Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. It is a weighted average of the time kept by about 200 caesium atomic clocks in over 50 national laboratories worldwide. It has been available since 1955, and became the international standard on which UTC is based on January 1, 1972. The United States is the single largest contributor to TAI, with clocks maintained at both the National Institute of Standards and Technology and the United States Naval Observatory.

The highest precision realization of TAI times can only be determined retrospectively, as the timescale is defined by periodic comparisons among its participating atomic clocks, under the auspices of the International Bureau of Weights and Measures. However, these corrections are usually only needed for applications that require nanosecond-scale accuracy. Most time service users use realtime estimates of TAI provided by atomic clocks that have been previously referenced to the composite timescale. GPS is a commonly-used realtime source of time tracable back to TAI.

Coordinated Universal Time (UTC) is the basis for legal time worldwide, and always differs from TAI by an integral number of seconds. In mid 2005, UTC was behind TAI by 32 seconds. The difference is due to leap seconds, which are periodically inserted into UTC due to slight irregularities in the Earth's rate of rotation. While TAI is a continuous and stable timescale, UTC has intentional discontinuities to keep it from drifting more than 0.9 seconds from UT1, a timescale defined by the Earth's rotation. Roughly speaking, solar noon (the time at which the sun is directly overhead) would drift away from 12:00:00 without leap second corrections. UT1 is computed by the International Earth Rotation and Reference Systems Service (IERS). TAI was defined such that TAI = UT1 on January 1 1958.

Because UTC is a discontinuous timescale, it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval. Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that can not handle leap seconds.

Civil time in various regions around the Earth generally differs from UTC according to the time zone; mostly the increments are in whole hours, but in a few regions, such as parts of India, French Polynesia, and Newfoundland, half-hour increments are used. Nepal and parts of Australia and New Zealand have whole hour plus 45 minutes offsets from UTC.

See also

• Terrestrial Time
• Coordinated Universal Time
• Universal Time
• Sidereal Time
• Time and frequency transfer

External links

• Bureau International des Poids et Mesures
• http://hpiers.obspm.fr website
• NIST Time and Frequency FAQs