Today’s picture comes from the San Diego Natural History Museum — it’s a Foucault Pendulum:
The Foucault Pendulum is simply a weight free to swing in any direction (usually hung from a cable), and is named after its inventor, the French physicist Léon Foucault. Foucault built the first such device in 1851 as a relatively simple demonstration of the rotation of the Earth.
It’s not as though many people in the 1850’s doubted that the Earth rotated — but Foucault’s pendulum was the first device that could show this rotation in a graphic way. As the earth rotates, the plane of the pendulum’s oscillation also rotates (although, at a rate that’s also a function of the latitude of the pendulum’s site). A number of museums now host Foucault pendula for this very reason, many augmented with dominoes or other light objects that the pendulum can duly knock over at hourly intervals.