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Earth is spinning faster this summer, making the days marginally shorter and attracting the attention of scientists and timekeepers.
July 10 was the shortest day of the year so far, lasting 1.36 milliseconds less than 24 hours, according to data from the International Earth Rotation and Reference Systems Service and the US Naval Observatory, compiled by timeanddate.com. More exceptionally short days are coming on July 22 and August 5, currently predicted to be 1.34 and 1.25 milliseconds shorter than 24 hours, respectively.
The length of a day is the time it takes for the planet to complete one full rotation on its axis —24 hours or 86,400 seconds on average. But in reality, each rotation is slightly irregular due to a variety of factors, such as the gravitational pull of the moon, seasonal changes in the atmosphere and the influence of Earth’s liquid core. As a result, a full rotation usually takes slightly less or slightly more than 86,400 seconds — a discrepancy of just milliseconds that doesn’t have any obvious effect on everyday life.
However these discrepancies can, in the long run, affect computers, satellites and telecommunications, which is why even the smallest time deviations are tracked using atomic clocks, which were introduced in 1955. Some experts believe this could lead to a scenario similar to the Y2K problem, which threatened to bring modern civilization to a halt.
Atomic clocks count the oscillations of atoms held in a vacuum chamber within the clock itself to calculate 24 hours to the utmost degree of precision. We call the resulting time UTC, or Coordinated Universal Time, which is based on around 450 atomic clocks and is the global standard for timekeeping, as well as the time to which all our phones and computers are set.
Astronomers also keep track of Earth’s rotation — using satellites that check the position of the planet relative to fixed stars, for example — and can detect minute differences between the atomic clocks’ time and the amount of time it actually takes Earth to complete a full rotation. Last year, on July 5, 2024, Earth experienced the shortest day ever recorded since the advent of the atomic clock 65 years ago, at 1.66 milliseconds less than 24 hours.
“We’ve been on a trend toward slightly faster days since 1972,” said Duncan Agnew, a professor emeritus of geophysics at the Scripps Institution of Oceanography and a research geophysicist at the University of California, San Diego. “But there are fluctuations. It’s like watching the stock market, really. There are long-term trends, and then there are peaks and falls.”
In 1972, after decades of rotating relatively slowly, Earth’s spin had accumulated such a delay relative to atomic time that the International Earth Rotation and Reference Systems Service mandated the addition of a “leap second” to the UTC. This is similar to the leap year, which adds an extra day to February every four years to account for the discrepancy between the Gregorian calendar and the time it takes Earth to complete one orbit around the sun.
Since 1972, a total of 27 leap seconds have been added to the UTC, but the rate of addition has increasingly slowed, due to Earth speeding up; nine leap seconds were added throughout the 1970s while no new leap seconds have been added since 2016.
In 2022, the General Conference on Weights and Measures (CGPM) voted to retire the leap second by 2035, meaning we may never see another one added to the clocks. But if Earth keeps spinning faster for several more years, according to Agnew, eventually one second might need to be removed from the UTC. “There’s never been a negative leap second,” he said, “but the probability of having one between now and 2035 is about 40%.”
The shortest-term changes in Earth’s rotation, Agnew said, come from the moon and the tides, which make it spin slower when the satellite is over the equator and faster when it’s at higher or lower altitudes. This effect compounds with the fact that during the summer Earth naturally spins faster — the result of the atmosphere itself slowing down due to seasonal changes, such as the jet stream moving north or south; the laws of physics dictate that the overall angular momentum of Earth and its atmosphere must remain constant, so the rotation speed lost by the atmosphere is picked up by the planet itself. Similarly, for the past 50 years Earth’s liquid core has also been slowing down, with the solid Earth around it speeding up.
By looking at the combination of these effects, scientists can predict if an upcoming day could be particularly short. “These fluctuations have short-period correlations, which means that if Earth is speeding up on one day, it tends to be speeding up the next day, too,” said Judah Levine, a physicist and a fellow of the National Institute of Standards and Technology in the time and frequency division. “But that correlation disappears as you go to longer and longer intervals. And when you get to a year, the prediction becomes quite uncertain. In fact, the International Earth Rotation and Reference Systems Service doesn’t predict further in advance than a year.”
While one short day doesn’t make any difference, Levine said, the recent trend of shorter days is increasing the possibility of a negative leap second. “When the leap second system was defined in 1972, nobody ever really thought that the negative second would ever happen,” he noted. “It was just something that was put into the standard because you had to do it for completeness. Everybody assumed that only positive leap seconds would ever be needed, but now the shortening of the days makes (negative leap seconds) in danger of happening, so to speak.”
The prospect of a negative leap second raises concerns because there are still ongoing problems with positive leap seconds after 50 years, explained Levine. “There are still places that do it wrong or do it at the wrong time, or do it (with) the wrong number, and so on. And that’s with a positive leap second, which has been done over and over. There’s a much greater concern about the negative leap second, because it’s never been tested, never been tried.”
Because so many fundamental technologies systems rely on clocks and time to function, such as telecommunications, financial transactions, electric grids and GPS satellites just to name a few, the advent of the negative leap second is, according to Levine, somewhat akin to the Y2K problem — the moment at the turn of the last century when the world thought a kind of doomsday would ensue because computers might have been unable to negotiate the new date format, going from ’99’ to ’00.’
Climate change is also a contributing factor to the issue of the leap second, but in a surprising way. While global warming has had considerable negative impacts on Earth, when it comes to our timekeeping, it has served to counteract the forces that are speeding up Earth’s spin. A study published last year by Agnew in the journal Nature details how ice melting in Antarctica and Greenland is spreading over the oceans, slowing down Earth’s rotation — much like a skater spinning with their arms over their head, but spinning slower if the arms are tucked along the body.
“If that ice had not melted, if we had not had global warming, then we would already be having a leap negative leap second, or we would be very close to having it,” Agnew said. Meltwater from Greenland and Antarctica ice sheets has is responsible for a third of the global sea level rise since 1993, according to NASA.
The mass shift of this melting ice is not only causing changes in Earth’s rotation speed, but also in its rotation axis, according to research led by Benedikt Soja, an assistant professor at the department of civil, environmental and geomatic engineering of The Swiss Federal Institute of Technology in Zurich, Switzerland. If warming continues, its effect might become dominant. “By the end of this century, in a pessimistic scenario (in which humans continue to emit more greenhouse gases) the effect of climate change could surpass the effect of the moon, which has been really driving Earth’s rotation for the past few billions of years,” Soja said.
At the moment, potentially having more time to prepare for action is helpful, given the uncertainty of long-term predictions on Earth’s spinning behavior. “I think the (faster spinning) is still within reasonable boundaries, so it could be natural variability,” Soja said. “Maybe in a few years, we could see again a different situation, and long term, we could see the planet slowing down again. That would be my intuition, but you never know.”
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