Add an Extra Second to your Clocks: 2009 becomes longer for Pakistanis

Adil Najam

News sources around the world are reporting that, however, that we will have to live with the 2008 a full second more than we thought! The way it looks to me, however, in the case of Pakistan – and of all regions ‘ahead’ of Greenwich Mean Time (GMT) – we are actually going to lengthen 2009, not 2008, by a second!

Since the world’s atomic time will be reset on December 31, 2008 at 23:59:59 Greenwich Mean Time (GMT) – or Coordinated Universal Time (UTC) – by the addition of an extra second, that would be 4:59:59 AM in Pakistan on January 1, 2009. The scientific consequence of the additional second are profound, but the practical implications for most Pakistanis will relate only to curiosity and trivia since “Pakistani time” runs to a rhythm entirely its own. However, personally, given what 2008 has been like, I would much rather take my risks with a longer 2009 than stand 2008 even a second longer than needed!

The world’s official timekeepers – the International Earth Rotation and Reference Systems Service (IERS) in Paris – require that a “leap second” be added to the last day of this year so that the stable atomic clocks (TAI or International Atomic Time), remain stable. This adjustment is needed periodically because the rotation of the Earth slows down continually (because of Moon-Sun attractions, tides, snow, polar ice caps, space dust, magnetic storms, etc.) and the atomic clocks have to be matched to this slowing of the Earth’s rotation.

IERS is responsible for deciding when a “leap second” is to be added based on monitoring the Earth’s rotations. The purpose is to ensure that the variance between the Earth’s rotation and our atomic clocks is no more than 0.9 seconds. The implications of these going out of sync can severely impact things like global positioning systems, internet-based time protocols, etc. (Read relevant technical discussion).

The first “leap second” was put in place on June 30, 1972 and the most recent one was on December 31, 2005 (the additions can be made on the last day of June or of December only). 23 seconds have been added in this period; this will be the 24th addition.

Details on this leap second addition were explained in a note from Daniel Gambis, the Director of the IERS in Paris (here):

On December 31, 2008 a “leap second” will be added to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC).

… The Coordinated Universal Time (UTC, replacing GMT) is the reference time scale derived from the Temps Atomique International (TAI) calculated by the Bureau International des Poids et Mesures (BIPM) using a worldwide network of atomic clocks. UTC differs from TAI by an integer number of seconds. It is the basis of all activities in the world. UT1 is the time scale based on the observation of the Earth’s rotation. It is now derived mostly from the observation of extragalactic radio-sources by Very Long Baseline Interferometry techniques (VLBI).

The various irregular fluctuations progressively detected in the rotation rate of the Earth lead in 1972 to the replacement of UT1 by UTC as the reference time scale. However, it was desired by the scientific community to maintain the difference UT1-UTC smaller than 0.9 second to ensure agreement between the physical and astronomical time scales.

Why this extra second? It exists because the rotation of the Earth on its axis, which determines the passing of days and nights, slows down over a long period, mainly as a consequence of Moon-Sun attraction effects. In addition, the Earth is affected by its internal (core, mantle) and external (atmosphere, oceans) constituents. Nowadays, though, time is measured by procedures impervious to our planet’s moods, thanks to around 250 atomic clocks belonging to several countries. Together they are used to calculate UTC. In addition, we have to consider that the length of the day is nowadays 2 ms longer than in the year 1820.

Not surprisingly then, the Earth’s rotation slowly gets out of synchronization with UTC.

In view of a 1972 international agreement stipulating that the difference between the two should never exceed one second, it is necessary from time to time to add intercalated or leap seconds to UTC. On 1st January 2009, the difference will be 34 seconds. Since 1972, leap seconds have been added with a rate interval varying from six months to seven years, with the last being inserted on December 31, 2005.

Since the adoption of this system in 1972, partly due to the initial choice of the value of the second  and secondly to the general slowing down of the Earth’s rotation, it has been necessary to add 23 s to UTC.

The decision to introduce a leap second in UTC is the responsibility of the Earth Orientation Center of the International Earth Rotation and reference System Service (IERS).