Understanding the Phenomenon
On July 22, Earth will complete a full rotation in slightly less time than usual, making it one of the shortest days ever recorded. The difference will be a mere 1.34 milliseconds less than the standard 24 hours, but it’s part of an unusual trend in Earth’s rotational behavior observed over recent years.
If this trend continues, it might necessitate subtracting a negative atomic second from global clocks by 2029, according to Space.com. This unprecedented event, known as the negative subliminal second, has never occurred before.
Earth’s Rotational History
The Earth’s rotational speed isn’t constant. Long ago, a day was much shorter than the current 24 hours due to the balance between solar atmospheric tides and oceanic lunar tides. However, over time, the primary culprit for lengthening days has been the friction from lunar tides, gradually moving the Moon away from Earth. As it moves further, the Moon absorbs Earth’s rotational energy, slowing its rotation and lengthening days.
Since the advent of atomic clocks in 1973, the shortest day ever recorded was 1.05 milliseconds shorter than 24 hours by 2020, according to Timeanddate.com. However, since 2020, Earth has repeatedly broken its own speed records. The shortest day ever measured occurred on July 5, 2024, when Earth’s rotation completed 1.66 milliseconds faster than usual.
Predicted Shortest Days in 2025
Looking ahead to 2025, scientists predicted that July 9, 22, and August 5 could be the shortest days of the year. However, new data suggests that July 10 became the shortest day recorded so far in 2025, with 1.36 milliseconds less than 24 hours. July 22 is expected to complete its rotation 1.34 milliseconds earlier, placing it in a close second position. If current predictions hold, August 5 will be approximately 1.25 milliseconds shorter than usual, making July 22 the second-shortest day of the year.
Polar Ice Melt and Its Impact
There are indications that the acceleration might be slowing down. The rate of decrease in day length appears to be decelerating, but the underlying cause of recent rotational changes remains uncertain.
A 2024 study suggests that polar ice melt and rising sea levels might be influencing Earth’s rotation. However, instead of driving acceleration, this mass redistribution could be moderating it.
A more likely factor lies beneath our feet: the slowdown of Earth’s liquid core, which might be redistributing angular momentum so that the mantle and crust rotate slightly faster.
Leonid Zotov, Earth rotation expert at Moscow State University, stated to Timeanddate: “Most scientists believe this is something internal to Earth. Oceanic and atmospheric models do not explain this enormous acceleration.”
Zotov predicts that Earth’s rotation might slow down again soon. If accurate, this sudden acceleration could merely be a temporary anomaly in Earth’s long-term trend towards slower rotation and longer days.
Key Questions and Answers
- What is happening on July 22? Earth will complete a full rotation slightly faster than usual, making it one of the shortest days ever recorded.
- Why is this happening? The cause remains uncertain, with theories pointing to internal Earth factors like the slowdown of the liquid core or external influences such as polar ice melt and rising sea levels.
- How significant is this change? The difference is only 1.34 milliseconds less than the standard 24 hours, but it’s part of a broader trend in Earth’s rotational behavior.
- What does this mean for future days? If the current acceleration continues, it might necessitate subtracting a negative atomic second from global clocks by 2029.
