(单词翻译:单击)
Here's a question for you: Which looks bigger, the full moon on the horizon, or your index fingernail held at arm's length?
提问！下面这两个，哪个看起来更大呢？是地平线上的满月还是距离人眼一臂距离的食指甲呢？
Without actually going outside to check, most of us would probably think the Moon is bigger.
大多数人很可能会不假思索地说前者更大。
I mean, it looks huge on the horizon.
也就是说，很多人觉得月亮在地平线上的时候看起来很大。
But from your perspective on the ground, the Moon and your fingernail are actually about the same size, no matter where the Moon is in the sky.
但对于站在地面上的我们来讲，月亮和手指甲的大小其实差不多，这跟月亮在天空中的位置无关。
It also doesn't matter how big your hands are, because if you have giant hands like mine you probably have longer arms, too, so it balances out.
同样无关的是手的大小，因为像我一样手很大的人，手臂也很可能更长了，所以水涨船高，相当于没变。
And yet, it really doesn't seem like the moon and your fingernail should be the same size. But they are.
虽然怎么看都不会觉得月亮和手指甲一样大，但实践起来确实如此。
The Moon looks bigger when it's on the horizon because of something called the Moon illusion, which has been puzzling people for thousands of years.
月亮在地平线上的时候看起来更大，这是因为月径幻觉的存在。数千年来，月径幻觉已经迷惑了很多人。
Seriously, Aristotle worked on this problem.
亚里士多德曾认真研究过这个问题。
Before we get our minds too twisted, let's review what we know for sure.
先别慌，我们来缕缕已知项。
When we talk about the size of something in the sky, we're talking about its angular size, or what fraction of the celestial sphere it takes up.
我们讨论天空中某个物体的大小时，其实是在讨论它的角距大小，也就是它在天球中所占据的部分有多大。
On average, the Moon is about half a degree across, meaning you'd have to line up about 695 Moons to make one 360-degree circle around the whole sky.
平均来看，月亮占了大概0.5度，也就是说，要将大概695个月亮排在一排才能排满整个天空中360度的圈。
And it's true that the Moon does actually get larger and smaller in angular size.
确实，用角距来看的时候，月亮会看起来或大或小。
In fact, it can change by up to 13% in diameter, mostly because its elliptical orbit takes it closer or farther from the Earth over time.
实际上，月亮的直径大小最多可变化13%，这主要是因为：随着时间的推移，月亮的椭圆轨迹让它离地球或近或远。
During the course of a night, though, it's a really tiny change.
不过，一个晚上的时间，发生不了太大的变化。
And despite what our brains try to tell us, the Moon is actually about 2% smaller when it's on the horizon compared to when it's high in the sky.
而虽然有月径幻觉的存在，但其实月亮在地平线上的时候要比在高空中的时候小2%。
To see why this happens, think about the distance from the center of the Earth to the center of the Moon.
怎么回事呢？问题的关键在于地心和月心之间的距离。
When the Moon is directly overhead, the radius of the Earth is boosting us a little closer, making it seem a little larger.
当月亮正好在我们头顶上的时候，地球的半径让我们离月球更近了，这样月亮看起来就更大些。
But when the Moon's on the horizon, it's basically at a right angle to us, and the Earth's boost doesn't help as much.
而当月亮在地平线上的时候，基本上跟我们呈直角，所以地球的半径起不到什么作用。
So back to the problem at hand. If the Moon is really farther away when it's on the horizon, why does it seem so big?
那么回到刚才的问题：如果月亮在地平线上的时候真的更远的话，那为什么会看起来那么大呢？
Well, we know physics isn't the answer, so it must have to do with biology.
这个嘛，我们都知道这个问题是物理学解释不了的，那就一定和生物学有关。
Over the centuries, two main hypotheses have emerged.
过去几个世纪里，主要有两种假说。
The first, sometimes called the relative distance hypothesis, is based on a shortcut used by our brains.
第一种假说，有时也称相对距离假说。这种假说基于人脑用到的一种捷径。
Over millions of years of evolution, our visual systems developed to associate things near the horizon with being far away.
经过了几百万年的进化，人类的视觉系统现在可以将地平线附近的东西确认为距离远的东西饿了。
After all, when you're standing on the ground, the horizon is literally the farthest thing you can see.
毕竟，当我们站在地面上的时候，地平线是我们能看见的最远的地方。
But stuff that's far away might still be important, so your brain kind of cheats and makes that stuff seem bigger than it really is.
但距离远的物体可能很重要，所以我们的大脑就有点自我欺骗的感觉，让又远又重要的物体看起来比实际要大。
And when you see the Moon near the horizon, that little cheat makes it look a bit bigger than it should.
当我们看到月亮在地平线上的时候，这种错觉让我们看到的月亮比本应该看到的月亮大。
There's just one problem with this idea:
第一种假说还提出了一个问题：
when you ask them, most people will say the Moon seems closer the nearer to the horizon it is, not farther away.
