The Leap Second: Patching Earth's Slowing Spin

Two clocks that refuse to agree

Before 1967, a "second" was simply a slice of the day — 1/86,400 of one Earth rotation. That definition had a fatal flaw: the day itself is not constant. So in 1967 the international community redefined the second physically, not astronomically. One second is now 9,192,631,770 periods of the microwave radiation emitted when a caesium-133 atom flips between its two hyperfine ground states. A good caesium fountain clock keeps this beat to better than one part in 10¹⁵ — it would drift by less than a second over tens of millions of years.

This gave humanity a clock that ignores the Earth entirely. Stack up 9,192,631,770 caesium cycles 86,400 times and you get an atomic day of exactly 86,400 SI seconds. The trouble is that the real Earth rarely spins in exactly 86,400 SI seconds. It usually takes a hair longer — a couple of milliseconds — because the planet has been slowing down. Those milliseconds are invisible in a single day, but they accumulate. After about 500 to 800 days, the gap between atomic time and rotation time swells past a full second.

Coordinated Universal Time (UTC) is the diplomatic compromise between the two. UTC ticks with atomic precision — its seconds are pure caesium seconds — but whenever it threatens to drift more than 0.9 seconds away from UT1, the time scale tied to Earth's actual rotation angle, timekeepers insert a leap second to yank it back into alignment. The leap second is the seam where a perfect clock is stitched onto an imperfect planet.

Why Earth's day keeps getting longer

The dominant reason the day lengthens is the Moon. Lunar and solar gravity raise tidal bulges in the oceans and, more subtly, in the solid Earth. Because the planet spins faster (once every ~24 hours) than the Moon orbits (once every ~27.3 days), friction drags these bulges slightly ahead of the Earth–Moon line. The misaligned bulge exerts a gravitational torque that acts like a brake on the spin — and, by conservation of angular momentum, it flings the Moon outward.

The numbers are astonishingly consistent. Lunar laser ranging shows the Moon receding at about 3.8 cm per year, and tidal braking lengthens the day by roughly 2.3 milliseconds per century. That figure is partly offset by glacial isostatic adjustment — the solid Earth still rebounding from the melting of the last ice age's ice sheets, which redistributes mass toward the poles and speeds the spin slightly, like a skater pulling in their arms. Netting these effects, atomic clocks and ancient eclipse records agree that the mean day has grown by about 1.7 ms per century over the past few thousand years.

It sounds trivial, but integrate it. If Earth had kept perfect 86,400-second days since Roman times, our clocks would now be running several hours ahead of the Sun. Babylonian and Chinese records of total solar eclipses — where the shadow track lands only if you know exactly how far Earth turned — are among the best evidence we have for this multi-millennium slowdown, a rare case where a 2,500-year-old astronomer's note constrains a modern geophysical constant.

How a leap second is actually inserted

The custodian of the leap second is the International Earth Rotation and Reference Systems Service (IERS), headquartered in Paris. Using Very Long Baseline Interferometry on distant quasars and global satellite navigation data, the IERS measures the Earth's rotation angle to microsecond precision and tracks the running quantity UT1 − UTC. When that difference is heading toward the 0.9-second limit, the IERS issues Bulletin C, its twice-yearly decree, about six months in advance.

A positive leap second is added at the end of June 30 or December 31, expressed in UTC. The last second of the day is not the usual 23:59:59 → 00:00:00; instead the clock reads:

• 23:59:59
• 23:59:60 — the leap second, a legal 61st second of the minute
• 00:00:00 of the next day

Every leap second inserted so far has been positive — the mechanism can in principle skip a second (a "negative" leap second, jumping 23:59:58 straight to 00:00:00), but that has never been needed. Each insertion widens the fixed offset between International Atomic Time (TAI) and UTC. TAI ran 10 seconds ahead of UTC when the modern system began in 1972; after 27 leap seconds it now runs a full 37 seconds ahead. GPS time, frozen to TAI − 19 s in 1980, is likewise 18 seconds ahead of UTC and completely ignores leap seconds — which is precisely why satellite navigation quietly carries its own second-count in the broadcast message.

