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.