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Why Does the Moon Look Bigger on the Horizon?

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(Image credit: raatcc36 | sxc.hu)

You might have heard it is magnified by the atmosphere just as it rises. Not so. Instead, it's your brain on Moon drugs.

The mind believes things on the horizon to be farther away than stuff overhead. Why? We're used to seeing clouds just a few miles above us, but the clouds on the horizon can be hundreds of miles away. So if we think it's farther away, and it's not, then it seems larger.

The effect (which also occurs with the setting Sun) is known as the Ponzo illusion. Try this at home whenever the Moon is full: When the Moon first comes up, hold something small in front of you and vary the distance until it appears the same size as the Moon, then repeat the experiment a few hours later.

The Moon generally appears full one or two nights before and after it's technical fullness. Oh, and technically, it's never really full.

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Robert Roy Britt
Rob was a writer and editor at Space.com starting in 1999. He served as managing editor of Live Science at its launch in 2004. He is now Chief Content Officer overseeing media properties for the sites’ parent company, Purch. Prior to joining the company, Rob was an editor at The Star-Ledger in New Jersey, and in 1998 he was founder and editor of the science news website ExploreZone. He has a journalism degree from Humboldt State University in California.