The names of 72 giants of French science, engineering, and mathematics are engraved on the Eiffel Tower. Two of them were born less than a week apart. For a while, they were colleagues. In scientific circles, though, they’re probably best known for similar experiments they conducted apart: measuring the speed of light.

León Foucault was born 200 years ago today. Hippolyte Fizeau came along five days later.

Foucault originally planned to become a doctor. But he fainted at the sight of blood. So he switched to physics. He was especially interested in the properties of light. And he devised a basic device which confirmed that Earth turns on its axis: the Foucault pendulum.

Fizeau was planning to become a doctor as well. In medical school, though, he developed severe migraines. He traveled for a while, which helped his health. Then, like Foucault, he turned to physics.

Foucault and Fizeau began working together in the late 1830s. They improved the photographic technique of the day, and used it to snap the first photo of the Sun.

Working independently, the two scientists worked on measuring the speed of light. In 1849, Fizeau came up with a number that was the best of its time — off by only five percent.

A couple of years later, Foucault proved that light travels faster through air than through water. And in 1862, he made his own measurement of lightspeed. It was off by only half a percent — by far the best measurement of the time.

Script by Damond Benningfield