An 8.6 kilometer ruler
In 1849, the French physicist Hippolyte Fizeau devised the first completely terrestrial method for measuring the speed of light. Previous estimates, like Ole Rømer's in 1676, relied on astronomical observations and were consequently less precise. Fizeau’s experiment used the terrain of Paris itself as a laboratory. The two points of his measurement were his father's house on the hill of Montmartre and the fortress of Mont Valérien in the suburb of Suresnes, a distance of 8,633 meters.
The setup was ingenious. At his home, Fizeau placed a light source and a rapidly spinning cogwheel. A beam of light was directed through the gaps between the wheel's teeth toward the distant fortress. At Mont Valérien, a mirror reflected the beam directly back to its origin point in Montmartre. An observer looking through an eyepiece could see the reflected light passing back through the same spinning wheel.
Racing a beam of light
The core of the experiment was timing the light's round trip. The total path was twice the distance between the hills, a trip of 17,266 meters. As the toothed wheel spun, it chopped the light beam into a series of short pulses. Fizeau's insight was that if the wheel spun fast enough, a tooth could rotate into the light's return path just in time to block it.
He used a wheel with 720 teeth. By gradually increasing the wheel's rotation speed, he found the exact point where the returning light was completely eclipsed by the adjacent tooth. This first occurred at a speed of 12.6 rotations per second. Knowing the rotational speed, the number of teeth, and the distance, Fizeau could calculate how long it took the light to travel to the fortress and back. His calculation produced a value of approximately 313,300 kilometers per second. This result was about 4.5% higher than the modern, accepted value of 299,792.458 km/s, an impressive degree of accuracy for the time. The main error source was the difficulty in judging by eye the precise moment of minimum light intensity.