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.