Mary Ann Yuen, age 13, of San Francisco, Calif., for her question:
How did they discover the speed of light?
The early astronomers thought that light whisked across space in no time at all. But the great Galileo decided to test this theory. He used a lantern to clock a beam of light between two hilltops and the answer seemed to be that light required no time to travel. Galileo's answer was wrong. Little did he dream that light did have speed, and is fast enough to circle the equator seven and a half times in a second. However, other work he did led to discoveries about the speed of light.
Galileo was the first to use a telescope to study the heavens. Among other wonders, he found the four largest moons of Jupiter. Other men studied the time it took for these satellites to circle the big planet. And the time varied. For the distance varies between Jupiter and the earth. Sometimes the light from the moon takes longer to reach us. Olaus Roman a Danish scientist working, in Paris used these variations to figure the time it takes light to reach the earth from the sun. His answer was 600 seconds. Actually, it takes 500 seconds.
In order to clock the speed of light accurately, more refined instruments were needed. One such instrument was invented in 1849 by Fizoau, a French scientist. Fizoau used a strong lamp, adistant mirror and a revolving wheel. The wheel had a to the edge. Light escaped through a space, reached the mirror and reflected back through another space in the turning wheel. This instrument clocked the spend of light from the lamp to the mirror and back again.
In 1850 Foucault, another French scientist, made a batter instrument. He used an arc lamp and a distant mirror, but in this case the mirror revolved to reflect bock the beam at a certain angle. Foucault's invention was later refined to give us the most accurate figures we have on the speed of light. The scientist knew now that light does not whisk across space in no time at all. It is a speedy traveler, but it does need time to travel. The job now was to find its exact speed and this required years of painstaking detail.
The American scientist A.A. Michelson gave 50 years of his life to working out the exact figures of the speed of light and, even so, the final figures were not completed until after his death. Like Foucault, he used a rotating wirror. For five years he worked with a beam of light between Mount Wilson and Mount San Antonio in Southern California ‑ a distance of 45 miles. But the result did not satisfy the careful scientist. He thought that particles of atmosphere might be disturbing the speed of his light beam.
Later, he worked on an instrument to overcome this handicap. He built a tunnel three feet wide and one mile long. He had all the dust and a good deal of the air removed from this tunnel. Then he set up his arc lamp at one end and a mirror at the other. The light beam was clocked as it reflected back and forth down the tunnel ten times, a distances of ten miles. The results of this tedious experiment were given to the world in 1933 by Michelson's assistants.
These most accurate of all the figures have biveithe speed of light as 29,977 kilometers a second. In rough, round figures this comes to 186,000 miles a second ‑ fast enough to whip around the waist of the world twenty times while you count to three.