Are you running out of time? The Earth has sped up its rotation and the days are getting shorter.

Earth is spinning faster this summer, slightly shortening the length of days and attracting the attention of scientists and timekeepers. July 10 was the shortest day of the year so far, measuring 1.36 milliseconds short of 24 hours, according to data from the International Earth Rotation and Reference Systems Service and the U.S. Naval Observatory, compiled by timeanddate.com. Similarly short days are forecast for July 22 and August 5, at 1.34 and 1.25 milliseconds shorter , respectively.

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A day is defined as the time it takes for the planet to complete one full rotation on its axis, equivalent to 24 hours or 86,400 seconds on average. However, this rotation is not completely uniform. Factors such as the gravitational pull of the Moon, seasonal atmospheric changes, and the dynamics of the Earth's liquid core cause the duration of each rotation to vary slightly.

Although variations are almost imperceptible in everyday life, technological systems that depend on precise timing—such as computers, satellites, and telecommunications networks—can be affected. For this reason, even the smallest deviations have been recorded by atomic clocks since 1955.

Atomic clocks use the oscillations of atoms in vacuum chambers to measure time with extreme precision. These devices define Coordinated Universal Time (UTC), the global standard for timekeeping based on approximately 450 atomic clocks. Astronomers compare UTC with the Earth's actual rotation time using satellites that observe the planet's position relative to fixed stars. These measurements make it possible to detect small differences between the two.

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July 5, 2024, was the shortest day since atomic clocks began, 1.66 milliseconds shorter than 24 hours . “We’ve been on a trend toward slightly faster days since 1972,” noted Duncan Agnew, professor emeritus of geophysics at 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 troughs.”

In 1972, due to the accumulation of rotational delays with respect to atomic time, the concept of the leap second was introduced to keep the two systems synchronized. Since then, 27 leap seconds have been added to UTC. None have been added since 2016, due to the recent acceleration of the Earth's rotation.

In 2022, the General Conference on Weights and Measures voted to retire the leap second by 2035. However, if the acceleration continues, there may be a need to remove a leap second, rather than adding it . “There has never been a negative leap second,” Agnew said , “but the probability of having one between now and 2035 is about 40%.”

The acceleration of Earth's rotation is influenced by factors such as lunar tides and seasonal changes. "During the summer, the Earth naturally rotates faster," Agnew explained, referring to the transfer of angular momentum from the atmosphere to the solid planet due to the movement of atmospheric currents. Added to this is the slowing of Earth's liquid core, while the solid Earth accelerates.

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“The leap second system was designed in 1972 without really considering the possibility of subtracting time,” Levine explained. “It was simply included in the standard because it was necessary for its completeness. Everyone assumed that only positive leap seconds would be needed.”

Levine also warned about the risks of a negative leap second. “There are still places where it’s done wrong—at the wrong time, with the wrong number, etc. And that happens with a positive leap second, something that has been repeated time and time again. There’s a much bigger concern with the negative leap second because it’s never been tested.”

The implications of introducing a negative leap second are worrying because of its impact on time-dependent systems such as telecommunications, financial transactions, power grids, and GPS systems. “It’s similar to the Y2K problem,” Levine said, referring to the computer scare that arose when the time shift from 1999 to 2000.

Climate change also plays an unexpected role. According to a study by Agnew published in Nature, ice melt in Greenland and Antarctica is spreading across the oceans, slowing the Earth's rotation. "If that ice hadn't melted, if we hadn't experienced global warming, we would already be experiencing a negative leap second, or very close to it," Agnew noted.

The shifting of this water mass also causes changes in the planet's rotational axis, according to Benedikt Soja, an assistant professor at the Swiss Federal Institute of Technology in Zurich. "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 exceed the effect of the moon, which has driven the Earth's rotation for the past billions of years," he noted.

Paula Galeano Balaguera

Portfolio Journalist