Léon Foucault was born in Paris on September 19, 1819. Due to his poor health, he was privately educated at home. He chose medicine as his vocation and practiced as a physician, but he soon left the profession because he could not stand the sight of blood. He became a science theorist, producing textbooks of arithmetic, geometry and chemistry and writing a column covering scientific principles for a newspaper. Foucault's interest in science and technology was unquenchable, and he collaborated with French physicist Armand Fizeau in a number of ventures and discoveries.

Foucault and Fizeau were the first to use the daguerreotype to photograph the surface of the Sun in 1845. Such photographs required long exposures from Foucault's camera, so he invented a pendulum-driven device with the ability to track the sun (or stars, for that matter) as their positions in the sky changed through the day. He noticed...