YSK: the story that Earth is spinning faster and may need…
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YSK: the story that Earth is spinning faster and may need a negative leap second quietly reversed this year
For about five years the running headline has been that Earth is spinning faster than it should, that the record for the shortest day keeps falling, and that for the first time in history we might have to subtract a second from the clock instead of adding one. The 2026 numbers no longer say that, and the turn has barely been reported.
The measure behind all of it is length of day, or LOD, which is simply how many milliseconds a given rotation runs above or below 86,400 seconds. The fast phase was real and it lasted years. 2020 produced the 28 shortest days of the atomic clock era up to that point, 2022 set a new record, and 2025 delivered another run of unprecedentedly short days, the shortest being -1.37 ms on 10 July. Then comes this year: working from IERS data in mid July, the shortest day of 2026 looks like about -0.65 ms on 26 July, roughly half of last year's shortfall.
The average matters more than the record, and the average has changed sign. Across 2025 the mean LOD was -0.08 ms, so a typical day ran slightly short. The current estimate for 2026 is +0.3 ms, meaning a typical day now runs long again. Leonid Zotov, a geodesist at Moscow State University, put it plainly: Earth has passed its highest velocity, and the deepest minimum of the past decade is behind us.
Source for all of the above: https://www.timeanddate.com/news/astronomy/earth-rotation-2026
Here is the part that makes this worth knowing rather than trivia. The world's timekeepers meet at Versailles from 13 to 15 October this year for the 28th General Conference on Weights and Measures, and the continuity of UTC is on the agenda: https://www.bipm.org/en/cgpm-2026
That process was set in motion by the 27th conference in 2022, and the acceleration scare was one of its stated motivations. So a decision that is weeks away was driven in part by an emergency that appears to have already peaked.
That does not make the decision wrong, and this is the bit I would not want anyone to take the other way. What frightens engineers about a negative leap second was never its size. It is that in more than fifty years of leap seconds every single one has been positive, so the negative branch of the code exists in a great many systems and has never once run in production anywhere. An untested time step is worth removing regardless of which way the rotation happens to be drifting this decade.
I would also not read the slowdown as settled. Forecasts more than about a year ahead are not reliable, because the largest wobbles come from the motion of the liquid core and from the atmosphere and oceans, and those are not understood well enough to predict. The 2026 figure is a mid year projection, not a closed book.
**Why YSK:** "Earth breaks record for shortest day" is one of those stories that runs every July almost regardless of what the underlying trend is doing, and this year the trend went the other way. If you would rather check than trust the news cycle, the IERS publishes the raw earth orientation data, and the timeanddate page linked above keeps a readable table of the shortest, longest and average day length for every year back to 1973.
#education
For about five years the running headline has been that Earth is spinning faster than it should, that the record for the shortest day keeps falling, and that for the first time in history we might have to subtract a second from the clock instead of adding one. The 2026 numbers no longer say that, and the turn has barely been reported.
The measure behind all of it is length of day, or LOD, which is simply how many milliseconds a given rotation runs above or below 86,400 seconds. The fast phase was real and it lasted years. 2020 produced the 28 shortest days of the atomic clock era up to that point, 2022 set a new record, and 2025 delivered another run of unprecedentedly short days, the shortest being -1.37 ms on 10 July. Then comes this year: working from IERS data in mid July, the shortest day of 2026 looks like about -0.65 ms on 26 July, roughly half of last year's shortfall.
The average matters more than the record, and the average has changed sign. Across 2025 the mean LOD was -0.08 ms, so a typical day ran slightly short. The current estimate for 2026 is +0.3 ms, meaning a typical day now runs long again. Leonid Zotov, a geodesist at Moscow State University, put it plainly: Earth has passed its highest velocity, and the deepest minimum of the past decade is behind us.
Source for all of the above: https://www.timeanddate.com/news/astronomy/earth-rotation-2026
Here is the part that makes this worth knowing rather than trivia. The world's timekeepers meet at Versailles from 13 to 15 October this year for the 28th General Conference on Weights and Measures, and the continuity of UTC is on the agenda: https://www.bipm.org/en/cgpm-2026
That process was set in motion by the 27th conference in 2022, and the acceleration scare was one of its stated motivations. So a decision that is weeks away was driven in part by an emergency that appears to have already peaked.
That does not make the decision wrong, and this is the bit I would not want anyone to take the other way. What frightens engineers about a negative leap second was never its size. It is that in more than fifty years of leap seconds every single one has been positive, so the negative branch of the code exists in a great many systems and has never once run in production anywhere. An untested time step is worth removing regardless of which way the rotation happens to be drifting this decade.
I would also not read the slowdown as settled. Forecasts more than about a year ahead are not reliable, because the largest wobbles come from the motion of the liquid core and from the atmosphere and oceans, and those are not understood well enough to predict. The 2026 figure is a mid year projection, not a closed book.
**Why YSK:** "Earth breaks record for shortest day" is one of those stories that runs every July almost regardless of what the underlying trend is doing, and this year the trend went the other way. If you would rather check than trust the news cycle, the IERS publishes the raw earth orientation data, and the timeanddate page linked above keeps a readable table of the shortest, longest and average day length for every year back to 1973.
#education
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