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, and sometimes even appeared 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 earliest known reference to this myth appears in a letter written in 1754 by the English antiquary William Stukeley. Stukeley wrote about Hadrian's Wall that, "This mighty wall of four score miles in length 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 9.1m (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 Great Wall is of course not a disc but more like a thread—it can be seen from much further than would be possible if it were simply a 30 foot disc. Still, 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. Not surprisingly, no lunar astronaut has ever claimed seeing the Great Wall from the moon.
Incidentially, if one could have seen the Great Wall from the moon, one ought to be able to see most of the roads in the world as well, given that they in total length far surpasses that of the great wall, and commonly are even wider. This is again not the case.