You could say that French physicist Leon Foucault (full name: Jean Bernard Leon Foucault) made discoveries that moved heaven and earth. Most famous for the Foucault pendulum, which demonstrates how the Earth rotates, he also proved that light moves more slowly through water than through air.
Son of a French publisher, Foucault was educated primarily at home and intended to study medicine until he discovered that he couldn’t bear the sign of blood. Instead, with little additional training, he went to work and learn from accomplished scientists and spearheaded some of the most important inventions in the history of in physics.
Foucault started out working as a laboratory assistant to Alfred Donne, a physician and bacteriologist who invented the photoelectric microscope. Donne was a mentor and supporter of Foucault throughout his career. He used the just-invented Daguerreotype photographic process to take the first photograph of the sun.
Foucault went on to work with Hippolyte Fizeau, and together they invented the Fizeau-Foucault apparatus, a device that uses rotating and stationary mirrors 20 miles apart to measure the speed of light. It was the Fizeau-Foucault apparatus that allowed Foucault to prove that light moves at different speeds through water than through air.
Scientists had been working unsuccessfully to find a way to demonstrate the Earth’s rotation, and it occurred to Foucault pendulum hanging on a wire and swinging directly north and south would appear to slowly move its plane of oscillation as the Earth turned underneath it.
Emperor Napoleon III was impressed by the public exhibition at the Paris Pantheon in 1851 where Foucault had invited people “to see the Earth turn.” The dramatic demonstration, with its 220 foot-long wire and 62-pound weight swinging over a circle of sand (Foucault attached a stylus to the ball so that the rotation would appear as lines in the sand) proved to Napoleon and the Parisian elite that the Earth revolves on its axis. Dr. Amir D. Aczel, Research Fellow in the History of Science at Boston University, calls Foucault’s pendulum “certainly one of the most important historical instruments of all time.” Dr. Aczel said that the invention and the demonstration brought “closure for Galileo” and led the Church to accept the rotation of the Earth.
The New York Times included Foucault’s Pendulum in it’s list of Science’s 10 Most Beautiful Experiments, noting that in 2001, scientists constructed a pendulum at the South Pole, confirming that a full rotation takes 24 hours.
Today, there are Foucault pendulums all over the world that employ various methods to mark how the Earth moves beneath us. Some have the pendulum make pathways in smooth sand; others use small markers that the pendulum knocks over as it makes its way around the circle. The California Academy of Sciences has a great tutorial about Foucault pendulums – how they work and how they’re built.