A lunar illusion you'll flip over

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May 19 2010 9:26 AM

A lunar illusion you'll flip over

The Lunar Reconnaissance Orbiter has a great blog where they post images from the hi-res camera onboard. I was perusing a recent image, and was a bit befuddled:

LRO_dome_crater

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies! Follow him on Twitter.

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What the heck? Is this a plateau of some kind? Is that a small dome just below the center of it? The whole thing looks pitted around the edge, too, like some sort of erosion has taken place. But that can't be right!

Happily, being an old hand with optical illusions, I knew exactly what to do. I flipped the image over, and all became clear: LRO_dome_crater_flipped

Ah, that's better. Now you can see what's what: it's a crater with boulders in it. The small dome is now clearly a tiny meteorite impact crater. What looked like pitting is now obviously rocks and rubble that have slid down the slope of the crater wall.

This is an old illusion. Having evolved on the surface of a planet, we interpret our surroundings assuming sunlight is coming from above. If we see a picture rotated such that the sunlight is coming from below, it plays tricks on our perception. Shadows point the wrong way, making craters look like domes. Flip the picture over, and voila! All is as it should be.

This image is a wonderful example of this illusion (though I've never run across a name for it. I suggest "Plait's Plateau"). You can take lots of lunar pictures and see it if you rotate them. I suggest you check out the LRO archive, because it's a terrific lesson that what you see is not always what you get (and also because the images are simply too cool). It's incredibly easy to fool our brains, and if more people realized that then it would be a lot easier for them to be skeptical of what they see, and of claims from other people about what they see!



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