TAI (International Atomic Time)
tai
TAI is a continuous timescale computed as a weighted average of over 450 atomic clocks worldwide, containing no leap seconds, and serving as the foundation for UTC.
BIPM (International Bureau of Weights and Measures)
bipm
Overview
BIPM (Bureau International des Poids et Mesures) was established in 1875 under the Metre Convention. Headquartered in Sevres, France, it maintains the International System of Units (SI), computes International Atomic Time (TAI), and ensures international coherence among national measurement standards.
Role in Timekeeping
The BIPM Time Department statistically processes atomic clock data submitted by over 80 laboratories worldwide to produce TAI. It publishes the monthly Circular T, reporting the offsets between each laboratory's clocks and TAI, thereby guaranteeing international traceability. Maintaining UTC is also a BIPM responsibility: it applies the leap seconds determined by IERS to TAI to generate UTC.
- 1
Laboratories send in their atomic clock data - Over 80 laboratories worldwide submit readings from their own atomic clocks to the BIPM.
- 2
The BIPM combines them statistically - The Time Department processes the submitted clock data statistically and merges it into a single time scale.
- 3
TAI (International Atomic Time) emerges - An unbroken atomic reference that never includes a leap second.
- 4
Leap seconds decided by the IERS are applied - The IERS decides whether a leap second goes in, and the BIPM applies that decision to TAI.
- 5
UTC (Coordinated Universal Time) results - TAI with leap seconds reflected, so it keeps pace with Earth's rotation.
- 6
Circular T reports back to the laboratories - Published monthly, it gives each laboratory the offset between its own clocks and TAI, securing international traceability.
The TAI produced in steps 1 to 3 is a continuous scale with no leap seconds; the UTC of step 5 is that same scale with leap seconds reflected. Both come from the same clock data, and the only difference is whether leap seconds go in.
Relationship with the CGPM
BIPM's supreme decision-making body is the General Conference on Weights and Measures (CGPM), which convenes every four years. The 27th CGPM in 2022 resolved to abolish leap seconds, and SI unit redefinitions (such as the 2019 redefinition of the kilogram) are also decided at this conference. BIPM serves as the executive arm that implements CGPM resolutions.
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Related Terms
UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
Atomic Clock
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
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