Todays pic is an optical illusion that is only normally visible to those lucky enough to go into space.
Moon Illusion
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.
Photo credit: The crew of ISS Expedition 7, NASA
Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson
Space Center. http://eol.jsc.nasa.gov/
A larger version of this picture can be found here, this is a 987K Jpeg image.
Previous Pic Of The Day posts
07th Jun 03
02nd Jun 03
31st May 03
29th May 03
27th May 03
25th May 03
20th May 03
Moon Illusion
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.
Photo credit: The crew of ISS Expedition 7, NASA
Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson
Space Center. http://eol.jsc.nasa.gov/
A larger version of this picture can be found here, this is a 987K Jpeg image.
Previous Pic Of The Day posts
07th Jun 03
02nd Jun 03
31st May 03
29th May 03
27th May 03
25th May 03
20th May 03
