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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.

  • So how do we know what sea level is? Well, water is held on earth by gravity, so we could

  • model the earth as a flattened & stretched spinning sphere and then calculate what height

  • the oceans would settle to when pulled by gravity onto the surface of that ellipsoid.

  • Except the interior of the earth doesn't have the same density everywhere, which means gravity

  • is slightly stronger or weaker at different points around the globe, and the oceans tend

  • "puddle" more nearer to the dense spots. These aren't small changes, either - the level of

  • the sea can vary by up to 100m from a uniform ellipsoid depending on the density of the

  • earth beneath it. And on top of that, literally, there are those pesky things called continents

  • moving around on the earth's surface. These dense lumps of rock bump out from the ellipsoid

  • and their mass gravitationally attracts oceans, while valleys in the ocean floor have less

  • mass and the oceans flow away, shallower.

  • And this is the real conundrum, because the very presence of a mountain (& continent on

  • which it sits) changes the level of the sea: the gravitational attraction of land pulls

  • more water nearby, raising the sea around it. So, to determine the height of a mountain

  • above sea level, should we use the height the sea would be if the mountain weren't there

  • at all? Or the height the sea would be if the mountain weren't there but its gravity

  • were?

  • The people who worry about such things, called geodetic scientists or geodesists, decided

  • that we should indeed define sea level using the strength of gravity, so they went about

  • creating an incredibly detailed model of the earth's gravitational field, called, creatively,

  • the Earth Gravitational Model. It's incorporated into modern GPS receivers so they won't tell

  • you you're 100m below sea level when you're in fact sitting on the beach in Sri Lanka

  • which has weak gravity, and has allowed geodesists themselves to correctly predict the average

  • level of the ocean to within a meter everywhere on earth. Which is why we also use it to define

  • what sea level would be underneath mountains... if they weren't there... but their gravity

  • was.

Sea level seems like a pretty easy concept, right? You just measure the average level

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B1 中級

海面とは? (What is Sea Level?)

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    Bing-Je に公開 2021 年 01 月 14 日
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