Light dawns

If you visit the Paris Observatory on the left bank of the Seine, you’ll see a plaque on its wall announcing that the speed of light was first measured there in 1676. The odd thing is, this result came about unintentionally. Ole Rømer, a Dane who was working as an assistant to the Italian astronomer Giovanni Domenico Cassini, was trying to account for certain discrepancies in eclipses of one of the moons of Jupiter. Rømer and Cassini discussed the possibility that light has a finite speed (it had typically been thought to move instantaneously). Eventually, following some rough calculations, Rømer concluded that light rays must take 10 or 11 minutes to cross a distance ‘equal to the half-diameter of the terrestrial orbit’.

Cassini himself had had second thoughts about the whole idea. He argued that if finite speed was the problem, and light really did take time to get around, the same delay ought to be visible in measurements of Jupiter’s other moons – and it wasn’t. The ensuing controversy came to an end only in 1728, when the English astronomer James Bradley found an alternative way to take the measurement. And as many subsequent experiments have confirmed, the estimate that came out of Rømer’s original observations was about 25 per cent off. We have now fixed the speed of light in a vacuum at exactly 299,792.458 kilometres per second.

Why is the speed of light the speed of light?
What might we do with the genomics of the entire planet?
What can paleogenetics tell us about our earliest ancestors?
Why this particular speed and not something else? Or, to put it another way, where does the speed of light come from?...

http://aeon.co/magazine/science/the-universal-constants-that-drive-physicists-mad/