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Here's a neat question you may have never thought to ask. How far away is the horizon?
你可能從未思考過這個簡單的問題，地平線有多遠？
Think about it.
想想這個問題，
The Earth seems flat from our viewpoint, and if you are in a desert or out at sea you get a sense that the earth extends indefinitely in every direction.
從我們的視角看來，地球似乎是平的，但是要是你待在沙漠里或是在海上的話，你會感覺地球在向四面八方無限地延伸。
This isn't correct, though; the planet is round, so at some point the ground must drop away.
然而這是不正確的，地球是圓的，因此陸地在某一點必下沉 。
When it drops below your angle of vision, you can't see it any more, and that's what we call the horizon.
當地面下沉到你的視角之下時，你就無法再看到，這就是我們所說的地平線。
So how far can you see before the planet curves away?
那么在地球發生彎曲之前，你可以看多遠呢？
If you're six feet tall, the answer is about three miles. Surprised?
如果你有六尺高的話，你可以看差不多三英里遠。震驚嗎？
You aren't alone.
可是并不是只有你一個人感到震驚。
Try asking a friend next time you're at the beach on a clear day to guess how far out across the ocean you are looking.
嘗試著下次問下你朋友這個問題：在一個晴天，你站在一個沙灘上，猜猜你可以看到多遠的海面。
She's more likely to say twenty or fifty miles than just three.
她很有可能會說二十或五十英里，而不是三英里。
Now that we're thinking about horizons, here's another cool thing to think about.
既然我們在思考關于地平線的問題，我們要思考另一個很酷的問題。
The horizon is three miles away on earth. But elsewhere in the solar system it's closer or farther, depending on the size of the body you're standing on!
地平線在地球上的三英里之外，但是在太陽系的其他星球上地平線是更近還是更遠呢，這取決于你所站立的星球的體積大小。
Take the moon.
以月球為例，
The moon is two thousand, one hundred and sixty miles in diameter, only about one quarter the size of the earth.
月球直徑為2160英里，只有地球的四分之一。
So the curvature at its surface is much more noticeable, and the horizon is much closer. It's only about 1.5 miles away.
因此，月球表面的曲率更明顯，地平線也更近，只有大約1.5英里遠。
One cool result?
這是不是一個很酷的答案？
The surface bends so quickly on the moon that it's possible to stand inside some of the largest impact craters and not know you're in one.
在月球上，地表彎曲得很快，因此當你站在一個撞擊坑中，很可能身處其中而不自知，
The walls of the crater are below the horizon on all sides!
因為撞擊坑的巖壁處在地平線以下。
