In ancient times, many scientists believed the speed of light was infinite and could travel any distance instantaneously. The Italian physicist Galileo Galilee was among the first to try to measure the speed of light. In the early 17th century, he devised an experiment in which two people with covered lanterns stood a known distance apart. One person uncovered his lantern and as soon as the other person saw the light, he uncovered his own lantern. Galileo attempted to record the time between lantern signals but was unsuccessful because the distance involved was too small and light simply moved too fast to be measured this way.
Around 1676, Danish astronomer Ole Roemer became the first person to prove that light travels at a finite speed. He studied Jupiter’s moons and noted that their eclipses took place sooner than predicted when Earth was nearer to Jupiter and happened later when Earth was farther away from Jupiter. Roemer reasoned this was the result of light moving at a finite speed; it took longer to make it to Earth when Jupiter was a greater distance away.
In the ensuing centuries, a number of other scientists worked to determine the speed of light and, using improved techniques, came up with increasingly accurate calculations. French physicist Hippolyte Fizeau is credited with making the first non-astronomical measurement, in 1849, using a method that involved sending light through a rotating toothed wheel then reflecting it back with a mirror located a significant distance away. One of the first precise calculations of light’s velocity was made in the 1920s by American physicist Albert Michelson, who carried out his research in the mountains of Southern California using an eight-sided rotating mirror apparatus. In 1983, an international commission on weights and measures set the speed of light in a vacuum at the calculation we use today: 299,792,458 meters per second (186,282 miles per second)—a speed that could circle the equator 7.5 times in a single second.