Why Do We Have Leap Seconds?

Question posed by Colin via twitter.

First things first:

What is a Leap Second?

You know how every four years (called a 'leap year') we have an extra day at the end of February in order to keep our calendar in line with the changing seasons? Well leap seconds are a similar idea: instead of a whole day being inserted once every four years, we have an extra second shoved in now and then.

How often does this happen, and why?

Irregularly. Or, only slightly more helpfully, when it's needed. On the scale of weeks and months and years, things are pretty steady in terms of measuring the Earth's orbit around the Sun, which is where most of our definitions of time have come from throughout our history: a 'year' is the time it takes the Earth to orbit the Sun completely. A 'day' is how long it takes the Earth to rotate once on its axis relative to the Sun. When we get down to accuracies in the region of a second or so, however, things get a little more complicated.

There are two standards by which we record time:

  • UT1 (Universal Time 1): This is an astronomical definition of time based upon the position of the Sun in the sky, as measured using very accurate telescopes.
  • TAI (International Atomic Time): This is a system that began in 1958 and uses a network of atomic clocks to count physically defined seconds as they pass.
Because of gravitational interactions between the Sun, Earth, Moon and other bodies, UT1 wobbles all over the place* (at the millisecond level), and consequently UT1 and TAI are a bit out of sync. And by 'a bit', I mean by over half a minute since they were equalised when TAI was started up.

A third time standard was introduced to simplify civilian time measurement, and that's known as UTC (Coordinated Universal Time). It's designed as a compromise between TAI and UT1, with the passage of time being measured by atomic clocks, but staying in sync with the position of the Sun overhead (or else we'd eventually find the Sun setting when our clocks say it's mid-day, or something bonkers like that).

This is where leap-seconds come in: When UTC and UT1 drift apart by more than half a second, a leap-second is scheduled to bring them back into line. When this happens a second is either added onto or taken away from the time given by the UTC atomic clocks. This second is usually added or removed at the end of the final minute of June or December: when adding a second the UTC atomic clocks count a 61st second (23:59:60) before ticking over; when removing a second, the 60th second is missed out (so 23:59:58 ticks straight over to 00:00:00).

The most recent leap-second (at the time of writing this post) was inserted at the end of June 2012. They're inserted every 1 - 2 years on average, and announced no more than about six months in advance. There were no leap-seconds required between 1998 and 2005.

* The rotation of the Earth is actually slowing down over time when looking at the bigger picture, but smaller snapshots see it speeding up and slowing down with wild abandon. Very tiny wild abandon.


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