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A 23-Hour and 56 Minutes Day

Sugarland, TX



Foucault’s pendulum is intriguing proof of the Earth’s rotation. A pendulum is something suspended from a set point that swings back and forth due to gravity. I saw the Foucault pendulum, a special kind of pendulum, at my local museum, and I was captivated by watching it knock down all the pins (very slowly). Later, I researched more about it and was surprised to see its true importance and how it relates to Earth’s rotation.

The Foucault Pendulum began with a French physicist named Jean-Bernard-Léon Foucault (1819-1868). He created the first Foucault pendulum, a sixty-two-pound iron ball hanging from the inside of a dome with a two hundred twenty feet long wire. It was first displayed at the World’s Fair in Paris. Foucault proved the Earth’s rotation with this pendulum. He received the Copley Medal of the Royal Society in London and accomplished many other things.


The pendulum would move in a fixed plane at the North Pole or the South Pole. The plane would seem to rotate through three hundred sixty degrees as the Earth makes a rotation (a day.) However, on any other point on Earth, the point where the pendulum is attached is not a fixed point because it’s moving as Earth rotates on its axis. The plane where the pendulum swings are in motion too. Therefore, the time it takes for the pendulum to make a complete rotation is a sidereal day, which is about twenty-three hours and fifty-six minutes, divided by the sine of the latitude of its location.


A Foucault pendulum rotates clockwise in the Northern Hemisphere. At the Equator, it will not rotate (because sine(zero) is zero); likewise, it will rotate counterclockwise in the Southern Hemisphere. In Paris, where Foucault first created his pendulum, the period of total rotation was about thirty-two hours, while the rotation was clockwise. A Foucault pendulum is a marvel worth viewing to discover Earth’s laws of physics in motion.


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