On Tuesday, August 5, 2025, Earth will complete its rotation 1.33 milliseconds earlier than the standard 24-hour day, making it one of the shortest days of the year at 86,399.99867 seconds. The change is too small to be noticed without highly precise instruments, but it is measurable with modern technology.
A sidereal day — Earth’s true rotation period relative to distant stars — lasts 23 hours, 56 minutes, 4 seconds and 90.5 milliseconds. The solar day, which is the basis for civil time, is about four minutes longer because Earth must rotate slightly more than 360 degrees for the Sun to appear in the same position in the sky.
On August 5, both the sidereal and solar days will be slightly shorter than average. Scientists attribute this to a combination of factors, including atmospheric winds, ocean currents, movements within Earth’s molten interior, and the gravitational pull of the Moon.
Since the 1970s, deviations in Earth’s rotation have been measured using atomic clocks and astronomical observations. In 1973, for example, the cumulative lag over the year was +1,106 milliseconds, prompting the introduction of leap seconds to keep atomic time aligned with Earth’s rotation.
Even small deviations matter for systems like GPS. A 1.33 millisecond difference can cause a positional error of about 62 centimetres at the equator. Without corrections, the drift in 1973 would have led to GPS errors of around half a kilometre.
To measure Earth’s rotation with such precision, scientists use quasars — extremely bright, distant objects powered by supermassive black holes. Because they are billions of light years away, quasars appear essentially stationary, making them ideal reference points. Radio telescopes track Earth’s position relative to these cosmic beacons, achieving sub-millisecond accuracy.
These measurements feed into computer models that account for atmospheric, oceanic, and celestial influences to predict day length. Winds are the largest short-term factor, as they interact with Earth’s surface and can slow its spin. In the northern hemisphere, winds are fastest in winter and slowest in summer, making June to August the period with the shortest days.
Long-term changes also affect rotation. Melting polar ice redistributes mass toward the equator, slowing Earth’s spin in the same way a spinning skater slows when extending their arms. Earthquakes and other geological events can also shift mass and alter rotation.
The Moon’s tidal pull has been gradually slowing Earth for billions of years. As tidal bulges are carried ahead of the Moon’s orbit, the Moon’s gravity drags them back, transferring rotational energy from Earth to the Moon and causing it to drift away at 3.8 centimetres per year.
From 1973 to 2020, Earth’s rotation slowed each year, requiring 27 leap seconds in total. Since 2020, however, Earth has been spinning slightly faster, likely due to interactions between its core and mantle.
August 5 was identified in advance as one of the year’s fastest days, along with July 5 and July 22, due to a combination of internal dynamics, seasonal wind patterns, and the Moon’s position reducing tidal drag.
While the change will not be perceptible to human senses, “to precision atomic clocks, quasar‐referenced astronomical measurements, it will be obvious.”
Source: The Conversation
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