A reader in Australia writes with a problem:

So, supposedly, the Great Wall of China is visible from space. But apparently it's not. Recently, I've discovered that both sides claim credibility (rather than one being an urban legend like the slowly boiling frog). I want to know if it's true.

The tallest and widest part of the wall is 9.1m wide (tapered to 3.7m) and 8m high. It's 6400km long, but much of that is significantly thinner than 9.1m. (According to Wikipedia).

Anyway, would such an object be discernible with the unassisted eye, on a clear, cloudless, pollutionless day, from Low Earth Orbit (2000km), or Geostationary Orbit (26000km) or somewhere in between (say 12000km)?

The most plausible answer I've been given is that you can see the Wall in the sense that because it's such a long continuous feature, especially with the correct shadow conditions, you can easily ascertain where it is if you know it's there somewhere. However, there was some debate as to whether this counted as 'seeing' - if an adult human that knew nothing about the Wall were in a shuttle looking out the window, would they go "Oh hey look! A wall!"?

The Great Wall, in Gansu province, not from space but from ground level.

Thanks to the Miracle of Technology, there is actually a way to answer this question beyond a reasonable doubt in the comfort of your home. I'll mention it as an aside in a day or two. Determining the truth of the boiling frogs was more difficult.