Why on Earth is the planet’s day getting shorter?

On Tuesday, August 5, 2025, Earth will complete its rotation 1.33 milliseconds earlier than average, resulting in one of the shortest days of the year. This subtle but significant change in day length is primarily influenced by natural processes such as atmospheric wind patterns, ocean currents, magma movement, and the gravitational pull of the Moon. Scientists can measure such minute variations with extreme precision using atomic clocks and astronomical observations.

The planet’s true rotation, known as a sidereal day, lasts just under 24 hours relative to distant space objects, while the solar day — from sunrise to sunrise — is roughly 24 hours due to Earth's orbit around the Sun. The shortening of both the sidereal and solar days on this date is attributed largely to seasonal shifts in winds, oceanic and geological fluid flows, and lunar tidal forces.

To achieve the extraordinary accuracy required for these measurements, researchers rely on quasars — immensely bright and distant cosmic sources — as fixed points in the sky. By referencing Earth's rotational position relative to these celestial beacons, astronomers determine variations in Earth’s spin down to sub-millisecond levels, allowing them to predict deviations in day length and adjust global systems such as GPS accordingly.

Besides short-term daily fluctuations influenced by winds that physically slow Earth's rotation, longer-term trends reveal a gradual slowing over decades caused by redistribution of mass on Earth due to melting polar ice caps and rising sea levels. This phenomenon shifts the planet’s mass away from its axis, akin to a spinning ballerina extending their arms, which reduces rotational speed. Additionally, tectonic events and lunar tidal friction contribute to changes in day length over geological timescales.

Interestingly, since 2020, Earth’s rotation has occasionally sped up, due to complex exchanges of angular momentum between the Earth's core and mantle. Specific dates in 2025, such as July 5, July 22, and August 5, have been identified as some of the year's fastest days, influenced by the Moon's orbital position and its interaction with Earth's rotation via tidal forces. These subtle variations, while imperceptible to daily life, are critical for atomic clocks and technologies that require precise timing.

Overall, the shortening of Earth's day highlights the dynamic and interconnected systems influencing our planet, from atmospheric conditions and oceanic flows to deep Earth processes and celestial mechanics. Understanding these changes allows scientists to maintain the accuracy of timekeeping systems fundamental to navigation and communication worldwide.