Guide to Space

Why Does The Moon Look So Big Tonight?

19 Feb , 2015 by Video

Look at the Moon. Have you ever noticed the Moon looks so big when it’s down on the horizon, but way smaller when it’s nearly overhead? What’s going on here? Turns out, you fell for the oldest trick in the book: the Moon Illusion.

Look at that Moon. It looks so big and full. Way bigger than it normally does. I wonder what’s going on to make it look so big? Maybe it’s closer and brighter? Maybe the atmosphere is distorting it like a lens? Or maybe, I’m just a human being, and I just fell for the oldest trick in the book: the Moon Illusion. Which really sounds more like a 80’s spy thriller novel than anything else. What I’m saying is, don’t believe your eyes.

The Moon is always the same size, and the distance varies by only a small amount during its orbit. As a result, the Moon is roughly the same size in the sky every night. Even though it looks huge on the horizon, it’s identical to when it’s directly overhead.

Don’t believe me? The Moon and your pinky fingernail when you hold your arm out at length, are about the same size. Next time the Moon’s in the sky, try it out, and you’ll see. Then try this out on one of those nights when the Moon just looks so big and fat. It’ll be it’s exactly the same size as it was before.

Look at this picture. Look at this collection of Moons, taken one after the other from Moonrise until the Moon is high in the sky. Exactly the same size! Every time! So what’s going on here?

The problem is up here, in my meat-thinky parts. For some reason, when the Moon is down on the horizon, we think it’s larger than when it’s directly overhead. But why? Bad news, we’re not actually sure yet. We’re still piling up the list of cognitive biases that make us think it’s a good idea to stay on an airplane that’s on fire or convince us to wait it out in our homes when there’s a tornado headed straight for us instead of evacuating like the nice people on the radio say.

How we perceive the moon's size may have much to do with what's around it. In this illustration, most of us seen the bottom moon as smaller, but they're both exactly the same size. Crazy, isn't it? Credit: NASA

How we perceive the moon’s size may have much to do with what’s around it. In this illustration, most of us seen the bottom moon as smaller, but they’re both exactly the same size. Crazy, isn’t it? Credit: NASA

One idea is that the Moon looks bigger on the horizon because it looks farther away. When we see stuff in the sky, like clouds, birds or airplanes, they seem tiny. But when we see the Moon, compared to closer objects on the horizon, like trees and buildings, our brain freaks out and decides that it’s actually larger.

Fun fact! It turns out our brain is really bad at knowing how big things actually are, and it’s easily tricked by the stuff around it. Here’s an optical illusion called the Ebbinghaus illusion. See those circles in the middle? They’re the same size in each example. But because of the other circles around them, our brain can’t deal. Normally buildings and trees are big. And yet they seem tiny compared to the Moon on the horizon.

I did say that it’s mostly the same distance, every night but the Moon actually does get bigger and smaller in the sky. It’s following an elliptical orbit around the Earth. At its closest point, the Moon gets about 363,000 km. And then at its furthest point, it’s about 405,000 km. So that is a bit of a difference, but seriously, you’d need a really good telescope to be able to tell, and it takes almost a month to make this journey from one end to the other.

Moon timelapse. Credit: Cory Schmitz

Moon timelapse. Credit: Cory Schmitz

Trust me, you can’t tell. Or you know what, you can tell, you’re right. It’s just me, and everyone else, for us regular mortals, our brains are fooled. So next time your friend mentions how huge the Moon looks, feel free to explain the cold hard facts to them. Let them know that their brain is lying to them, and how they’re easily deceived. Then laugh and mock them for their amusing little human frailties. Then, I suppose you might be looking for new friends… but you will have enlightened them to the way of their wrongness, and that’s a gift that keeps on giving.

Well, did you fall for this? Did you think the Moon looks huge on the horizon, or are you somehow immune to the Moon illusion? If so, tell us your secret in the comments below.

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By  -          
Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.



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Abes48
Member
Abes48
February 19, 2015 7:00 PM

Fraser, use FARTHER for distance. Use further for figurative:
Stand farther from the camera.
You’re smart and further you’re a good teacher.

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