Earth Is Spinning Faster: The Shortest Days on Record
For most of history the Earth has been slowing down, and clocks added leap seconds to keep up. Since about 2020 it has been speeding up instead — fast enough that timekeepers feared the first negative leap second ever.
8 November 2026 · 9 min read · Science
For more than fifty years the story of the Earth's spin ran one way: the planet was slowing down, the day was slowly getting longer, and every few years timekeepers added a leap second to let the clocks wait for it. Then, around 2020, the trend flipped. The Earth started turning faster, and in the summers since it has set one short-day record after another.
That reversal is the hidden reason the world voted in October 2026 to abolish the leap second. The fear was not another slow year — it was the opposite: a planet spinning fast enough to need a negative leap second, a second removed from the clock, something that has never been done.
The shortest day on record
A mean solar day is meant to be 86,400 seconds long. It almost never is exactly that. The International Earth Rotation and Reference Systems Service (IERS), which measures the planet's spin against distant quasars, records a tiny daily surplus or shortfall, usually under a couple of milliseconds.
On 5 July 2024, that shortfall hit 1.66 milliseconds — the shortest day since precise atomic measurements began in the 1960s. It beat a run of records set over the previous four years.
| Date | Day shorter than 86,400 s by | Note |
|---|---|---|
| 19 July 2020 | about 1.47 ms | Record at the time (IERS: 1.4602 ms) |
| 29 June 2022 | about 1.59 ms | New atomic-era record |
| 5 July 2024 | about 1.66 ms | Shortest day on record |
| 10 July 2025 | about 1.36 ms | One of several near-record short days in 2025 |
The summer of 2025 produced several more unusually short days — around 9 July, 22 July and 5 August — but none quite matched the 2024 mark. The headline figures come from IERS data, reported by Sky at Night, Space.com and timeanddate.com among others.
A word on wording: these are the shortest days since atomic clocks began keeping daily records, not the shortest in the planet's history. Geologically the opposite is true — see below.
The long game: the Moon has been braking us for aeons
Zoom out, and the Earth is unmistakably slowing. The Moon raises tides, and the friction of those tides drags on the planet's rotation, bleeding spin into the Moon's orbit. The Moon drifts away about 3.8 centimetres a year, and in exchange the Earth's day lengthens.
Records of ancient solar eclipses, whose timing and location only make sense if the day was shorter long ago, put this braking at roughly 2.3 milliseconds per century averaged over the last few thousand years. Hundreds of millions of years ago a day was closer to 22 or 23 hours; corals and shells from that era carry daily growth bands that record it.
That long slowdown is why, when atomic time was defined in 1967, the Earth was already lagging behind — and why every leap second ever added has been a positive one. More on those in leap seconds are ending.
So why is it suddenly speeding up?
This is the honest part: nobody is sure. The recent acceleration is small, real and not fully explained. Scientists point to several candidates, none of them settled:
- The core. The Earth's liquid outer core and solid inner core can exchange angular momentum with the rocky mantle we live on. If the core slows, the surface speeds up. This core–mantle coupling is the leading suspect for the multi-year trend.
- Winds and weather. Seasonal shifts in the atmosphere, especially the jet streams, move enough mass to change the day by fractions of a millisecond. NASA researchers attribute roughly 90 per cent of the fastest short-term wobbles to the atmosphere. This is what makes the record short days cluster in July and August.
- The Moon's position. On days when the Moon sits far from the equator, it tugs the spin slightly faster — which is why specific dates could be forecast in advance.
Notably, one familiar culprit is ruled out: no great earthquake coincided with the speed-up. And a second, climate change, actually pushes the other way. NASA-funded studies find that melting polar ice and shifting groundwater have moved water towards the equator, slowing the planet by about 1.33 milliseconds per century since 2000 — the spin-skater effect, arms spreading out. Without that drag, the Earth would be turning faster still.
What a negative leap second is
Here is the mechanism that worried timekeepers. Civil time (UTC) ticks at the steady atomic rate. Astronomical time (UT1) follows the real planet. The two are kept within 0.9 seconds of each other.
- If the Earth runs slow, UT1 falls behind UTC, and a positive leap second is inserted to let civil time pause. This has happened 27 times since 1972; the last was 31 December 2016.
- If the Earth runs fast, UT1 pulls ahead of UTC, and the only fix is to remove a second — a negative leap second. It has never happened.
A negative leap second would mean a minute running 23:59:58 → 00:00:00, skipping the 59th second entirely. Software that has never once seen a missing second — and barely survived the extra ones — would be tested in a way it never has been.
Worked example: how a millisecond a day adds up
Why does a shortfall of a millisecond matter? Because it accumulates.
- Suppose the day ran a steady 1 millisecond short of 86,400 seconds.
- Each day, astronomical time gains 1 ms on the atomic clock.
- To reach the 0.9-second trigger: 0.9 s ÷ 0.001 s/day = 900 days, about two and a half years.
Real days vary too much for so clean a sum, but the logic is exactly what the IERS watches: enough fast days in a row, and the drift would cross the line the wrong way.
Why the world chose to abolish it instead
In 2024, geophysicist Duncan Agnew of the Scripps Institution of Oceanography published a study in Nature projecting that, on current trends, a negative leap second would be needed around 2029 — and that, were it not for the slowing effect of melted ice, it would already have been needed by 2026. The BIPM's Patrizia Tavella warned that waiting until 2035 to act carried about a 30 per cent chance of facing one first.
Rather than attempt a dangerous subtraction, the 28th General Conference on Weights and Measures, meeting in Versailles on 13–15 October 2026, voted to retire the leap second altogether. From 20 May 2027, UTC becomes continuous and the allowed gap to astronomical time is widened to one hour. The full decision is covered in our guide to the 28th CGPM.
Can you feel it?
No. A millisecond is a thousandth of a second; the records of the last few years are smaller than a blink. Your watch, your calendar and your sense of a day are untouched.
Where it matters is at the top of the chain — satellite navigation, telecoms, financial trading timestamps and the networks that assume a second is a second. For them, the Earth's restless spin is not a curiosity but a thing to be measured, to the millisecond, every single day.
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