Metrology
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Day 26: The Astronomical Anchor Problem
If Einstein’s gift to us yesterday was that there is no universal “now,” today I want to come back down to Earth and revisit something more practical: how we measure time, and how we define the second.
We dug into the second itself back on Day 8. The short version is that there are two competing definitions, one astronomical and one atomic, and since 1972 they’ve drifted about 37 seconds apart. The gap is small, but it grows, and the system we built to manage it (leap seconds) is being dismantled. Today I want to look at what that means, and whether we should replace it with anything.
As you read this, I want you to sit with one question: what is civil time actually for? This isn’t a technical problem, it’s a values problem. What we anchor civil time to depends entirely on what we decide it’s for, and our civilization has only recently started changing its mind about that, mostly without noticing. More people should be paying attention.
Two definitions of the second
There are two ways to define a second.
The first is astronomical. A day is one rotation of the Earth, a second is 1/86,400 of that, and you divide the sun-up-sun-down cycle into uniform pieces. This is UT1, the modern name for “time measured by Earth’s rotation.” It’s what humans used for most of history.
The second is atomic. Since 1967 the official second has been 9,192,631,770 transitions between two energy levels of cesium-133 (we covered this on Day 8). This is TAI, International Atomic Time, and it’s defined without any reference to the Earth at all.
We know the Earth is a bad clock. Its rotation slows over geologic time, wobbles with the atmosphere and oceans, and drifts unpredictably on decade scales for reasons involving the molten core that we don’t fully understand. The astronomical second is not the same length from one day to the next.
The atomic second is, by construction, exactly the same length. It’s the most precisely defined quantity in human civilization, and it does not care what the Earth is doing.
So the atomic second became the official one, and UT1 has been drifting against it ever since. We invented UTC, the civil time on your phone, as a compromise: tick at the atomic rate, but insert a leap second whenever the gap from UT1 approaches 0.9 seconds. I wrote about the mess that caused on Day 19 and Day 20.
In 2035, leap seconds are being abolished… probably. After that, UTC will be allowed to drift from solar time by somewhere between one minute and one hour, to be decided by a 2026 vote. The astronomical anchor is being loosened, and maybe eventually cut entirely.
What “anchored to the sun” actually means
Civil time should be anchored to Earth’s rotation means two completely different things depending on who says it.
If a chronobiologist says it, they mean morning light should arrive at roughly the clock time when people wake up. That’s a circadian-health argument, the mechanism is real, and I dug into it on Day 21. But it’s an easy bar to clear: civil time only has to stay within about an hour of the sun. We already break that constantly. Time zones and daylight saving routinely shove the clock an hour or more off solar noon and mess with everyone’s circadian rhythm far more than a slow drift ever would, and society carries on. If anything, that’s a better argument that DST is dumb than that the anchor matters.
If an astronomer says it, they mean something more literal. Picture the Earth as a spinning sphere: at any instant you can calculate its orientation relative to the sun, and for most of history that was time. The official clock pointed at the Earth’s angular position because that’s all “time” ever meant. It’s a clean, logical definition, and it made complete sense for thousands of years. But it’s a definitional and cultural argument, not a biological one, and where the chronobiologist’s bar is easy to clear, this one is strict and expensive. With atomic clocks keeping time and UT1 tracking the Earth separately, anchoring civil time to that geometry makes far less sense now than it did at the dawn of civilization.
The metrologists' argument is that we can drop the astronomical anchor without losing anything humans actually depend on. We’ve all grown used to solar noon being noon, and that attachment is understandable, but in their view it’s swappable: let it go and you still have a stable, well-defined, engineering-grade measurement of time.
I don’t think they’re crazy. The tradeoff is real: we accept that civil time slowly comes unmoored from its position in the sky, and in exchange we get a clock that never needs correcting.
