Finding your position
GPS broadcasts precise timing information from satellite atomic clocks. A receiver uses signal timings to work out position and time. Ground control monitors clocks and provides corrections.
GPS.gov: timing← All toolsSmall atoms. A shared second.Count one second
Atomic clocks: the beat behind modern time
Your phone, a satellite and a science laboratory need a way to agree on a second. Atoms provide a remarkably repeatable reference.
From a steady beat to a clock
A clock needs something that repeats and a way to count repetitions. A pendulum swings; a quartz crystal oscillates. An atomic clock checks an electromagnetic oscillator against a particular transition between atomic energy states. Feedback keeps its frequency close to that atomic reference.
Generate a signal
An oscillator produces electromagnetic waves.
Ask the atoms
Measure how atoms respond near a chosen resonance.
Correct and count
Adjust the signal, count cycles, and turn the count into time.
Original process schematic, not a drawing of a particular clock. An atom does not contain tiny clock hands, and electrons are not little planets circling on fixed tracks. The useful reference is a quantum transition.
9,192,631,770 cycles
The SI second fixes a specified caesium-133 transition frequency at exactly 9,192,631,770 hertz. One hertz means one cycle per second. A clock realising that definition must control and correct the conditions affecting its measurement.
A six-second illustration represents your selected interval. The illustrated count is slowed down for learning.
1 second corresponds to 9,192,631,770 cycles at this reference frequency.
This is exact arithmetic using the defined frequency, not a live connection to an atomic clock. A second is a unit of elapsed time; the date and time of day require a time scale as well.
Why such precise time matters
Keeping networks coordinated
Communications and data networks need agreed timing to coordinate operations. Precise timestamps also help people compare records made in different places.
NPL: applications of precise timeTesting nature
Comparing clocks at different heights and speeds tests how gravity and motion affect elapsed time. Better clocks make smaller differences measurable.
NIST: testing relativityAccurate clocks still experience relativity
An atomic clock’s stability does not make it immune to motion or gravity. GPS needs relativistic corrections; a clock in an ISS-like orbit has a different balance of effects. Compare ISS and GPS clock rates and learn how the clocks are coordinated.
UTC provides a shared civil time scale. It is not simply the reading of one perfect master clock. Learn about UTC, GMT and leap seconds.
Ideas built over time
- 1949: an early ammonia-based atomic clock at the US National Bureau of Standards.
- 1955: NPL’s operational caesium-beam clock.
- 1967: the second receives an atomic definition.
These are different milestones, rather than competing claims for one invention day. NIST’s clock history · Explore scientific milestones in Time Machine.
Can atomic clocks ever be wrong?
Yes. A real instrument has measurement uncertainty and can be affected by its environment and electronics. Laboratories evaluate those effects and compare clocks. Different clock technologies use different atomic transitions; optical clocks use much higher frequencies than microwave caesium clocks. A higher frequency alone is not a guarantee of a better complete instrument.
NIST’s introduction · NPL’s overview. Reviewed 25 October 2026. Explanations and diagrams are original.