The plot twist: Earth is now spinning faster

For decades the leap-second machinery only ever ran one way — adding seconds to keep pace with a slowing planet. Then, around 2020, something reversed. Earth began setting speed records. On June 29, 2022 the planet completed a rotation about 1.59 ms short of 86,400 seconds; on July 5, 2024 it shaved off 1.66 ms — the shortest day ever measured with atomic clocks. The summers of 2025 and beyond continued to produce unusually fast days.

The causes of this recent burst are only partly understood and genuinely debated. Contributors likely include the sloshing of Earth's molten outer core exchanging angular momentum with the mantle, shifts in ocean and atmospheric circulation, and the poleward redistribution of mass. Superimposed on all of it are predictable short-period wobbles driven by the Moon's changing declination — the planet spins slightly quicker when the Moon rides far north or south of the equator.

The consequence is unprecedented: for the first time, timekeepers face the prospect of a negative leap second — deleting a second rather than adding one. Geophysicist Duncan Agnew argued in a 2024 Nature paper that the first negative leap second could be needed around 2029. Strikingly, he noted that melting polar ice — moving mass from the poles toward the equator and thus slowing the spin — has been counteracting the core's acceleration and has likely delayed the negative leap second by about three years. Climate change, in other words, is subtly rewriting our clocks.

Why one second breaks so much software

A single leap second seems harmless. For computer networks it has been a recurring catastrophe. Most software assumes minutes have exactly 60 seconds and time never runs backward. Confront it with a 23:59:60 or an unexpectedly repeated timestamp and things break in interesting ways.

The June 30, 2012 leap second was the worst episode: a Linux kernel bug sent CPUs into a spin-loop when the extra second arrived, and services including Reddit, LinkedIn, Yelp, Mozilla, and the Qantas airline reservation system stumbled or crashed simultaneously. The industry's workaround is the leap smear — pioneered publicly by Google — in which the extra second is not inserted all at once but spread thinly across many hours, slightly stretching each second so servers never see a 60th tick. Amazon, Meta, and others adopted variants. The catch: during the smear, those clocks are deliberately wrong by up to a second and disagree with everyone using standard UTC, which is intolerable for finance, radio astronomy, and navigation.

These recurring failures are the practical argument that finally killed the leap second. If a positive leap second could take down major websites, a never-before-attempted negative one — code paths that have literally never executed in production — terrified system engineers. "We do not know how to cope," as some timekeepers bluntly put it.

The end of an era — and what replaces it

In November 2022 the 27th General Conference on Weights and Measures (CGPM) — the treaty body that governs the SI units — passed Resolution 4, voting to increase the maximum allowed UT1−UTC tolerance no later than 2035, a change that effectively ends regular leap seconds. The exact new tolerance is to be proposed at the 28th CGPM in 2026; "abolish the leap second by 2035" is the popular shorthand for it. Once the tolerance is widened, UTC will be allowed to drift well beyond one second from UT1, and the correction will be batched up and applied only rarely — perhaps as a "leap minute" once a century, or whatever larger tolerance is finalized. Continuity of civil time will be preserved; the ragged, unpredictable one-second jolts will not.

The decision was contentious. Astronomers, satellite operators, and some national timing labs had defended the leap second precisely because it keeps clock time locked to the sky — pointing a telescope or a deep-space antenna requires knowing Earth's rotation angle to the millisecond. Their concern is that once UTC and UT1 are permitted to diverge freely, every application that needs true rotation angle must separately track UT1 − UTC, reintroducing exactly the bookkeeping the leap second was invented to hide.

There is a deeper irony. The leap second was created to defend the ancient link between the clock on the wall and the Sun in the sky. Abolishing it severs that link for the first time in history: from 2035, civil time will be a pure atomic construct, and the spinning Earth — the original clock, the thing that gave us the word "day" — will finally be demoted to a rough approximation. The planet's wobbly, tide-braked rotation will still be measured obsessively by the IERS. It just won't be allowed to set your watch anymore.

The three time scales the leap second knits together

Time scale	What it counts	Behavior
TAI (International Atomic Time)	Weighted average of ~450 atomic clocks worldwide	Perfectly uniform; never adjusted; runs 37 s ahead of UTC
UT1 (Universal Time)	Earth's actual rotation angle, from VLBI + GNSS	Wanders as the planet's spin speeds up and slows
UTC (Coordinated Universal Time)	TAI ticks, but with leap seconds inserted	Atomic-precise yet kept within 0.9 s of UT1