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Horizon
地平线
At a beach, you may see a line between the sea and the sky.
在海滩上，你可能会看到大海与天空之间的一条线。
It is called the horizon.
这叫做地平线。
Why do we see the horizon?
为什么我们会看到地平线呢？
Is it because the sky really meets the ground at that point?
那是因为天空真的在那点碰到了地面吗？
No, it's because the Earth is not flat.
不是，是因为地球不是平的。
Because the Earth is round, we cannot see line forever in any one direction.
因为地球是圆的，我们在任何一个方向上都看不到直线。
Even if there are no trees or buildings around us.
即使我们周围没有树木或建筑物。
Rather we see the horizon.
相反，我们看到了地平线。
Hundreds of years ago, people thought the Earth ended at the horizon.
几百年前，人们认为地球在地平线上结束了。
So they thought they would die if they went beyond the horizon.
所以他们以为一旦越过了地平线就会死。
Now, we know that's not true.
现在，我们知道那不是真的。
