While Aristotle believed that light could travel instantaneously, the first experimental attempt to measure the speed of light came from Galileo Galilei in 1667. He placed two people with covered lights on the top of hills that were about a mile apart. The first person was instructed to uncover his light, and when the second person observed that light, he was to uncover his own. Galileo was able to determine that light traveled at least 10 times faster than sound.
The first numerical estimate for the speed of light came just 10 years later from Ole Römer, who observed that the eclipses of Jupiter’s moon always came later than mathematically anticipated. He estimated that light took 10 – 11 minutes to travel from the sun to Earth. In 1728, James Bradley estimated the speed of light to be 185,000 miles per second, using the apparent change in position of the stars due to Earth’s motion.
In the mid-1800s, two French physicists narrowed the estimate of the speed of light to about 1,000 miles per second of what is accepted today. Hippolyte Fizeau used a spinning wheel with prongs and a mirror set up five miles away, varying the speed of the wheel to measure how long the light took to travel to the mirror and back to the wheel. Leon Foucault used a rotating mirror.
The story seems to end with Albert Michelson, who aimed to repeat Foucault’s experiment in 1879, but with longer distance and higher quality mirrors and lenses. He measured 186,355 miles per second: which was accepted as the most accurate measurement into the 1920s. Michelson’s number comes very close to today’s accepted speed of 186,282 miles per second—an impressive feat that earned him a Nobel Prize!
Source: SPACE.com
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