It was believed earlier that light can travel any distance in no time, i.e. that light had infinite speed. In 1676 Ole Rømer first demonstrated that light traveled at a finite speed by studying the apparent motion of Jupiter’s moon. So this settled all doubts about the speed of light is infinite or too fast to measure. Finally, people accepted that the speed of light was very fast but finite.

So how did we measure the speed of light so accurately?

Additionally, based on this how accurate is the standard unit of distance – “a meter,” we have defined?

Measuring the speed of light has a series of experiments which started before the time of Aristotle. Back then, the scientists made observations entirely based on their perception. Observing the light traveling from burning torch to the stone manuscript quickly, people believed that the velocity of light is infinite and the light was so swift that there was no observed lag in time for the position of earth shadows on the moon during an eclipse.

However, the Aristotle disproved this claim but later lost the debate. In the 17th century, Galileo undermined the velocity of light dramatically and attempted to measure it terrestrially. He and his assistant stood on different distanced hills holding the bright source of light. They covered and uncovered the bright light and calculated the speed of light by measuring the elapsed time until he saw the light from his assistant’s side. He made a calculation and made a remark that the speed of light is not instantaneous but extremely rapid.

Observing moon and stars

In 1676, Ole Roemer calculated the speed of light; however, he didn’t have any incentive to measure it. Roemer carefully observed the fourth moon of the Jupiter i.e. Lo, which disappeared during Jupiter’s gigantic shadow while orbiting it and was later illuminated by the light of the sun. He predicted that there was a difference in the predicted time of eclipse and the real-time they were observing. He told the reason for light reaching our telescope stretched and contracted both, is because the Earth and Jupiter moved closer or father towards each other in their orbits and this made the light to travel an elongated distance.

Later, Roemer approximated the speed of light to be 214,000 km/s, which was 80000 Km/s slower to what is actual. However, he reached a result which was quite near the original speed of light 300 years ago of the time when the true value was calculated.

The astronomers constantly worked upon to minimize the error and in the year 1728, James Bradley observed an aberration and made an estimation. An aberration is a small change in the position of any heavenly bodies by the connivance of the motion of light and the observer. James observed a star in a constellation called Draco in the northern sky and got to know that there is a change in the position throughout the year as earth revolved around the sun.

By using the angle of deviation of the light coming from stars, along with the velocity of earth moving around the sun, he calculated the speed of light to be 301,000 km/s.

Closing in,

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.