Here's a question for you:

Which looks bigger, the full moon on the horizon, or your index fingernail held at arm's length?

Without actually going outside to check, most of us would probably think the Moon is bigger.

I mean, it looks huge on the horizon.

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.

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.

And yet, it really doesn't seem like the moon and your fingernail should be the same size.

But they are.

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.

Seriously, Aristotle worked on this problem.

Before we get our minds too twisted, let's review what we know for sure.

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.

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.

And it's true that the Moon does actually get larger and smaller in angular size.

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.

During the course of a night, though, it's a really tiny change.

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.

To see why this happens, think about the distance from the center of the Earth

to the center of the Moon.

When the Moon is directly overhead, the radius of the Earth is boosting us a little closer,

making it seem a little larger.

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.

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?

Well, we know physics isn't the answer, so it must have to do with biology.

Over the centuries, two main hypotheses have emerged.

The first, sometimes called the relative distance hypothesis,

is based on a shortcut used by our brains.

Over millions of years of evolution, our visual systems developed to associate

things near the horizon with being far away.

After all, when you're standing on the ground,

the horizon is literally the farthest thing you can see.

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.

And when you see the Moon near the horizon,

that little cheat makes it look a bit bigger than it should.

There's just one problem with this idea: when you ask them, most people will say the

Moon seems closer the nearer to the horizon it is, not farther away.

That's where this whole question came up in the first place:

the Moon seems bigger and closer, even though it's not.

Which brings us to the relative size hypothesis,

which claims that it's not just the horizon that matters.

Unless you're in the ocean or the middle of the desert right now, when you look off

into the distance, you'll probably see a bunch of other stuff, like trees or rooftops.

And you often have to look past all of that to see the rising Moon.

That's where another trick of the eyes,

called the Ebbinghaus illusion, comes into play.

The Ebbinghaus illusion is what it's called when an object seen surrounded by smaller

things appears larger than an identical object surrounded by bigger ones.

Take these orange circles, for example.

They're exactly the same size, but for most people they don't look like it,

all because of the grey circles around them.

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.

And when the Moon is overhead, surrounded by the vastness of the sky,

it shrinks in your mind's eye.

Unfortunately, there's a problem with this hypothesis, too.

Airline pilots, who see the Moon against the horizon with nothing framing it,

also report the Moon illusion.

So could either of these ideas actually be right?

It's hard to know for sure, but modern neuroscience suggests it might be the first one,

the relative distance hypothesis, that's correct.

We now know that our brain's vision system is probably made up of two separate pieces,

the ventral stream and the dorsal stream.

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.

The Moon illusion happens when the combination of the two streams breaks down.

First, your ventral stream sees a white thing on the far-off horizon and thinks,

“Hey, this is an instance of the thing 'Moon,'

and it must be really big because it's far away like the horizon is.”

Then, the dorsal stream takes over and says

“Ah, this Moon thing is really big and so it must be pretty close.”

In the end, you end up thinking the Moon is both bigger and closer than it really is;

exactly what most people report!

On the other hand, when you see the Moon high in the sky,

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.

It's probably not too surprising that our visual systems, well-adjusted for threats

here on Earth, aren't really equipped to deal with the vast distances of space.

But it does mean that we don't always see the sky as it really is.

Thanks for watching this episode of SciShow Space!

If you're interested in learning more about skywatching,

you can check out our episode about body hacks you can use to make yourself a better stargazer.

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