Who first calculated the speed of light?

speed of light photo

Question from Ed Clarke

A fairly accurate calculation for the speed of light was derived by Danish astronomer Olaus Roemer all the way back in 1676. Roemer was monitoring the eclipses of the moons of Jupiter and noticed that when the Earth was moving further away from the gas giant, the time interval between successive eclipses seemed to increase. He surmised that this must be because the light had further to travel when the Earth was further away and so it was taking longer to  reach his observatory. From this he was able to estimate the speed of light and arrived at a respectable 225,000 kilometres per second. Others would later refine his estimate to reach the accurate 299,792.458 kilometres per second value.


Answered by James Horton for Brain Dump in How It Works issue 177 

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