What is not decided yet
There are decisions still on the table. The 2022 vote killed the 0.9-second tolerance by 2035 but left the replacement open. After three years of the standards bodies working through options, the call lands this October: the 28th meeting of the CGPM, the General Conference on Weights and Measures, convenes in Versailles, France, and one item on the table is the new limit, how far UTC will be allowed to drift from solar time. The candidates range from keeping it nearly as tight as today to abandoning the anchor completely:
- 1 minute (no correction needed for about a century)
- 1 hour (a correction roughly every several thousand years)
- No limit at all (let civil time drift from the sun forever)
There is no neutral position here. The choice is really a vote on how much we still care about the sun.
Either way, astronomers lose the easy version of this. Today UTC doesn’t stray much from Earth’s rotation, close enough to read orientation straight off the civil clock. Once the tolerance loosens to a minute or more, that stops being true under every option on the table, so we will have to wire up UT1 directly to its own time signal.
The clock keeps clocking
Future generations will probably look back and ask why we did it this way. The honest answer is that time is complicated. Leap seconds made sense for part of our history, and they probably don’t make sense forever. The clocks will keep clocking either way.
Tomorrow: a survey of the prior attempts at fixing civil time. It has happened before, and we can learn a lot from how they failed.
Sources
- Atomic vs. Astronomical Time (TAI & UT1): International Earth Rotation and Reference Systems Service (IERS) and BIPM definitions.
- The Abolition of the Leap Second: Resolution 4 of the 27th General Conference on Weights and Measures (CGPM), 2022.
- Who is deciding the new tolerance: The CCTF Task Group on a continuous UTC, established by the BIPM in 2023 to draft the new maximum UT1−UTC value for the 28th CGPM (2026).
- Where it gets decided: The 28th General Conference on Weights and Measures, Versailles, 13–15 October 2026 — the meeting set to vote on the new maximum UT1−UTC value.
- The one-minute proposal: Levine, Tavella & Milton, “Towards a consensus on a continuous Coordinated Universal Time,” Metrologia, 2022 — argues a tolerance of about one minute keeps UTC within UT1 for roughly a century.
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Day 20: The Leap Second Is No More, Huzzah!
I know yesterday’s post was a real cliffhanger. Talking about leap seconds. I hate to burst your bubble, but today there’s going to be more of that when we cover the exciting and well-documented, internationally renowned meeting in November 2022.
I’m sure everyone totally knows about the General Conference on Weights and Measures (CGPM). Well, it turns out they have an important role as the world’s final authority on the metric system since 1875, a big deal. On that 18th in November 2022, they passed Resolution 4. It did one thing:
By or before 2035, the maximum value of the difference (UT1 − UTC) will be increased.
That means: we are abolishing the leap second.
This is a big deal. The leap second has been a feature of civil time for fifty years. Getting rid of it is the biggest change to “what time is it” since UTC itself got adopted in 1972. And it took almost twenty years of “discussion” to get there.
That “battle” is what today’s post is about. What got won and what got lost is way more interesting than the resolution text.
Two tribes, two ideas of “right”
Here’s the thing nobody tells you. There are two whole tribes of people whose job it is to care about what time it is. And they want completely different things.
Team Metrologists care about precision. Stability. The boring stuff. Their crowd is physics experiments, GPS satellites, financial exchanges, telecom networks, cloud data centers. To them, time is a standard of unit and measurement, like the kilogram or the meter. A measurement standard that inserts a random extra second is not a measurement standard. You’d never tolerate a gram that occasionally weighed an extra gram.
Team Astronomers care about Earth’s rotation. We should be on team astronomers every day, all the time. Their crowd is observatories, and the very old ideas that the day should obey man. To them, civil time should mean solar time. To them, UTC was a deliberate compromise. Atomic precision for the other team, plus a periodic correction so “noon” stayed put near the sun. Take away the correction and you’ve broken a contract that Noon is always in the middle of the day and midnight is always solar midnight. The fear is that the sun might rise at midnight at some point in the future.
Both are defensible? You can imagine serious people talking about serious things for decades at a time.
Well, guess what, the metrologists won, for now.
What actually moved the needle
The first real push to abolish the leap second came at the ITU-R (the radio-spectrum arm of the International Telecommunication Union) in 2004. The US, Japan, France, and Italy were the early supporters. The UK, China, and Russia opposed. The vote was delayed. And then delayed again. And then delayed again. Through 2012, 2015, 2019, the ITU-R kept punting because nobody could agree.
