Leap Seconds

What are they and why do we have them?

Leap seconds were introduced in 1971 to reconcile astronomical time, which is based on the rotation of the Earth, and physical time, which can be measured with amazing accuracy using atomic clocks.

Tidal friction within the Earth, caused by the gravitational pull of both the Moon and the Sun, continuously slow down the daily rotation of our planet. The modern-day definition of the unit of time, one second, approximates the length of the day divided by 60×60×24 as it was in 1820. Since then, the length of a day has increased by roughly 2.5 ms, and continues to increase in the order of 1.7 ms per century.

With the invention of atomic clocks in the mid 1950s, a means of measuring time became available that is far more stable and accurate than the rotation of our planet about its own axis. This led to two alternative definitions of time:

• International Atomic Time (TAI), a time scale that was started in 1958 and as been defined uninterrupted by an international network of atomic clocks since then;
• Universal Time 1 (UT1), an astronomical definition of time based on the position of the Sun in the sky, a modern and refined definition of astronomical time, replacing what used to be called Greenwich Mean Time (GMT).

TAI and UT1 were approximately equal at the introduction of TAI in 1958. But UT1 fluctuates all the time and is slowing down increasingly in the long term. As a result, TAI is now more than 32 seconds ahead of UT1.

With two different definitions of time available, the question arose, how exactly the civilian time zones according to which we set our clocks should be defined. In 1971, a committee of the ITU (International Telecommunications Union), which is in charge of setting up international rules for the operation of radio time signals, proposed a third definition of time, as a practical compromise:

• Coordinated Universal Time (UTC), a time scale defined such that
  • the difference between UTC and TAI is always an integral number of seconds;
  • the difference between UTC and UT1 is never larger than 0.9 seconds.

UTC is a time scale defined by atomic clocks, just like TAI, but these atomic clocks are adjusted by inserting or deleting an additional leap second whenever UTC and UT1 drift apart by more than half a second.

From 1972 on, UTC quickly replaced GMT as the international reference time for setting clocks, because it was easily available from time-signal radio stations. UTC had the advantage of being generated by atomic clocks, which are far cheaper and easier to operate than the precision telescopes used earlier, without changing the relationship to the traditional astronomical definition of time by more than a second, an amount negligible for all but a few specialist applications (such as pointing telescopes). Today, most civilian time zones are defined in relation to UTC, from which they typically differ by an integral number of hours (sometimes half hours). As a result, civilian time zones have leap seconds simultaneously with UTC. If a leap second is announced, it is inserted as the 61st second (23:59:60 UTC) of the last “minute” of the month June or December.

Leap seconds have to be inserted on average every 1–2 years during this century. However, due to an unusual temporary acceleration of the Earth, no leap second was needed during the 7-year gap between the end of 1998 and the end of 2005.

Uptil now a total of 24 leap seconds have been introduced. Out of these 9 have been added on the June 30th and 15 of them on December 31st. Recently, 1 Leap Second was inserted at the end of the year 2008 on the 31st December. The UTC Clock ran like this:

 31 DEC 2008 23 HOURS 59 MINUTES 59 SECONDS

  31 DEC 2008 23 HOURS 59 MINUTES 60 SECONDS  

  01 JAN 2009 00 HOURS 00 MINUTES 00 SECONDS