How was the speed of light determined?

The speed of light is a fundamental constant in physics that is used to measure distances across the universe and to develop modern technologies. But how was this speed determined?

The first successful measurement of the speed of light was conducted by Danish astronomer Ole Rømer in 1676. Rømer observed the orbit of Jupiter's moon Io and noticed that the times at which Io disappeared behind Jupiter and reappeared on the other side varied as Jupiter moved in its orbit around the Sun. Rømer concluded that the variations were due to the time it took light to travel the increasing and decreasing distance between Earth and Jupiter. He estimated that the speed of light was about 220,000 kilometers per second.

Another method to determine the speed of light was developed by French physicist Armand Fizeau in 1849. Fizeau used a rapidly rotating toothed wheel and a mirror placed several kilometers away. A beam of light was sent through a hole in the wheel, and the light was reflected back by the mirror. By adjusting the speed of the wheel, Fizeau was able to determine the minimum rotation rate at which the light could pass through the holes in the wheel and return through the same hole. From this, he calculated the speed of light to be about 315,000 kilometers per second.

The most accurate method for measuring the speed of light was developed in 1983 by the International Committee for Weights and Measures. They defined the speed of light to be exactly 299,792,458 meters per second, based on the distance traveled by light in a vacuum over a certain amount of time.

The speed of light was first measured by Ole Roemer in 1676. He noticed that the time between eclipses of Jupiter's moons varied depending on the position of Earth in its orbit around the Sun. This was because the Earth's motion towards or away from Jupiter affected the amount of time it took for light from the moons to reach Earth. From this, Roemer was able to calculate that the speed of light was about 225,000 kilometers per second.

In the 19th century, several scientists improved on Roemer's measurement, including Hippolyte Fizeau and Léon Foucault. Fizeau used a rotating mirror to measure the speed of light, while Foucault used a rotating mirror and a beam of light to measure the speed of light around the Earth. By the end of the 19th century, the speed of light was known to within a few kilometers per second.

In the 20th century, the speed of light was measured with even greater accuracy using a variety of methods. In 1972, the United States National Bureau of Standards measured the speed of light to be 299,792,458 meters per second. This value is now used as the definition of the meter.

The speed of light is a fundamental constant of nature. It is the fastest speed at which information can travel. This has important implications for our understanding of the universe. For example, it means that we can never see events that occur before the light from those events reaches us.