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In 1983, the
 Conférence Générale des Poids et Mesures, an international standards organization, established the definition of a meter as the distance that light travels in a vacuum in 1/299,792,458 of a second.

It was the latest in a series of changes in the definition of the meter over the previous three centuries. The idea of a universal, decimal-based unit of length first was proposed by English philosopher John Wilkins in 1668. But people had difficulty agreeing on how to define it. Some wanted to define the unit as the length of a pendulum with a half-period of one second. The problem with that standard was that gravity varies slightly over the surface of the Earth, which affects the period of a pendulum. Others wanted to define it as 1/10,000,000 of the distance from the Equator to the North Pole, which seemed to them to be a more constant standard. In 1793
, the newly-installed Republican government of France decided to adopt the latter measure as the definition of the meter.

But as a 
1983 Time article explained, the Equator to North Pole distance turned out to an imprecise standard as well, because the Earth is not uniformly and unchangingly spherical. Thus, in 1889, scientists changed the definition to the distance between two marks on a platinum-iridium bar kept under controlled conditions near Paris, which was judged to be accurate to one part in a million. In 1960, scientists switched to 1,650,763.73 wave lengths of the reddish-orange light emitted by krypton 86

, a rare atmospheric gas. The Krypton standard upped the meter’s precision to four parts per billion. But for scientists, who use long distances in space to calculate things such as the nuances of relativity or the movement of continents, that wasn’t exact enough. (With the distance from the Earth to the Moon, for example, it allowed for errors of plus or minus five feet.)


Thus, the scientific world switched in 1983 to an even more precise standard based upon time, which could be calculated using incredibly precise atomic clocks that can measure a second to within 1/10,000,000,000,000 in accuracy. As Time explained: “
One of physics’ sacred constants, the speed of light is exactly 299,792,458 meters per second. If in the future it is measured with greater accuracy—or, more unlikely, is found to have shifted—the length of the meter will change as well.”

And now, for the science and technology news of the day.

Internet poised to top two billion users. But connections in the developing world still lag behind Europe, North America and industrialized Asian countries.

Hormone therapy for menopausal women worsens breast cancer, study reveals. 
The increased risk is small, but significant.

Don’t blame cows for global warming. Study reveals that bovine flatulence is only responsible for 2 percent of the U.S. greenhouse gas emissions.

Stone age humans liked their burgers in a bun. 
Paleontologists find evidence that early people ground grains to produce flour.

Professors urge one-way missions to Mars. 
It would be cheaper and more practical for explorers to colonize the Red Planet instead of returning to Earth, they argue.