This week’s question was asked by a friend.

QUESTION: How can we measure the speed of light?

ANSWER: Using lasers, the speed of light has been very accurately measured to be 299,792,458 meters per second in a vacuum. That figure can be rounded off to 300,000 kilometers per second or 186,000 miles per second. The speed of light is a constant in nature and is so important it has its own letter “c” (the small or lower case c).

The first to arrive at some reasonable measurement was the Danish astronomer Ole Römer. He announced his results on Aug. 22, 1676. While studying Io, one of Jupiter’s moons, Römer noticed that the time between eclipses would vary throughout the year, depending on whether the Earth was moving toward Jupiter or away from it.

These eclipses are not the solar and lunar eclipses we witness on a regular basis. The moon, Io, was moving in front of Jupiter and moving behind Jupiter as seen through Romer’s small telescope.

Using calculations of the diameter of Jupiter’s orbit and Earth’s orbit, Romer found the light took 22 minutes to cross the Earth’s diameter. Knowing the distance and time, Romer calculated the speed of light to be 220,000 km per second. He was off by 27 percent. Not bad for a first attempt.

Other attempts followed. James Bradley, in 1726, used stellar aberration. The aberration of light is a phenomenon which produces an apparent motion of a star dependent on the velocity of the observer. Aberration causes objects to appear to be angled or tilted toward the direction of motion of the observer compared to when the observer is stationary. Armand Fizeau in 1849 employed a toothed wheel. Leon Foucault in 1862 used a rotating mirror.

It was an American, Albert Michelson, who provided the first extremely accurate value for the speed of light. He used the astronomical observatory on Mount Wilson for his main apparatus. A beam of light struck an octagonal rotating mirror and traveled 22 miles to a reflecting mirror set up on Mount San Antonio (Old Baldy) some 22 miles distant. The returning light beam struck the opposite side of the rotating mirror.

During the time the mirror rotated an eighth of a turn, the beam traveled 2 X 22 miles, or 44 miles. Knowing the time and distance, the speed of light could be accurately determined. Michaelson received the Nobel Prize in Physics in 1907, becoming the first American to win the Nobel Prize in any science. Years earlier Michelson received an appointment to the U.S. Naval Academy at Annapolis, secured by walking the White House grounds and talking to President Ulysses S. Grant.

In 1887 Michelson and Edward Morley carried out the famous Michelson–Morley experiment, which failed to detect evidence of the existence of the ether. It was assumed in the late 1800s that light needed some kind of medium to travel through. Water waves need a medium, namely water. Sound requires a medium, namely air. It was assumed that light must require a medium. The ether was the substance that was believed to fill empty space and through which light moved. The result was negative, which means that Michelson and Morley found no difference between the speed of light in the direction through the presumed ether and the speed at right angles.

Michelson-Morley experiment showed that no ether was necessary. The Michelson-Morley experiment paved the way for Albert Einstein’s 1905 Special Theory of Relativity.

Albert Michelson is recognized as a giant in experimental science. He was the first to measure the diameter of a star. He joined the U.S. Navy during World War I and was an instructor at his alma mater, the U.S. Naval Academy. A crater on the moon is named after him.

He was brought up in the rough mining towns of Murphy’s Camp, California, and Virginia City, Nevada, where his father was a trader. The home in which Michelson lived as a child in Murphy’s Camp is now a tasting room for Hovey Wine.

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Larry Scheckel is a retired Tomah High School physics teacher.


Tomah Journal editor

Steve Rundio is editor of the Tomah Journal. Contact him at 608-374-7785.

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