This summer has witnessed an intriguing phenomenon: Earth has been spinning faster, leading to shorter days, which has captured the attention of scientists and timekeepers alike. On July 10, 2023, Earth experienced its shortest day recorded thus far, clocking in at 1.36 milliseconds less than the standard 24 hours. However, this unusual trend isn’t expected to end anytime soon, with predictions indicating even shorter days on July 22 and August 5, which are expected to be 1.34 and 1.25 milliseconds shorter than 24 hours, respectively.
The length of a day is generally understood as the time it takes for Earth to make one full rotation on its axis — theoretically comprising 24 hours or 86,400 seconds. In reality, this rotation isn’t uniform, as several factors come into play, including the moon’s gravitational influence, seasonal atmospheric variation, and even the dynamics of Earth’s liquid core. Consequently, such irregularities can lead to days being slightly longer or shorter than 86,400 seconds, although these variations typically go unnoticed in everyday life.
Despite their minuscule nature, these variations in Earth’s rotation can accumulate over time, potentially affecting technologies such as computers, satellites, and telecommunications. To monitor this, scientists employ atomic clocks, a technology developed in 1955 that measures time with unprecedented accuracy. The standard time derived from these atomic clocks is known as Coordinated Universal Time (UTC), which serves as the global reference by which all electronic devices, including smartphones and computers, are synchronized.
Astrophysicists diligently track the fluctuations in Earth’s rotation using satellites that assess its positioning relative to distant stars. They have observed minute discrepancies between the time kept by atomic clocks and the actual duration of Earth’s rotational cycle. Remarkably, on July 5, 2024, Earth recorded the shortest day since the inception of atomic clocks, registering a remarkable 1.66 milliseconds shorter than the established 24-hour benchmark.
Duncan Agnew, a professor emeritus of geophysics at the Scripps Institution of Oceanography and a research geophysicist at the University of California, San Diego, noted a consistent trend toward slightly faster days beginning in 1972. Nevertheless, these variations are not linear; rather, they resemble fluctuations in the stock market—with long-term trends countered by short-term ups and downs.
The introduction of a “leap second” in 1972, which compensated for delays incurred in Earth’s rotation relative to atomic time, illustrates how much Earth’s rotational speed has varied. Leap seconds, analogous to leap years, are additions made to keep our clocks in sync with astronomical time. Since then, 27 leap seconds have been appended to UTC, although the frequency of such additions has slowed, correlating with Earth’s increasing rotational speed. Notably, no leap seconds have been added since 2016.
Looking ahead, the General Conference on Weights and Measures (CGPM) has proposed to phase out leap seconds entirely by 2035. As Earth’s rotation continues to accelerate in the coming years, there exists a possibility that, instead of adding seconds, one could be removed from UTC, a situation termed a “negative leap second.” Agnew estimates a 40% chance of this occurring before the 2035 deadline.
The nuances of Earth’s rotation are affected by various natural forces, including lunar gravitational pulls and tidal actions, which tend to decelerate the planet’s spin when the moon hovers over the equator and speed it up at polar heights. These mechanics are further complicated by seasonal variations; during summer, Earth’s rotation tends to quicken due to shifts in atmospheric dynamics. The overall impact of these changes is reflected in a notable pattern of acceleration over the years.
Judah Levine, a physicist affiliated with the National Institute of Standards and Technology, explained that tracking these fluctuations can be complex. While short-term predictions can be measured effectively, uncertainties increase when forecasts extend over longer durations, rendering prophesies about Earth’s spin unpredictable past a year.
While the implications of shorter days may appear trivial, the ongoing trend raises the specter of a potential negative leap second. Levine emphasized that the conceptualization of negative leap seconds was merely a theoretical safeguard when the system was defined in 1972, as it was universally assumed that only positive additions would be necessary. However, the current acceleration of Earth’s spin presents a new set of challenges regarding timekeeping systems, paralleling the concerns that surrounded the Y2K incident.
Intriguingly, climate change, known for its devastating impacts on Earth, serves to counterbalance the dynamics affecting Earth’s rotation. Research led by Agnew elucidates that glacial melting from Antarctica and Greenland has contributed to an overall slowdown in Earth’s spin by displacing mass into the oceans—akin to how a figure skater slows down when tucking their arms in. Should climate change persist, this influence might become increasingly substantial.
Lastly, the shifting mass from melting polar ice
