Atomic clock: the exact time now
The clock above shows atomic clock time: your device clock corrected against an internet time server that is synced to the atomic clocks behind UTC. The line under the time shows how far your device is off and the margin of error.
How an atomic clock works
Every clock counts something that repeats: a swinging pendulum, a vibrating quartz crystal. An atomic clock counts the vibration of electromagnetic waves that make electrons in cesium atoms flip between two energy levels. That frequency is a property of nature, identical for every cesium atom, so it never wears out or drifts. Since 1967 the official definition of the second has been 9,192,631,770 cycles of that radiation. How the second is defined explains more.
How atomic time reaches your screen
- National labs such as NIST (USA), PTB (Germany) and NPL (UK) run atomic clocks.
- The BIPM near Paris combines hundreds of clocks in about 80 laboratories into Coordinated Universal Time (UTC).
- Time servers pass it on over the internet with the Network Time Protocol (NTP), radio stations broadcast it, and GPS satellites carry their own atomic clocks.
- Your device syncs with a time server from time to time, and this page measures and corrects any remaining error.
Radio time signals
| Station | Location | Frequency | Used by |
|---|---|---|---|
| WWVB | Fort Collins, Colorado, USA | 60 kHz | "Atomic" wall clocks and watches in North America |
| DCF77 | Mainflingen, Germany | 77.5 kHz | Radio clocks in central Europe |
| MSF | Anthorn, UK | 60 kHz | Radio clocks in the UK |
| JJY | Japan (two stations) | 40 and 60 kHz | Radio clocks in Japan |
An "atomic" wall clock from a store doesn't contain an atomic clock. It holds a quartz movement and a small radio that listens to one of these stations, usually at night, and resets itself.
Milestones
- 1949: the first atomic clock (ammonia), US National Bureau of Standards.
- 1955: the first accurate cesium clock, by Louis Essen and Jack Parry at the National Physical Laboratory, UK.
- 1967: the second is redefined using cesium.
- 1972: UTC with leap seconds begins.
- Today: optical clocks are so precise that the second is expected to be redefined around them in the 2030s.
Questions people ask
What is an atomic clock?
A clock that counts the natural vibrations of atoms. Since 1967 the second has been defined as 9,192,631,770 cycles of the radiation from cesium-133 atoms, so a cesium clock measures the second directly.
How accurate is an atomic clock?
The best cesium fountain clocks, such as NIST-F2, would gain or lose about one second in 300 million years. Newer optical clocks using strontium or ytterbium are about 100 times better still.
How do I get atomic clock time?
This page shows the time corrected against an internet server that is synced, through the Network Time Protocol, to atomic clock time. Radio-controlled clocks receive it directly from stations such as WWVB in Colorado, and GPS receivers get it from the atomic clocks on GPS satellites.
Where is the atomic clock for the United States?
The US official time is kept by NIST in Boulder, Colorado, and the US Naval Observatory in Washington, DC. NIST broadcasts it from radio station WWVB near Fort Collins, Colorado, and online at time.gov.
Who made the first atomic clock?
The US National Bureau of Standards built an ammonia clock in 1949, but the first accurate atomic clock was the cesium clock built by Louis Essen and Jack Parry at the UK's National Physical Laboratory in 1955.
See also: exact time with milliseconds · leap seconds · the history of timekeeping