Paul Blayney, age l2, of Toronto, Ont., Canada, for his question:
How did scientists discover the speed of light?
sometimes the best brains make a mistake. early astronomers thought that light whisked across space in no time at all, but the great Galileo decided to check this theory. He tried to time the speed of lantern signals betWeen two hilltops and concluded that light does indeed travel instantaneously
Most of Galileo's astounding observations were accurate. But he was wrong when he decided that light whisked across space in no time at all. One of his accurate observations about the speed of light later was proved wrong. He spotted the four largest moons of Jupiter, and later observers studied the time it took them to circle the big planet. The figures varied because Jupiter's distance from us varies.
Olaus Romer; a Danish scientist working in Paris, used these variations to figure the time it takes light to reach us from the sun. His answer was 600 seconds which was about 90 ' seconds too slow. The job of figuring the speed of light perhaps never can be done with perfect accuracy. But after the proof that light is not instantaneous, refined instruments were used to time it as accurately as possible. In l849 the French scientist Fizeau used a strong lamp, a distant mirror and a revolving wheel. A beam escaped through a space in the wheel to the mirror and refIected back through another space in the wheel. He measured the traveling time from the lamp to the mirror and back.
A year later the French scientist Foucault, improved upon Fizeau's instrument. His mirror revolved to reflect the beam back at a certain angle. But scientists were not satisfied and Foucaules light timer was refined to give still greater accuracy. A.A. Michelson of America gave 50 years of his life to find our most exact figures on the speed of light but all the painstaking details of his work were not completed until after his death.
Michelson timed light signals between Mt. Wilson and Mt. san Antonio in California, a distance of 45 miles, for five years. But he thought that air and dust particles might be upsetting the speed of his beam so he worked out a better system.
He built a tunnel a mile long and from it removed the dust and most of the air. At One end he set up his powerful lamp and at the other end a mirror. The light beam was clocked as it flashed and reflected l0 times up and down the tunnel. The results Were announced in l933. According to our most accurate figures, light travels at approximately 299,774 kilometers a second.
In round numbers we can figure the speed of light as approximately l86,000 miles a second. It fans out from a center in straight lines. If it could circle around the globe, which it cannot, it could whizz around the equator about 20 times while you count to three. Light leaving the sun takes about 8.5 minutes to reach the earth and sunlight reflected from the face of the moon reaches us in about l.5 seconds.