What actually changed the politics were the outages.
The 2012 Linux kernel meltdown that took Reddit, LinkedIn, and Qantas offline was the first global-scale “oh no this is real” moment. Then 2015 was quiet (smearing was working). Then 2016/2017 was Cloudflare. Then we had a long stretch with no leap seconds at all, the Earth started speeding up, and metrologists started seriously talking about the negative leap second. Nobody thought this was a good idea.
The fight stopped being “two professions disagreeing” and started being “metrologists vs a problem that might kill somebody.”
The vote got moved to the CGPM, a higher-status body than the ITU-R, sitting under the BIPM in Paris. In November 2022, in a single afternoon, the resolution passed.
The vote did not specify how leap seconds would be abolished. It just said the threshold would be increased, meaning UTC will be allowed to drift further from UT1 than the current 0.9-second limit, and that the new threshold would be decided by or before 2035.
The question that is up for discussion this year at the 28th CGPM in 2026 is: how much drift do we put up with before we have to do something about it?
Options on the table:
- One minute. UTC drifts from solar time by up to a minute, which takes roughly 100 years at current rates. Then we insert a “leap minute” once, in some coordinated worldwide event. Still sounds dangerous.
- 256 seconds. A computing-friendly number (it fits in an 8-bit field). Drift accumulates over ~400 years.
- One hour. Roughly 5,000 years out. By that point the leap is just “everybody shift one time zone for a few years until civil time re-syncs.”
- No threshold at all. Let UTC and UT1 diverge forever. Civil time eventually has nothing to do with the sun. The option astronomers hate most, and engineers find most tempting.
This is genuinely undecided. The drafting task force is still working on it. The 2026 vote will probably set the threshold, but the mechanism (how an eventual leap minute or hour actually gets inserted, how it’s announced, how software handles it) is still an open problem.
What happens when (or did they) metrologists win
The biggest immediate win is that POSIX becomes correct.
Quick recall from Day 13: Unix time is defined as “seconds since 1970-01-01 UTC,” with the deeply weird caveat that it pretends leap seconds don’t exist.
So now every day is exactly 86,400 seconds, full stop. This made Unix time a lie for decades. A lie that all of computing depends on.
By killing the leap second, the lie becomes the truth. UTC will, after 2035, actually tick uniformly.
POSIX’s definition starts being an honest description of what’s happening. Every piece of date-arithmetic code ever written that assumed
86400 * daysequals a stretch of days will be correct.It’s maybe a boring victory, if it’s just the absence of a problem. In 2035, time finally heals.
What astronomers lost
The astronomers, the people who built the leap-second system in 1972, lost a principle. A simple and very old one: noon should be when the sun is overhead. Midnight should be the middle of the night. That contract is older than civilization, and UTC was the deal that kept it intact through the atomic age.
Abolishing the leap second breaks that contract. Slowly. Over centuries. But permanently. Their fear is that eventually, not tomorrow, not in 100 years, but someday, the sun will rise at midnight.
Here’s what’s not lost. The astronomers' toolkit for actually measuring where the Earth is is excellent and getting better. They watch distant quasars with radio telescopes spread across continents and know where the Earth is to the millisecond. They’ll still know exactly when solar noon happens. They’ll just have to broadcast that signal themselves, almost certainly a UT1 service separate from UTC, instead of getting it baked into civil time.
What gets lost is the default. After 2035, civilization no longer says with “noon-is-noon” as a built-in promise. If you want it, it’s there, you just have to opt in.
So what’s the big deal?
The disagreement is just about time.
If civil time is a measurement standard: you optimize for precision and stability and you remove the ambiguity. The metrologists' answer is correct.
If civil time is a human social agreement about when noon is: you accept a little time wiggle, every now and then and who isn’t a fan of a little wiggle, in exchange for keeping noon attached to the sun. Well then, the astronomers' answer is correct.
I can see how both answers are answers, but I think they are answers to different things.
Computers and the people who use them.
