The Foucault Pendulum — one of the most popular displays in the original Planetarium lobby at the Calfiornia Academy of Sciences — is one of the most difficult to explain properly.

As the 236-pound brass ball swung, it gradually seemed to change the direction of its swing due to the effects of Earth’s rotation, knocking down a small peg every 33 minutes. This Academy-made exhibit, installed in 1951, never failed to attract attention, stimulate conversation, generate anticipation, and — when a peg finally got knocked down — get a round of applause.

One problem with it — and a frequent source of amusement for visitors — was that the space between the slanted vertical bars surrounding the pendulum was wider at the top, and when kids stuck their heads through and knelt down to get a closer look at their reflections in the shiny brass ball…they learned what it must’ve been like to be imprisoned in a set of medieval stocks — though not for long, to everyone’s relief, as fleet-footed staff moved quickly to the rescue. A similar Academy-made pendulum was installed — with more closely-spaced bars — at the renovated Academy, continuing to swing, mesmerize, and mystify. The Academy continues to produce Foucault Pendulums.