Light

The measurement of the speed of light, a fundamental constant in physics, began with Ole Rømer in the 17th century, who used observations of Jupiter's moons. Later, in the 19th century, Fizeau and Michelson devised more refined techniques involving mirrors and rapidly rotating gears. Eventually, in 1973, the definition of the meter was based on the speed of light, leading to the highly accurate value we use today: approximately 299,792,458 meters per second.