June 6, 2003

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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.