The Earth is spinning faster than it has in decades, with some days lasting slightly less than the standard 24 hours. According to data from the International Earth Rotation and Reference Systems Service and the US Naval Observatory, July 10 was the shortest day of the year so far, lasting 1.36 milliseconds less than 24 hours. This phenomenon has sparked interest among scientists and timekeepers, who are considering an unprecedented move to account for the planet’s accelerating spin.
The length of a day is determined by the time it takes the Earth to complete one full rotation on its axis, which is approximately 24 hours or 86,400 seconds. However, due to various factors such as the gravitational pull of the moon, seasonal changes in the atmosphere, and the influence of Earth’s liquid core, each rotation is slightly irregular. These discrepancies can affect computers, satellites, and telecommunications, which is why atomic clocks are used to track even the smallest time deviations.
Atomic clocks count the oscillations of atoms held in a vacuum chamber to calculate 24 hours with extreme precision. The resulting time is called Coordinated Universal Time (UTC), which is the global standard for timekeeping. However, Earth’s rotation is not perfectly uniform, and astronomers have to account for the difference between atomic time and the planet’s actual rotation. To address this issue, leap seconds are added to UTC to keep it in sync with Earth’s rotation.
Since 1972, a total of 27 leap seconds have been added to UTC, but the rate of addition has slowed down due to Earth’s speeding up. In 2022, the General Conference on Weights and Measures voted to retire the leap second by 2035, meaning we may never see another one added to the clocks. However, if Earth continues to spin faster, it’s possible that a negative leap second might need to be introduced, which would involve subtracting a second from UTC. According to Duncan Agnew, a professor emeritus of geophysics at the Scripps Institution of Oceanography, “There’s never been a negative leap second, but the probability of having one between now and 2035 is about 40%.”
Agnew explains that the shortest-term changes in Earth’s rotation 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 due to seasonal changes in the atmosphere. Judah Levine, a physicist at the National Institute of Standards and Technology, notes that “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.”
Climate change is also playing a role in Earth’s rotation, albeit in a surprising way. According to Agnew’s research, melting ice in Antarctica and Greenland is spreading over the oceans, slowing down Earth’s rotation. “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. Benedikt Soja, an assistant professor at the Swiss Federal Institute of Technology, adds that if warming continues, its effect might become dominant, potentially surpassing the effect of the moon on Earth’s rotation.