什么是海平面

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? Sea level seems like a pretty easy concept, right?

You just measure the average level of the oceans and that's that.

But what about parts of the earth where there aren't oceans?

For example, when we say that Mt. Everest is 8850m above sea level, how do we know what sea level would be beneath Mt. Everest, since there's no sea for hundreds of kilometers?

If the earth were flat then things would be easy - we'd just draw a straight line through the average height of the oceans and be done with it. But the earth isn't flat.

If the earth were spherical, it would be easy, too, because we could just measure the average distance from the center of the earth to the surface of the ocean.

But the earth isn't spherical - it's spinning, so bits closer to the equator are 'thrown out' by centrifugal effects, and the poles get squashed in a bit.

In fact, the earth is so non-spherical that it's 42km farther across at the equator than from pole to pole.

That means if you thought earth were a sphere and defined sea level by standing on the sea ice at the north pole, then the surface of the ocean at the equator would be 21km above sea level.

This bulging is also why the Chimborazo volcano in Ecuador, and not Mount Everest, is the peak that's actually farthest from the center of the earth.

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