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Have you ever noticed

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how the full moon looks bigger

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when it's near the horizon

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than when it's high over head?

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If so, you're not alone.

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People have wondered about this strange effect

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since ancient times,

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and surprisingly, we still don't have

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a great explanation,

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but that's not for lack of trying.

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Some of the greatest minds of history --

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Aristotle,

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Ptolemy,

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Da Vinci,

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Decartes

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-- have all wrestled with this problem

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and failed to generate an adequate explanation.

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One of the first ideas suggested

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was that the image of the moon in the sky

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really is bigger near the horizon.

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Perhaps the Earth's atmosphere acts

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as a giant lens,

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magnifying the moon as it rises and sets.

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But this explanation doesn't cut it.

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If anything, the refraction of the atmosphere

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would make the moon look slightly smaller.

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Plus, if you actually measure

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the size of the visible moon

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at different positions,

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it doesn't change at all.

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But then, why does it still seem bigger

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when it's rising?

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This must be some kind of optical illusion.

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The question is, which one?

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One explanation is the Ebbinghaus Illusion,

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where two identical objects look different

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because of the relative size

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of the objects they're surrounded by.

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Here the two center circles are actually the same size.

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Maybe the moon looks bigger near the horizon

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because it's next to tiny trees,

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houses,

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and towers in the distance.

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But when the moon is higher up,

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it's surrounded by the vast darkness of the night sky

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and looks tiny by comparison.

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Another possibility is the famous Ponzo Illusion.

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If you've ever tried to draw in perspective,

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you know that the closer something is to the horizon,

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the smaller you should draw it.

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Our brain compensates automatically for this

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by perceiving objects near the horizon

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as larger than they actually appear.

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The two yellow lines in this drawing

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are the same size,

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but the upper one seems bigger

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because we interpret it as receding

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farther into the horizon.

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So, between Ponzo and Ebbinghaus,

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it seems like we've solved the mystery

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of the moon illusion,

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but, unfortunately, there are a few details

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that complicate things.

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For one thing, if this was just the Ebbinghaus effect,

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then we would expect the moon illusion

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to disappear for pilots flying high above the clouds

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since there wouldn't be any other smaller objects

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near the horizon.

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But in fact, pilots and sailors out on the ocean

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still claim to see the moon illusion.

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On the other hand, if it's just our brain's autocorrecting

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the size of objects near the horizon,

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then we'd expect the moon illusion

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to be visible inside a planetarium,

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where the whole sky,

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including the horizon,

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is displayed on a spherical dome overhead.

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Studies have shown, though,

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that this is not the case.

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To make matters worse,

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it seems the moon illusion disappears entirely

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if you just bend over

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and look at the moon between your legs.

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Now, this is just getting ridiculous!

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One of the most promising explanations today

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is known as Convergence Micropsia.

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Our brains judge the distance to objects

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and their apparent size

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by the focus of our eyes.

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When looking at the horizon,

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your eyes focus far-off into the distance

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so your brain knows you're looking far away.

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The moon appears a certain size.

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Your brain thinks it's far away,

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which it is,

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so you naturally conclude

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that the moon must be big.

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But when looking up at the night sky,

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there's nothing for your eyes to focus on,

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so they default to their rest focus,

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which is point just a few meters away.

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Now your brain thinks the moon is much closer

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than it really is,

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so you naturally conclude

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that the moon is not as big as you thought it was.

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Rather than explain why the moon

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looks so big near the horizon,

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Convergence Microspia explains

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why the moon looks so small when overhead.

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Still not satisfied?

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Well, frankly, neither are many scientists

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so the debate over the moon illusion still rages on

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and may continue as long as we still see it

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in the night sky.