For systems, all our ways of tracking time, the metrologists are correct. End of story. All the GPS satellites, financial exchanges, telecom networks, cloud data centers need a clock that just ticks; uniformity, no surprises, no funny business. This is something the 2022 vote got correct.
But what is Civil time if its' not for the people? And maybe there’s still a way for everybody to win. Keep the systems on uniform atomic time, no discontinuities, no outages, and let civil time keep its old promise. Noon stays near the sun. Even if we have to do something clever every century to make it work. A leap minute. A coordinated time-zone shift. Whatever it takes.
The systems fight are over but boy do we got some stuff to figure out with that civil time.
Tomorrow we get into the real mud, the dirt of it. What everyone hates….
The case against daylight saving time.
Sources
- Resolutions of the 27th CGPM (2022) — BIPM. The official text of Resolution 4, which increases the tolerance for UT1-UTC and effectively abolishes the leap second by 2035.
- The End of the Leap Second — Nature (2022). Comprehensive coverage of the historic vote at Versailles and the twenty-year debate between metrologists and astronomers.
- Abolishing the Leap Second — The New York Times (2022). Explains the tension between the ITU-R, the CGPM, and the risks of the negative leap second.
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Day 9: A Clock You Can Use to Measure a Mountain
Yesterday I said cesium was about to be deposed. Today’s the deposition.
The successor is the optical lattice clock. It is so precise that, it can detect the altitude difference between two desks in the same office.
That last sentence sounds crazy but Its what optical clocks do.
Here’s how.
More ticks per second
The trick of the optical lattice clock is simple in concept: use a finer ruler.
A cesium atom oscillates at about 9 × 10⁹ Hz (~9 GHz, microwave). That’s how many “ticks” per second the clock has to count to keep time.
A strontium-87 atom has a clock transition at about 4 × 10¹⁴ Hz (~430 THz, visible light). That’s 50,000 times more ticks per second.
Same second. More tick marks on the ruler.
Faster clock makes better time.
Faster clocks are better than slower ones.
Faster clocks make for better timekeeping.
See the difference and why we need to upgrade the second?
Today’s best optical clocks drift about one second over the age of the universe. 13.8 billion years. Our best cesium clocks are nowhere near that, drifting the same second in roughly 300 million years instead. About a hundred times worse.
The magic wavelength trick
So if optical is obviously better, why didn’t we do this first?
Because it’s hard. Two reasons, really.
First, you can’t just point an optical clock at a free-floating atom. Atoms in flight have all sorts of velocity, Doppler-shifting the frequency you measure. You have to hold them still. But the only way to hold an atom still is to grip it with something, and gripping it with a laser changes the energy levels you’re trying to measure. Catch-22.
Second, even if you could pin the atom, you couldn’t count that fast. Microwave at 9 GHz fits inside what electronics can divide down and tick off. Visible light at 400+ THz does not. We had no way to count optical-frequency oscillations until the optical frequency comb showed up in 1999, work that won Hänsch and Hall the 2005 Nobel.
In 1967 when the CGPM picked cesium, none of this existed. The laser was seven years old. Laser cooling wouldn’t be demonstrated until the late 1970s. Frequency combs were 30+ years away. Cesium at 9 GHz wasn’t the best clock you could imagine. It was the best clock you could build.
The fix came in 2003 from a Japanese physicist named Hidetoshi Katori. He proposed trapping the atoms in a standing-wave laser pattern, a 3D optical lattice, like an egg crate made of light, and tuning the lattice laser to a specific frequency called the magic wavelength.
At the magic wavelength, the lattice light affects both energy levels of the clock transition by exactly the same amount. The trap is invisible to the clock. The atoms are held still, but the energy levels they emit at are unperturbed. You get to measure the atomic transition cleanly while the atom sits frozen in midair.
Optical lattice clocks are a beautiful piece of physics. Do not ask me about the math behind them.
Which atom?
Two main candidates have been thinking about for the title of “next SI second”:
- Strontium-87. Run in optical lattices by labs around the world: NIST/JILA in Boulder, NPL in the UK, RIKEN in Japan, SYRTE in Paris. About 1 part in 10¹⁸ uncertainty. Currently the favorite.
