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The pulsing Moon

Thoughts on the ever-so-puzzling Moon illusion.

The classic Moon illusion — that it appears larger when seen near the horizon than when overhead — is both a personal and complicated affair.

It’s personal because not everyone notices it; some can’t see it even if you point it out to them. It’s complicated because for those who do see it, the reasons why this psychological phenomenon occurs are vast: the perceived shape of the sky, the direction of our gaze, the influence of nearby objects on our distance estimates, atmospheric refraction, framing effects, and linear perspective, to name but a few. Now I’d like to add another dimension to this already perplexing affair.

Bad Moon rising

This past August 15, I watched the 13-day-old gibbous Moon rise above a distant horizon lined with houses and trees. The Moon loomed large in my mind’s eye, and I chalked it up to the Moon illusion. When I looked again around sunset, our satellite, as expected, appeared noticeably smaller midway up the sky. What I didn’t expect was that about an hour later, the disk had swollen once again — though not to the size it was at the horizon. So I spent the next few nights leading up to Full Moon watching for this unusual event. It happened repeatedly.

It didn’t take long for me to realize what was going on. Most people enjoy looking for the Moon illusion around the time of Full Moon. When the Full Moon rises, the sky is already on its way to becoming dark; we never do get to see a Full Moon midway up the sky in the daylight or twilight.

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