Moon Illusion

You’ve heard the saying, “seeing is believing,” right? The moon will be full next Wednesday, July 24th. Look at it when it’s just rising above the horizon at 9:30pm. The moon will be so big “you won’t believe your eyes”. So don’t trust them because the moon really won’t be any bigger.

This is the time of year when the sun is high in the sky and as the saying goes, “when the sun rises high, the moon hangs low” and a low hanging moon looks unusually large. This is the moon illusion.

Moonlight shining in your eyes creates an image in the back of your eye about six millionths of an inch wide. It doesn’t matter if the moon you’re looking at it high about your head or near the horizon, it’s the same size. Then why do summertime full moons look so big?

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Scientists aren’t quite sure, but the best known explanation is Ponzo’s Illusion. Ponzo drew two lines over a picture of railroad tracks. The lines were parallel and of equal length and width. The line farther away from you, where the tracks converge, appears larger because it seems to span a greater distance. When the moon is near the horizon, buildings and trees play the role of the converging railroad tracks making the moon appear bigger than it is.

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The problem with Ponzo’s Illusion is this, airline pilots report the moon illusion too. So perhaps part of the explanation lies in our perception of the sky itself. People perceive the sky to be a flattened dome, the zenith appears to be fairly close overhead and the horizon far away in the distance. When the moon is overhead we perceive it to be closer and smaller than it really is. This illusion is called oculomotor micropsia. On the flip side, when the moon is close to the horizon, landscape is a reference and we perceive the moon to be farther away and larger than it is. This illusion is called oculomotor macropsia.

But the moon is beyond the sky
and so far away that it doesn’t change size when on the horizon or overhead. But we tend to think of the moon as a flattened disk stuck on the dome of the sky. This is how Ponzo’s Illusion comes into play. Our minds conclude that when the moon is near the horizon it must be far away, but since it isn’t smaller, as we think it should be on the horizon, we perceive it to be larger than it really is. When the moon is low, we think it is far away. But in order to keep it in size with everything on the horizon, our minds make it bigger.

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Time-lapse of the rising moon over Seattle, Washington. Photography and copyright by Shay Stephens.

If you don’t believe me, look at this picture of the moon rising over Seattle, WA. Camera and film don’t lie. What do you see? You’re right, the moon is the same size. So there must be something fooling your brain and tricking your eyes.

If you still don’t believe me, try this. You’ll need a sheet of paper and some tape. Roll the paper into a long tube. Go outside when the moon is large and just off the horizon. Looking through the tube, adjust its size until it’s just wrapped around the moon and tape it. Go back outside several hours later when the moon is high in the sky and look at it through the tube. Does the moon still fit in the tube? You bet!
Now you’re weather wise.

By the way, low moons often appear orange in color. That’s no illusion. The color changes because pollution scatters the shorter wavelengths of light (blue, green, and violet) away from our eyes leaving the longer wavelengths of yellow, orange, and red. Just look at the photograph. The moon is orange near the horizon and slowly turns whiter and whiter as it climbs higher in the sky. Visit Shay Stephens at www.shaystephens.com/purchase.asp to see more of his photography.