International Atomic Time

International Atomic Time (TAI) is a continuous time scale computed from a weighted average of atomic clocks maintained by the International Bureau of Weights and Measures. It has no leap seconds.

Atomic clock: A clock that measures time using the frequency of radiation associated with transitions in atoms. TAI draws on measurements from many atomic clocks around the world.
Frequency: The number of repetitions of a periodic event per unit time. Atomic clocks infer elapsed time by counting a highly stable periodic frequency.
Leap second: A one-second adjustment inserted into UTC to keep it near Earth’s irregular rotation-based time. Leap seconds change UTC’s offset from continuous TAI without interrupting TAI.
Louis Essen: A British physicist who built an early practical atomic clock using caesium. Essen’s clock demonstrated the precision that made atomic time scales feasible.
International Bureau of Weights and Measures: An intergovernmental organization that coordinates international measurement standards. The bureau calculates and publishes TAI from participating clock data.
Caesium standard: A frequency standard that realizes the SI second using the caesium-133 hyperfine transition. Primary caesium standards help calibrate the rates of clocks contributing to TAI.
Global Navigation Satellite System: A satellite-based system that provides positioning, navigation, and timing services worldwide. Satellite timing systems use atomic timekeeping and relate their scales to international references.
Harold Lyons: An American physicist who developed an early atomic clock based on ammonia. Lyons’s work helped establish atomic clocks as practical timekeeping instruments.
Coordinated Universal Time: The global civil time standard, based on atomic seconds and adjusted with leap seconds to track Earth rotation. UTC shares TAI’s atomic basis but inserts leap seconds; their offset changes over time.
Ephemeris time: A historical uniform time scale based on the orbital motions of celestial bodies. Ephemeris time was an earlier route to uniform time before atomic standards prevailed.

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