How an Atomic Clock Defines the Second

One second is the time it takes an atomic clock to count 9,192,631,770 microwave oscillations of a cesium atom. NIST says the second is currently defined this way, and the cesium resonance has defined it since 1967. NIST says atomic clocks define the second in a precise, measurable way that does not change over time.

1. Where the second came from

NIST explains that time cannot be measured directly. A clock measures intervals between two events, and "time" is the accumulation of those intervals.

The minute and second began in astronomy, not clockmaking. The Sumerians divided a circle into 360 degrees about four thousand years ago, and Babylonian astronomers and then the Greeks applied this to the sky. Around 150 C.E., Ptolemy proposed splitting each degree into 60 minutes and each minute into 60 seconds in his Almagest. Those units reached mechanical clocks in medieval Europe. Before that, people used hours of uneven length, such as Egyptian day and night periods that were longer in summer daytime, until monks built mechanical clocks to set fixed prayer and work periods.

2. What "counting cesium" means

Cesium atoms absorb and emit microwave radiation at one specific frequency. An atomic clock tunes a microwave signal until it matches that frequency, then counts the cycles. NIST states the number: 9,192,631,770 cycles make one second. The measured resonance is 9,192,631,770 Hz, so one hertz is one cycle per second.

3. How a fountain clock works

The United States' primary frequency standard is a cesium fountain clock at NIST in Boulder, Colorado, which contributes to calibrating Coordinated Universal Time. NIST currently operates two fountains, NIST-F3 and NIST-F4. F3 is built as a stable reference, and F4 is under evaluation. The steps NIST describes for these fountains are:

1. Cesium gas enters a vacuum chamber and six infrared laser beams slow the atoms into a cold cloud, near absolute zero.
2. Two vertical lasers gently toss the cloud about a meter upward, then all lasers are turned off.
3. The cloud rises and falls under gravity, passing through a microwave cavity twice, once up and once down. The round trip takes about half a second.
4. Atoms that changed their internal state are detected by laser-induced fluorescence.
5. The process repeats while the microwave frequency is tuned until the most atoms change state. That peak is 9,192,631,770 Hz.

The cooling is what makes it precise. Traditional cesium clocks measure room-temperature atoms moving at several hundred meters per second, so each observation lasts only a few milliseconds. Laser cooling reduces the speed to a few centimeters per second, giving about one second of observation, limited only by gravity pulling the atoms down. Longer observation makes it easier to tune the microwave frequency.

4. How accurate is it?

NIST says commercially available cesium clocks keep time to within 1/3,000,000 of a second per year. That is a drift of about 0.33 microseconds per year, so such a clock would take around 3 million years to gain or lose a full second.

Cesium fountains at metrology institutes can realize the definition with an uncertainty approaching 1 part in 10 to the 16th. Run for a year (about 31.6 million seconds), a clock with that fractional error would be off by roughly 3 billionths of a second. NIST adds that the next generation of frequency references, using laser cooling and ultra-stable lasers, approaches 1 part in 10 to the 18th, and that new experimental clocks based on other atoms are ten thousand times more precise than commercial cesium clocks.

5. Why it matters day to day

NIST notes that time organizes when buses, trains and planes arrive and lets scientists measure and better understand countless other things.

6. Key takeaways

• The second is 9,192,631,770 oscillations of a cesium atom's microwave resonance, a definition in use since 1967.
• The 60-minute, 60-second split goes back to Ptolemy, who divided each degree of a circle that way.
• Fountain clocks cool atoms with lasers and toss them up through a microwave cavity to get about a second of observation.
• Commercial cesium clocks stay within 1/3,000,000 of a second per year (NIST, as of October 2026).
• The next generation of frequency references approaches 1 part in 10 to the 18th (NIST).