International Atomic Time

International Atomic Time (TAI, from the French Temps Atomique International) is the internationally recognized, high-precision time scale computed by the International Bureau of Weights and Measures (BIPM) from the combined readings of hundreds of atomic clocks operated by timing laboratories around the world. Its scale unit is the SI second, and it is the continuous reference from which Coordinated Universal Time (UTC), the basis of civil timekeeping worldwide, is derived; since 1 January 2017, TAI has stood exactly 37 seconds ahead of UTC. As a distributed, ensemble-based standard maintained by dozens of national institutes, TAI represents one of the most demanding achievements of modern metrology, providing a uniform time reference whose stability and accuracy underpin navigation, telecommunications, fundamental physics, and everyday life.

Background

Until the mid-twentieth century, time was defined astronomically. The apparent rotation of the Earth provided universal time (UT), while the orbital motion of the Earth around the Sun provided ephemeris time (ET), adopted in 1952 as a theoretically more uniform standard. Observations with precision quartz clocks in the 1930s and 1940s, however, revealed that the Earth's rotation is irregular: it varies seasonally, fluctuates unpredictably, and is gradually slowed by tidal friction. A time scale tied to the Earth's rotation therefore could not be perfectly uniform, and ephemeris time, although uniform in principle, was difficult to realize in practice because it depended on laborious astronomical observations.

The solution came from atomic physics. In 1949 the United States National Bureau of Standards (NBS, now NIST) demonstrated an ammonia absorption clock, and in 1955 Louis Essen and Jack Parry at the National Physical Laboratory (NPL) in the United Kingdom brought into operation the first practical caesium-beam atomic frequency standard. In collaboration with the United States Naval Observatory (USNO), the NPL instrument was used to measure the frequency of the caesium transition relative to the ephemeris second, yielding the value 9,192,631,770 cycles—later