Earth’s Rotation Speeds Up
Recent observations show that Earth’s rotation is accelerating. On 9 July 2025, the planet completed one full spin faster than ever before. This day was shorter by approximately 1.6 milliseconds. Scientists attribute this change to the gravitational influence of the Moon. The phenomenon has implications for timekeeping and scientific study.
Earth’s Rotation and Day Length
Earth’s day length is not constant. It varies due to factors such as gravitational forces, seismic activity, and changes in atmospheric conditions. The Moon’s position affects Earth’s spin by exerting tidal forces. These forces can speed up or slow down the rotation slightly. The 9 July 2025 event marks the shortest day since records began.
Shorter Days in 2025
Following 9 July, other days like 22 July and 5 August 2025 are also expected to be shorter. These repeated speed-ups show a pattern of Earth’s rotation accelerating temporarily. Scientists monitor these changes closely to understand their causes and predict future variations.
Leap Seconds and Timekeeping
To keep clocks aligned with Earth’s rotation, leap seconds are added or subtracted. Since 1972, 27 positive leap seconds have been inserted to compensate for Earth’s slowing spin. A leap second is usually added at the end of June or December. However, due to the recent acceleration, a negative leap second is planned for 2029. This will be the first time a second is removed from Coordinated Universal Time (UTC).
Negative Leap Second Explained
A negative leap second subtracts one second from the official time to correct for faster Earth rotation. This adjustment ensures that atomic clocks remain in sync with astronomical time (UT1), which is based on Earth’s actual spin. The International Earth Rotation and Reference Systems Service (IERS) is responsible for monitoring Earth’s rotation and deciding on leap second adjustments.
Scientific Significance
While the shorter days do not pose any danger, they challenge scientists and timekeepers. Precise timekeeping is critical for navigation, communication, and scientific research. Understanding the factors behind Earth’s rotation changes helps improve models of Earth’s interior and its interaction with the Moon and Sun.
Atomic Time vs Astronomical Time
Atomic time (UTC) is measured using highly stable atomic clocks worldwide. Astronomical time (UT1) depends on Earth’s rotation. When these two time scales diverge by 0.9 seconds or more, leap seconds are introduced to realign them. Recent faster rotation means UT1 is ahead of UTC, prompting the need for a negative leap second.
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