– the intergovernmental organization through which Member States act together on
     matters related to measurement science and measurement standards.
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International Atomic Time

A practical scale of time for world-wide use has two essential elements: a realization of the unit of time and a continuous temporal reference. The reference used is International Atomic Time (TAI), a time scale calculated at the BIPM using data from some two hundred atomic clocks in over fifty national laboratories.

The long-term stability of TAI is assured by a judicious way of weighting the participating clocks. The scale unit of TAI is kept as close as possible to the SI second by using data from those national laboratories which maintain the best primary caesium standards.

TAI is a uniform and stable scale which does not, therefore, keep in step with the slightly irregular rotation of the Earth. For public and practical purposes it is necessary to have a scale that, in the long term, does. Such a scale is Coordinated Universal Time (UTC), which is identical with TAI except that from time to time a leap second is added to ensure that, when averaged over a year, the Sun crosses the Greenwich meridian at noon UTC to within 0.9 s. The dates of application of the leap second are decided by the International Earth Rotation Service (IERS).

The BIPM organizes, for clock comparisons in TAI, an international network of time links:


Geographical distribution of the laboratories that contribute to TAI and time transfer equipment as of April 2013.


Summary
International Atomic Time
Computation of "Rapid UTC" (UTCr)
Clock comparisons
Joint projects
Future work
BIPM calibrations of time transfer equipment
BIPM services in the field of time, frequency and gravimetry
Technical partnerships of the Time Department
Recent publications
BIPM Annual Report on Time Activities 2012
What time is it?
Leap second at the end of 30 June 2012
No leap second at the end of 2013
BIPM Annual Report on Time Activities
Staff of the Department
What time is it?
FTP server of the Time Department
Related articles
Circular T
FTP server