How do you measure the fastest thing known in the universe?
Fizeau, a French physicist who was born on this date in 1819, was the first person to successfully (although approximately) measure the speed of light without using astronomical calculations. How did he do it?
Fizeau sent a narrow beam of light between gear teeth on the edge of a rotating wheel. The beam of light traveled 8 kilometers (about 5 miles) to a mirror and then bounced back to the wheel. If the spin was fast enough, a tooth would block the light. Because Fizeau knew the rotational speed of the wheel and the mirror's distance, Fizeau was able to directly measure that light travels 299,792,458 meters per second (approximately 186,000 miles per second). Actually, that is the speed of light in a vacuum (no air or other material). Light travels slightly slower in air—ONLY 299,792,368 meters per second—and slightly slower yet through glass or water. Fizeau's measurements were good enough to show the difference between light traveling in air and in water.
Fizeau also made discoveries about polarization of light, the expansion of crystals, the Doppler effect, and even daguerreotypes (early photography).