Measuring the speed of light
How do you measure the speed of light?
Since light moves very fast, you either need to measure its motion over a
very long distance, or else you need to be very fast about it. The first
measurements (including one based on the orbits of the moons of Jupiter)
used the long-distance approach, but with modern very fast clocks and other
technology it is not that hard to measure the few nanoseconds it takes for
light to travel a certain distance. Actually, nobody ever has to measure
the speed of light again because it is been established as a constant that
is used to define the metric unit of length (the meter) in terms of the unit
of time (the second) which we can measure much more precisely.
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Update: June 2012