How Did We Measure The Speed Of Light?

In the year 1849, Armand Fizeau measured the speed of light terrestrially, disregarding the isolation of the heavens. He used the setup consisting of a mirror, light source and wheel through which light beam would transverse. The wheel was placed between the light source and the reflecting mirror. The beams of light transversed mirror two-time first while going towards the mirror and second while coming back from the mirror.

Fizeau calibrated the rate of rotation in a way that the light passed through the gaps while going towards the mirror and passed through consecutive gaps while returning from the mirror. By knowing the distance between mirror & light source and rate of rotation of the wheel, he calculated the speed of light to be 315,000 km/s.

Later, Leon Foucault used improved apparatus which consists of a rotating mirror, thus bringing down the speed of light to 298000km/s. After Maxwell introduced the laws of electromagnetism, the speed of light could be calculated by the reciprocal of the square root of the product of electric permittivity of free space and magnetic permeability.

Then, in the year 1907, Dorsey and Rosa calculated the speed of light to be 299,788 km/s. It was the most accurate value of light speed at that time. Gradually, as the technology advanced, Froome in the year 1958 calculated the speed of light to be 299,792.5 km/s  by laser interferometers.

The National Bureau of Standard in Boulder Colorado used cesium clocks and helium-neon lasers to calculate the speed of light and defined the meter as the distance traveled by light in vacuum for 1/299,792,458th of a second such that the speed of light in the vacuum is 299,792,458 m/s or 299,792.458 km/s.