Monday, September 20, 2010


Kaboom. Another of my beliefs bites the dust - this one involving the moon.

Of course the moon has regularly been the subject of delusions. Probably the most common has been that a full moon somehow causes insanity, or at least wild, irrational behavior. (I once knew a very capable county sheriff who was absolutely convinced that crime rose sharply on the night of a full moon.)

But my comeuppance involves an illusion common to us all: The fact that the moon just above the horizon appears to be far larger than the moon at its zenith. We know that the moon remains the same size, and that its orbit keeps it pretty much the same distance from us at all times, so this apparent difference in size does indeed have to be an illusion of some kind. But what causes it?

Like many of us, I came to conclude that it's a matter of context. We know the size of big buildings or mountains beyond which the low moon can be seen, and somehow that makes us exaggerate the moon's size. Straight up in the sky, there is no context, so the illusion is lost. (Put more scientifically, the difference is using "distal" cues - we know the sizes of buildings and mountains - and "proximal" cues from the retina, because we have no other cues.)

Other explanations have included the idea that looking at the moon near the horizon involves seeing through a longer column of atmosphere (or smog), thus somehow scattering its light and making it look bigger, or the suggestion that the "angle of regard" (looking straight ahead at the moon, or looking way up) has something to do with it.

Well, according to Professor Daniel N. Robertson, in a section on studies of perception in his course on "Great Ideas in Psychology," all these ideas are ruled out by studies that eliminated all such elements as context, atmosphere and angle of view. He says the damn moon STILL looks bigger on the horizon than high in the sky.

Obviously there remains a lot to be learned about this sort of thing. For now, I think I'll just call it a mild kind of, well, lunacy.

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