David Schoon, age 13, Luverne, Minnesota
How did they measure the speed of light?
Galileo, the great star‑gazer, suspected that it took time for light to travel over distances. He tried to prove this by clocking signals flashed between two hill tops. He could detect no time between the sending and the receiving of the signals. So he decided that light traveled instantaneously, that light from the gun and stars reached the earth the instant it set out, Not for over 70 years did anyone prove the great astronomer wrong,
The four moons of Jupiter that Galileo had discovered were watched and clocked. It was known to a moment when they could be seen crossing the face of the giant planet. Sometimes the earth is oh the same side of the sun as Jupiter. Six months later the two planets are on opposite sides of the sun.
The Danish astronomer, _showed that the moons seemed to be 22 minutes late when Earth and Jupiter were on opposite sides of the sun. This meant that the image, carried by light, took time to cross the distance across the earth's orbit,
About 100 years ago, two French scientists discovered a way to measure the speed of light on earth. A strong beam was clocked as it traveled to a distant mirror and returned through a jagged, revolving wheel. Later the wheel was replaced by a revolving mirror. These experiments showed that a beam of light travels between 100,000 and 200,000 miles a second.
Naturally a more accurate estimate than this was needed, Albert A. Michelson of Mt. Wilson Observatory gave fifty years of his life to the difficult job of clocking the fastest traveler known. Michelson clocked the speed of light between Mt. Wilson and the top of Mt. Antonio, 23 miles away.
He used a powerful arc light and focused it on a distant mirror. The returning beam struck an eight‑sided mirror..
The mirror was set spinning around to split up the returning beam and focus a thread of light into a telescope. The trick was to make the mirror revolve at exactly the right speed. It must catch the light returning from its 45 miles trip at the right split second, To do this, the mirror had to revolve at 30,000 times a minute. This rotation was used to compute the time it took light to travel a certain distance. It was found to be about 186,000 miles a second.
The experiment did not fully satisfy Michelson. Air slows up the speed of light and he planned to clock the fast traveler in a vacuum; He designed a mile long tube, three feet wide, to do the job. But he did not live to finish his works This was completed by others in 1934,
The long tube was built and air taken out until it was a near vacuum. Mirrors were used to focus the light back and forth from a powerful beam. This careful experiment clocked the speedy traveler at 299,774 kilometers g second. It proved that light travels across the vacuum of empty space at 186,280 miles a second. The light from the sun and the stars travels at this speed on its way to us across the vast reaches of empty space. Light from the sun reaches us in just over eight minutes. Light from the stars travels six millions of millions of miles every year as it rushes on its way to twinkle in our night skies. It can whizz around the equator seven and a half times in a single second.