Why does the moon look bigger when it’s just rising or setting, but smaller when it’s high up in the sky?

This perplexing phenomenon, often called the ‘moon illusion’, has puzzled observers for centuries. But science has an explanation, and it involves something called atmospheric refraction.

When the moon is low on the horizon, it appears alongside familiar objects like trees, buildings, or mountains.

These nearby objects act as reference points for our brains to gauge the moon’s size.

Because the moon appears so much larger than these objects, our brains perceive it as being bigger too.

It’s like when you hold a small object close to your face – it seems larger in comparison to your hand.

But what about when the moon is high up in the sky, seemingly all by itself?

Without any nearby objects for comparison, our brains struggle to judge its size accurately.

This is where atmospheric refraction comes into play.

As the moon rises or sets, its light travels through a thicker layer of earth’s atmosphere near the horizon.

This denser atmosphere can bend the moon’s light slightly, making it appear larger than it really is.

It’s similar to how a straw looks bent when you place it in a glass of water – the light bends as it passes through different mediums.

So, when you combine the effects of atmospheric refraction with the presence of nearby objects, you get the illusion of a larger moon on the horizon.

It’s like nature’s own optical illusion!

Next time you catch a glimpse of the moon rising or setting, remember that its apparent size is not actually changing – it’s just your brain playing tricks on you, influenced by atmospheric conditions and the surrounding landscape.

Send your science questions in to Dr Bec via [email protected]

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