The Moon Illusion: Why does the moon look so huge?

RALEIGH, N.C. (WTVD) -- This came up today with my coworkers as we were talking about the supermoon last night. On TV, we were showing a giant moon rising over the Golden Gate Bridge in San Francisco, and one of them stated it was SO big over Raleigh, too. Here's the thing: That giant moon you see, as you look at the horizon, is the same size as when you see it in the sky. It's called The Moon Illusion.



Here's what it looked like as it was rising over Raleigh last night. Thanks to one of my favorite photographers, Matt Robinson, for letting me share the pic. You can see more of his work here.

This is a trick our brains like to play on us when we see the moon close to the horizon. Pictures prove that it's the same size near the horizon as when it's in the sky. Our brains don't perceive it that way, and the debate among scientists on why this happens has raged for years.

Don't believe me? Try this next time you see a giant moon on the horizon: Look at it through a paper towel tube. It won't seem as big without all the objects in your peripheral vision. Or cover the moon with your thumb. They should be roughly the same size. A couple of hours later, when the moon is in the sky, your thumb will cover it up EXACTLY as it did earlier.



One of the more widely accepted explanations is the "Apparent Distance Hypothesis." It suggests that we think the moon is farther away when it is near the horizon. We view it in relation to objects on the ground, such as trees or buildings. The brain adjusts our perception to make the moon more prominent than it does when it's seen high in the sky without reference points. Lloyd and James H. Kaufmann, in their research article "Explaining the Moon Illusion," show this diagram to help explain it. You can read the article here.



Another explanation looks at how our eyes and brains process visual information. The eye sees the sky as a flattened dome rather than a perfect sphere. When the moon is up high, it appears closer, as seen in the "dome" that is the sky, leading to a smaller size. When the moon is near the horizon, it's seen in a different context against the hugeness of the Earth's surface, and our brain sees it as larger.



The funny thing about those explanations, they all deal with our perspective on Earth. NASA astronauts, far above the surface, have also seen the Moon Illusion, from a very different, and unique, perspective. It's weird, that in a time when the answers to most questions are just a click away, there are still things we're trying to figure out. So next time you wonder just how big that Moon is, there are lots of people around the globe, scholarly brains, and young children, all wondering the same thing!