Sapere

By Iqra Sharjeel

Abstract

In the summer of 2025, Earth exhibited an unexpected acceleration in its rotation, shortening the length of day (LOD) by approximately 1.5 milliseconds on three separate dates: July 9, July 22, and August 5. This phenomenon has reignited discussions about the possibility of a “negative leap second”—a time correction that would subtract a second from Coordinated Universal Time (UTC). This article reviews the geophysical causes, measurement techniques, and long-term implications of this rotational anomaly for global timekeeping and technology systems.

Introduction

Earth’s rotation is not constant—it varies subtly due to interactions between the planet’s core, atmosphere, oceans, and even human activities. For decades, scientists have tracked these minute variations using atomic clocks and astronomical observations. Typically, Earth’s rotation is slowing over time due to tidal friction with the Moon. However, in recent years, anomalous patterns—including brief accelerations—have emerged, culminating in a measurable speed-up during summer 2025.

Observational Findings: Summer 2025

Researchers at the International Earth Rotation and Reference Systems Service (IERS) and other geophysical observatories recorded shortened days by approximately 1.5 milliseconds on:

• July 9, 2025
• July 22, 2025
• August 5, 2025

This change was identified using a combination of very-long-baseline interferometry (VLBI), laser ranging, and atomic time comparisons from over 200 international observatories.

The current definition of one day (86,400 seconds) is calibrated based on the average rotation of the Earth. When the actual rotation deviates, small corrections are needed to keep atomic time (UTC) synchronized with Universal Time 1 (UT1)—a measure of Earth’s rotation angle.

Historical Context: Has This Happened Before?

Although the recent anomaly is surprising, Earth’s rotation has accelerated temporarily before:

• 2020–2022: A series of record-short days were observed, with the shortest day recorded on June 29, 2022 at 1.59 milliseconds shorter than 24 hours.
• Pre-1972: Earth’s rotation was generally slowing, and 27 leap seconds have been added to UTC since 1972 to account for this.

However, this 2025 pattern is distinctive for its frequency and consistency, indicating an underlying geophysical process that may be new or poorly understood.

Causes of Rotation Acceleration:

Multiple overlapping geophysical and climatic processes influence Earth’s rotation. Potential contributors to the 2025 anomaly include:

a. Core-Mantle Coupling

Interactions between the Earth’s solid mantle and its fluid outer core can create torque that temporarily speeds up or slows down the planet’s rotation.

b. Post-Glacial Rebound

As polar ice continues to melt from climate change, mass redistribution causes the Earth’s shape to adjust—particularly affecting the equatorial bulge and rotation rate.

C. Atmospheric and Oceanic Dynamics

Changes in jet streams and ocean currents redistribute mass across Earth’s surface. In El Niño or La Niña years, atmospheric angular momentum can increase or decrease significantly.

D. Seismic Activity

Large-scale earthquakes, especially megathrust events, can slightly alter Earth’s moment of inertia, leading to minuscule changes in LOD.

E. Hydrological Changes

Massive reservoirs, floods, or droughts can affect Earth’s angular momentum distribution.

The Case for a Negative Leap Second

To synchronize UTC with UT1, leap seconds are occasionally added. However, the current acceleration could mean UTC is running behind Earth’s actual rotation, necessitating a subtraction—i.e., a negative leap second.

a. Precedent

No negative leap second has ever been implemented since the system was introduced in 1972. The idea was long considered unlikely due to Earth’s general deceleration.

b. Technical Implications

Negative leap seconds would pose unique challenges:

• Computing: Many systems struggle to handle time rollbacks, risking data loss or time drift.
• GPS and telecom: Satellite systems rely on precise synchronization, and subtracting a second can introduce discrepancies.
• Finance: High-frequency trading systems depend on nanosecond accuracy and must be adapted for time subtraction.

As of mid-2025, the IERS has not officially proposed a negative leap second but has acknowledged the need to study the phenomenon closely.

Broader Implications

The 2025 acceleration event is not just a curiosity; it highlights the delicate interplay between planetary geophysics and human technological systems.

• Climate science: Tracking LOD changes helps validate Earth system models and monitor climate-induced mass redistribution.
• Geophysics: Insights into the Earth’s deep interior, including core dynamics, can be inferred from rotational anomalies.
• Timekeeping: The future of civil time—whether we continue using leap seconds or transition to a continuous atomic timescale—is being actively debated.

In fact, the International Telecommunication Union (ITU) has proposed abolishing leap seconds by 2035, favoring a long-term divergence between atomic and astronomical time.

Conclusion

Earth’s rotational acceleration in summer 2025—marked by shortened days and measurable LOD shifts—has captured global scientific attention. While the exact geophysical causes remain under investigation, the event has important implications for both fundamental Earth science and the future of timekeeping.

As technology grows ever more dependent on precise synchronization, events like these underscore the need for robust, adaptive global standards in chronometry. The possibility of a negative leap second may soon move from theoretical to practical—a momentous step in the evolving relationship between Earth and time.

References

1. IERS Bulletin A (2025). Earth Orientation Parameters Updates.
2. Gross, R. (2007). Earth Rotation Variations—Long Period. In Treatise on Geophysics.
3. Capitaine, N., & Bizouard, C. (2023). The Future of Leap Seconds. Journal of Geophysical Timekeeping.
4. NASA Earth Observatory (2022). Shortest Day on Record: Earth’s Accelerated Spin.
5. International Telecommunication Union (2022). Resolution D: Leap Seconds and UTC Standardization.
6. Seitz, F. et al. (2023). Climate-driven angular momentum changes. Geophysical Research Letters.
7. Clark, P. U. et al. (2020). Postglacial rebound and Earth rotation. Nature Geoscience.