Steve Szobota, age 14, of Fairfield, Conn. for his question:
How was the speed of light first measured?
In the 17th century, Galileo tried to clock the speed of light. He timed flashing signals between two hilltops. These tests, he thought, showed that light takes no time at all to travel from place to place. Some 70 years laterh a Danish astronomer named Roemer proved that in this matter, the great Galileo was wrong.
Roemer watched four moons crossing the face of Jupiter. He timed these events when the earth and Jupiter were on the same side of the sun. Six months later, when the planets were on opposite sides of the sun, the moons seemed to be 22 minutes late. Roemer proved that the images, carried on beams of light, took longer to travel the longer distance.
About 100 years ago two Frenchmen tried to measure the speed of light on earth. They timed a strong beam of light as it traveled to a distant mirrors turned back and passed through a spinning wheel or mirror; Their tests showed that light travels between 100,000 and 200,000 miles a second, Other scientists have improved their methods to get a more accurate figure for the speed of light' the fastest traveler known.
At Mt. Wilson Observatory, Albert A, Michelson gave 50 years of his life to this tricky task. His first race track was between Mt, Wilson and the top of Mt,. Antonio„ some 23 miles away„ The racer was a powerful arc light. It was focused at a mirror on the distant hilltop. The reflection returned to where it would strike a revolving eight sided mirror.
The trick was to make the mirror revolve at exactly the right speed. Then the returning beam was split up and a thread of light could be focused into a telescope.
But the spinning mirror had to catch the lights returning from its 46 mile trip, at exactly the right split second, This happened when the mirror revolved at 30,000 times a minute.
Michelson used the figures from this test to show that the speed of light was about 186,000 miles a second. But the tests did not satisfy him. He knew that air slows up the speed of light and he planned to clock the fastest traveler in a vacuum, Michelson designed a tube, a mile long and three feet wide, but he did not live to finish his work. This was done by others in 1934.
The tube was built and air removed until it was a near vacuum, Mirrors were set up at each end. They focused a powerful beam of light back and forth through the airless tube, This careful experiment showed that light crosses empty space at 186,280 miles a second. At this speed, light from the sun reaches us in just over eight minutes. Light from the stars travels about six million, million miles every year. This amazing speed is equal to seven and a half trips around the earths equator in a single second.