The following got printed in New Scientist, and I got a 10 pound book token.

Is the Great Wall of China visible to the unaided eye from outer space?

No. It is well known as one of the most widely believed urban legends, perhaps second only to the one about mass suicide by lemmings.

A person with perfect eyesight is able to resolve up to about one minute of arc without binoculars or a telescope. The Great Wall of China is, very approximately, 20 feet (6 metres) wide. This means that it is not directly visible above about 70,000 feet (20Km), or just over twice the height of Mount Everest. Even if its shadow is taken into account, this would only make it visible, in places, up to perhaps about 200,000 feet (60km) at the most which, because of atmospheric drag, is still below the height necessary for a stable orbit.

There are, however, many man-made objects which are visible from outer space, the largest being the Dutch polders. Cities can also be seen at night (because of the street lights).

D. Fisk.

Notes

It is, of course, visible using imaging equipment. Cameras on spy satellites would have no trouble in resolving it, just as Earth-bound telescopes can pick up Saturn's rings and Jupiter's red spot but the naked eye cannot. It is also true that some astronauts thought that they could see it. However, at the limits of resolution, the eye can be deceived. That is why Lowell and others at the turn of the century thought they saw canals on the surface of Mars.

What the Astronauts Actually Say

"We spent several passes looking for the Great Wall of China with no luck. Although we could see things as small as airport runways, the Great Wall seems to be made largely of materials that have the same color as the surrounding soil. Despite persistent stories that it can be seen from the Moon, the Great Wall is almost (sic) invisible from 180 miles up." -- Page 80, ORBIT (NASA Astronauts Photograph the Earth), by Jerry Apt, Michael Heifert & Justin Wilkinson. National Geographical Society.


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