← Daily DoseThe Slipping Second — Daily Dose No. 096
Daily Dose / Oct 22, 2026 / Earth's Spin
The Slipping Second
The Moon is braking the Earth, a few milliseconds at a time. Over deep time that has stretched an 18-hour day into a 24-hour one. Lately, though, the planet has been spinning faster, and melting ice is the only thing slowing it back down.
+1.8ms
Average lengthening of the day per century, measured from eclipse records 720 BC – AD 2015 — Stephenson, Morrison & Hohenkerk, Proc. R. Soc. A, 2016
01 · Deep time
A day that keeps getting longer
Fossil rocks and corals record a faster Earth: about 18.7 hours a day 1.4 billion years ago, about 22 hours when Devonian corals laid down ~400 daily rings a year.
*Illustrative extrapolation: +1.8 ms per century × 2 million centuries ≈ 3,600 s, about one extra hour. Real rates vary over geological time.
02 · The clock fix
Twenty-seven leap seconds, then silence
Since 1972, timekeepers have added a second to UTC 27 times to wait for the Earth to catch up. None has been needed since 31 Dec 2016.
Cumulative positive leap seconds inserted into UTC (excludes the initial 10 s offset set in 1972). Source: IERS Bulletin C. Dashed marker: Agnew (Nature, 2024) projects the first-ever negative leap second could be needed around 2029.
03 · Who's pushing the clock
The Moon brakes, and now the ice does too
Each force changes the day's length by a few milliseconds per century. Climate-driven melt is catching up with the Moon.
ms per century. Tidal and observed figures: Stephenson et al. 2016. Climate figures: Kiani Shahvandi et al., PNAS 2024. The 2100 value is a projection (±0.79) under high emissions. “Other” is the gap between tidal and observed, which the authors attribute to post-glacial rebound and core–mantle coupling.
04 · The jolts
Earthquakes and dams nudge the spin, in microseconds
Shifting mass toward Earth's axis makes it spin faster, like a skater pulling in their arms. Shifting mass outward slows it down.
−2.68 µs
Day shortened by the 2004 Sumatra–Andaman earthquake (M9.1)
NASA JPL · R. Gross, 2005 (modeled)
−1.8 µs
Day shortened by the 2011 Tōhoku earthquake, Japan (M9.0)
NASA JPL · R. Gross, 2011 (modeled)
+0.06 µs
Day lengthened by water held behind China's Three Gorges Dam
NASA · B. Chao, 2005 (modeled)
1 µs = one millionth of a second. All three are model estimates, too small to measure directly.
05 · The stakes
One second can take down a network
Software assumes every minute has 60 seconds. The planet doesn't keep that promise.
−1.66 ms
The shortest day on record, 5 July 2024. The record before 2020 was about −1.05 ms.
timeanddate.com / IERS data, 2024
~2029
The first negative leap second could be needed. Without polar melt it would have come in 2026.
Agnew, Nature, 2024
2035
The year leap seconds are to stop, after a vote by the world's metrology body
CGPM Resolution 4, Nov 2022
30 June 2012: one extra second
~1 HRThe Amadeus Altea airline booking system went offline, and Qantas and Virgin Australia checked passengers in by hand.
LINUXA timer livelock in the Linux kernel took down Reddit, LinkedIn, Mozilla, Yelp and Foursquare servers.
1 January 2017: the 27th second
DNSCloudflare code assumed time never runs backward. It hit a negative duration and some of its DNS resolution failed.
NEXTA negative leap second has never happened before, so a lot of time-handling code has never been tested against one.
06 · Why it matters
What you can do
If you build software
Use a monotonic clock for any time interval. Use leap-smeared NTP (Google, AWS and Meta all smear) so a leap second can't arrive as a sudden jump. Test what your code does with a 59-second minute.
If you're just curious
IERS Bulletin C announces every leap second about six months ahead. The length of the day is also a climate signal: the fact that ice melt is measurable in Earth's spin shows how much mass is moving from the poles to the oceans.