Have you ever been floating in a swimming pool, all comfy and warm, thinking,
你是否曾经漂浮在游泳池，非常惬意，暖洋洋，遐想不断，
"Man, it'd be cool to be an astronaut!
“天啊，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.
只是事实并不如此。
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!"
好吧，就此考虑一会儿：你得到的感觉是这样的，就像你坐上云霄飞车，即将爬过顶端，冲吧，“喔喔！”
Only you're doing this the whole time you're orbiting the Earth, for two, three, four hours, days.
只是这回你将一直这样，你将围绕着地球转动两个，三个，四个小时，好几天。
Whatever it takes, right? So, how does orbiting work? Let's take a page from Isaac Newton.
要几天就几天，对吧？那么，为什么会绕行地球呢？有请牛顿先生出场。
He had this idea, a little mental experiment:
他想法是这样的，这是一个小小的思想实验，
You take a cannon, you put it on top of a hill.
你将一门大炮放在一个小山顶上。
If you shoot the cannonball, it goes a little bit away.
如果你发射炮弹，飞到不远处就落地了。
But if you shoot it harder, it goes far enough so that it lands a little bit past the curvature of Earth.
但是如果你非常非常猛地发射，炮弹落地前会飞较长的距离，落在地表曲面的较远处。
Well, you can imagine if you shot it really, really, hard,
那么，想象一下，如果你把炮弹很快、很快的射出去，
it would go all the way around the Earth and come back -- boom! -- and hit you in the backside or something.
它会一路绕着地球走，最后绕回来，咚！比如说，打到你后背或命中什么东西。
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，并把你放进位于北极上空的人造卫星中，并设北方为向上方向。
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.
但实际上你是快速向一旁移动。
So when you fall down, you're going to miss.
所以，当你落下时，你会与地球擦身而过。
You're going to end up on the side of the Earth, falling down, and now the Earth is pulling you back in sideways.
你将最终移动到地球的一边，你往下落，而地球把你往一边拉。
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，你被地球往下拉，同时你往正下方降落，你也再度与地球擦身而过，现在，你在地球的下方。
The Earth is going to pull you up, but you're moving sideways still.
地球要把你往上拉，但你依然向侧边移动。
So you're going to miss the Earth again.
所以你还是与地球无缘。
Now you're on the other side of the Earth, moving upward, and the Earth's pulling you sideways.
现在，你在地球的另一边了，向上移动的同时，地球把你向一边拉。
So you're going to fall sideways, but you're going to be moving up and so you'll miss.
于是，你持续往一边偏。但是，因正在向上移动，所以你再次与地球擦身而过。
Now you're back on top of the Earth again, over the North Pole, going sideways and falling down, and yep -- you guessed it.
你终于再次回到地球上方，在北极上空，持续往侧边移动并且往地球落下。没错，正如你所预测的一样。
You'll keep missing because you're moving so fast.
因为你一直飞快地移动，所以你将一直与地球擦身而过。
In this way, astronauts orbit the Earth.
航天员就是以这种方式绕着地球转。
They're always falling towards the Earth, but they're always missing, and therefore, they're falling all the time.
他们持续朝地球落下，但始终错过地球，因此，他们一直在下落。
They feel like they're falling, so you just have to get over it.
他们也感觉自己正在下落，所以多少都会习惯这种感觉。
So technically, if you ran fast enough and tripped, you could miss the Earth.
在技术上来说，如果你跑得够快，突然被绊一脚而跳起来，你就可能对地球说再见了。
But there's a big problem. First, you have to be going eight kilometers a second.
但这里面藏着一个大问题。首先，你必须每秒狂奔8公里。
That's 18,000 miles an hour, just over Mach 23!
相当于每小时18000英里，超过23马赫！
The second problem: If you're going that fast, yes, you would orbit the Earth and come back where you came from,
其次是，如果你真的跑那么快，虽然你可以绕着地球转并且回到起点，
but there's a lot of air in the way, much less people and things.
但是一路上会有大量的空气等着你，更不用说还有人和其他东西。
So you would burn up due to atmospheric friction. So, I do not recommend this.
所以，大气摩擦将导致你燃烧至尽。因此，我并不推荐这样做。