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[Phys-L] It's the shape of the sky.



HUGE MOONRISE THIS WEEK

NASA - Step outside any evening at sunset and look around. You'll see a
giant moon rising in the east. It looks like Earth's moon, round and
cratered; the Man in the Moon is in his usual place. But something's wrong.
This full moon is strangely inflated. It's huge. You've just experienced the
moon illusion.

This week's full moon hangs lower in the sky than any full moon since June
1987, so the moon illusion is going to be extra strong.

What makes the moon so low? It's summer. The sun and the full Moon are on
opposite sides of the sky. During summer the sun is high, which means the
full moon must be low. This week's full moon occurs on June 22nd, barely a
day after the summer solstice on June 21st -- perfect timing for the moon
illusion.

So why does your brain think one is bigger than the other? After all these
years, scientists still aren't sure why. A similar illusion was discovered
in 1913 by Mario Ponzo, who drew two identical bars across a pair of
converging lines. The upper 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.

But there's a problem. Airline pilots flying at very high altitudes
sometimes experience the moon illusion without any objects in the
foreground. What tricks their eyes?

Maybe it's the shape of the sky. Humans perceive the sky as a flattened
dome, with the zenith nearby and the horizon far away. It makes sense: Birds
flying overhead are closer than birds on the horizon. When the moon is near
the horizon, your brain, trained by watching birds, miscalculates the moon's
true distance and size.

http://science.nasa.gov/headlines/y2005/20jun_moonillusion.htm

TIME OF MOONRISE IN YOUR TOWN
http://aa.usno.navy.mil/data/docs/RS_OneDay.html


UnderNews, 050620
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