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That's One Big Moon!

Posted by
Don Smith (California, United States) on 18 January 2009 in Landscape & Rural.

As I mentioned in my last post, the moon appears larger on the horizon to our eyes due to an optical illusion called the Ponzo Illusion. Named after scientist, Mario Ponzo, the illusion is similar to how our brain perceives railroad tracks converging as we look straight down them. As scientists understand it, our brain needs visual reference clues in order to make sense of the size of an object. When the moon is on the horizon, we have visual clues, such as these large oaks, which are seemingly dwarfed by this incredibly large moon. When the moon climbs higher in the sky, our visual clues go away and our brain percieves it as being smaller, when in reality, it is the same size!

The next time you are out photographing a full moon, hold a pencil's eraser away from your dominant eye until it covers the moon (note your distance from the eraser to your eye). Let the moon climb into the sky until it looks noticably smaller, then retry the eraser at the same distance from your eye - it will be the same! When you look at the moon, regardless of its position in the sky, rays of moonlight converge and from an image about 0.15mm wide in the back of your eye. High moons and low moons make the same sized spot. Illinois State University physics professor, Carl Wenning, concluded that humans judge objects on the horizon as being further away, thus a moon on the horizon must be really big to span half-a-degree across the sky, so our brain inflates the moon accordingly. Illusion's are cool and have been used in photography and filmmaking for years. As always, I invite you to visit my website at

Canon EOS-1Ds Mark III 3/10 second F/9.0 ISO 200 400 mm

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