Have you ever noticed how the full moon looks bigger when it's near the horizon than when it's high over head?
你注意到了吗，满月在接近地平线时比高挂于空中时看起来大一些。
If so, you're not alone.
如果注意到了这点，那你不是唯一一个人。
People have wondered about this strange effect since ancient times,
人们长久以来对这个奇怪的现象惊奇不已，
and surprisingly, we still don't have a great explanation, but that's not for lack of trying.
惊讶的是，我们仍然没有一个很好的解释，但这不是因为我们没有努力去寻找解释。
Some of the greatest minds in history -- Aristotle, Ptolemy, Da Vinci, Decartes
历史上一些最伟大的头脑--亚里士多德，托勒密，达芬奇，笛卡尔，
have all wrestled with this problem and failed to generate an adequate explanation.
他们都曾经尝试解释这个问题，却都铩羽而归。
One of the first ideas suggested was that the image of the moon in the sky really is bigger near the horizon.
最早的想法中有这样一种：月亮在空中成的像确实在接近地平线时大一些。
Perhaps the Earth's atmosphere acts like a giant lens, magnifying the moon as it rises and sets.
或许地球的大气层就像一面巨大的透镜，在月亮升起和降落时放大它的图像。
But this explanation doesn't cut it.
但是这个解释并不合理。
If anything, the refraction of the atmosphere would make the moon look slightly smaller.
如果真有大气层折射，那也应该让月亮看起来小一点。
Plus, if you actually measure the size of the visible moon at different positions, it doesn't change at all.
而且，如果你真的在不同地点测量月亮可见部分的尺寸，会发现其实都是一样的。
But then, why does it still seem bigger when it's rising?
那么，究竟是为什么当月亮升起时会显得大一些？
This must be some kind of optical illusion. The question is, which one?
一定是一些光学上的错觉。问题是，是哪一种错觉？
One explanation is the Ebbinghaus Illusion,
一种解释是艾宾浩斯错觉，
where two identical objects look different because of the relative size of the objects they're surrounded by.
这是指两个相同的物体会因为它们和周围物体的相对尺寸而显得不一样大。
Here the two center circles are actually the same size.
这里，中心的两个圆事实上是一样大的。
Maybe the moon looks bigger near the horizon because it's next to tiny trees, houses, and towers in the distance.
或许月亮在接近地面时显得大是因为它边上有小树，房子和远处的塔楼。
But when the moon is higher up, it's surrounded by the vast darkness of the night sky and looks tiny by comparison.
而当月亮在高空中时，它被茫茫黑夜环绕，所以相对来说显得小了。
Another possibility is the famous Ponzo Illusion.
另一种解释是著名的庞佐错觉。
If you've ever tried to draw in perspective,
如果你曾经试过透视画法，
you know that the closer something is to the horizon, the smaller you should draw it.
那你就会知道，一样东西越靠近地平线就该画得越小。

Our brain compensates automatically for this by perceiving objects near the horizon as larger than they actually appear.
我们的大脑自动弥补了这样的透视效果，使得地平线附近的物体在我们脑海中的形象比实际更大。
The two yellow lines in this drawing are the same size,
这幅画里两条黄线是一样的尺寸，
but the upper one seems bigger because we interpret it as receding farther into the horizon.
但上面那一条看起来更长，因为我们觉得它更接近地平线，所以应该比画里面显得更长。
So, between Ponzo and Ebbinghaus, it seems like we've solved the mystery of the moon illusion,
那么，通过庞佐和艾宾浩斯错觉，我们似乎已经解决了这个月球错觉问题，
but, unfortunately, there are a few details that complicate things.
但不幸的是，还有一些细节让事情不那么简单。
For one thing, if this was just the Ebbinghaus effect,
举例说，如果真是艾宾浩斯效应，
then we would expect the moon illusion to disappear for pilots flying high above the clouds
那么我们应该预期到月球错觉对于在空中的飞行员不起作用，
since there wouldn't be any other smaller objects near the horizon.
因为对他们来说，在地平线附近没有什么小物体可以用来作比较。
But in fact, pilots and sailors out on the ocean still claim to see the moon illusion.
但事实上，飞行员和大洋上的水手仍声称看到了这个月球错觉。
On the other hand, if it's just our brain's autocorrecting the size of objects near the horizon,
另一方面，如果这只是因为我们大脑的自动纠正功能使得地平线附近的物体尺寸有变化，
then we'd expect the moon illusion to be visible inside a planetarium,
那么我们应该预期到月球错觉在天文观测台内仍然有效，
where the whole sky, including the horizon, is displayed on a spherical dome overhead.
在那里，整个天空，包括地平线，都在头顶的球形屋顶上。
Studies have shown, though, that this is not the case.
但是研究表明并不是这样。
To make matters worse, it seems the moon illusion disappears entirely if you just bend over and look at the moon between your legs.
更糟的是，如果你弯下腰，从两腿之间看月亮，月球效应似乎完全消失了。
Now, this is just getting ridiculous!
这岂不是荒谬！
One of the most promising explanations today is known as Convergence Micropsia.
目前最有希望的解释之一是聚焦视野显小症。
Our brains judge the distance to objects and their apparent size by the focus of our eyes.
我们的大脑通过眼睛的聚焦来决定与物体的距离以及他们的尺寸。
When looking at the horizon, your eyes focus far off into the distance so your brain knows you're looking far away.
当我们看地平线时，眼睛聚焦在极远的地方，这样你的大脑就知道你在看远方。
The moon appears a certain size.
这时月亮有一个固定的尺寸。
Your brain thinks it's far away, which it is, so you naturally conclude the moon must be big.
你的大脑认为月亮在很远的地方，它确实是很远，所以你自然认为月球一定很大。
But when looking up at the night sky, there's nothing for your eyes to focus on,
但是当你看天空的时候，没有东西可以让你的眼睛聚焦，
so they default to their rest focus, which is a point just a few meters away.
所以眼睛自动聚焦到了平常的焦点，这距离你只有几米。
Now your brain thinks the moon is much closer than it really is,
现在你的大脑认为月球距离你很近，远比实际距离要近，
so you naturally conclude the moon's not as big as you thought it was.
所以你的大脑自然认为月球并没有你想象的那么大。
Rather than explain why the moon looks so big near the horizon,
尽管它并没有解释为什么月球在地平线附近显得大，
Convergence Microspia explains why the moon looks so small when overhead.
但聚焦视野显小症解释了为什么月球在空中看起来显得小。
Still not satisfied? Well, frankly, neither are many scientists,
还是不满意？好吧，老实说，很多科学家也不满意，
so the debate over the moon illusion still rages on and may continue as long as we still see it in the night sky.
所以关于这个现象的争论仍在继续，说不定只要我们一日能见到夜空中的月球，这争论就一日都不会停止。