Astronomy EssentialsSpace

Moon illusion makes the moon look big!

The moon illusion is a trick of the mind

We’ve all seen a full moon looming large shortly after it rises, when it’s still hugging the horizon. And it’s true that the moon is sometimes closer to Earth than at other times, so that it’s minutely larger in our sky. But those large supermoons aren’t noticeably large. The difference is barely discernable. Meanwhile, a large moon viewed near the horizon is an illusion, a trick your brain is playing. It’s called the moon illusion.

Weirdly, even now, the causes of the moon illusion aren’t precisely known. But the video above, from AsapSCIENCE, offers some explanation.

The main one is that, when the moon is near the horizon, you’re gazing at it in the company of many familiar visual reference points: trees, buildings, mountains and so on. Your brain automatically compares the moon to these reference points. But when the moon is higher up, there’s nothing to compare it to. As ASAPScience says:

The moon seems smaller against the vastness of the night sky.

A red or orange moon is a physical effect

By the way, there’s a second phenomenon that the moon is subject to when it’s seen near the horizon. That is, a low moon often appears red or orange in color. That reddish color is not an illusion. It’s a true physical effect, caused by the fact that – when the moon is low in the sky – you’re seeing it through a greater thickness of Earth’s atmosphere than when it’s overhead. The atmosphere filters out the bluer wavelengths of white moonlight (which is really reflected sunlight). Meanwhile, it allows the red component of moonlight to travel straight through to your eyes. So a low moon is likely to look red or orange to you.

How do people get those photos of extra big moons seen near a horizon? They’re the result of photographic tricks and techniques, which you can read about here.

More photography tips: How to shoot the moon

Full moon names by month and by season

Moon illusion: Bright golden moon rising over dark trees.
View at EarthSky Community Photos. | A great example of the moon illusion from Nathaniel Adam Cruz in Cagayan de Oro City, Philippines. He captured a supermoon – a close, full moon, in this case the biggest of 2022 – on July 14, 2022. Thank you, Nathaniel. Yes, supermoons are bigger than normal, but not so big you can notice their extra large size in the sky. On the other hand, a rising full moon might appear very large, due to a psychological effect called the moon illusion.

Bottom line: It’s nearly full moon. So you might see an extra-large-looking moon low in the sky. Why does the moon look so big? It’s because of the moon illusion.

November 4, 2022
Astronomy Essentials

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Deborah Byrd

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