In 1638 Galileo (1564-1642) proposed a method of measuring the speed of light. Galileo would have one lamp and an assistant a great distance away would have a second lamp. The assistant was to uncover his lamp immediately when he saw Galileo uncover his own lamp. The speed could then be determined by measuring the time it would take the light to travel from Galileo to the assistant and back again. Galileo claimed to have done the experiment several years before 1638 but there was no record of his results. In 1667 the academy of sciences in Florence, Italy, carried it out between two observers a mile apart. They reported there was no measurable delay, showing that the speed of light must be extremely rapid.

The first measurement of the speed of light in a laboratory was by Hippolyte Armand Fizeau (1819-1896) in 1849. He used a beam of light that passed through the gaps between teeth of a rapidly rotating wheel, was reflected from a mirror 8 kilometers away, and returned to the wheel. The speed of the wheel was increased until the returning light passed through the next gap and could be seen. The speed was calculated to be 315,000 kilometers per second. Leon Foucault (1819-1868) improved on this a year later by using a rotating mirror in place of the wheel and found the speed to be 298,000 kilometers per second. He also used this technique to determine that light travels slower in water than in air.

The American physicist Albert Michelson (1852-1931) greatly improved Foucault’s measurement using an eight-sided rotating mirror and a plane mirror located on Mount San Antonio, 35 kilometers (114,800 feet) away from the source on Mount Wilson in California. By measuring the speed of the rotating mirror and the distance between the mirrors, Michelson made the most accurate measurement of the speed of light to that date. In 1907, he was honored by being the first American to win the Nobel Prize in physics. In 1926 he made a new measurement that yielded 299,796 kilometers per second with an uncertainty of 4 kilometers per second.

After Maxwell published his theory of electromagnetism it became possible to calculate the speed of light indirectly from the relationships between an electric charge and its electric field and an electric current and the magnetic field it produces. In 1907 Rosa and Dorsey obtained 299,788 kilometers per second in this way. The uncertainty of 30 kilometers per second made it the most precise measurement to that date.

Research on microwaves used in radar during World War II led to a new method of measuring the speed of light. By 1950 Louis Essen reported a result of 299,792 kilometers per second, slightly more precise than Michelson’s result. In the 1970s scientists at the National Institute of Standards and Technology in Boulder Colorado succeeded in directly measuring simultaneously the wavelength and the frequency of an infrared laser. From these two measurements they could calculate the speed of light: 299,792.4562 kilometers per second with an uncertainty of only 1.1 meters per second!

This new technique prompted an investigation of the length standard, the wavelength of light from the gas krypton. The new techniques discovered that the standard was “fuzzy” and had to be replaced. In 1983 the Conférence Générale des Poids et Mesures decided to fix the speed of light in a vacuum at 299,792.458 kilometers per second and define the meter as the distance light travels in 1/299792458 of a second.

Here is a summary of the history of measurements of the speed of light.