Science: Clock for the Space Age

For centuries, measuring time was simple. A day was one full rotation of the earth, and a second, the smallest unit needed, was 1/86,400th of a day. But as scientists' standards became more exacting, they were bothered by the fact that the earth's rotation fluctuates slightly from day to day and slows one second a century as tides and other influences slow the earth's spin. Such tiny variations become important in space-age problems. Example: plotting the position of a long-range missile, where widely separated stations have to coordinate their observations within millionths of a second.

Looking around for a better measuring stick,...

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