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Space Station Science

Picture of the Day

June 6, 2003

Moon Illusion
Photo credit: The crew of ISS Expedition 7, NASA

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? 

Today's picture, ISS007-E-05379, was captured using a digital camera
and an 50 mm lens. Additional images taken by astronauts and
cosmonauts may be viewed at the NASA-JSC Gateway to Astronaut
Photography of Earth.

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