The Foucault Pendulum, a long-time favorite at the old Academy, returns as the first exhibit to be installed in the new Academy’s east wing.

Its first swings demonstrated that yes, in fact, Earth is still rotating (whew!). So how exactly does a pendulum demonstrate Earth’s rotation? Find out here.

In the video clip below, the 235-pound brass ball (called a “bob”) has been set in motion for its first test run. It swings constantly in the same direction, while the Earth rotates beneath it. So as the day goes by, the pendulum knocks down pins one by one (seen in the foreground), demonstrating that over the course of 24 hours, the Academy’s floor turns about 220 degrees. At the North Pole, the floor would turn a full 360 degrees in 24 hours, but at San Francisco’s latitude, we see a 220 degree turn. To get your mind around that one, check out this helpful illustration.

Below is the Academy’s original Foucault Pendulum, built in 1951. Since then, the Academy has built 97 other pendulums for museums, universities, etc. all over the world.