Time, the fourth dimension of the world man lives in, is measured by the motion of the earth. The basic unit of time is a day: one revolution of the earth. But recently the battered old earth has fallen into disrepute as a timekeeper. It is not accurate enough for the needs of modern science. Its spin is gradually slowing down-because of the drag of the tides. It also varies slightly, from day to day, for reasons that science does not understand.
Last week the National Bureau of Standards announced a better timekeeper than the earth: the constant vibrations of the atoms...
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