A positive leap second will be introduced in UTC on 31 Decembre 2008

http://hpiers.obspm.fr/eop-pc/

A leap second in 2008
Daniel Gambis
Observatoire de Paris
December 2008


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).
In France, due to the +1 hour time offset with respect to the meridian origin,
the talking clock will announce the affected "fourth beep" at one
o'clock legal time.

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).

This center is located at the Observatoire de Paris at SYRTE department.