This summer, Earth's rotation has accelerated, causing some days to be slightly shorter than the usual 24 hours. Scientists and timekeeping systems are closely monitoring this small but critical change.
According to the International Earth Rotation and Reference Systems Service and the U.S. Naval Observatory, July 10 was the shortest day so far this year, lasting 1.36 milliseconds less than 24 hours. Similar slight shortenings of 1.34 milliseconds on July 22 and 1.25 milliseconds on August 5 are also predicted.
A day is defined as the time Earth takes to complete one full rotation on its axis—approximately 86,400 seconds or 24 hours. However, each rotation varies slightly due to factors like the Moon's gravity, seasonal atmospheric changes, and movements within Earth's liquid core.
These variations, though imperceptible in daily life, are crucial for computers, satellites, and communication systems, which rely on extremely precise timing.
ATOMIC CLOCKS COMPENSATE FOR TIME DIFFERENCES
Atomic clocks, first used in 1955, measure time with extraordinary accuracy by counting vibrations of atoms held in vacuum chambers. Coordinated Universal Time (UTC), determined by about 450 atomic clocks worldwide, is the standard for devices like phones, computers, and GPS.
EARTH LEAVING TIME BEHIND AS IT SPEEDS UP
Astronomers also track Earth's rotation using satellites measuring positions against fixed stars, revealing microsecond differences from atomic clock time. On July 5, 2024, the shortest day ever recorded since atomic clocks were invented occurred—lasting 1.66 milliseconds less than 24 hours.
According to geophysicist Prof. Duncan Agnew of the University of California's Scripps Institution of Oceanography, "Since 1972, there's been a trend toward faster days, though with fluctuations, much like the stock market."
By 1972, Earth's rotation had slowed enough that a leap second was added to UTC to realign atomic time with astronomical time. Since then, 27 leap seconds have been added, though fewer in recent years—with none added since 2016.
In 2022, the International Bureau of Weights and Measures decided to abolish leap seconds by 2035. However, Agnew warns if Earth continues speeding up, a negative leap second (removing a second) might be necessary—something never done before but now estimated as 40% likely by 2035.
WHY IS EARTH SPEEDING UP?
Short-term changes mostly arise from the Moon and tides. When the Moon is above the equator, it slows Earth's rotation; when farther north or south, it speeds it up.
Atmospheric conditions in summer also speed Earth up. Shifts in jet streams slow the atmosphere, and by conservation of angular momentum, Earth spins faster. Similarly, movements in Earth's liquid core affect rotation speed.
Combining these factors allows prediction of short days. Physicist Judah Levine of the National Institute of Standards and Technology notes, "If Earth speeds up one day, it tends to keep speeding up the next day," though this correlation fades long-term.
THE PROBLEM WITH NEGATIVE LEAP SECONDS
Levine says a single short day isn't problematic, but persistent trends may force a negative leap second. When leap seconds were introduced in 1972, no one anticipated needing to remove seconds. While positive leap seconds have caused system errors—wrong timing or values—negative leap seconds are untested and pose greater risks.
Critical systems like finance, power grids, telecommunications, and GPS depend on precise timing, and a negative leap second could cause disruptions comparable to the Y2K bug in 2000.
MELTING ICE SLOWS EARTH'S SPIN
Climate change also affects Earth's rotation—but surprisingly, it slows it down. Melting ice from Antarctica and Greenland spreads mass into oceans, reducing rotation speed much like a figure skater slowing down with arms extended.
If these ice masses hadn't melted, a negative leap second would be an imminent issue today, says Agnew.
According to NASA, ice melt from Greenland and Antarctica accounts for about one-third of sea level rise since 1993.
Research by Benedikt Soja of the Swiss Federal Institute of Technology in Zurich finds this melt also shifts Earth's rotation axis. Soja warns that if greenhouse gas emissions remain high, climate change's impact by century's end may surpass the Moon's billions-year influence.
He adds, "Currently, all changes are within natural variability, but a long-term slowdown cannot be ruled out. Still, we can never be certain."
