英语听力：自然百科 地球自转的奥秘

Well, the story of the earth spin goes back all the way to the origin of the solar system, four and a half billion years ago. The solar system started out as a cloud of gas and dust. And these gas and dust clumped together to form the sun and the various planets. And the way the earth grew was to cannibalize smaller clumps that were also growing. So if you were sitting on the surface of the earth, you would see a bombardment of particles all the way from dust particles to perhaps Mars-size objects. It turns out that it’s this bombardment which also gave the earth its spin. The earth that’s sitting out in space is bombarded by all these particles. It’s gonna have somerotation. So that's where the rotation of the earth comes from.

But that’s not the end of the story. It turns out in the early days the earth was rotating much faster than it is now. So it was perhaps rotating five times faster. So the question is ‘why has it slowed down?

Yeah.

And it turns out the main culprit is the moon. The moon raises tides on the earth, and it turns out that the moon attracts the ocean tides, and it attracts them in such a way that the gravity of the moon tries to stop the earth in its rotation. Well this is a very small effect, because the tides aren’t very high and the moon is very far away, so its gravity is weak. But this effect keeps on going on century after century and it’s been doing that for billions of years. So originally the earth started out rotating much faster, and now it’s slowed down quite a bit. The earth will continue to slow its rotation in the future. What’s peculiar is that the moon will move away from the earth for a while, the moon will start coming back and towards the earth and eventually will collide with the earth. But that’s not for billions of years, uh, into the future.