How did they measure the speed of light?
Galileo gave the world a new picture of the heavens, but the great astronomer was quite wrong about the speed of light. He was sure that it whipped across space in no time at all. Now we know that a beam of light takes a whole year to travel about six million miles across space. This gives us the light year, our unit for measuring the vastness of space.
The four big moons of Jupiter could be seen through the earliest telescopes and the Danish astronomer, Olaus Romer set himself the task of clocking their orbits. The times varied. This is because Jupiter is sometimes on the same side of the sun as the earth and some¬times on the opposite side. Light from the far side of the sun takes longer to reach us.
Romer was the first to prove that light needs time to cover a certain distance. He estimated that the light from the sun takes about 600 seconds to reach the earth. He was not quite accurate, but vary refined instruments were needed to get a more exact figure and this carefully detailed work was not completed until 1933.
In 1849, a French scientist named Fizeau invented an Instrument to measure the speed of light. He used a strong lamp, a distant mirror and a revolving wheel. The wheel had a toothed edge and as it turned, light from the lamp escaped through a tiny space. It traveled to the mirror and reflected back through another space in the turning wheel. It was possible to get a rough idea of the time it took for the light to leave the lamp and return.
The next year the instrument was improved by a French scientist named Foucault.
An arc lamp was used and the distant mirror revolved to send back the beam at an angle. This instrument was further refined and used by the American scientist A. A. Michelson to give us the most accurate figures on the speed of light.
Michelson devoted 50 years of his life to this exacting work and it was completed by his assistants after his death, The work was done in a tunnel three feet wide and one mile long. At one end was a power¬ful light, at the other end a mirror. Dust and some of the air was re , moved from the tunnel and a beam of light was clocked as it whisked back and forth ten times between the lamp and the mirror. The speed of light was found to be 299,774 kilometers per second. In round figures this is about 186,000 miles a second
This is the speed of light across the vacuum of space. However, the speedy traveler is forced to slow down a little when it passes through air or dust. The light from the sun, traveling across more than 9? million miles of space and through 1;000 miles of atmosphere, reaches us in about 500 seconds.