How is the speed of light measured?
(Lansing State Journal, February 14, 1996)

The first measurement of the speed of light was made by the Danish astronomer Roemer in 1676.  He noticed that the time it seemed to take for one of Junipers moons, Io, to orbit Jupiter changed depending upon the time of year.

He argued that the time increased when the earth was moving away from Jupiter and decreased when it was moving toward Jupiter, due to the speed with which light traveled from Jupiter to Earth.

He calculated a speed of light that turned out to be about two-thirds too small.

In 1849 the French scientist Fizcau devised an ingenious method to measure the speed.  He placed a light source behind a rapidly rotating wheel with notches cut in it.  This created a flickering light which traveled more than 8 kilometers to a mirror and was then reflected back to the spinning wheel.

In the late 1800s and early 1900s, American physicist Michelson improved upon this idea and used rapidly rotating mirrors and found a speed of 299,800 kilometers per second.

Modern measuring methods use the fact that light is a form of electromagnetic wave.  The speed of any wave is the product of its frequency and its wavelength.

Using pure laser light with a single known frequency, the wavelength of the light is measured and from this measurement the speed of light is calculated.

Such measurements are so accurate that in 1983 it was decided to simply choose the speed of light to be 299,792.458 kilometers in one second, and define the unit of length, the meter, to be distance traveled by light in a vacuum in (1/299,792,458) of a second.

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