Earth Spun Faster Today. Here's How We Know.

Earth will finish a full spin 1.33 milliseconds sooner than normal on Tuesday, 5 August 2025. That puts it among 2025’s shortest days, with a duration of 86,399.99867 seconds.

Understanding why this happens - and how scientists can time it so precisely - is enough to make your head whirl as well.

On average, the planet’s physical rotation takes 23 hours, 56 minutes, 4 seconds and 90.5 milliseconds. This is known as a sidereal day, meaning Earth’s “true” spin when measured against distant deep‑space objects such as stars.

Most of us, however, live by a solar day, which is 24 hours: the interval between two sunrises (or from one noon to the next). The additional 4 minutes comes from the fact that, as Earth travels around the Sun, it must rotate an extra 1 degree - to 361 degrees - for the Sun to appear in the same position in the sky again.

On 5 August 2025, both the sidereal day and the solar day are a little shorter than usual. That is mainly linked to changes in atmospheric winds, ocean circulation and magma movement - plus the gravitational pull of the Moon.

Since the 1970s, departures from a perfectly regular 24‑hour day have been measured with high accuracy using atomic clocks alongside astronomical observations.

Across an entire year, tiny day‑to‑day differences add up. In 1973, for instance, the total of these deviations reached +1,106 milliseconds, indicating Earth’s rotation was lagging by a little over a second. Leap seconds were brought in that same year to compensate, by inserting an extra second at the end of a day - 23:59:60.

Such seemingly ridiculous precision is essential for modern time‑keeping. Global positioning systems (more commonly called GPS) can determine your location in space accurately - that part is straightforward. The difficulty is that if the planet’s surface has rotated slightly faster or slower than expected, an uncorrected GPS system cannot account for it, and your calculated position will not line up properly with your map.

A deviation of 1.33 millisecond corresponds to an error of roughly 62 cm at the equator. Left uncorrected over a year, the cumulative drift seen in 1973 would have produced GPS inaccuracies of about half a kilometre.

Why doesn’t Earth’s rotation stay perfectly steady?

To measure how quickly Earth is spinning at all, you need a reference frame in which, ideally, nothing is moving. In reality, everything in the universe moves relative to everything else - but the farther away you look, the more motion seems to disappear. It is like watching scenery from a train: distant hills appear to shift slowly, while nearby fields streak past.

Fortunately, there are objects so extraordinarily luminous that they can outshine entire galaxies: quasars, visible across the universe from billions of light years away.

Quasars are supermassive black holes with masses up to billions of times that of our Sun. They emit between 100 and 10,000 times more light than our whole galaxy, the Milky Way. Because they can be seen from billions of light years away - where the universe is effectively stationary from our viewpoint - they serve as reliable cosmic signposts.

Using radio telescopes, scientists measure Earth’s position relative to these quasars, allowing Earth’s true rotation period to be determined with sub‑millisecond accuracy.

Those ultra‑precise measurements also feed into computer models that incorporate atmospheric motion, ocean dynamics, celestial movements and other effects to forecast the length of day. That is how we can predict in advance when a day will be shorter - and apply the corrections needed for GPS.

The dominant day‑to‑day influence on day length is wind in Earth’s atmosphere. As air masses move and interact with the planet’s surface - especially when they collide with mountain ranges - they exchange momentum. Remarkably, this process can slow Earth’s spin.

The planet’s prevailing winds reach their highest speeds in northern hemisphere winter, and are generally slowest from June to August. As a result, the summer months tend to produce the shortest days of the year (even though these are often described as the "longest" days in the northern hemisphere, because they bring more hours of daylight).

These daily and seasonal swings are short‑term fluctuations on top of longer‑term trends. Over decades, the melting of the polar ice caps has been contributing to a slowdown in Earth’s rotation. The reason is similar to what happens to a spinning ballerina: pull the arms in and the spin speeds up; stretch them out and the spin slows. A rotating body such as Earth follows the same physics.

Earth is oblate, which means the surface at the equator lies 21.5 km farther from the planet’s centre than the surface at the poles. As climate change melts polar ice, the resulting water shifts from the poles towards the equator through the oceans. As sea levels rise, more mass ends up farther from the axis of rotation - like the ballerina extending their arms again - helping to slow Earth’s spin. Similar redistribution of mass can alter rotation as well, including effects triggered by earthquakes.

The Moon, for all its beauty, has also acted as a powerful brake over billions of years. Lunar gravity raises tides in Earth’s oceans, and as Earth rotates, these tidal bulges are carried slightly ahead of the Moon along its orbit. The Moon continues to tug on the displaced water, pulling it back against Earth’s anticlockwise rotation, which reduces our spin rate.

Earth’s rotational energy is not destroyed; instead, it is transferred to the Moon. The Moon gains orbital energy, improving its ability to escape Earth’s gravity - which is why it is receding from us by 3.8 cm a year. Over geological time, this has lengthened our day: it has increased from 17 hours 2.5 billion years ago, largely because the Moon has been draining Earth’s angular momentum over the ages.

From 1973 through to 2020 (the period for which precise measurements are available), Earth’s rotation slowed each year, with annual lags accumulating by hundreds of milliseconds - a drift that has already been compensated for by adding 27 leap seconds.

From 2020, the pattern shifted: instead of slowing year after year, Earth began rotating faster annually. This is thought to be linked to exchanges of angular momentum between Earth’s core and mantle, although it is also shaped by the many other influences described above.

The dates 5 July, 22 July and 5 August were identified well ahead of time as some of the year’s fastest days because, alongside internal Earth processes and seasonal changes in winds, the Moon’s orbital position also affects rotation by slowing Earth twice per orbit (every two weeks).

This happens because when the Moon sits directly over the equator, all of its tidal drag acts from east to west. On the dates listed above, however, it lies farthest north or south, which weakens that braking effect.

You will not perceive sunrise arriving 1.33 milliseconds earlier - but to atomic clocks and quasar‑referenced astronomical measurements, the difference will be unmistakable.

James O'Donoghue, Research Associate Professor in Planetary Astronomy, Meteorology, University of Reading

This article is republished from The Conversation under a Creative Commons licence. Read the original article.