Today is the birthday of Jean Foucault, born in 1821 to a publisher in Paris. In addition to defining and inventing the Foucault pendulum, Foucault is credited with naming the gyroscope. But first, the pendulum. Since the time of Galileo who defined the laws governing the motion of pendulums, but Foucault was the first to use the pendulum to show the rotation of the earth independent of celestial observation. Before he was thirty he devised an experiment to measure the speed of light. Today he is known more for the pendulum that bears his name than any of his other achievements. The word pendulum is a New Latin neuter of the noun pendulus meaning hanging down from the verb pendere meaning to hang.
Image of a Foucault pendulum at the Pantheon in Paris.