As light is tremendously fast, measuring its speed was almost an impossible task. It was easier to calculate when light had travelled a long way.
Ole Romer, a Danish astronomer accidentally found this in 1676. He noticed that the time between eclipses of Jupiter’s moons when they are hidden behind the planet varied throughout the year. Romer realised that it was because of variation in two things: the distance from Earth to Jupiter throughout the year and the distance travelled by light from the moons. His initial calculations estimated the speed of light to be about 220,000 km per second. Though this was 25 per cent slower than the correct speed, it was a significant start in the right direction. Christiaan Huygens later deduced that the speed of light is approximately 212,000 km/s.
In 1809, astronomer Jean Baptiste Joseph Delambre estimated the time taken by light to travel from the Sun to Earth as 8 minutes and 12 seconds. This is quite close to the modern value, which is 8 minutes and 19 seconds, at a speed of 299,792.458 metres per second.
Picture Credit : Google