How does conservation of angular momentum affect Earth's rotation?

When the 2011 Japan earthquake shoved a tectonic plate deep into the mantle, Earth's mass shifted inward. To conserve angular momentum, the planet spun faster, shortening our day.

The same physical law that makes a figure skater spin faster when they pull their arms in governs the Earth. Our world is not a perfectly rigid, solid sphere; it is a dynamic, squishy planet that constantly rearranges its mass through geological and atmospheric processes. When that mass shifts closer to or farther from the planet's axis of rotation, the Earth must speed up or slow down to keep its total angular momentum constant.

Here is how this mass redistribution changes the length of our day:

Melting Polar IceGlobal climate change is currently melting vast ice sheets in Greenland and Antarctica. These ice masses are located near the poles—right on Earth’s axis of rotation. As the ice melts, the resulting water flows into the global oceans and bulges around the equator. In the figure skater analogy, this is the Earth throwing its arms out. Moving that mass away from the rotation axis increases Earth's moment of inertia, forcing the planet to spin slower. Because of this, our days are currently getting longer by tiny fractions of a millisecond.

Glacial Rebound
During the last Ice Age, heavy ice sheets covered regions like Canada and Northern Europe, pressing down on the Earth's crust and squeezing mantle material outward. When the ice melted, that immense weight vanished. Ever since, the depressed land has been slowly rebounding, pulling dense, deep mantle material back up toward the poles. Because this dense material is moving closer to the Earth's axis of rotation, it acts like a skater pulling their arms in. For thousands of years, this post-glacial rebound has been steadily speeding up Earth's rotation.

Massive Earthquakes
When tectonic plates violently shift during huge earthquakes, they can reorganize vast sections of the crust in an instant. As seen in the 2011 Japan quake, when a dense plate dives deeper toward the center of the Earth through subduction, mass moves closer to the rotational axis. This sudden inward shift acts like a skater sharply pulling their arms in, speeding up the planet's rotation by microscopic but measurable amounts, such as the 1.8 microseconds shaved off the day in 2011.

Earth's rotational speed is constantly fluctuating as the planet redistributes its mass. Source: Wikimedia Commons.

Why was Earth's rotation faster hundreds of millions of years ago, and what evidence supports this?

 This is how close the Moon was to the Earth about four billion years ago. Today’s view of the Moon today is to the left for comparison.

The Moon affects the Earth with tides, in the water and in the crust. This gravitational energy slows the Earth’s rotation. Being 81 times more massive, the Earth slowed the Moon’ s rotation to once every 27.8 days, called tidal lock. This lost energy had to go somewhere, so, being less massive, the Moon’s distance from the Earth had to take up the slack; energy is conserved. A higher orbit takes more energy, so the Moon moves away about one inch/2 cm with every 27.8 day orbit.

This has been measured very precisely. There is a laser reflector on the Moon set there by the Apollo astronauts used to measure this.

Why does the Chinese-made Three Gorges Dam slow down the Earth's rotation? What are the side effects?

As the world's largest dam, this reservoir can hold up to 42 billion tons of water.

Now, you might think that water is just water and it doesn't matter where it is. But that's wrong.

Water has mass, and mass has gravity. And gravity influences the speed at which the Earth rotates around its axis.

As you know, the Earth is not a perfect sphere. It is slightly flatter at the poles and bulges at the equator.

This is due to centrifugal force caused by rotation. The faster the Earth rotates, the flatter the Earth becomes.

However, moving large amounts of water from one place to another changes the distribution of mass on the Earth's surface.

And that changes the amount of centrifugal force present in different locations on Earth.

The Three Gorges Dam moved a massive amount of water from a low altitude to a high altitude close to the Earth's axis of rotation.

This means that the centrifugal force of the water in the reservoir will be weaker than before, pulling more strongly on the Earth's core. As a result, the Earth's center will become rounder and the top will become flatter.

What would happen if we made something even rounder?

The rotation will slow down.

That's right, the Three Gorges Dam slowed down the Earth's rotation by increasing its moment of inertia.

The moment of inertia is a measure of how difficult it is to change the angular motion of an object.

The greater the mass near the axis of rotation of an object, the larger its moment of inertia becomes, and the slower its rotation.

Now, the spin won't slow down that much. According to NASA scientists, the Three Gorges Dam will only shorten the length of a day by a few hours.It was extended by 0.06 microseconds.。

That's 0.00000006 seconds. You can't perceive it with the naked eye.

Perhaps, in the long run, we'll have another second to spare. Thank you, China!