Posted on 09/20/2015 1:35:08 PM PDT by SunkenCiv
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 Romer, 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. Romer and Cassini discussed the possibility that light has a finite speed (it had typically been thought to move instantaneously). Eventually, following some rough calculations, Romer 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 Romer'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...
The Scottish physicist James Clerk Maxwell showed that when electric and magnetic fields change in time, they interact to produce a travelling electromagnetic wave. Maxwell calculated the speed of the wave from his equations and found it to be exactly the known speed of light. This strongly suggested that light was an electromagnetic wave -- as was soon definitively confirmed.
(Excerpt) Read more at aeon.co ...
That’s the book. He also had a cranky newsletter I used to get, it was a blast.
Thanks ETL.
It fluctuates, doesn't it? Like I really know.
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