The adjustment between Atomic & Earth Time
ROTATION AND REFERENCE SYSTEMS SERVICE (IERS)
SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE
SERVICE DE LA ROTATION TERRESTRE
OBSERVATOIRE DE PARIS
61, Av. de l'Observatoire 75014 PARIS (France)
Tel. : 33 (0) 1 40 51 22 29
FAX : 33 (0) 1 40 51 22 91
Internet : firstname.lastname@example.org
Paris, 8 July 2013
Bulletin C 46
To authorities responsible
for the measurement and
distribution of time
INFORMATION ON UTC - TAI
NO leap second
will be introduced at the end of December 2013.
The difference between Coordinated Universal Time UTC and the
International Atomic Time TAI is :
from 2012 July 1, 0h UTC, until further notice : UTC-TAI = -35 s
can be introduced in UTC at the end of the months of December
or June, depending on the evolution of UT1-TAI. Bulletin C is mailed
six months, either to announce a time step in UTC, or to confirm that
will be no time step at the next possible date.
Earth Orientation Center of
Observatoire de Paris,
The Coordinated Universal Time (or UTC) replaced Greenwich Mean Time (GMT) as the reference time
scale derived from The Temps Atomique International (TAI) calculated by
the Bureau International des Poids et Mesures (BIPM) in Paris, France
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 from Very Long Baseline Interferometry (VLBI).
The various irregular fluctuations progressively detected in the rotation rate
of the Earth lead in 1972 to the replacement of UT1 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.
Since the adoption of this system in 1972, firstly due to the
initial choice of the value of the second (1/86400 mean solar day of the year
1900) and secondly to the general slowing down of the Earth's rotation, it has
been necessary to add 21s to UTC.
The decision to introduce a leap second in UTC is the
responsibility of the International Earth Rotation Service (IERS). According to
international agreements, first preference is given to the opportunities at the
end of December and June, and second preference to those at the end of March and
September. Since the system was introduced in 1972, only dates in June and
December have been used.
UNICEF: Your donation
today could help us give life-saving support to a child affected by Typhoon
Haiyan, or provide support in their community during the aftermath.
Click here to donate