You may have learned that the speed of light is 186,282 miles per second. (299,792,458 meters per second.) You may also know that Alpha Centauri is in the nearest stellar system which is also a part of our galaxy and it is about 4.35 light years away. But how do we know this?
Before it was proven by Thomas Young that light traveled in waves just like sound it was believed that light was composed of particles (Newton's idea) which traveled through the "ether". Now we know there is no ether as it was presented and light travels in waves. Recently scientists were able to greatly slow the speed of light by transmitting it through an optically dense substance called Bose-Einstein Condensate. The Condensate is made up of atoms of rubidium cooled to almost absolute zero. There is a lot more information about bosons and condensates but most of them are far above the basics of physics needed to be known to understand how to measure the speed of light.
Albert Einstein's Theory of Relativity uses "c" to represent the constancy of the speed of light. It is now believed that light itself may be able to travel faster than the speed of light at least within light (not important for this discussion) and through experiments we have been able to slow down and even stop (freeze) light. But for all current and practical knowledge we believe that the speed of light through a non-vacuum is 3 x 10(8th) meters per second.
Galileo may have been the first scientist to try and measure the speed of light and he used a very rudimentary set of tools: two lanterns. He and an assistant separated themselves by about a mile Galileo would uncover his lamp allowing light to shine and as soon as his assistant saw the light from Galileo's lamp he would uncover his. Galileo judged this to be instantaneous. Well, he was almost right. It takes about 1700 billionths of a second for light to travel one mile.
Ole Roemer used the moons of Jupiter to prove that the speed of light could be measured (that it was not infinite). He noticed that when the orbits of Jupiter and Earth were closer he could see the moons of Jupiter much earlier than when Earth and Jupiter were at their farthest distance. In fact that amount of time turned out to be 16 minutes and 40 seconds though Roemer evidently held it to be 22 minutes and a few seconds. While Roemer did calculate the speed of light from these numbers he was actually of by 50%. Roemer calculated the speed of light to be 1.5 x 10(8th) meters per second - 1/2 slower than actual. The formula he used was to calculate the time it took for the light of Io to reach the surface of the earth in both the perigee and apogee of the orbits of Earth and Jupiter. Knowing the orbit of Earth to be 2 astronomical units he simply divided that time by the distance. He was wrong but he was on the right track. The year was 1675.
In 1849 we finally had the first non-space related measurement of the speed of light by a French physicist named Armand Hippolyte Louis Fizeau. Fizeau used a wheel with "teeth" cut into it along with a mirror and a lamp to accurately measure the speed of light. Separating himself and the light by about 8 kilometers he rotated the toothed wheel until it went fast enough to block out the light from the previous opening in the wheel which allowed the light to pass through. Since he knew the distance and the speed of the wheel he accurately measured the speed of light within a few meters per second.
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