foucault Pendulums exist all over the world to measure, track and explain how the Earth’s rotation can be measured and observed in the natural world. Not only can the speed of the Earth’s rotation be determined, but scientists can determine their precise latitudinal coordinates based on how the pendulum behaves.

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