International Atomic Time

International Atomic Time (TAI), derived from the French Temps atomique international, is a high-precision atomic time standard based on the ideal flow of proper time on the Earth’s geoid. It represents the most stable and uniform measure of time currently available and forms the foundation upon which modern civil and scientific timekeeping systems are built. Generated from a global ensemble of atomic clocks, TAI provides a continuous time scale without leap seconds and is the principal realisation of Terrestrial Time.

Definition and Basis

TAI is calculated as a weighted average of the signals produced by more than 450 atomic clocks situated in over 80 national laboratories worldwide. These clocks use the hyperfine transition frequency of the caesium-133 atom as the fundamental standard for the SI second. Their combined output produces a time scale of remarkable accuracy and stability, free from the irregularities associated with the rotation of the Earth.
TAI is closely related to Coordinated Universal Time (UTC), which is the primary standard for civil timekeeping across the world. Unlike TAI, UTC includes leap seconds inserted at irregular intervals to keep it aligned with the Earth’s gradually slowing rotation. Since 1972, UTC has accumulated 27 leap seconds, in addition to an initial 10-second difference at the adoption of TAI–UTC coordination, producing a current offset of 37 seconds. In 2022, the General Conference on Weights and Measures approved a plan to eliminate leap seconds by or before 2035, after which UTC and TAI will diverge by a fixed, permanent offset.

Operation and Realisation

Each participating laboratory broadcasts a frequency signal accompanied by time codes representing its realisation of atomic time. These signals are often expressed in the form of UTC for public dissemination; for example, UTC(NPL) corresponds to the National Physical Laboratory’s time signal in the United Kingdom. When expressed directly as atomic time without leap seconds, the scale may be denoted TAI(NPL). Laboratories may also maintain independent local atomic time scales that are not synchronised with TAI.
The International Bureau of Weights and Measures (BIPM), based in France, collects time comparison data from all participating clocks. Using this information, it retrospectively computes the weighted average that constitutes TAI. The official results appear monthly in Circular T, which provides authoritative tables of differences between TAI and the participating institutions’ time scales. When necessary, revisions or errata are published in subsequent circulars, but the canonical TAI scale remains fixed once issued.
The BIPM also publishes the Free Atomic Scale (EAL), an uncorrected version of the atomic time ensemble that lacks the relativistic adjustments applied to TAI.

Historical Development

Atomic timekeeping began experimentally in the 1950s. Early atomic clocks were not operated continuously but were used to calibrate quartz oscillators. The National Physical Laboratory in the United Kingdom first developed a caesium atomic clock in 1955, forming the basis of the Greenwich Atomic time scale. In the United States, the Naval Observatory initiated the A1 time scale in 1956, while the National Bureau of Standards began the NBSA scale in 1957.
The International Time Bureau (BIH) established its own atomic time in 1955 using caesium clocks and low-frequency radio signal comparisons. These early scales were synchronised to a common epoch at the start of 1958. The ensuing decades saw refinement in atomic timekeeping methods, including the adoption of the SI second in 1967, defined using the caesium-133 atom.
Between 1971 and 1975, international resolutions established TAI as the standard atomic time scale maintained by the BIPM. During the 1970s, relativistic effects proved significant: clocks at different altitudes tick at different rates due to gravitational time dilation. In response, from 1 January 1977, corrections were applied so that TAI corresponded to proper time at the Earth’s geoid. The moment of this adjustment marks the epoch for the definitions of Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT), all of which were defined to have specific values at that instant. Since then, TAI has been regarded as a realisation of TT, related through the fixed offset:
TT = TAI + 32.184 s
Discussions in the early twenty-first century occasionally questioned whether TAI should continue to exist separately from UTC, particularly in light of proposals to abolish leap seconds. The retention of TAI, however, remains integral to high-precision scientific timekeeping.

Relationship to Coordinated Universal Time

TAI is a continuous time scale, whereas UTC is discontinuous due to the insertion of leap seconds. Between leap seconds, UTC differs from TAI by a constant whole-second offset. Before 1972, UTC used fractional adjustments to stay close to Universal Time 2 (UT2), but these were replaced by whole-second modifications to maintain UT1 within a defined tolerance. This compromise made international synchronisation easier while preserving alignment with Earth rotation, which remains essential for navigation and astronomical applications.
UTC therefore serves as a bridge between atomic time and rotational time, while TAI provides the underlying uniform metric essential for scientific measurement.

Significance

International Atomic Time underpins modern systems requiring precise and stable timekeeping, including satellite navigation, telecommunications, metrology, and high-energy physics. Its long-term stability and global coordination ensure consistency across national laboratories and international scientific programmes. By providing a time scale independent of the Earth’s variable rotation, TAI represents one of the most reliable standards in contemporary science, enabling measurements that require accuracy at the level of nanoseconds and beyond.