Here's a neat question you may have never thought to ask. How far away is the horizon?
你可能从未思考过这个简单的问题，地平线有多远？
Think about it.
想想这个问题，
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.
从我们的视角看来，地球似乎是平的，但是要是你待在沙漠里或是在海上的话，你会感觉地球在向四面八方无限地延伸。
This isn't correct, though; the planet is round, so at some point the ground must drop away.
然而这是不正确的，地球是圆的，因此陆地在某一点必下沉 。

When it drops below your angle of vision, you can't see it any more, and that's what we call the horizon.
当地面下沉到你的视角之下时，你就无法再看到，这就是我们所说的地平线。
So how far can you see before the planet curves away?
那么在地球发生弯曲之前，你可以看多远呢？
If you're six feet tall, the answer is about three miles. Surprised?
如果你有六尺高的话，你可以看差不多三英里远。震惊吗？
You aren't alone.
可是并不是只有你一个人感到震惊。
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.
尝试着下次问下你朋友这个问题：在一个晴天，你站在一个沙滩上，猜猜你可以看到多远的海面。
She's more likely to say twenty or fifty miles than just three.
她很有可能会说二十或五十英里，而不是三英里。
Now that we're thinking about horizons, here's another cool thing to think about.
既然我们在思考关于地平线的问题，我们要思考另一个很酷的问题。
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!
地平线在地球上的三英里之外，但是在太阳系的其他星球上地平线是更近还是更远呢，这取决于你所站立的星球的体积大小。
Take the moon.
以月球为例，
The moon is two thousand, one hundred and sixty miles in diameter, only about one quarter the size of the earth.
月球直径为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显，地平线也更近，只有大约1.5英里远。
One cool result?
这是不是一个很酷的答案？
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，地表弯曲得很快，因此当你站在一个撞击坑中，很可能身处其中而不自知，
The walls of the crater are below the horizon on all sides!
因为撞击坑的岩壁处在地平线以下。