- Ytterbium-171. Comparable accuracy, different sensitivity profile (different ways the clock can go wrong, which is good for cross-checking against strontium).
There are also trapped-ion optical clocks, which hold a single ion in an electric field instead of a lattice of neutral atoms. The aluminum-ion logic clock at NIST has hit about 1 part in 10¹⁹. That’s one second of drift in 300 billion years. The amount of drift is older than the universe itself. That sounds pretty good to me.
What this gets you
Optical clocks are sensitive enough to see general relativity at human scale.
What does that mean? It means you can put two clocks on different shelves and watch one tick slower than the other.
Einstein said clocks deeper in a gravitational field run slower than clocks higher up. Near Earth, the effect is about 1 part in 10¹⁶ per meter of altitude. A clock on the floor runs slower than a clock on the table. For most of human history this was a theoretical curiosity. Now it’s a measurement.
In 2022, a JILA strontium clock measured a 1 millimeter height difference as a frequency shift. One millimeter. The thickness of a credit card. The clock could tell which side of the card it was sitting on, from gravitational time dilation alone.
This opened a new field: relativistic geodesy. Put an optical clock at two locations, compare frequencies, and you’ve directly measured the gravitational potential difference between them. Detect underground oil, magma movement, ice sheet melt, anything that shifts mass around the planet.
Cesium can’t do this.
The next “second”
The BIPM is planning to redefine the SI second around an optical transition by 2030. Same dance as 1955: measure the new number against the current cesium standard, freeze it at that precision, declare it the new second.
Four layers of backwards-compatibility instead of three. The fingerprints accumulate.
Where this goes
We’ve now spent three days on how a second is built. The next question is the one the calendar dodges: which second?
You need a reference point. A zero. An origin.
Tomorrow we’ll talk about where you start counting from, and why astronomers picked noon on January 1, 2000.
Sources
- Optical clock — Wikipedia
- Hidetoshi Katori — Wikipedia
- Quantum logic clock — Wikipedia
- Relativistic geodesy — Wikipedia
- Bothwell et al., Nature 2022 — millimeter-scale gravitational redshift
- Gravitational time dilation — Wikipedia
- Jun Ye — Wikipedia
- Epoch (astronomy) — Wikipedia
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Day 8: 9,192,631,770
Today’s number is 9,192,631,770.
That’s how many times a cesium-133 atom oscillates between two specific energy states in one second. Or, more precisely, it’s how many times we decided a cesium atom does that in one second. The number is doing some heavy lifting that the universe didn’t ask for.
This post is about the number, how it got picked, and how we use it to measure time.
The chain
To know what a second is, you need a way to measure one. For most of human history, the answer was: “look at Earth and count” or earth++ for the programmers reading this.
One second = 1/86,400 of a mean solar day.
That worked for sundials and pendulum clocks. It stopped working in the 1930s, when good quartz clocks revealed that Earth’s rotation isn’t uniform. Yesterday, we talked about how. Earth’s rotation is not a good measurement of time. I mean good in the sense of precise, because there are so many variables that go into the rotation and impact it. We don’t have an accurate way to measure and predict how those variables change.
In 1960, astronomers tried again. Because why not? Wheel, reinvented.
They picked Earth’s orbit instead of its rotation, which is far more stable:
One second = 1/31,556,925.9747 of the tropical year for 1900.
That weird denominator was chosen specifically so the new “ephemeris second” came out the same length as the old mean solar second. Don’t break clocks … like ever.
Great in principle, terrible in practice. To know what time it was, you had to consult a 60-year-old astronomical table. It doesn’t work with the clock in the lab. The ephemeris second won the philosophy and lost the engineering.
But wait, maybe there’s a third option.
How I wish it was Cesium-123 and not Cesium-133, but alas. The 133 is the mass number: 55 protons + 78 neutrons. Cesium-133 also happens to be the only stable isotope of cesium, so it kind of picked itself.
