The Three Gorges Dam was built to control a dangerous river and boost a fast-growing economy. NASA scientists say it is so big that it changes how the Earth rotates a little bit. The effect is very small, almost funny small, but it shows that human engineering can now be seen on a planetary scale.
The mega-dam that slightly bends time
The Three Gorges Dam is in Hubei province, China, and it crosses the Yangtze River. It is now the largest hydroelectric dam in the world in terms of installed capacity, and it is one of the most ambitious infrastructure projects ever attempted.
The building started in the 1990s and went on in stages until about 2012. The dam caused flooding in whole towns, forced millions of people to move, and made an artificial lake that was hundreds of kilometers long.
Three main goals guided Chinese officials in building it: to produce much-needed electricity, to stop deadly floods from happening downstream, and to show the country’s industrial and political strength. The reservoir looks like a blue scar on the continent when seen from space.
The Three Gorges Dam holds so much water that it has a small effect on how the Earth spins on its axis.
What amount of water are we talking about?
The Three Gorges reservoir can hold about 40 cubic kilometers of water when it is full. That is about 10 trillion gallons, which is enough to fill millions of Olympic-sized swimming pools.
There is a place for this water to go. It is kept at a higher level than it would naturally be, thanks to concrete and gravity. That means that a lot of mass has moved in relation to the center of the Earth, according to physics.
- Hubei province on the Yangtze River.
- Reservoir volume full: about 40 km³ of water
- The building took about 18 years and happened in stages.
- China’s share of electricity demand is only about 3% instead of the 10% that was promised.
This moving of mass is what makes the strange idea that a dam can change the length of a day possible.
NASA says yes, the day on Earth really does change.
NASA scientists have been looking into how mass movements on and inside Earth change the planet’s rotation for a long time. They said in 2005 that big things like mega-earthquakes and filling up reservoirs leave small marks on the way the Earth spins.
The earthquake and tsunami in the Indian Ocean in 2004 was a very clear example. That event was so strong that it changed the way mass is spread out in the crust and mantle of the Earth.
NASA’s math shows that the 2004 earthquake moved Earth’s internal mass, which made the day about 2.68 microseconds shorter.
A huge body of water can slow down the planet, just like a sudden tectonic shift can speed it up. A dam like Three Gorges slightly increases Earth’s moment of inertia by moving a lot of water farther from the center of the Earth. In short, it makes the spinning skater move their arms out a little.
The analogy of an ice skater
Imagine a figure skater spinning with their arms close to their body. The skater’s rotation slows down when they stretch their arms out. The same physics works for Earth.
The Earth is not a perfectly hard ball. It acts more like a complicated spinning top made of rock, metal, water, and air. The balance of that spinning top changes when big things move, like when ice sheets melt, groundwater is pumped, or huge reservoirs are filled.
| Effect | Estimated change |
|---|---|
| Change in the length of the day | Add about 0.06 microseconds to the time. |
| The shape of the Earth | A little rounder at the equator and a little flatter at the poles |
A millionth of a second is one microsecond. That means that 0.06 microseconds is the same as 0.00000006 seconds. No one will notice this at breakfast, but modern geophysical tools can measure the change.
Projects made by people that push a planet
This is not the only thing that the Three Gorges Dam does. Theoretically, any big piece of infrastructure that moves a lot of water or rock has an effect on the rotation of the Earth.
Benjamin Fong Chao, a NASA scientist, once said that every global event that involves mass movement is important, from seasonal weather patterns to something as simple as driving a car. Most of these changes are so small that only very accurate satellite measurements and long-term observations can find them.
Human activity has progressed to a point where our most significant endeavors are comparable to earthquakes, ice sheets, and ocean currents.
Not all sources of change are the same, though. Tectonic events and the gradual reconfiguration of ice, oceans, and continents persist as the primary influences over geological timeframes.
Changes in the climate and the movement of large groups
Climate change makes things even more complicated. When ice sheets in Greenland and Antarctica melt, more water flows into the oceans. This moves mass from high latitudes toward the oceans and lower latitudes. That can also change the way the Earth spins and even move the axis of rotation a little bit.
Satellites that keep an eye on changes in Earth’s gravity field are already keeping track of these small changes. They help scientists figure out how fast ice is melting and where water is moving around the world.
Does a longer day make a difference for us?
It sounds like a lot when you see it in a headline and a dam, but in real life, it doesn’t really change anything.
Atomic clocks, which set our official time standards, are precise enough to pick up on these differences. To keep our clocks in sync with the planet’s slightly wobbly rotation, timekeepers already make changes every now and then, like leap seconds. These changes are caused by a number of things, including tides, interactions between the core and mantle, winds in the atmosphere, ocean currents, and, on the edges, big reservoirs like this one.
These numbers are important in fields that need very high accuracy, like satellite navigation, Earth observation, and communication with deep space. When plotting spacecraft trajectories or comparing decades of climate data, engineers and scientists need to take small changes in rotation into account.
In simple terms, what is “moment of inertia”?
“Moment of inertia” is a good word to use here. It tells you how hard it is to change the way something spins. The moment of inertia is bigger when mass is far from the center, making it harder to speed up the spin.
The Three Gorges project raises the Earth’s moment of inertia a little bit by lifting trillions of gallons of water higher up and spreading it out over a long reservoir. The speed of rotation slows down just enough to add a tiny bit of time to the day.
Engineers already work with this idea on a smaller scale. For instance, when designing wind turbines, spinning machinery, or even sports equipment, the way the weight is spread out affects how well they work and how stable they are.
A look at engineering on a planetary scale in the future
The story of the Three Gorges Dam and the Earth’s rotation is a taste of bigger arguments that may get worse this century. Our physical footprint on the planet will keep getting bigger as societies build bigger dams artificial islands, underground cities, and coastal defenses.
This huge dam in China doesn’t threaten the stability of the Earth or change the way time is kept in a big way. Its effect on rotation is an interesting scientific question, not a disaster waiting to happen. But it shows how closely human choices are connected to planetary systems that once seemed untouchable.
Future projects, like huge pumped-storage systems or geoengineering ideas that change the way water flows or reflect sunlight, will raise the same kinds of questions. To weigh the pros and cons of things like clean power or flood control against small, long-term side effects, scientists will need clear models and the public will need clear language.
The Three Gorges Dam is more than just a power plant in that way. It shows how you can follow a country’s infrastructure goals all the way to the rotation of the whole planet, down to the last 0.06 microseconds of a day.
