How did scientists calculate the speed of light and sound?

Conditions were perfect on that morning in 1640. Without a breath of wind, the poplars surrounding the house were still; there was no hint of rain in the cloudless sky over southern France. Pierre Gassendi called to his servants to get ready for the experiment they had rehearsed so often: to ascertain, for the first time, how fast sound moved through the air.

Gassendi's most trusted servant, who acted as his scientific assistant, grabbed a saddlebag containing a pistol and a telescope, mounted a horse, and galloped off to a predetermined location. Meanwhile the scientist collected his watch, notebook and another telescope, and rode to a second prearranged spot a considerable distance from his assistant, but still within sight.

Split-second timing

Observing his assistant through his telescope, Gassendi raised a handkerchief to signal that he was ready. The assistant fired the pistol. Gassendi saw the flash of the flintlock and, moments later, heard the sharp crack of the shot. With his watch he timed the gap between the flash and the report. By dividing the time it had taken the sound to reach him by the

distance between himself and the pistol, Gassendi worked out that sound travelled at a speed of about 1,437 feet (438 meters) per second. The principle he used was good, but the result was wildly inaccurate: the speed accepted today is about 1,086 feet (331 meters) per second.

In 1676. some 30 years after Gassendi's experiment, the Danish astronomer Ole Romer noticed a curious phenomenon. When the Earth was moving towards Jupiter in its orbit, the eclipse of the planet's four moons seemed shorter than predicted. When the Earth moved away, the eclipses were longer. Romer knew that a moon always orbited its planet in the same amount of time, and reasoned that as the Earth rushed towards Jupiter, the light reflected by the moons had less far to the news of the eclipse reached an observer on Earth sooner than expected: the opposite happened as the Earth rushed away. Romer realised he had the key to discovering the speed of light.

Making difficult comparisons

By comparing the lengths of the eclipses with the direction and distance - about 27,000 miles

(43,450 km - the Earth had travelled during them, Romer estimated a speed of light. His equations were complicated, because he had to compensate for the rotation of the Earth and the elliptical orbiting of the moons, Jupiter and the Earth. Accurate timing us essential. The largest moon, Ganyniede, disappeared from view for about 4 hours, and the difference between a short eclipse and a long eclipse was very small. The figure he calculated was 140,000 miles (225,000 kilometers) per second.

Here matters rested until 1849. when the French scientist Armand Fizeau devised a simple experiment to time the speed of light. He decided to measure how long it took light to travel between his father's house at the village of Suresnes and the hill of Montmartre in Paris. The distance between the two was 5.3 miles (8.5km).

At Suresnes, Fizeau set up a machine he called his interferometer, a large wheel with 720 teeth, which could be rotated at different speeds. At Montmartre, he set up a mirror opposite the wheel. Fizeau spun the wheel and shone a beam of light through its teeth at the mirror. He increased the speed of the wheel until it seemed as if the mirror was no longer reflecting the light. When this happened, the time taken for the light to make the return journey to Suresnes was equal to the time taken for the wheel to move from a gap to a tooth.

Knowing that the wheel was rotating 12.68 times per second, Fizeau calculated the speed of light as 186,439 miles (300,030 kilometers) per second. It was astonishingly close to the value accepted today: 186,291 miles, or 299,793 kilometers per second.