On July 22, Earth is expected to spin a little faster than usual, skipping 1.34 milliseconds of the usual 24-hour day. This will make it the second shortest day of the year so far. 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. August 5 is predicted to be 1.25 milliseconds shorter than 24 hours. How is the length of a day calculated? 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. Why do shorter days matter? 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. CNN, in its report, highlighted how 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. An atomic clock in the time laboratory of the Physikalisch-Technische Bundesanstalt (PTB) in Germany. These devices use lasers and atoms to calculate time with extreme precision. – Julian Stratenschulte/picture alliance/dpa/Getty Images 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. Is it becoming a trend? 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,” Duncan Agnew, a professor emeritus of geophysics at the Scripps Institution of Oceanography and a research geophysicist at the University of California, San Diego told CNN. “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%.” What is causing Earth to spin faster? 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. The role of melting ice 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. With CNN inputs