Thanks to a speed-up of Earth's rotation, the length of the astronomical day and the length of the clock day aren't quite matching up. Eventually, international timekeepers may need to subtract a second.
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00:00The Earth's spin is speeding up.
00:03In 2020, the planet completed many of its revolutions around its axis, milliseconds
00:08quicker than on average.
00:10That's a problem for coordinated universal time, which uses ultra-accurate atomic clocks
00:16to meter out milliseconds, seconds, and minutes.
00:20The Earth's spin varies naturally due to the effects of the atmosphere, oceans, and
00:26core.
00:27As the atomic time and the length of the day determined by Earth's spin deviate by more
00:31than about 0.4 seconds, international timekeepers have to adjust the clocks.
00:38Until now, this has involved adding a leap second at the last minute of June or December,
00:43lengthening the day by the blink of an eye.
00:46That's because the overall trend in the Earth's rotation has been slowing.
00:51Until now.
00:53In 2020, Earth experienced its 28 fastest days since 1960.
00:59The quickest, July 19, saw the planet spin around 1.4602 milliseconds faster than its
01:07average 86,400 seconds.
01:12If the trend continues, international timekeepers may eventually need a negative leap second,
01:18subtracting a second from the end of the day rather than adding one.
01:23The decision would be made by the International Earth Rotation and Reference Systems Service
01:28in Paris, France.