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I t’s hard to miss the Foucault pendulum that dominates the lobby of the Physics Building on campus. One of only about 10 such publicly accessible pendulums in California, it is popular especially with visiting middle and high school students. Invented in 1851 by French physicist Jean Foucault, the Foucault pendulum demonstrates the rotation of the Earth. The Physics Building first acquired one in the 1970s. Its renovation in 2016 was initiated and commissioned by Umar Mohideen, a distinguished professor and then-chair of the Department of Physics and Astronomy. Mohideen also came up with the world map and clock dial motif that designers Emilie Bean and Samuel Odiakosa used to create the mosaic artwork now gracing the pendulum pit.

The Foucault pendulum consists of a heavy ball suspended by a long wire, which swings freely. According to Newton’s laws, an object in motion will continue in a straight line unless acted upon by an external force. No force alters the Foucault pendulum’s plane of swing; instead, the Earth’s rotation causes the pendulum’s apparent precession. The result is the Physics Building, the campus, and beyond move around the pendulum, creating the illusion that the pendulum’s plane is rotating. To counteract air friction, an electromagnet lies buried in the center of the pendulum pit. Upgraded in 2016 by UCR physicist Allen Mills, it gives the pendulum ball a small push each swing, keeping it in motion.

The time it takes for the pendulum’s plane of oscillation to complete a full rotation depends on the pendulum’s latitudinal location. The closer the pendulum is to the poles, the faster the rotation. At the poles, the precession period is 24 hours. At the equator, the plane of swing does not rotate. The precession period exceeds 24 hours at intermediate latitudes.