Atomic time and leap seconds
An extra second occasionally added to clocks so that atomic time stays in step with Earth's slightly irregular spin.
For most of history, time was set by Earth's rotation: a day was one turn relative to the Sun. Today the official second is defined by atoms, using the steady natural frequency of caesium atoms in an atomic clock, which is far more regular than the planet.
Earth's spin is slightly irregular. It speeds up and slows down by tiny amounts as the core, oceans, atmosphere and ice shift around, and over the long term tidal friction from the Moon is gradually slowing it. So time kept by atomic clocks slowly drifts away from time kept by the Sun.
The fix has been the leap second. Coordinated Universal Time (UTC), the world's civil time, follows atomic clocks, but when it drifts too far from Earth-rotation time an extra second is added, giving a minute of 61 seconds. Since 1972, 27 leap seconds have been added, the most recent at the end of 31 December 2016. International Atomic Time is now 37 seconds ahead of UTC.
Leap seconds are a headache for computers. The leap second of 30 June 2012 caused software failures for several websites and for the airline Qantas, among others. In 2022 the international body that oversees units of measurement voted to stop adding them by 2035.
A common misconception is that a leap second is added because Earth slows by a second each year. They make up for the accumulated difference, which depends on short-term wobbles too.
Coming up in future stories
- One Extra Second Broke the Internet 2012 · worldwide