This is lunar illusion at work: A trick of perception seems to exaggerate the size of the moon when it is near the horizon. While it is no closer or farther away from the viewer on Earth, the low-hanging moon appears a good deal larger than it seems when elevated higher in the nighttime sky.
The popular explanation is that the moon looks bigger when you have something to compare it to. Seeing the moon compared to other objects -- trees, buildings, mountains -- makes it appear larger. The moon alone in the middle of the sky has no surrounding frame of reference, so it appears smaller.
Certainly this is the case, but it doesn't explain what happens in the human brain to change the viewer's perception. Why should a comparison to features on the ground change the way we perceive the moon? What trick is the brain playing that makes the moon appear to change size?
Thinkers throughout history have struggled to explain the illusion, and today the little-understood phenomenon is still a subject of controversy.
Two opposing theories vie for acceptance, but they are contradictory: One suggests that the horizon moon appears large because visual cues in the intervening landscape make the moon seem far away. The other idea says those same cues make the same moon appear closer.
Now a father-son research team's experiments may settle the dispute. Lloyd Kaufman, a professor emeritus of psychology and neural science at New York University, and James Kaufman, a physicist at IBM's Almaden Research Center in San Jose, California, explain their work in the January 4 issue of the Proceedings of the National Academy of Sciences.
The Kaufmans argue that the explanation of the illusion lies in the fact that viewers judge a horizon moon to be much farther away than the overhead moon. The brain then exaggerates the perceived size of the moon as if to drive home this conclusion: the moon is so far away, therefore it must really be huge if it takes up so much space in the sky.
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