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The main reason the Earth is rotating now, is that there is nothing to stop it. The Earth and everything on it, including you, me, the ocean, and the atmosphere is spinning, revolving, and moving through the emptiness of space. Our planet moves through a vacuum. There is hardly anything out there to drag on us. The air is held here by gravity. Above it, space gets emptier and emptier.
The Earth got started spinning a little over four and a half billion years ago, when it formed from cosmic dust. Tiny as they may be, specks of dust have gravity. Check out a dusty shelf sometime. The Earth isn't just pulling the dust down. The dust is, ever so slightly pulling the Earth up. (Weird, yes?) The particles left over from the explosions of distant stars pulled themselves together and formed our Sun, the asteroids, the other planets and moons, and our own Earth.
The Earth and the other planets are enormous balls of deep-space dust. The dust particles came from all different directions in space. We can see stars in every direction, and with sensitive enough telescopes, we can see exploding stars in every direction. The process of cosmic dust-making is still going on. When the Earth formed, the dust didn't come together exactly evenly -- almost, but not exactly. This unevenness made the Earth take on a slight spin.
Try this. Put a quarter on a smooth table. Now, slide pennies or dimes right at the quarter, bumping and nudging it. Watch the picture (the eagle or George Washington). You'll notice that it's very hard to bump the quarter without making it rotate just a little bit. The same thing happened with the dust. As it came together it took on a little spin, a little rotation. And, that motion is still with us today. It makes the world go round. Whee!
http://www.ccmr.cornell.edu/education/ask/?quid=118
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