The cesium handoff
Cesium-133 has two slightly different ground-state energies. Drop an atom from one to the other and it spits out a photon at a specific frequency, call it f. If you can build a clock that locks itself to that frequency, you have a timekeeper that doesn’t depend on the Earth, the Sun, or any astronomer’s table. Every cesium-133 atom in the universe agrees on f to absurd precision, because quantum mechanics doesn’t have local variants.
The number for f is approximately 9.192631770 GHz. So if you count 9,192,631,770 oscillations, exactly one second has elapsed.
In 1967, the General Conference on Weights and Measures (the CGPM, the body that gets to define units) voted to make that the official definition. The SI second became:
The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
No Earth, no Sun, no tables. Just an atom.
Why that exact number
The number 9,192,631,770 is not a fundamental constant of the universe. It was measured. By two specific guys. In 1955.
Their names were Louis Essen and Jack Parry, and they worked at the National Physical Laboratory in the UK. They had just built the first cesium clock that actually worked. The question was: how many cesium ticks fit in one ephemeris second? They measured it carefully. Their answer, published in 1958 with William Markowitz and Robert Hall:
1 ephemeris second = 9,192,631,770 ± 20 cesium periods.
Nine years later, the CGPM took that number, dropped the uncertainty, and made it the definition. From then on, the second was defined as exactly 9,192,631,770 ticks. The plus-or-minus twenty disappeared.
This means three things:
- The cesium second was chosen so it would equal the ephemeris second.
- The ephemeris second was chosen so it would equal the mean solar second.
- The mean solar second was 1/86,400 of an Earth day.
The three layers of don’t break the previous standard. If Essen and Parry had been off by one in their measurement, the entire SI second would be slightly different today, and every clock and computer and GPS satellite would be calibrated to that other version.
The number is a historical fingerprint, not a physical constant.
It records, with eleven decimal-digit precision, exactly how well two physicists in 1955 could compare a cesium clock to an Earth-orbit calculation.
Every GPS satellite, every NTP server, every timestamp on every photo you’ve ever taken, they’re all calibrated to the limits of what Essen and Parry pulled off with 1955 equipment. The universe doesn’t care about your iPhone. Your iPhone cares about two guys at the NPL.
How precise are we now
Modern cesium fountains, the latest generation, where atoms are laser-cooled and tossed gently upward through a microwave cavity, falling back down under gravity like a slow ballistic juggling act, hit accuracies of about one part in 10¹⁶. That’s roughly one second of drift per 300 million years.
NIST-F2 in Boulder, the U.S. primary clock, is one of these. About a dozen others sit in metrology labs across France, Germany, the UK, Japan, and China. They all report their measurements to the BIPM in Paris, which combines them into a weighted average called TAI, International Atomic Time. TAI is what your phone’s clock is ultimately disciplined to, through a long chain of NTP servers and GPS signals.
Everything you do that involves time:
- sending a message,
- taking a photo with a timestamp,
- syncing with a calendar,
- getting a stock trade priced
It all traces back to a few hundred atomic clocks averaging each other in real time.
What comes next
Cesium won’t be the way we measure the second forever. It’s about to be deposed.
Optical lattice clocks, using strontium or ytterbium atoms instead of cesium, operating at visible-light frequencies instead of microwave, are now about a hundred times more precise. They lose one second per about 30 billion years, which is roughly twice the age of the universe.
The BIPM is planning to redefine the SI second around an optical transition, possibly by 2030.
When that happens:
- Mean solar second = 1/86,400 of an Earth day
- Ephemeris second chosen to match the mean solar second
- Cesium second chosen to match the ephemeris second
- Optical second chosen to match the cesium second ← the new one
Three layers of backwards-compatibility will become four.
Tomorrow we’ll look at those optical clocks, what they are, how they work, and how they let us measure what cesium can’t.
Sources
- Louis Essen — Wikipedia
- William Markowitz — Wikipedia
- Second — Wikipedia
- Ephemeris time — Wikipedia
- Atomic clock — Wikipedia
- International Atomic Time — Wikipedia
- NIST-F2 — Wikipedia
- Optical clock — Wikipedia
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