The full of water reservoir of China's Three Gorges Dam can slow down Earth's rotation

The reservoir of China's Three Gorges Dam is so large that when it is full of water, the Earth's rotation will slow down and the daytime will be extended.
July 02, 2020 | 08:53
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It is undeniable that the reservoir can influence on the Earth's rotation despite these are small changes.

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Three Gorges Dam. Photo:

Here's the truth: Large earthquakes can affect the Earth's orbital motion, reduce the length of the day, deform the planet's surface or even cause the Arctic to move slightly. However, all these changes are extremely small and not serious enough for us to feel. Despite this fact, science still has a way to calculate the changes, no matter how small the number is.

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Photo: Xinhua

"Any action involving the movement of a large amount of matter can affect the rotation of the Earth, from seasonal weather to land vehicles," Professor Benjamin Fong Chao from Goddard Space Travel Center, NASA said.

Therefore, it's not uncommon for the Three Gorges Dam, the world's largest hydroelectric project (based on an installed capacity of 22,500 megawatts), to be able to affect the orbital motion of the Earth. When the reservoir is full, the water level of this dam will be 175m above sea level. The reservoir has 660 km in length and 1.12 km in width. It's volume is up to 39.3 km3, meaning that the water of the Tam Hiep lake will be 42 billion tons when it is full.

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Photo: Xinhua

Such a big volume will affect the rotation of the Earth, specifically the moment of inertia of rotational motion. Take the example of an ice skater rotating on the spot: when their arms is close to the reel, the moment of inertia decreases and their rotation speed increases; When stretching out their hands, the rotation speed will slow down.

The full reservoir of Three Gorges Dam is like stretching an ice skater with his arms out; due to a huge amount of water accumulated in one place, the mass of the Earth moves further away from the axis of rotation and thus the Earth's rotation speed slows down. This is the thing occurs in theory, but the actual number is not too large because 42 billion tons of water is no matter to the Earth's mass of 5.972 billion tons.

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Photo: Xinhua

NASA scientists calculated that the large volume of water in reservoir Three Gorges Dam will increase the day length to 0.0000000006 seconds. If there were no other changes, and the assumption of the reservoir's water level would remain the same so that the continuous length of the day would add up to 0.00000006 seconds, then in 47,650, the time of day would be increased by 1 second.

The influence is too small for us to feel, but at least math can show us the answer.

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