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last mo.

Earth Sets Second-Shortest Day of 2025 as Spin Continues to Accelerate

Ruling out a 2025 leap second, international timekeepers prepare protocols for a possible negative adjustment by 2029.

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This stack composite photo shows a paddy rice field under the starry sky in Shuangyashan City, northeast China's Heilongjiang Province, May 25, 2025. With paddy rice transplanting largely concluded in Heilongjiang, the vast black soil in the province is now dressed in green, hinting the hope for a bountiful harvest. The Milky Way spreading across a starry sky is mirrored in waterlogged paddy rice fields, making a breathtaking scene in the early summer night. (Photo by Qu Yubao/Xinhua via Getty Images)
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.

Overview

  • IERS data show July 22 completed a full rotation 1.34 milliseconds faster than 24 hours, making it the second-shortest day recorded this year.
  • Earth’s rotation has been accelerating since 2020, shattering successive millisecond-scale day-length records and peaking at 1.66 ms short on July 5, 2024.
  • Short-term spikes in spin link to the Moon’s maximum declination, while atmospheric shifts, melting ice and core-dynamics changes also tweak Earth’s moment of inertia.
  • The International Earth Rotation and Reference Systems Service confirmed no leap second will be added in 2025 and is drafting procedures for a first-ever negative leap second by 2029.
  • These minute variations, imperceptible in daily life, can disrupt satellite navigation, GPS precision and the synchronization of global atomic-clock systems.