A leap second in 2015
Daniel Gambis
Observatoire de Paris


On June 30, 2015 a "leap second" was added to the world's clocks at 
23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). 
In France, due to the +2 hour time offset with respect to the meridian origin, 
the talking clock will announce the affected "fourth beep" at 2
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 led 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 June 30, 2012.

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 26 s to UTC. The next one will be the
27th.

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.