Earth’s Rotation Accelerates: Negative Leap Second Looms

On July 10, 2025, Earth’s rotation speed reached this year’s fastest rate, with the day being 1.38 milliseconds shorter than the standard 24 hours, according to observations by the International Earth Rotation and Reference Systems Service (IERS) and the U.S. Naval Observatory. July 9 and July 22 also showed shortening trends, with August 5 predicted to be another historically short day.

The Truth About Earth’s Rotation Revealed by Atomic Clocks

Earth’s rotation speed refers to the time it takes for Earth to complete one rotation on its axis, approximately 86,400 seconds (24 hours). However, this value is only an average, and it actually varies constantly.

Accurate measurement of a day’s length using atomic clocks began in 1955. Atomic clocks use the spectrum emitted by atoms as a frequency standard, based on the principle that the discrete energy intervals of atoms, molecules, or ions remain constant and unchanging.

Record-Breaking Shortest Days Continue in Unprecedented Situation

The recent acceleration of Earth’s rotation speed is truly an unprecedented phenomenon.

“2020 is considered the year with the fastest rotation since recording began in the 1970s. Despite the long-term tendency for rotation speed to slow due to lunar tidal forces, the rapid acceleration of rotation speed in recent years is historically abnormal, and scientists are perplexed, unable to identify clear causes.”

Complex Mechanisms Behind Earth’s Changing Rotation Speed

Earth’s rotation speed varies due to complex interactions between various Earth components (solid Earth, liquid core, oceans, atmosphere, ice sheets) and influences from external celestial bodies like the Moon and Sun. This is based on the law of conservation of Earth’s rotational angular momentum, where changes in mass distribution in one part cause compensatory changes in rotation speed in other parts.

1. Lunar Gravity and Tidal Friction – The Main Cause of Long-term Deceleration

The primary reason Earth’s rotation is gradually slowing is “tidal friction” caused by lunar gravity. Since Earth rotates faster than the Moon’s orbital speed, lunar gravity acts like a “brake” on Earth.

2. The “Boring Billion” and Thermal Tidal Torque

Interestingly, during the period called the “Boring Billion” from about 2 billion to 1 billion years ago, research has revealed that oceanic tidal torque and thermal tidal torque balanced out, and Earth’s rotation period remained nearly fixed at about 19 hours.

Solar “thermal tidal torque” occurs when the Sun heats water vapor and ozone in the atmosphere, causing atmospheric expansion. While this effect is weaker than oceanic tidal torque, it works to accelerate Earth’s rotation.

3. Earth’s Inner Core Motion – Key to Recent Acceleration?

Changes in Earth’s inner core motion are thought to affect rotation speed, and the coupling state between the fluid core and mantle also provides information about rotation speed variations. When the mantle and fluid core rotate around different axes, they can exert torque on each other through the core-mantle boundary.

Many scientists believe this “core lag” is not unrelated to the recent rapid rotation acceleration, but why the core’s speed is changing so suddenly now remains completely unexplained.

4. Atmospheric and Oceanic Motion

Earth’s atmosphere and oceans rotate with Earth, and changes in their motion affect Earth’s overall rotation speed.

5. Changes in Earth’s Mass Distribution

When Earth’s mass distribution changes, similar to how figure skaters adjust rotation speed by pulling in or extending their arms during spins, Earth’s rotation speed also changes.

The 2011 Great East Japan Earthquake changed Earth’s mass distribution, shortening the day by 1.8 microseconds (0.0000018 seconds).

Historical Changes in Earth’s Rotation Speed

Earth’s rotation speed has changed dramatically since its birth.

These historical records are estimated by examining growth lines in coral and shell fossils (similar to tree rings, showing daily and annual growth patterns). Shells form daily layers by secreting calcium carbonate, allowing inference of shorter days from thinner shell layers.

Cutting-edge Observation Technology: VLBI and Space Geodesy

While Earth rotation observations in the Meiji era used visual observations with optical telescopes, current mainstream technology uses space geodetic techniques represented by radio telescope interferometry (VLBI: Very Long Baseline Interferometry).

Other precision Earth measurement technologies include volume strain meters (measuring underground volume changes), tide gauges (measuring sea level changes), and seismic tomography (inferring convection from mantle heterogeneity).

The Crisis of the First-Ever “Negative Leap Second”

When Earth’s rotation speed diverges from atomic clock time (International Atomic Time: TAI), “leap seconds” are introduced to adjust the discrepancy. Since Earth’s rotation has tended to lag, a total of 27 “positive leap seconds” (adding 1 second) have been inserted since 1972.

Past System Failures from Leap Seconds

Even past “positive leap second” adjustments have caused serious problems:

“Major IT companies like Meta, Google, and Amazon strongly oppose introducing ‘negative leap seconds.’ Many systems can handle adding time but aren’t designed to handle time deletion.”

International Movement Toward Leap Second Abolition

In response to these concerns, the General Conference on Weights and Measures (CGPM) adopted a policy in 2022 to stop leap second adjustments by 2035. This effectively means “abolishing leap seconds.”

Note that “leap years” adjust for discrepancies between Earth’s orbital period and the calendar, a completely different mechanism from “leap seconds.” Even if leap seconds are abolished, leap years will remain.

Short-term Effects of Lunar Position Changes

While lunar gravity causes long-term deceleration, from a short-term perspective, changes in the Moon’s position in its orbit also affect Earth’s rotation speed.

Human Activity’s Impact on Earth’s Rotation

“Humans are having a greater impact on Earth than we realize, and naturally, humans bear great responsibility for Earth’s future.”

Glacier melting and groundwater movement due to climate change show that human activity indirectly affects Earth’s rotation speed. This demonstrates how changes in Earth’s rotation speed symbolize the magnitude of human impact on the entire Earth system.

Impact on Daily Life and the Concept of Time

Even if day length changes by milliseconds, we cannot perceive it in daily life. Since eye blinks take 100-400 milliseconds, a change of about 1 millisecond is imperceptible.

However, in fields like astronomy, communications, and precision engineering, such small errors can lead to significant problems, making this an important issue that cannot be ignored.

Earth’s Internal Structure Reveals Rotation Mysteries

Earth’s internal structure is essential for understanding its rotational behavior.

The mantle behaves as a “fluid” on long timescales, undergoing thermal convection. Surface motion of this mantle convection is plate tectonics. The phenomenon of “seismic anisotropy” is also observed, where seismic waves passing through the core vary greatly depending on direction.

Conclusion: A Living Planet in Constant Change

Changes in Earth’s rotation speed result from complex interactions among lunar gravity, atmospheric and oceanic motion, and Earth’s internal dynamics. While the long-term trend is deceleration due to lunar tidal forces, recent observations show temporary acceleration due to influences like inner core motion and atmospheric variations.

Understanding variations in Earth’s rotation speed is important for elucidating deep Earth structure, climate change relationships, and the evolution of other solar system bodies.

Our Earth is a living planet in constant change, full of mysteries.