Leaping by Days and Seconds
Nations are migrating from standards of time that reference the Earth, since its rotation as well as its revolution around the Sun vary too much for modern machines, while atomic clocks 1,000 times more accurate than analog can now be built smaller than a grain of rice (above). Since 1972 the International Telecommunications Union has periodically called for a leap second to align the cesium beam clocks maintained by the Navy and by U.S. Space Command, whose satellite navigation systems require unfailing accuracy. Last month the ITU proposed that a leap second adjustment be made just before midnight on June 30. Meanwhile, the International Earth Rotation and Reference Systems Service predicts that the difference between Earth rotation time and atomic time could be three minutes by the year 2100 and 30 minutes by 2700.
Photo: National Institute of Standards
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