http://eclipse.gsfc.nasa.gov/SEatlas/SE ... as2001.GIF
http://en.wikipedia.org/wiki/Great_Wall_of_China#Visibility_from_space wrote:
The Great Wall of China
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Visibility from the moon
<<Popular beliefs ranging from Ripley's Believe It or Not!'s cartoons from 1930s, which claimed that the Great Wall is "the mightiest work of man, the only one that would be visible to the human eye from the moon," to Richard Halliburton's 1938 book Second Book of Marvels which makes a similar claim, have persisted, assuming urban legend status, and sometimes even appearing in school textbooks. Arthur Waldron, author of The Great Wall of China: From History to Myth, has speculated that the belief might go back to the fascination with the "canals" once believed to exist on Mars.
One of the earliest known references to this myth appears in a letter written in 1754 by the English antiquary William Stukeley. Stukeley wrote that, "This mighty wall of four score miles in length (Hadrian's Wall) is only exceeded by the Chinese Wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the moon."
The Great Wall is a maximum 9.1 m (30 ft) wide and is about the same color as the soil surrounding it. Based on the optics of resolving power (distance versus the width of the iris: a few millimetres for the human eye, metres for large telescopes) an object of reasonable contrast to its surroundings some 70 miles in diameter (1 arc-minute) would be visible to the unaided eye from the moon, whose average distance from Earth is 384,393 km (238,857 miles). The apparent width of the Great Wall from the moon is the same as that of a human hair viewed from 2 miles away. To see the wall from the moon would require spatial resolution 17,000 times better than normal (20/20) vision. No lunar astronaut has ever claimed seeing the Great Wall from the moon.
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Visibility from low earth orbit
A more controversial question is whether the Wall is visible from low earth orbit, i.e. an altitude of as little as 100 miles (160 km). NASA claims that it is barely visible, and only under nearly perfect conditions; it is no more conspicuous than many other man-made objects. Other authors have argued that due to limitations of the optics of the eye and the spacing of photoreceptors on the retina, it is impossible to see the wall with the naked eye, even from low orbit, and would require visual acuity of 20/3 (7.7 times better than normal).
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Anecdotal reports
Astronaut William Pogue thought he had seen it from Skylab but discovered he was actually looking at the Grand Canal of China near Beijing. He spotted the Great Wall with binoculars, but said that "it wasn't visible to the unaided eye." U.S. Senator Jake Garn claimed to be able to see the Great Wall with the naked eye from a space shuttle orbit in the early 1980s, but his claim has been disputed by several U.S. astronauts. Veteran U.S. astronaut Gene Cernan has stated: "At Earth orbit of 100 miles (160 km) to 200 miles (320 km) high, the Great Wall of China is, indeed, visible to the naked eye." Ed Lu, Expedition 7 Science Officer aboard the International Space Station, adds that, "it's less visible than a lot of other objects. And you have to know where to look."
In October 2003, Chinese astronaut Yang Liwei stated that he had not been able to see the Great Wall of China. In response, the European Space Agency (ESA) issued a press release reporting that from an orbit between 160 and 320 km, the Great Wall is visible to the naked eye. In an attempt to further clarify things, the ESA published a picture of a part of the “Great Wall” photographed from Space. However, in a press release a week later (no longer available in the ESA’s website), they acknowledged that the "Great Wall" in the picture was actually a river.
Topographic maps put together showing the location of the eastern parts of the wall between the Yellow River and the Bohai Sea.
Neil Armstrong stated about the view from Apollo 11: "I do not believe that, at least with my eyes, there would be any man-made object that I could see. I have not yet found somebody who has told me they've seen the Wall of China from Earth orbit. ...I've asked various people, particularly Shuttle guys, that have been many orbits around China in the daytime, and the ones I've talked to didn't see it.
Leroy Chiao, a Chinese-American astronaut, took a photograph from the International Space Station that shows the wall. It was so indistinct that the photographer was not certain he had actually captured it. Based on the photograph, the China Daily later reported that the Great Wall can be seen from space with the naked eye, under favorable viewing conditions, if one knows exactly where to look. However, the resolution of a camera can be much higher than the human visual system, rendering this irrelevant to the issue of whether it is visible to the naked eye.>>