Matt Jones

Why the Moon Looks Bigger on The Horizon

  • Posted by Matt (Permalink)
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In keeping with today's Moon theme, here's some interesting information I've learnt about why the Moon and the Sun always look bigger when they are low in the sky.

I'd never really given it much thought and on the rare occasion that I did, I assumed that it was the result of some kind of distorting effect by the Earth's atmosphere. In fact, the atmosphere does distort the Sun and Moon slightly, as is evident when the setting Sun appears to bulge slightly and doesn't appear as a perfect disc. However, this doesn't explain why the Sun and Moon are so much bigger when they're hanging low in the sky. The following explanation is much more interesting (although not 100% proven), and reveals the odd way in which we percieve distance and the space around us. One thing is for sure, when the moon low in the sky, it's no bigger at all.

Imagine a plane in the distance and heading in your direction. At first it's near the horizon, and because it's far away, it's very small. Eventually, it flies directly over-head and when you look up, it's much bigger because it's much closer to you. We're used to things being smaller on the horizon than when they're directly above us. That makes perfect sense to our brains because, of course, things get smaller the further away they are from us.

The Moon is another object that appears in the sky, but as it's in orbit around the Earth, it stays a constant distance from us no matter where it is in the sky (more or less). Now this really confuses the brain because the rule of something being smaller when it's in the distance does not apply here. When we see the Moon across in the distance, we expect it to be smaller… but it isn't, so therefore it just seems bigger. It's just one big optical illusion.

When you see a large moon low in the sky, try closing one eye and holding your thumb out at arms length towards it. The diameter of the moon is never greater than about the half the width of your thumbnail.

Just thought I'd share that with you.