HowDoWhy

First of all, let’s clear up that the question is all wrong. The moon doesn’t shrink as it passes over us. It isn’t actually bigger on the horizon. It does appear to be, though, doesn’t it? A rising moon on the horizon looks huge compared to one hanging high in the heavens. Almost double the size by some accounts. A common myth is that atmospheric distortion accounts for the apparent difference. If your friends insist that’s the case, challenge them to measure its size on the horizon using the eraser of a pencil or a ruler held at arm’s length and then again a few hours later when the moon is higher in the sky. They’ll find it’s the same size!Obviously what we’re dealing with is an optical illusion. Want a better explanation?

Ok, it comes down to the way your brain interprets what it sees. Without visual queues against which to compare the moon when it’s hanging up in the sky, your brain sees the moon as one size. When it is low on the horizon, your brain places it in context against the surrounding landscape which recedes from you to the horizon. The result is that you perceive the moon to be larger. You can see a great example of this illusion in the following illustration of a classic illusion of bars over tracks.

See the big bar up top? It’s a little longer than the bar at the bottom, right? Nope. They are identical in size. Framing the bottom bar inside the tracks “convinces” your brain that it is a bit smaller than the bar above which clearly stretches well across the tracks. Your brain reasons it must be bigger even though the bars are exactly the same size.

For some more REALLY cool optical illusions, check out the links below. These will absolutely blow your mind.

http://www.michaelbach.de/ot/mot_rotsnake/index.html

http://www.michaelbach.de/ot/mot_ske/index.html

http://www.michaelbach.de/ot/ Pick your own favorites!