How atomic clocks work
Updated
Every clock counts something that repeats. A pendulum swings and a quartz crystal vibrates; an atomic clock counts a natural frequency of an atom. Since 1967 that count has been the definition of the second itself.
What a second is
The international definition fixes the frequency of one transition in the caesium-133 atom — the unperturbed ground-state hyperfine transition — at exactly 9,192,631,770 hertz. One second is the time taken by 9,192,631,770 cycles of the radiation from that transition. The General Conference on Weights and Measures adopted the caesium second in 1967, making it the first time standard based on atoms rather than on the motion of the Earth and the sky.
How the clock counts
An atomic clock is an oscillator steered by atoms. Microwaves close to the caesium frequency are sent into a sample of caesium atoms, and a detector counts how many atoms the microwaves changed. Those counts are used to tune the microwave frequency towards the exact atomic resonance. An electronic counter then counts 9,192,631,770 wave peaks of the tuned microwaves; the time that takes is one second.
Fountains of atoms
At room temperature atoms move fast, which leaves very little time to measure them. A fountain clock slows them first: laser beams from six directions cool a cloud of caesium atoms to near absolute zero, then a short pulse from two vertical beams tosses the ball of atoms about a metre upward. The microwaves then act on a slow-moving cloud instead of a buzzing swarm, which leaves far longer to measure.
How good they are
The first caesium clock stable enough to serve as a time standard was built by Louis Essen and Jack Parry at the UK's National Physical Laboratory in 1955. Essen called it "the death of the astronomical second and the birth of atomic time". The US fountain clock NIST-F1, from 1999, was accurate to within one second in 20 million years; NIST-F2, from 2014, to about one second in 300 million years. Optical lattice clocks using strontium set records in 2024 that would gain or lose less than one second in 30 billion years.
From hundreds of clocks to one time
No single clock keeps the world's time. International Atomic Time (TAI) takes its stability from more than 450 atomic clocks around the world, and its accuracy from a small group of frequency standards — in November 2021, sixteen of them in eleven laboratories. Coordinated Universal Time, UTC, is TAI with leap seconds inserted on the advice of the International Earth Rotation and Reference Systems Service, to keep it close to the Earth's rotation. Since 1 January 2017 UTC has been 37 seconds behind TAI, and no leap second will be added at the end of December 2026. More in UTC, GMT and leap seconds.
What it means for the countdown
Every time zone is defined as an offset from UTC, so midnight on 1 January anywhere is a UTC instant, and the seconds on the board are these atomic seconds. The board reads your device's clock: if the clock is right, the countdown is right.
Sources
- SI base unit: second — BIPM (definition, 9 192 631 770 Hz)
- A Brief History of Atomic Time — NIST (Essen and Parry 1955, 1967 definition, NIST-F1, NIST-F2, 2024 optical clocks)
- Fountains of Atoms: Exquisite Timekeepers — NIST, updated 22 August 2025 (how the counting and the fountain work)
- TAI news, 21 December 2021 — BIPM (more than 450 clocks; sixteen standards in eleven laboratories)
- Time metrology — BIPM (UTC from TAI by leap seconds on IERS advice)
- Bulletin C 72 — IERS, 6 July 2026 (UTC−TAI = −37 s since 1 January 2017; no leap second at the end of December 2026)