You might have noticed that some days feel just a little different, but the truth is far more precise than any human can sense. Earth has been turning faster than usual since 2020, shaving milliseconds off each day, and on July 22, 2025, the day will be 1.34 milliseconds shorter than the standard 24 hours. That tiny gap has big implications for the atomic clocks that run modern life, and it could force the first-ever negative leap second by 2029. Here’s what’s happening, why it matters, and what to expect next.

4 related posts

Shortest day ever measured: 1.66 ms faster on July 5, 2024 ·
July 22, 2025 day length: 1.34 ms shy of 24 hours ·
Potential first negative leap second: As soon as 2029 ·
Earth’s rotation variation: Milliseconds per day since 2020

Quick snapshot

1Confirmed facts
2What’s unclear
  • Exact cause of the speed-up is still debated (Space.com)
  • Whether a negative leap second will actually be enacted in 2029 (Organiser)
3Timeline signal
  • 2020: Earth’s rotation begins to speed up after decades of slowing (Space.com)
  • July 2024: Shortest day ever recorded (Space.com)
  • 2029: Potential negative leap second (Organiser)
4What’s next
  • First negative leap second could occur in 2029 (Organiser)
  • Long-term Earth rotation is still slowing (Space.com)
  • Scientists continue to monitor and model changes (Space.com)
Why this matters

The millisecond difference is invisible to us, but it forces atomic timekeepers to decide: skip a second for the first time, or risk a growing gap between human time and Earth time.

Key facts at a glance

Here are the essential numbers that define Earth’s accelerating spin.

Fact Value
Shortest day ever measured July 5, 2024 (1.66 ms faster) Source: Space.com
July 22, 2025 day length 1.34 ms shy of 24 hours Source: Organiser
Potential first negative leap second As soon as 2029 Source: Organiser
Speed trend since 2020 Increasing rotational speed (shortening days) Source: Space.com

Is spinning faster days getting shorter?

Yes, and the change is measurable: each rotation now finishes a few milliseconds ahead of schedule.

How much shorter are days?

  • The shortest day so far was July 5, 2024, when Earth spun 1.66 milliseconds faster than the standard 86,400 seconds (Space.com).
  • On July 22, 2025, the day will be 1.34 milliseconds short (Organiser).
  • Another projected short day on August 5, 2025 is estimated at 1.51 milliseconds fast (Organiser).

The pattern: While a millisecond is barely noticeable, the cumulative effect over months and years forces clock adjustments that modern infrastructure depends on.

What is the record shortest day?

The current record is July 5, 2024, with a rotation 1.66 milliseconds faster than 24 hours (Space.com). That day beat the previous record from July 2020 by a tenth of a millisecond.

This marks a dramatic reversal: for decades, Earth’s rotation had been gradually slowing, requiring the addition of leap seconds. Now timekeepers face the opposite problem.

Bottom line: Days are shorter by about 1–2 milliseconds, small enough for humans to miss but large enough to disrupt atomic timekeeping.

The implication: even a millisecond shift forces global timekeepers to consider unprecedented clock adjustments.

What is causing the Earth to spin faster?

The speed-up stems from several geophysical forces that can accelerate or decelerate the planet’s spin on shorter timescales.

Core-mantle coupling

  • Fluctuations in the flow of Earth’s liquid outer core can exchange angular momentum with the mantle, speeding up or slowing down rotation (Space.com).
  • This “core-mantle coupling” is considered a primary driver of multidecadal rotation changes.

Climate change and glacial rebound

  • Melting ice sheets redistribute mass from land to oceans, altering Earth’s moment of inertia—a process that can affect spin (Space.com).
  • Glacial isostatic rebound, where continents rise after being weighed down by ice, also subtly changes the planet’s shape and rotation.

Ocean currents and atmospheric pressure

  • Large-scale ocean currents and atmospheric circulation patterns can push or slow the planet by transferring angular momentum (Space.com).
  • The Chandler wobble—a small oscillation of Earth’s rotational axis—also contributes to millisecond-level variations.

The trade-off: The same geophysical system that produces the current speed-up will eventually slow Earth down again, but the timing is uncertain.

The paradox

Earth is both speeding up in the short term and slowing down over millennia due to tidal friction from the moon (Space.com). The net effect for coming decades is the one that matters for timekeeping.

The catch: predicting the net direction over the next decade remains a scientific challenge.

Will we lose a second in 2029?

Possibly—the first “negative leap second” would remove a second from Coordinated Universal Time (UTC), a change that has never been implemented before.

What is a negative leap second?

Since 1972, leap seconds have been added to UTC to keep atomic time in sync with Earth’s slowing rotation (Organiser). A negative leap second would subtract a second instead—essentially skipping one second on our clocks.

Why 2029 is significant

  • IERS, the International Earth Rotation and Reference Systems Service, predicts that if the current speed-up continues, Earth and atomic time will diverge by more than 0.9 seconds by 2029 (Organiser).
  • The threshold for a leap second adjustment is 0.9 seconds; crossing it would trigger the first negative leap second.

How timekeepers decide

The IERS monitors Earth’s rotation using a global network of telescopes, satellites, and atomic clocks (Organiser). They announce leap seconds months in advance, typically for June 30 or December 31.

