HOW COME? When the Earth moves, we don't

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The next time you get on an elevator in the lobby of a tall building, close your eyes. As the box you're riding in smoothly ascends, you may feel like you're not moving at all -- at least, until it slows to a stop at your floor. Think about it, and you'll realize you've had the same experience in a train or a car. Or even on a jet plane, traveling through the clouds at more than 500 mph. We're traveling on our planet, too, circling the sun, journeying through space with the rest of the solar system, all while Earth spins on its axis like a top.

In fact, our planet's rotational speed at the equator is higher than a commercial jet's cruising speed. The Earth measures about 24,900 miles around at its widest. Divide that by the 24 hours it takes to turn once, and we get the Earth's speed at the equator: a dizzying 1,040 mph.

But since the distance around the planet shrinks as we travel toward the poles, the relative speed changes, too. So at the latitude of New York City, the Earth's rotational speed is about 783 mph. Which means that each second (one hippopotamus, two hippopotamus) you've traveled 1,148 feet forward on your planetary merry-go-round. And as in a plane at constant speed, you're just not feelin' it.

Physicists discovered this principle centuries ago: In a closed box, no windows to peek from, there's no way to tell whether we're stopped still or moving at an unvarying speed. But if the "box" (or elevator, or plane) speeds up or slows down, the feeling of movement suddenly appears, too. We experience motion when it's changing.

Since Earth's rotational speed is so constant, we can't feel how fast we're really spinning. (The same goes for our 365-day trip around the sun, which our speedy planet whizzes through at 67,000 mph.) Although you're being spun to the east at nearly 800 mph, the matter in your body is strongly attracted to the much greater mass of matter of the planet. The centrifugal, outward "force" created by rotation is a tiny fraction of the strength of our planet's downward-directed gravitational force.

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But if the Earth's rotational speed changed suddenly, we'd realize we're moving at breakneck speed. If the Earth suddenly slowed, scientists say, we'd tumble forward; if it sped up, we'd fall over backward.

And if Earth's rotational speed increased to more than 18,000 mph (with a day lasting just 80 minutes), gravity could no longer keep us safely planted to the ground. And we would, indeed, go flying off into the dark.

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