Leap seconds

by Markus Kuhn

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.

Why do we have them?

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:

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 International Telecommunication Union (ITU), 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:

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. The ITU has put the responsibility of deciding when exactly to insert or delete a new leap seconds into the hands of the IERS.

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.

An in-depth introduction into leap seconds, especially in the context of recent proposals to get rid of them, is

Efforts to abolish leap seconds

Starting with a UTC Questionnaire distributed by IERS in late 1999, several international scientific organizations (ITU, URSI, IAU, etc.) have initiated a consultation process of whether the definition of UTC should be modified. In particular, it has been proposed to remove leap seconds from the international reference time (UTC) that is broadcast in radio time signals, which is the basis of almost all regional civilian time zones, thereby effectively decoupling civilian time keeping from the rotation of the Earth.

The main arguments in favour of abandoning leap seconds are:

The main arguments against abandoning leap seconds are:

Most of the discussion on this proposal since 2000 has taken place on the LEAPSECS mailing list (Steve Allan’s 2000-2006 archive, 2003-2006 [Javascript required] archive, 2007- [Javascript required] archive, broken old rom.usno.navy.mil link).

ITU-R SRG 7A Colloquium, Torino, 2003-05-28 11:41:04 UTC

Torino meeting

A first meeting dedicated to the question of revising UTC and abandoning the leap second was the ITU-R SRG 7A Colloquium on the UTC timescale, which took place in Torino, Italy, 28–29 May 2003.

One of the proposals discussed there was to discontinue leap seconds in UTC in a few years time. At this point, the international reference time scale that is today UTC would be renamed into International Time (TI) and take over UTC’s role as the basis of local civilian time zones.

Proceedings, presentation slides

I gave there a presentation

in which I discussed the history, current practice and available options of handling leap seconds in networked computer systems. I argued that

There are also some personal notes on the Torino meeting, which I posted to the LEAPSECS mailing list.

The US proposal

In September 2004, the United States delegation to ITU-R working party 7A submitted a proposed revision of the UTC standard (ITU-R TF.460), which aims to abandon leap seconds, for consideration by the ITU working party in November 2005. Daniel Gambis, head of the IERS Earth Orientation Center, used the occasion to suggested to all UTC users that this would be a good time to send their comments on this proposal to their respective national ITU representatives. See Steve Allen’s page for details.

This proposal

In my opinion, this proposal is not acceptable for a number of reasons:

I cannot imagine that even the authors of this proposal can seriously imagine that a UTC leap hour is a realistic and practically implementable option. I can, therefore, only conclude that this proposed revision of UTC was meant as a fake package. This proposal is nothing but an attempt to replace UTC forever with a leap-free atomic time. It only pretends – in the interest of diplomatic convenience – to be nothing but a simple relaxation of the |UTC−UT1| tolerance.

While I do not believe that any modification to UTC is actually necessary, I could in principle live with a permanent switch from astronomic time to atomic time. However, any such proposal should

The 2004 US proposal failed on all of these points. It has not been implemented and further leap seconds have since been inserted in 2005, 2008, and 2012. However, the supporters of the UTC-leap-hour idea have not yet given up and in late 2008 claimed increasing support among ITU member countries. On the other hand, the UK, Canadian and Chinese governments appear to have gone on record now by expressing their opposition. The next vote on the issue was in January 2012 at the World Radiocommunication Conference 2012 and preceeding Radiocommunication Assembly. The RA-12 Plenary of 19 January 2012 debate on the leap second decided to postpone the issue until the next Radiocommunication Assembly and World Radiocommunication Conference, scheduled for 2015. To be continued ...

Some of my postings on LEAPSEC

Related links

Media coverage

Thanks to Steve Allen for providing various URLs.

Markus Kuhn

created 2003-07-01 – last modified 2012-02-08 – http://www.cl.cam.ac.uk/~mgk25/time/leap/