A closer look
Before lasers and atomic clocks, measuring the speed of light seemed mathematically impossible, so Hippolyte Fizeau just used a really fast gear and a mirror. In 1849, he shot a beam of intense light through the teeth of a spinning cogwheel toward a mirror positioned five miles away. By spinning the wheel faster and faster, he found the exact speed where the returning light was blocked by the next tooth on the gear. It was an obscenely low-tech, brute-force mechanical solution to a profound cosmological problem, and he managed to calculate light speed to within an impressive five percent of the actual value.
