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Who first measured the speed of light?

Who discovered the speed of light? When was it discovered? How was it calculated or derived?

Scientists have been trying to study the speed of light since the ancient Greeks. Greek astronomers believed, amongst other things, that the speed of light was effectively infinite. They had no way to test this educated guess, however. Nevertheless, it was generally taken for granted that light-speed was infinite until the astronomer Galileo in 1600. One of Galileo's more unsuccessful experiments was his attempt to quantify the speed of light, by using distant lanterns with shutters, which an assistant opened at specified times. Galileo would try to record how long it took light to get to him from across the field on which the experiment was done. His only conclusion was that light-speed was "fast."

The first true measurement of light-speed came in 1676 by a fellow named Olaf Roemer. He noted that the time elapsed between eclipses of Jupiter with its moons became shorter as the Earth moved closer to Jupiter and became longer as the Earth and Jupiter drew farther apart. This anomalous behavior could be accounted for by a finite speed of light. He calculated that the speed of light was something like 2.14 x 108 meters per second. This measurement, considering its antiquity, method of measurement, and 17th century uncertainty in exactly how far Jupiter was from the Earth, is surprisingly close to the modern value of 2.99792458 x 108 meters per second. These modern values are obtained using devices called laser interferometers, which can very precicely pin down speeds and distances.

January 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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