The Great Wall cannot be seen from the Moon
The Great Wall of China is not visible to the naked eye from the Moon. From low orbit it is hard to pick out at all.
The Wall is long and it is narrow, often under ten metres wide. At the Moon's distance its angular width is a small fraction of an arcsecond. The naked eye resolves on the order of an arcminute. The Wall is hundreds of times too thin. Apollo crews did not see it, and were not surprised.
From low Earth orbit the problem is contrast, not theoretical resolution. The Wall is the colour of the ground it crosses. Roads, airports, and city lights are easier. Alan Bean and Yang Liwei, among others, said they could not find it unaided. Photographs that do show it are taken with telephoto lenses and a known location, which is a different sentence.
Caveats
- 'From space' is often used loosely for low orbit. The Moon is the claim that fails most clearly. Low orbit, naked eye, is already doubtful.
- The Wall is not one continuous structure of uniform width. None of its sections rescue the lunar claim.
Sources
- 1NASA. NASA, China's Wall Less Great in View from Space. NASA Earth Observatory, 2005.