What this means: A negative leap second would be a historic first—and a stress test for systems that were built assuming time only ever moves forward.

Bottom line: The decision rests with IERS. For network engineers and financial exchanges, the risk of a skipped second is already a contingency they need to plan for.

The pattern: the need for a negative leap second is unprecedented and will test global coordination.

Is July 22 the shortest day?

Not quite, but it will be one of the shortest—ranked second after the July 5, 2024 record.

How short is July 22, 2025?

That day will be 1.34 milliseconds shorter than the full 24-hour standard (Organiser). It’s the second-shortest day in the modern measurement era.

Comparison to previous records

  • July 5, 2024: 1.66 ms fast (record)
  • July 22, 2025: 1.34 ms fast (second place)
  • August 5, 2025: projected 1.51 ms fast (Organiser)

Why it’s happening

The clustering of short days in summer 2024–2025 reflects a temporary acceleration phase linked to the same core-mantle and atmospheric processes described above (Space.com).

The catch: The exact timing of these record days depends on complex, short-term fluctuations—scientists can forecast them weeks in advance but not years.

Is it bad that the Earth is spinning faster?

Not in a catastrophic sense, but the consequences for precision infrastructure are real.

Effects on timekeeping

Atomic clocks—which define the second based on cesium vibrations—keep time far more accurately than Earth’s rotation (Organiser). The growing discrepancy is what forces leap second decisions.

Impact on satellites and GPS

GPS satellites rely on onboard atomic clocks to triangulate positions. A leap second—whether added or subtracted—requires software updates to keep navigation systems accurate (Space.com).

Long-term rotation trends

Despite the current speed-up, Earth’s rotation is gradually slowing over geological time due to tidal friction from the moon (Space.com). The next few decades will be an anomaly within a longer deceleration.

Why this matters: Investors in telecom, finance, and critical infrastructure should watch the IERS bulletins—a negative leap second could disrupt everything from high-frequency trading to GPS-guided agriculture.

Bottom line: The risk to modern infrastructure is real, but manageable with advance planning.

The implication: industry must prepare for an unprecedented clock reversal.

Clarity: confirmed vs. unclear

Confirmed facts

  • Earth’s rotation is currently faster than average (Space.com)
  • Days have shortened by up to 1.66 milliseconds (Space.com)

What’s unclear

  • Exact cause of the speed-up is still debated (Space.com)
  • Whether a negative leap second will actually be enacted in 2029 (Organiser)
  • Whether July 22, 2025 will officially be one of the shortest days (depends on precise measurement) (Organiser)

What scientists are saying

“The leap second is a small adjustment, but it has a huge impact on systems that depend on precise timing.”

Judah Levine, physicist at the National Institute of Standards and Technology (NIST)

“A negative leap second would be unprecedented and requires careful coordination among all timekeeping organizations.”

Scientist from the International Earth Rotation and Reference Systems Service (IERS)

These perspectives from leading timekeeping bodies underscore that the shift from adding to subtracting seconds represents a fundamental operational challenge, not just a theoretical curiosity.

Timeline: Earth’s rotation speed changes

  • : Earth’s rotation begins to speed up after decades of slowing (Space.com)
  • : Shortest day ever recorded (1.66 ms faster) (Space.com)
  • : Second-shortest day (1.34 ms short) (Organiser)
  • : Potential negative leap second (Organiser)

For timekeepers and technology operators, the 2029 deadline is the most concrete milestone. The decision will be based on rotation data collected over the next three years.

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Frequently asked questions

Does Earth’s faster rotation affect gravity?

No, the change is far too small to affect gravity. Gravity variations due to rotation are negligible at millisecond scale.

How do scientists measure Earth’s rotation speed?

Using very long baseline interferometry (VLBI) — a network of radio telescopes that measure Earth’s orientation relative to quasars — supplemented by GPS and satellite laser ranging (Space.com).

Is this related to global warming?

Melting ice sheets and changes in ocean mass can affect Earth’s moment of inertia, but the current speed-up is not directly caused by global warming (Space.com).

Can humans feel the difference in day length?

No, a millisecond is imperceptible. Humans have no biological mechanism to detect such tiny changes.

How often does Earth’s rotation speed change?

Continuously — by milliseconds over days to months, and by larger amounts over decades and centuries. The long-term trend is slowing, but short-term accelerations are common (Space.com).

What is the Chandler wobble and does it matter?

The Chandler wobble is a small (<10 meters) oscillation of Earth’s rotational axis with a period of about 433 days. It contributes to millisecond-level variations in day length but is not the main driver of the current speed-up (Space.com).

Will days ever become significantly shorter or longer?

Over millions of years, tidal friction will slow Earth enough to add about 2 milliseconds per century (Space.com). But in our lifetimes, the variation will remain well under a tenth of a second.

For the world’s timekeepers and the industries that depend on them, the choice ahead is clear: either implement the first negative leap second in history, which carries operational risks, or gradually reset time standards to keep clocks aligned with a faster Earth. The decision will ripple through GPS, telecom, and financial networks — and it’s coming as soon as 2029.

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