After several years of the shortest days in the era of atomic clocks, new research points to a 70-year cycle that is likely related to movements in the Earth's core - and challenges predictions that we will soon have to erase a second from universal time.
In recent years we have become accustomed to a new type of headlines: Earth is spinning faster, the days are getting shorter, and perhaps for the first time in the history of time measurementזמן The modern will need to be carried out "Leap second "Negative" – meaning to skip one second from the clock.
But the geophysical reality is more complex. New research on fluctuations in Earth's rotation, accepted for publication in the journal Earth, Planets and Space, indicates that some of the changes we see may belong to a multi-decadal cycle of about 70 years related to movements inEarth's core.
This does not mean that the Earth has finally “returned to normal.” Its rotation rate is constantly changing under the influence of the moon, the atmosphere, the oceans, and the movements of material within the planet. However, according to the new study, the multi-decadal cycle itself does not indicate an imminent and sustained continuation of the shortening of the day to a level that would require a negative second.
A day is not always exactly 24 hours.
We are used to saying that a day lasts 24 hours, or 86,400 seconds. In reality, the time it takes the Earth to complete a rotation varies in millionths and thousandths of a second.
Since 2020, a series of exceptionally short days has been recorded, raising the possibility that if the acceleration continues over time, it will be necessary to subtract a second from Coordinated Universal Time for the first time.
But these changes do not occur in a straight line. סיבוב כדור הארץ It can accelerate for a period of time and then slow down again. That's why it's difficult to conclude from a few unusual years what the rotation trend will look like in the coming decades.
What happens inside the Earth's core?
In the new study, Ziwen Zhang and his colleagues examined data on the length of the day from 1883 to 2022. The researchers removed modelable effects from the data, including tides, the movement of fluids on the Earth's surface, and long-term trends.
They identified a prominent oscillation of about 70 years. One analysis yielded a 69.7-year cycle in the length of day data, and a 71.8-year cycle in data derived from the angular momentum of the nucleus.
The agreement between the two data sets was high. The researchers found that the findings are consistent with the possibility that the Earth's core has a significant contribution to the slow changes in the planet's rotation speed.
The possible explanation is related to the exchange of angular momentum between the core and the mantle. When one region inside the Earth changes its rotation speed slightly, another region can respond with an opposite change.
However, the researchers emphasize that the mechanism is not unambiguous. It may involve electromagnetic, gravitational, viscous forces, and mechanical interactions between the core and the mantle. The study does not claim to have found a single mechanism that explains all the changes.
The second fetus was born because of the clock mismatch.
Since 1967, the second has been defined by energy transfer in the cesium-133 atom rather than by the rotation of the Earth. Atomic clocks Much more stable than the planet's changing rotation.
To maintain consistency between atomic time and astronomical time, a "leap second" was occasionally added in 1972. In such cases, an unusual moment appears on the clock – 23:59:60 – before midnight.
We at Hidan already dealt with the phenomenon in 2015, when June 30th was extended by a second to compensate for changes in the Earth's rotation..
So far, all the seconds elapsed have been positive – meaning we added a second.
But if the Earth accelerates sufficiently over time, the opposite situation may arise: astronomical time will advance atomic time by an amount that requires removing one second.
Such a step was never taken.
Why does one second even bother anyone?
For the average person, a difference of a millisecond in the length of a day is of no practical importance. But computer systems, communication networks, satellite navigation, financial markets, and electrical infrastructure rely on extremely precise timing.
Even adding a leap second has caused problems in computer systems in the past, because not all software handles the leap second the same way. A negative second would be another challenge, because it has never been tried on a global scale.
So the debate over leap seconds is not just an astronomical question. It brings together two very different kinds of "clocks": atoms, which provide us with a highly accurate benchmark, and the Earth itself, whose rotation rate varies.
And in 2026? There is no leap second.
The practical decision is not made on the basis of a single study. The body responsible for this is the International Service for Earth Rotation and Reference Systems – IERS.
In Bulletin C No. 72 published by IERS on July 6, 2026 The organization officially announced that at the end of December 2026 You will not be driven for a second longer thanThe difference between Coordinated Universal Time UTC and the International Atomic Time (TAI) will remain 37 seconds.
The new study is not an operational forecast from IERS. The researchers explicitly emphasize that their model is a geophysical assessment and not a tool for predicting future UTC decisions.
However, within the model we examined, the multi-decadal oscillation does not in itself indicate a continued shortening of the day in the near future that would require Negative leap second.
In the long run, the Earth is actually slowing down.
There is an interesting irony here. Despite the acceleration recorded at certain times in recent years, the long-term trend for the Earth is actually a slowdown.
The Moon's gravitational pull creates tides, which gradually transfer angular momentum from the Earth to the Moon's orbit. As a result, the Moon slowly moves away from the Earth and the day lengthens over geological timescales.
On top of this slow trend, there are short and multi-decadal fluctuations caused, among other things, by the atmosphere, the oceans, seasonal mass changes, and deep movements within the Earth.
Therefore, there is no contradiction between the fact that the Earth is slowing down in the very long term and the fact that for a few years it can rotate slightly faster.
The internal motion of the Earth has also been of interest to researchers for decades. Back in 2005, we reported in the newspaper on studies that showed that Not all of the Earth rotates at the same speed, and that the inner core may move at a different rate than the rest of the planet.
So will you delete it for us for a second?
Right now – no.
The possibility of a negative second exists in principle as long as the leap second mechanism continues to operate. But the new study presents a more complex picture than simply stating that the Earth is "spinning too fast."
It is possible that some of the observed acceleration is an expression of a long-term natural oscillation related to processes occurring thousands of kilometers beneath our feet.
Ultimately, the episode illustrates how even a basic concept like "day" is not completely fixed. Our atomic clocks are incredibly accurate; the Earth, upon which we invented the clock, is much less so.
Questions and Answers
Does the Earth always rotate at the same speed? No. The length of a day varies by tiny fractions of a second due to the effects of the atmosphere, the oceans, the moon, and processes within the Earth.
What is a negative leap second? Instead of adding a second to the clock, one second is skipped to realign atomic time with the Earth's rotation. To date, a negative leap second has never been performed.
Will there be a leap second at the end of 2026? No. IERS officially announced that there will be no leap second at the end of December 2026.
What did the new study find? The researchers identified a 70-year oscillation in the length of the day, which matches the oscillation found in data related to the angular momentum of the Earth's core. They suggest that processes in the core contribute to the multidecadal changes in rotation speed.
More on the subject on the science website
Why was June 30th a second longer?
Not all of the Earth rotates at the same speed
For the scientific article: Opening the scientific article
