Earth’s rotation has accelerated this summer, resulting in marginally shorter days that are drawing close scrutiny from scientists and international timekeepers.
According to data compiled by timeanddate.com from the International Earth Rotation and Reference Systems Service and the US Naval Observatory, July 10 was the year’s shortest day so far, lasting 1.36 milliseconds less than 24 hours. The trend is expected to continue with exceptionally short days predicted for July 22 and August 5.
While a day is defined as 24 hours, or 86,400 seconds, the planet’s actual rotation on its axis is slightly irregular. Factors such as the moon’s gravitational pull, seasonal atmospheric changes, and the motion of Earth’s liquid core cause minute variations, making each day slightly longer or shorter. These millisecond discrepancies have no noticeable effect on daily life but can accumulate and impact critical technologies like computers, satellites, and telecommunication networks.
To manage this, global time is tracked with extreme precision by atomic clocks, which were introduced in 1955. These devices form the basis of Coordinated Universal Time (UTC), the international standard to which all modern digital clocks are set. Astronomers simultaneously monitor Earth’s actual rotation against the fixed stars, detecting any drift between planetary time and UTC.
This monitoring has revealed a long-term trend of acceleration. Duncan Agnew, a geophysics professor at the University of California, San Diego, notes that while days have generally been getting faster since 1972, the trend includes constant fluctuations.
Historically, Earth’s rotation was slowing relative to atomic time. To correct this, the first “leap second” was added to UTC in 1972, similar to how a leap year adds a day to the calendar. A total of 27 leap seconds have been added since then, but none have been required since 2016 as the planet’s spin has quickened.
This acceleration now raises the unprecedented prospect of a “negative leap second,” where a second might need to be removed from UTC. Agnew estimates a 40% probability of this occurring by 2035, the year the leap second is scheduled to be retired.
The possibility of a negative leap second is a significant concern for technologists. Judah Levine, a physicist at the National Institute of Standards and Technology, explained that even positive leap seconds, which have been implemented for 50 years, still cause problems. “There’s a much greater concern about the negative leap second, because it’s never been tested,” he said, comparing the potential disruption for systems in finance, energy grids, and GPS to the Y2K problem.
The planet’s rotational speed is influenced by multiple forces. Short-term changes are driven by the moon and tides, as well as seasonal atmospheric shifts. A more significant, long-term driver has been the slowing of Earth’s liquid core, which in turn speeds up the rotation of the solid planet to conserve angular momentum.
However, a surprising factor is counteracting this acceleration: climate change. A study published in Nature by Agnew details how melting ice in Greenland and Antarctica is redistributing mass across the oceans, slowing Earth’s rotation. “If that ice had not melted,” Agnew said, “we would already be having a negative leap second, or we would be very close to having it.”
According to Benedikt Soja, an assistant professor at ETH Zurich, if warming continues, its effect on Earth’s rotation could eventually surpass that of the moon. For now, the complexity of these interacting forces makes long-term predictions highly uncertain. “Maybe in a few years, we could see a different situation, and long term, we could see the planet slowing down again,” Soja said. “That would be my intuition, but you never know.”
