Our ancient ancestors divided their lives into days, following the natural rhythm of the rising and setting Sun. This system worked well for millennia but by the 20th century, scientists made a big discovery: the Earth is actually a terrible timekeeper.
Why? It turns out that no two rotations – no two days – are ever exactly the same length.
There are many factors that can affect the Earth’s spin, including earthquakes. The 2011 quake in Japan – the one that triggered the Fukushima nuclear accident – sped up Earth’s rotation by 1.8 millionths of a second.
The Moon’s gravity also contributes to all of this. It tries to hold our oceans in place, but the Earth continues to turn beneath them regardless. This tidal friction robs the Earth of a little rotational energy, meaning the day is getting longer by about two-thousandths of a second (2 milliseconds) per century.
Slowing may be the long-term trend, but there have been short-term records in the other direction. For instance, 29 June 2022 was the shortest day ever recorded, at 1.59 milliseconds faster than average, beating the previous record set in 2020. This has been put down to changes deep within Earth's core.
Scientists have also discovered that so-called megastructures built by humans can also affect the Earth’s rotation. Take the 185m (about 600 feet) tall Three Gorges Dam. Spanning the Yangtze River in Hubei province, Central China, it is the largest dam in the world and is over 2,300m (7,500 feet) in length.
Its vital statistics are dizzying. It was made using 28 million cubic metres of concrete and enough steel to build 63 copies of the Eiffel Tower. It took 40,000 people 17 years to construct, at a total cost of $37 billion (£28 billion). The dam can hold 40 billion cubic metres of water – about 16 million Olympic-sized swimming pools.
Back in 2005, NASA scientist Dr Benjamin Fong Chao calculated that when that amount of mass is concentrated into one spot it is enough to affect the Earth’s rotation. That claim has recently been doing the rounds again on social media, but how much of an effect does the dam really have and why?
Well, Chao calculated that, when full, the dam could increase the length of the day by 0.06 microseconds. That’s 60 billionths of a second.
Chao also calculated that the dam can move the poles of the Earth by around two centimetres. The actual amount is always changing, though. According to Prof Maik Thomas and Dr Robert Dill, from the German Research Centre for Geosciences, there's "a more or less seasonal variation in Earth's rotation due to seasonal changes in water level".
But how can something we’ve made affect the rotation of the entire planet?
How buildings can change Earth's spin
It all has to do with something called the 'moment of inertia'. This essentially how much an object resists changes in motion – the more mass an object has, and the further away the mass is from the centre of rotation, the more it resists spin.
The Three Gorges Dam sits 185m above sea level at its highest point. And when the dam is full, both the local mass and the distance of that mass from our rotational line have increased. In other words, the moment of inertia goes up, creating a (very very small) resistance to Earth's spin.
To see how this slows the Earth down, next we need to talk about angular momentum. It's basically the total 'quantity' of spinning an object has, which depends on both the speed of rotation and how the object's mass is spread out.
A key point about angular momentum is that it’s always conserved. This means the total angular momentum in a system stays the same and can't change.
Ice skaters are often used as a way to explain how this works. With their arms outstretched, they spin relatively slowly. Draw their arms in, however, and they dramatically speed up. This is because their total angular momentum has to stay the same.
If they become more compacted, they decreasing the distance of their mass to the axis of rotation. This means their speed must increase in order to maintain the same angular momentum.
The same angular momentum conservation applies to the Earth-Moon system, but the effect is the reverse. We saw earlier how the interaction between the Moon and the Earth is slowing the Earth’s rotation down. In order to maintain the total 'quantity of spin', that also means that the system has to become less compact.
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That’s exactly what’s happening – experiments left on the Moon by the Apollo astronauts show that the Moon is receding from the Earth by 3.8 centimetres per year (which, incidentally, is about the same speed that your fingernails grow). It's the cosmic equivalent of the ice skater spreading out their arms.
Angular momentum depends both on the moment of inertia (that's the resistance to spin, remember) and rotation speed. In the case of the Three Gorges Dam, we’ve seen that when it's full, the dam increases the local moment of inertia. So, in order to maintain the same overall angular momentum, Earth's rotation speed must go down. That’s why the dam slows the Earth’s rotation by 60 billionths of a second. “The effect of other megastructures might be even smaller,” say Thomas and Dill.
They also point out that other human activity has had a bigger impact on Earth's rotation than Three Gorges. “Lake Aral has lost more than quarters of its water volume since 1960,” they say.
Lying between Kazakhstan and Uzbekistan, Aral used to be the third-largest lake in the world before the Soviets diverted many of the rivers that once fed it for irrigation projects. Thomas and Dill estimate that the resulting slowdown in Earth's rotational speed was over three times greater than the effect of building Three Gorges.
“The shift of the Earth's rotational axis towards Canada due to ice mass losses in Greenland is significantly higher,” say Thomas and Dill. They estimate that the resulting changes to the length of the day are around ten times greater than when Three Gorges was first filled.
So, what's all the fuss?
Given the minuteness of these changes, why does it matter? Well, in order to control our satellites and successfully navigate the probes visiting the other planets of our solar system, space agencies need to know the orientation and rotation period of the Earth to a very high level of accuracy.
The subtle changes caused by both megastructures like Three Gorges – and, to an even greater extent, the way we are altering the natural world around us – are significant enough to throw the probes off if they aren't adequately accounted for.
Given these important effects – on top of the changes wrought by the Moon, earthquakes and other rumblings deep within the bowels of the Earth – it is no wonder scientists struggle to use Earth as a timepiece. The second is no longer defined as a fraction of Earth's rotation period. Instead is defined as the time it takes for a caesium atom to oscillate a little over nine billion times.
Our ancestors may have used the planet beneath their feet as a giant clock, but ultimately it's a very unreliable one. And the more we add to and take from the world, the more this will be true.
About our experts
Dr Benjamin Fong Chao is a geophysicist based at the Institute of Earth Sciences, Academia Sinica. His work has been published in IEEE Transactions on Geoscience and Remote Sensing, Surveys in Geophysics and Global and Planetary Change.
Prof Maik Thomas is a geologist based at the German Research Centre for Geosciences. His work has been published in the Journal of Geodesy, Geophysical Research Letters and Geoscientific Model Development and Global and Planetary Change.
Dr Robert Dill is a geologist based at the German Research Centre for Geosciences. His work has been published in Geophysical Journal International, the Journal of Advances in Modeling Earth Systems and Advances in Space Research.
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