Though the difference is microscopic, with each listed day shedding about 1.3 to 1.5 milliseconds, the phenomenon is caused by natural—but unexpected—changes within the Earth-Moon system.
What Is a "Shorter Day"?
One day equals approximately 86,400 seconds, but this isn’t exact. Scientists use atomic clocks and satellite data to track minute daily variations in Earth’s rotation—called the Length of Day (LOD).
While the overall trend for Earth has been a gradual slowdown due to tidal friction, since 2020 we’ve seen a surprising reversal: multiple days nearly half a millisecond shorter than the usual pre-2020 range.
Which Dates, and by How Much?
Researchers have identified three days this summer with notably fast spins:
Date - Estimated Shortening
July 9 –1.30 milliseconds
July 22 –1.38 milliseconds
August 5 –1.51 milliseconds
August 5 is expected to be one of the shortest days on record (excluding July 5, 2024, which saw –1.66 ms).
What’s Causing the Acceleration?
Experts are still investigating, but several factors are under consideration:
Lunar orbit alignment: The Moon being farthest from Earth’s equator may be nudging the spin faster.
Internal Earth dynamics: Changes in the molten core or tectonic shifts could be altering Earth’s rotation.
Mass redistribution: Melting glaciers and changing ocean currents can shift mass toward the equator, altering angular momentum.
However, Dr. Leonid Zotov notes that none of the models fully explain the phenomenon yet.
Why It Matters: Timekeeping and Technology
Though a millisecond isn’t noticeable in daily life, it matters for precision systems:
Atomic clocks and GPS systems rely on Earth’s spin.
Since rotation is speeding up, scientists are preparing for the first-ever negative leap second—removing a second—in 2029.
This adjustment ensures UTC (Coordinated Universal Time) stays in sync with the planet’s rotation.
It’s a fascinating reminder that even time—something we consider constant—is subject to Earth’s complex systems. The upcoming shorter days on July 9, 22 and August 5 offer a unique window into how planet and clock co-exist—and how science evolves to keep them in harmony.
