Is a second really 9.19 billion atomic ticks? How atomic clocks define time, and the 2030 redefinition

BOTH Chochim editorial team · November 12, 2026

Written as of November 12, 2026. The definition of the second and its history follow the BIPM's SI unit page and the resolutions of the 13th (1967) and 28th (2026) General Conference on Weights and Measures (English versions). How caesium fountain clocks work and how accurate they are come from NIST. The KRISS standard time service follows KRISS's official page; the ytterbium optical clock details come from press coverage, since we could not find them on an official page. The exact wording of the pre-1967 definition was not on the official pages we read, so we do not quote it.

A minute before tickets go on sale, you watch the seconds on your phone tick from 59 to 00. But who decided how long that second is, and with what? It is easy to assume it is the day divided into 86,400 pieces.

The answer is not in the sky but inside a tiny atom. The second the world uses today is the time it takes light from a caesium atom to oscillate 9,192,631,770 times, about 9.19 billion. Why caesium, how do you measure that, and why might even this definition change soon?

Why the second left the Earth for the atom

For a long time, timekeeping was based on the Earth's motion. The trouble is that the Earth is not a perfectly steady clock. As we saw in our leap second article, its rotation speeds up and slows down. A wobbly reference means a wobbly second.

So scientists looked for something that does not wobble. When an atom moves between set energy states, it always exchanges radiation at the same frequency. The same kind of atom behaves the same way in Seoul or in Paris. That is the starting point of the atomic clock.

According to NIST, the first atomic clock was an ammonia clock built at NIST in 1949, and the first practical caesium atomic clock was built at the National Physical Laboratory in England in 1955. In 1967, Resolution 1 of the CGPM defined the second using caesium, noting that the previous definition was inadequate for the needs of metrology at the time. The atom had become a better clock than the planet.

Counting 9.19 billion: inside a caesium fountain

"Counting 9 billion oscillations" sounds like someone counting very fast, but the idea is different. An atomic clock is closer to a device that finds the frequency caesium responds to best and locks onto it, like turning a radio dial until a station comes in perfectly clear.

Take NIST-F2, a U.S. time standard. Following NIST's description, six lasers gather about 10 million caesium atoms into a ball and cool them close to absolute zero. Lasers then toss the ball about 1.3 meters upward. The atoms pass through a microwave cavity on the way up, and gravity pulls them back through it on the way down. Because they rise and fall like water in a fountain, it is called a fountain clock.

Another laser then makes the atoms whose state was changed by the microwaves glow, and a detector counts that light. The frequency that changes the most atoms is caesium's natural frequency, 9,192,631,770 Hz. The cycle repeats thousands of times an hour. The atoms are cooled so they move slowly and spend longer with the microwaves; the longer you look, the more precisely you can tune.

How accurate is the result? When NIST launched NIST-F2 in 2014, it said the clock would neither gain nor lose one second in about 300 million years. Had it been running since before the first dinosaurs, it would only now be about a second off.

Then, in 2019, the definition was turned around

Here is a twist. Since 2019, the sentence that defines the second has a different structure. The 1967 version said, in effect, that the second is the duration of 9,192,631,770 periods of caesium radiation. The second came first, and caesium described it.

Today's BIPM definition works the other way. It fixes the caesium frequency at exactly 9,192,631,770 Hz, and the second follows from that number. Since hertz means oscillations per second, pinning the number pins the length of the second. According to the BIPM notes, this change was decided at the 26th CGPM in 2018 and took effect in 2019.

Did the second get longer or shorter? No. The same number is simply used from the other direction, so your everyday second is unchanged. What changed is the philosophy: the reference is now a fixed number, not an object or a measurement. And that approach leaves the door open for the next change.

Clocks 100 times better than caesium, and 2030

The microwaves in a caesium clock oscillate about 9.2 billion times a second. Visible light oscillates hundreds of trillions of times a second. Just as a ruler with finer marks measures length more precisely, a faster oscillation slices time more finely. That is the optical clock. In 2014, NIST already said optical clocks could lead to standards more than 100 times more accurate than caesium.

That has come true. Resolution 4 of the 28th CGPM in October 2026, "On the future definition of the second," notes that the best optical frequency standards surpass the accuracy of the current definition's realization by a factor of up to 100. Korea has one too: according to press reports, KRISS's ytterbium optical clock KRISS-Yb1, completed in 2021, contributes to world time, and in May 2026 KRISS began a study comparing it with the atomic clock on the International Space Station.

You may hear that "the second changes in 2030," but the resolution says something more careful. It asks for a proposal for the new definition and a proposal for its implementation date to be presented at the 29th CGPM in 2030. There is no consensus yet on which atom to use, or whether to combine several. Some mandatory criteria, such as optical clocks contributing to the calibration of International Atomic Time (TAI), are not yet met, though the resolution says they could realistically be met by 2030. So 2030 is a target for a proposal, not a done deal.

From the atomic clock to your phone

How does such a precise second reach your hand? National standards institutes run atomic clocks to keep their country's time and send their clock data to the BIPM. The BIPM combines them into TAI and Coordinated Universal Time (UTC), and its monthly Circular T reports, at five-day intervals, how far the time at about 80 institutes is from UTC. In Korea, KRISS keeps the national standard time.

And one last twist. A clock good to one second in 300 million years becomes much rougher by the time it reaches your phone over the internet. KRISS says that syncing to its time server (ntp.kriss.re.kr) is accurate to several milliseconds to hundreds of milliseconds, depending on the network. The path the signal travels matters far more than the atomic clock itself.

Chochim checkpoint For an on-the-dot click, what matters is not atomic precision but whether your device clock is seconds off standard time. See how to sync your phone and PC clock, and how Chochim measures server time for why the ticketing site's own clock is measured separately.

Frequently asked questions

What is the exact definition of a second?

Per the BIPM, it is defined by fixing the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom at 9,192,631,770 Hz. Put simply, one second is the time for that radiation to oscillate 9,192,631,770 times.

Since when has the second been defined by atoms?

Since Resolution 1 of the 13th CGPM in 1967. The current "fixed number" wording was decided in 2018 and took effect in 2019, without changing the length of the second.

Will the second change in 2030?

Not for certain. The 2026 Resolution 4 asks for a proposed new definition based on optical clocks, and a proposed implementation date, to be presented in 2030. Which atom to use is not yet agreed. Even if it changes, the everyday second will be effectively the same.

How accurate are atomic clocks?

NIST says the caesium fountain NIST-F2 is accurate to about one second in 300 million years. The 2026 resolution notes that the best optical clocks are up to 100 times better.

In short, the second you watch before a ticket drop comes not from the Earth's day but from caesium atoms cooled almost to absolute zero, and the standard is preparing to move to faster-oscillating light. On the clock in your hand, though, the network is a bigger variable than the atom. That is the one thing to remember before an on-the-dot click.

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