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Earth Spins Record-Short Day, Timekeepers Eye Negative Leap Second

July 9’s rotation, 1.30 ms below standard, marks the latest in a series of record-short days that may trigger the first-ever negative leap second by 2029

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"Nobody Expected This": Earth's Rotation Will Speed Up Tomorrow, Bucking The Downward Trend
Earth spinning faster on July 9, 2025 due to Moon’s position | Image: Shutterstock / X

Overview

  • International Earth Rotation and Reference Systems Service data show July 9 completed 1.30 milliseconds faster than the 86,400 seconds defining a standard day.
  • Forecasts based on IERS and United States Naval Observatory models predict further dips on July 22 (–1.38 ms) and August 5 (–1.50 ms), each poised to break new speed records.
  • Since 2020, Earth has consistently reversed its millennia-long tidal slowdown, breaking the annual shortest-day record each year.
  • Researchers have ruled out atmospheric and oceanic causes and suspect internal geophysical processes are driving the unexplained acceleration.
  • UTC has gone without a leap second since 2016, and timekeepers are planning the first negative leap second around 2029 to realign atomic time with Earth’s faster spin.