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21:52
September 21, 2011
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I guess most people know that the velocity of light in a vacuum is c = 2.99792458 x 10**8 metres/second, or about 186,282 miles per second. In materials the velocity is less, depending upon the refractive index of the material. And it's not just light that exhibits this fundamental property but all electromagnetic waves, irrespective of their frequency.
Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism. And in 1905, Albert Einstein postulated that the speed of light with respect to any inertial frame is independent of the motion of the light source, which led to his famous Special Theory of Relativity.
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s with a measurement uncertainty of 4 parts per billion. In 1983, the metre itself was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.
All interesting stuff, but what is a light year? It's simply the distance travelled by light in one year; basically, a heck of a long way!
As a close-to-home example consider a phone call which happens to go via a geostationary satellite as opposed to the more usual fibre optic link. These satellites sit in a circular orbit some 36,000km above the equator and the round-trip delay of the microwave signals from ground to satellite and back to ground is about a quarter of a second, which is why one hears that delay in speech.
But to really put the concept of a light year into perspective have a look at this blog post by Bruce McClure in EarthSky's Astronomy Essentials http://earthsky.org/astronomy-.....light-year
This short article really puts things into perspective and gives some useful "rules of thumb" which you can get your head around.
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