From: Sabine Atkins (sabine@posthuman.com)
Date: Sun Jun 08 2003 - 11:26:12 MDT
http://science.nasa.gov/ppod/y2003/06jun_moonset2.htm?list558649
From the article:
June 6, 2003: Astronauts onboard the International Space Station (ISS) have seen
some odd-looking moons before: red moons, squashed moons, vanishing moons.
They've learned to expect the unusual. Even so, today's picture, taken on May
11th while the ISS was above Russia, must have come as a surprise. The moon
seems to be floating inside Earth's atmosphere.
It's an illusion, of course. The moon is really a quarter of a million miles
away. The picture is tricky because of its uneven lighting. The sun's elevation
angle is only 6 degrees. On the left side of the image, night is falling; on the
right side, it's still broad daylight. This gradient of sunlight is the key to
the illusion.
Atmospheric optics expert Les Cowley explains: "The moon is close to Earth's
terminator, so the atmosphere in front of it is not receiving much sunlight."
Sunlit air scatters light and glows blue. "We call this 'airlight,'" says
Cowley. "But in this image the air in front of the moon is only dimly lit. The
moon is therefore clear and bright because it is not masked by superimposed
airlight."
"This makes the moon seem to be 'in front' of the atmosphere."
Uneven lighting reinforces the illusion in another way: On the sunlit side of
the image (right), the boundary between air and land is distinct. Our brains
extrapolate the direction of Earth's limb, right to left, using information
mostly from that sunlit area. But the brain does it wrong. The extrapolated limb
goes behind the moon; so the moon seems to be in the foreground, floating on
air. This idea, suggested by the author, is illustrated by a sequence of three
images: #1, #2, #3.
Whatever the explanation, the illusion is wonderful. And unexpected. It makes
you wonder ... what's next?
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-- Sabine Atkins www.posthuman.com
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