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Have you ever been floating in a swimming pool, all comfy and warm, thinking, "Man, it'd be cool to be an astronaut!
你是否曾经浮在游泳池里，感觉非常惬意，暖洋洋得，想着，“天啦，当一名宇航员太爽了！
You could float out in outer space, look down at the Earth and everything. It'd be so neat!"
你可以在外太空漂浮，俯瞰地球以及所有的一切。太棒了！“
Only that's not how it is at all.
但事实并不如此。
If you are in outer space, you are orbiting the Earth: it's called free fall.
如果你在外太空，你将会绕地球转动，这叫做自由落体。
You're actually falling towards the Earth.
事实上，你正朝着地球下落。
Think about this for a moment: that's the feeling you get if you're going over the top of a roller coaster, going, like, "Whoa!"
想一下这种感觉：你坐上云霄飞车，即将爬过顶端时会有 “喔哦！”的惊叹声。
Only you're doing this the whole time you're orbiting the Earth, for two, three, four hours, days. Whatever it takes, right?
只是这回，你将一直这样围绕着地球转，两个小时、三个小时、四个小时或好几天。无论多久都行，是吧？
So, how does orbiting work?
那么，怎么旋转的呢？
Let's take a page from Isaac Newton.
让我们请教一下牛顿吧。
He had this idea, a little mental experiment: You take a cannon, you put it on top of a hill.
他有这样一个想法，也是一个小的心理实验：你拿来一个大炮，把它摆在山顶上。
If you shoot the cannonball, it goes a little bit away.
如果你发射炮弹，它会射到不远处。
But if you shoot it harder, it goes far enough so that it lands a little bit past the curvature of Earth.
但是如果你更猛地发射，它将“嗖”的飞到足够远，以至于它降落在地球表面的曲度以外。
Well, you can imagine if you shot it really, really, hard, it would go all the way around the Earth and come back— boom! — and hit you in the backside or something.
那么，想象一下，倘若你非常非常猛地发射，它也许会绕着地球飞行很远，然后绕回来，砰一声，撞到你后背或什么东西。
Let's zoom way back and put you in a little satellite over the North Pole of the Earth and consider north to be up.
让我们把焦点拉回太空，并把你放在一个位于地球北极上方的小卫星里，假设北极方向为正向上。
You're going to fall down and hit the Earth.
你将会下落并撞向地球。
But you are actually moving sideways really fast.
然而，你实际正在非常快速地向一旁移动。
So when you fall down, you're going to miss.
所以，当你下落的时候，你将错过正下方。
You're going to end up on the side of the Earth, falling down, and now the Earth is pulling you back in sideways.
你将最终移到地球的一边，你往下落的同时地球把你往一旁拉。
So it's pulling you back in and you fall down, and so you miss the Earth again, and now you're under the Earth.
也就是说，你在下降时被地球往回拉，所以，你将再次错过地球。这时，你已经移动到地球的下面了。
The Earth is going to pull you up, but you're moving sideways still.
地球往上拉你，但是你仍旧往一旁移动。
So you're going to miss the Earth again.
所以，你不得不再次错过地球。
Now you're on the other side of the Earth, moving upward, and the Earth's pulling you sideways.
现在，你已经移动到地球的另一边，正往上移动，并且地球正把你拉向一边（垂直朝向地球）。
So you're going to fall sideways, but you're going to be moving up and so you'll miss.
也就是说，你在往一边下落，但是，你正在快速地向上移动，所以你再次错过地球。
Now you're back on top of the Earth again, over the North Pole, going sideways and falling down, and yep -- you guessed it.
现在，你再次回到了地球顶端，北极上方，往一旁移动，同时向下落，是的，你已经猜到了。
You'll keep missing because you're moving so fast.
你将一直错过（地球），因为你正在飞速移动。
In this way, astronauts orbit the Earth.
正是如此，宇航员围绕地球飞行。
They're always falling towards the Earth, but they're always missing, and therefore, they're falling all the time.
虽然他们一直朝地球下落，但他们却一直错过（落回地球），因此，他们一直在下落。
They feel like they're falling, so you just have to get over it.
他们也感觉自己在下落，你基本上只能接受这样一种感觉。
So technically, if you ran fast enough and tripped, you could miss the Earth.
所以，技术上说，如果你跑得足够快，突然绊一脚，你将脱离地球。
But there's a big problem.
但是，这里有个大问题。
First, you have to be going eight kilometers a second.
第一，你必须每秒跑8公里。
That's 18,000 miles an hour, just over Mach 23!
也就是每小时1万8000英里，超过23马赫（23倍音速）！
The second problem: If you're going that fast, yes, you would orbit the Earth and come back where you came from, but there's a lot of air in the way, much less people and things.
第二个问题：假设你能移动那么快，你将绕地球运动并且回到你的起点，但是一路上会有大量的空气，当然，人和其他事物极少。
So you would burn up due to atmospheric friction.
所以，大气摩擦将导致你燃烧至尽。
So, I do not recommend this.
因此，我并不推荐这样做。
