Hippolyte Fizeau

Armand Hippolyte Louis Fizeau was born 23 September 1819 in France, into a century hungry for precision. In 1849, he became the first person on earth to measure the speed of light using an apparatus built on solid ground: a rotating cogwheel that chopped a beam into pulses, eight kilometers of open air, and math sharp enough to land within 5% of the true value. Two years later, in 1851, he turned his attention to light moving through flowing water, an experiment that would bear his name and probe whether the medium dragged the beam along with it. He died 18 September 1896, five days short of seventy-seven, having translated the speed of the cosmos into something humans could write down.