Solstice moon illusion tomorrow night


Photo: Edmund E. Kasaitis.

Tomorrow night's full moon will be very low in the sky, and will give a strong illusion of being far bigger than it actually is.

Exactly why we experience this phenomenon is unclear; NASA provides several explanations, including this one:

A similar illusion was discovered in 1913 by Mario Ponzo, who drew two identical bars across a pair of converging lines [like railroad tracks]...The upper yellow bar looks wider because it spans a greater apparent distance between the rails. This is the "Ponzo Illusion."

Some researchers believe that the Moon Illusion is Ponzo's Illusion, with trees and houses playing the role of Ponzo's converging lines. Foreground objects trick your brain into thinking the Moon is bigger than it really is.

Talking of illusions, here's the winner of the 2008 Best Visual Illusion of the Year Contest.


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I don't think the illusion works for everybody. Having done tracking analysis on Lunar Prospector, I'm aware of the distance to the Moon. Maybe practical knowledge spoils the illusion.

I thought this was pretty straight-forward... Our mental model of the sky is a flattened dome, rather than a hemisphere. When judging the distance to something in the sky (clouds, birds, Moon, whatever), we model things overhead as fairly close, and things near the horizon as far away.

When it is close to the horizon we can see the moon in perspective to other objects so we realize how big it is. Naturally it will seem bigger when it is among houses and trees instead of the entire blue sky or the stars.