Earth will complete a rotation 1.33 milliseconds earlier than usual on Tuesday, August 5. That makes it one of the shortest days of 2025 at 86,399.99867 seconds long. How that happens, and how we can even measure it with such precision, might make your head spin faster too.
Author
- James O'Donoghue
Research Associate Professor in Planetary Astronomy, Meteorology, University of Reading
On average, Earth physically rotates in 23 hours, 56 minutes, 4 seconds and 90.5 milliseconds - this is called a sidereal day. It is Earth's "true" rotation relative to distant objects in deep space, like stars.
However, the kind of day most people go by is 24 hours long and that is called a solar day - it's the time between two sunrises, or consecutive noons. The extra 4 minutes comes from the fact that Earth has to rotate 1 more degree, to 361 degrees, for the Sun to appear in the same place again.
Both kinds of day are slightly shorter on August 5 2025, largely due to what is happening with winds in Earth's atmosphere, fluid circulation in the ocean and magma - and even the Moon's gravitational pull.
Deviations from 24 hours have been accurately measured since the 1970s using atomic clocks and astronomy. Over the course of a year, these changes build up: in 1973, for example, the sum of deviations added up to +1,106 milliseconds, meaning that the Earth lagged behind in its rotation by just over a second. Leap seconds were introduced in the same year to correct for this, with one second added to the clock at the end of the day - 23:59:60.
Absurd levels of accuracy are needed in time-keeping. Global positioning systems (more commonly called GPS) can pinpoint where you are in space, that's no problem. But if the planetary surface you are on has physically spun slightly faster or slower than expected that day, an uncorrected GPS won't know that, and your position won't match with your map.
A 1.33 millisecond deviation translates to a position error of about 62 cm at the equator, so 1973's cumulative drift would have caused GPS errors of around half a kilometre if left uncorrected over the year.
Why doesn't the Earth stay still?
To find out how fast the Earth is spinning at all, you need to find a reference frame in which, ideally, nothing is moving. Everything in space moves relative to everything else, but the farther we look, the more still things seem; just as distant hills appear to move slower while you're on a train, and nearby farms rush by.
Luckily, there are objects so magnificently bright that they outshine entire galaxies. These are quasars, and they are visible across the universe from billions of light years away.
Quasars are supermassive blackholes up to billions of times the mass of our Sun, which emit between 100 and 10,000 times more light than our entire galaxy, the Milky Way. Quasars are detectable from billions of light years across the universe, where things are essentially stationary, so they act as cosmic beacons.
Radio telescopes measure our position relative to these, yielding values of Earth's true rotation period to sub-millisecond accuracy.
Those ultra‑precise observations are also the starting point for computer models which include movements of the atmosphere, oceans, celestial motions and more to predict the length of day. This is how we know, in advance, when a day is shorter, and how to correct GPS as a result.
Winds in Earth's atmosphere are the biggest influence on the length of each day as a result of their collisions with the land surface, particularly when they hit mountain ranges. Incredible as it may sound, wind actually slows the spin of the Earth this way.
Earth's prevailing winds are fastest in the northern hemisphere winter, but slowest from June to August, so the summer months always bring the shortest days of the year (even though we tend to say these are the "longest" days in the northern hemisphere, because of their greater daylight duration).
These daily and seasonal changes are just short‑lived blips atop broader slowdowns. Over decades, the melting of the polar ice caps has been slowing the Earth's rotation. To understand why, consider a spinning ballerina retracting their outstretched arms - they begin to spin much faster. A spinning ball, like Earth, is no different.
Earth is oblate, meaning the surface at the equator is 21.5 km farther from the centre of the planet than the surface at the poles. As climate change melts the polar ice caps, meltwater moves from the poles to the equator via the ocean. Rising sea levels mean water is farther from the surface, and just like the ballerina moving their arms back out, it aids Earth's slowdown. Redistribution of Earth's mass changes our rotation in similar ways, including by earthquakes .
The Moon, while beautiful, can be a huge drag over billions of years. Earth's oceans are raised by the Moon's gravity, but as the Earth rotates, the raised oceans are carried slightly ahead of the Moon in its orbit. But the Moon continues pulling on those oceans, dragging them backwards against the Earth's anticlockwise rotation, which slows us down.
Earth's rotational energy isn't lost, it's transferred to the Moon, which gains orbital speed and causes it to escape Earth's gravity a little better - this is why it's moving away from us at 3.8 cm a year . Our length of day has increased from 17 hours 2.5 billion years ago largely due to the Moon sapping Earth's angular momentum over the eons.
Earth's rotation has slowed every year from 1973 to 2020 (where precise measurements exist), with each year accumulating hundreds of milliseconds of lag , which has already been accounted for by adding 27 leap seconds. Things changed from 2020 - the Earth started spinning faster instead of slower every year, probably the result of angular momentum exchange between the Earth's core and mantle , but modulated by the numerous other motions we've explored.
July 5, July 22 and August 5 were singled out as some of this year's fastest days far in advance, because on top of the Earth's internal motions and seasonal quirks in atmospheric winds, the Moon's position in orbit also slows the Earth twice per orbit (every two weeks). This is because when the Moon is directly above the equator, all of its tidal drag acts east to west, but on these dates, it is positioned farthest north and south, weakening that effect.
You won't notice the sunrise arrive 1.33 milliseconds sooner, but to precision atomic clocks, quasar‐referenced astronomical measurements, it will be obvious.
James O'Donoghue receives funding from the UK Science and Technology Facilities Council (STFC).
