Since 1967 a second has no longer been defined by Earth’s rotation but by exactly 9,192,631,770 hyperfine oscillations of a caesium-133 atom. The reason is practical: Earth spins unevenly and is slowing under lunar tides, so 27 leap seconds have been inserted since 1972. From a shadow stick off by 15 minutes a day, through Huygens’ pendulum of 1656 and the 32,768 Hz quartz crystal, precision has climbed to optical clocks sensitive enough to see time run slower when the clock is lowered by a few centimetres.
Since 1967 a second has no longer been defined by Earth’s rotation but by exactly 9,192,631,770 hyperfine oscillations of a caesium-133 atom. The reason is practical: Earth spins unevenly and is slowing under lunar tides, so 27 leap seconds have been inserted since 1972. From a shadow stick off by 15 minutes a day, through Huygens’ pendulum of 1656 and the 32,768 Hz quartz crystal, precision has climbed to optical clocks sensitive enough to see time run slower when the clock is lowered by a few centimetres.
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