On July 9, 2025, Earth experienced its shortest day ever recorded. Scientists noted that the day was 1.3 to 1.51 milliseconds shorter than the usual 24 hours. This remarkable change is due to the Moon’s position in orbit, which is subtly increasing Earth’s spin. The International Earth Rotation and Reference Systems Service (IERS) confirmed this occurrence, stating that similar brief days are expected on July 22 and August 5, 2025.
The IERS, the global authority on timekeeping, is planning to introduce a “negative leap second” in 2029. This will be the first time such a correction has been made to keep global time systems accurate. Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography, said, “This is an unprecedented situation and a big deal.” While the change in Earth’s rotation is not expected to cause any disasters, it is an important scientific development.
A day on Earth lasts about 86,400 seconds. However, this length can vary due to factors such as the positions of the Sun and Moon, magnetic shifts, and how Earth’s mass is distributed. In the past, Earth’s rotation was much faster. Scientists believe that 1 to 2 billion years ago, days lasted just 19 hours because the Moon was closer to Earth, exerting a stronger gravitational pull.
Over time, the Moon has moved farther away, causing Earth’s rotation to slow down. In 2020, however, scientists found that Earth was spinning faster than ever since they began precise recordings in the 1970s. The fastest rotation was noted on July 5, 2024, when Earth completed a full spin 1.66 milliseconds quicker than the usual 24 hours.
The reasons for the recent acceleration in Earth’s rotation are not entirely understood. On July 9, the Moon’s position was such that it was farther from the equator and closer to the poles. This change altered its gravitational pull on Earth, much like spinning a top faster by pushing it from the edge. The same conditions are expected to occur on July 22 and August 5, leading to two additional short days.
Although these milliseconds may not be noticeable in daily life, they are significant for scientific measurements and timekeeping systems. The adjustments are crucial for technologies such as GPS and satellite navigation, which rely on precise timing. Since 1972, leap seconds have been added to account for changes in Earth’s rotation speed. A total of 27 leap seconds have been added, with the last one occurring on December 31, 2016.
The anticipated negative leap second will adjust timekeeping systems to align with the changes in Earth’s rotation. Scientists are closely monitoring these developments to see if further records will be set in the near future. As Earth continues to rotate, its behavior remains a fascinating subject for study in the field of geophysics.