Visibility from space

The Great Wall of China as seen in a false-color radar image from the Space Shuttle, taken in April 1994

Visibility from the moon

A Ripley's Believe It or Not! cartoon from May 1932 claimed that the wall is "the mightiest work of man, the only one that would be visible to the human eye from the moon," and Richard Halliburton's 1938 book Second Book of Marvels makes a similar claim, but it is not true. This belief has persisted, assuming urban legend status, 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.

The Great Wall is a maximum 30 ft (9.1m) 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 four thousand miles in diameter (such as the Australian land mass) would be visible to the unaided eye from the moon (average distance from earth 238,857 miles (384,393 km)). But the Great Wall is of course not a disc but more like a thread, and a thread a foot (15 cm) long would not be visible from a hundred yards (90 m) away, even though a human head is. Not surprisingly, no lunar astronaut has ever claimed he could see the Great Wall from the moon.

Visibility from low earth orbit

A different question is whether the Wall is visible from low earth orbit, i.e an altitude of as little as 100 miles (160 km). The consensus is that it is barely visible, and only under nearly perfect conditions; it is no more conspicuous than many other manmade objects.

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." US 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 US astronauts. Chinese astronaut Yang Liwei said he could not see it at all.

Veteran US 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."

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.