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From the pages of

√°Science Geek√°

Issue Two

(P.O. Box 8641, Trenton, NJ 08650; $3 per issue) SECRETS OF THE MOON

By Doug Larkin

Sometimes I get asked questions that I think I know the answer to, and then I turn out to be very wrong. Such is the case with the moon. Everyone knows that when the moon (the sun does it too) is close to the horizon it is very large, and it gets smaller as it goes higher in the sky. It's especially noticeable during a full moon because it happens around sunset when you're most likely to be looking at the sky. I would get asked about this a lot. My stock answer up until about a year ago was that the curvature of the atmosphere as you looked across the horizon acted like a big magnifying glass. I had sort of figured that out myself and was very proud of my explanation. Unfortunately I was dead wrong--the size of the moon is an optical illusion--the size of the moon does not change! I had a very hard time believing this when I first read it.

One way to prove this to yourself is to do the following experiment. Hold an aspirin or a small washer at arm's length against the full moon at the horizon. You should just be able to eclipse or surround the moon. If the moon really does shrink as it rises, you should be able to notice it by the comparison. Go out a few hours later and check. Moonie no shrinkie.

Even armed with this knowledge, the moon still looks bigger on the horizon. Why do our brains continue to be tricked? The answer lies in the way we perceive the night sky. Instead of perceiving it as a hemi-spherical surface (which it isn't anyway--but if we did the size of the moon wouldn't change) we see it as a flattened dome. The horizon appears to be farther away than the "top" of the sky.

If you take two identical soccer balls and put one 10 feet away and the other 15 feet away, you perceive the one 10 feet away to be slightly larger. Now someone takes two more soccer balls and puts one closer and one farther. If they appear to be the same size, you conclude that the one that is farther is larger--its bigger size compensating for its distance. What our brain does is misjudge the distance to the moon. Since we think it is farther away at the horizon--and it is the same actual size as it is high in the sky--our brains conclude that it must be larger. By the way, in the physics class I teach, whenever we were doing stuff like this I always substituted the word "spheres" to avoid the distracting Beavis and Butt-head-type laughter--or if I was in a very good mood I would say "balls" a lot just for fun.

Art accompanying story in printed newspaper (not available in this archive): zine cover.

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