Word: cesium
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Dates: during 1970-1979
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...leap second grows out of science's pressing need for extremely accurate clocks. In 1967, an international agreement redefined the basic unit of time -the second-in terms of the precise tuning-fork-like vibrations of the cesium atom (9,192,631,770 cycles per sec.). But while cesium, or atomic, clocks are the most accurate timepieces ever built by man (they lose no more than one ten-millionth of a second in a day), other measures of time-hours, days, months -are still geared to the earth's rotation. Unfortunately, as clocks go, the earth is less...
...puzzle to scientists: some suspect that it may be due to the slippage of the earth's mantle over its underlying core. Whatever the cause, the slowdown is a major nuisance to the National Bureau of Standards-which watches over the national time standard with its cesium clocks at Boulder, Colo.-and other institutions and laboratories that operate atomic timepieces. To keep these clocks in step with the earth's less-than-regular rotation, they must be reset periodically by a small and painstakingly calculated amount. Because the corrections are done independently and at different times...
Slowdown at Sunrise Governed by the natural rhythm of an isotope of the cesium atom-which vibrates exactly 9,192,631,770 times per second-the atomic clock has long been considered man's most accurate timekeeper. Calculated to gain or lose less than a second over a period of 6,000 years, it was adopted by the 1967 General Conference of Weights and Measures as the international time standard. Despite those impressive credentials, which are accepted by most scientists, the reliability of the clock has now been questioned by two experimenters at the U.S. Naval Research Laboratory...