当被问及的时候，大多数人会认为，月亮离地平线的距离更近，而非更远。
That's where this whole question came up in the first place: the Moon seems bigger and closer, even though it's not.
这也是问题的起因：月亮看起来更大、更近，但其实并非如此。
Which brings us to the relative size hypothesis, which claims that it's not just the horizon that matters.
这就涉及到第二个假设，相对大小假设，该假设称，影响因素不仅限于地平线。
Unless you're in the ocean or the middle of the desert right now, when you look off into the distance,
只要不是在海洋里或者沙漠的中心，那么放眼望去，
you'll probably see a bunch of other stuff, like trees or rooftops.
会看到很多东西，比如树和屋顶。
And you often have to look past all of that to see the rising Moon.
要越过这些物体，才能看到慢慢爬上指头的月亮。
That's where another trick of the eyes, called the Ebbinghaus illusion, comes into play.
这时候就有第二种错觉在发挥作用了，即艾宾浩斯错觉。
The Ebbinghaus illusion is what it's called when an object seen surrounded by smaller things
艾宾浩斯错觉就是：一个物体，如果包围它的物体比它小，
appears larger than an identical object surrounded by bigger ones. Take these orange circles, for example.
那么它看起来就要比被更大物体包围起来的时候看起来更大些，就拿这些橙色的圆圈为例。
They're exactly the same size, but for most people they don't look like it, all because of the grey circles around them.
他们几乎是差不多大小的，但在大多数人看来并非如此，只是因为它们周围的灰色圆圈。
So when you see the Moon framed by things like trees that we know to be pretty small compared to the rest of the landscape, it makes the Moon seem big.
所以，当月亮周围的东西与周遭环境比起来显得很小的时候，月亮看起来就会很大。
And when the Moon is overhead, surrounded by the vastness of the sky, it shrinks in your mind's eye.
当月亮高挂天空、万里无云之时，肉眼就会觉得月亮变小了。
Unfortunately, there's a problem with this hypothesis, too.
不过，这个假说也存在问题。
Airline pilots, who see the Moon against the horizon with nothing framing it, also report the Moon illusion.
航空公司飞行员看到，地平线上的月亮，周围明明什么都没有，却还是有艾宾浩斯错觉。
So could either of these ideas actually be right?
那么这两种假设中有正确的版本吗？
It's hard to know for sure, but modern neuroscience suggests it might be the first one, the relative distance hypothesis, that's correct.
目前没有定论，但现代神经学认为，可能是第一种假设正确。
We now know that our brain's vision system is probably made up of two separate pieces, the ventral stream and the dorsal stream.
现在，我们都知道，人脑的视觉系统很可能有两个组成部分，即由腹侧通路和背侧流。
The ventral stream helps you figure out what you're looking at, and then the dorsal stream places that object in 3D space around you.
腹侧通路让我们知道自己在看什么，而背侧流可以将我们看到的物体置于三维空间里。
The Moon illusion happens when the combination of the two streams breaks down.
腹侧通路和背侧流两兄弟失效的时候，就会出现错觉。
First, your ventral stream sees a white thing on the far-off horizon and thinks,
首先，如果腹侧通路在遥远的地平线上看到了一个白色的物体，
Hey, this is an instance of the thing Moon and it must be really big because it's far away like the horizon is.
我们就会觉得这个物体一定像月亮一样大，因为它离得太远了，跟地平线一样远。
Then, the dorsal stream takes over and says Ah, this Moon thing is really big and so it must be pretty close.
然后，背侧流占据了主导地位。这时，我们又会觉得啊，这月亮好大啊，肯定离我很近。
In the end, you end up thinking the Moon is both bigger and closer than it really is; exactly what most people report!
最后，我们会认为月亮比实际上又大又近，这就是大多数人的感受！
On the other hand, when you see the Moon high in the sky,
另一方面，当我们看到月亮悬挂高空的时候，
your object recognition doesn't get tricked by the presence of the horizon and you end up with a better sense of the Moon's angular size.
我们的目标识别能力不会因为地平线的存在而出现问题，所以我们更加理解月亮的角距大小。
It's probably not too surprising that our visual systems, well-adjusted for threats here on Earth,
意料之内的是：我们的视觉系统对于在地球上可能面临的威胁做好了准备，
aren't really equipped to deal with the vast distances of space.
但还没有对广袤的太空做好准备。
But it does mean that we don't always see the sky as it really is. Thanks for watching this episode of SciShow Space!
但这并不是说，我们看到的天空就不是本来的样子，感谢收看本期的《太空科学秀》！
If you're interested in learning more about skywatching,
如果大家对天空观测还感兴趣的话，
you can check out our episode about body hacks you can use to make yourself a better stargazer.
可以查看我们的其他视频，你会更加精通与太空有关的内容。
