Sky Talk March 2008: The Moon Illusion

One of the abiding mysteries of the night sky is that of the “Moon Illusion” — the fact that our satellite looks much bigger when rising or setting than it does when seen overhead. Everyone has experienced this but no one really knows why this happens or what causes it! We offer some tips for experimenting with the illusion that will demonstrate just how truly mysterious it is while at the same time shedding some light on its possible source.

No matter where it is in the sky, most people greatly overestimate the apparent size of the Moon, most saying that it would take a quarter at arms length to cover its fully-illuminated disk. In fact, a dime more than does it! But it’s when the Moon is just rising or setting over the horizon that it often looks absolutely huge — like some giant celestial pumpkin, especially when there’s enough atmospheric haze to turn it orange!

Many different explanations have been forth to explain this fascinating phenomenon, all of them unsuccessfully. The most common of these is that when the Moon is near the horizon there are houses and trees and other ground objects to compare its size with, but when high in the sky it’s all by itself with no points of reference. For a dramatic demonstration of why this isn’t the answer to the illusion, try the following. Turn your back to the Moon, bend over and look at it through your legs. You’ll be surprised to find that it appears normal in size! A less ungainly method that gives the same result is to look directly at our satellite through a cardboard tube such as paper towels are rolled up on.

You may also be surprised to learn that if you take images of the Moon through a telescope (like any of those offered in the astronomy section of the Scientifics catalog) when it’s both rising and overhead, the images will appear the identical. In fact, if the telescope provides a large enough image-scale, you’ll find that the Moon actually appears slightly larger when overhead than when rising — just the opposite of what’s seen with the Moon Illusion! This is due to the fact that our satellite is physically closer to us when overhead than when on the horizon by the 4,000-mile radius of the Earth itself.

So look at the calendar to find when the next Full Moon is scheduled (and if you can the actual times of moonrise and moonset in the paper or an almanac, for the illusion is then at its very best). If the sky’s clear, head out to experience the illusion again for yourself. And here’s a suggestion when you do: take your trusty Edmund binoculars with you. First stare in wonderment at the Moon with your eyes alone. Then gaze at it through the glasses. Does it still look as big relative to nearby horizon objects? We’ll leave this for you to decide!

In taking leave of the Moon Illusion, it appears that this strange phenomenon arises in our perception of the Moon seen at differing elevations in the sky — somewhere in the eye-brain combination. But the actual mechanism still remains a mystery.

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.

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