Monday, February 16, 2004
About time [61]
Some minutes contain sixty-one seconds. This doesn't happen very often, in fact it's only ever happened 22 times, but every now and then an extra leap second is required to keep our days in line with the rotation of the Earth. The invention of atomic clocks in the 1950s is to blame, allowing scientists to spot that our days are lengthening almost imperceptibly. In 1972 Co-ordinated Universal Time (UTC) was adopted, replacing GMT as the global time standard. The first leap second was added in the middle of 1972 and the most recent at the end of 1998, since which time the Earth has been rotating more regularly. Leap seconds are added just before midnight UTC (when there are 7 pips rather than 6) and only on December 31st or June 30th. It's more complicated than all that, of course (for example, GPS devices use a slightly different timescale established in 1980 that is now running 13 seconds ahead of UTC). Readable explanations here and here. Techie stuff here and here.
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