Suppose you send a loved one on a small errand. Take out the trash. If you are concerned about his or her propensity to dawdle, it’s easy to calculate his or her average speed to acquire solid evidence. Just take the distance to the curb, multiply by 2 to account for the “to” and “from” portions of the trip. Divide this result by the total time it took and you’ll get the average speed. (If the result is comparable to the land speed of a box turtle, you are in the clear to berate your loved one.) In 1849, Armand Fizeau used this same principle to calculate the speed of light directly: He reflected light off of a mirror a fixed distance away and timed the round trip. This was the first measurement of the speed of light on Earth.
While Fizeau used the same method to calculate the speed of light, it wasn’t quite so easy.
