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The Great Wall of China is NOT the only manmade object visible from space
It's a piece of folklore that the only manmade object visible from space is The Great wall of China. A long time ago someone said it, and it got stuck in the popular imagination, and after that people have quoted it to each other over and over as if it's a fact and no-one stops to think whether it seriously is true.
The first thing to consider is how far away SPACE is. An aircraft flying at 35,000 feet is not in space, but a satellite 60 miles up IS in space. So, you don't need to be millions of miles away to be "in space". You just need to be outside almost all the atmosphere, and preferably going at some speed if in orbit, so as to remain in space and not end up arriving back on the earth in a big hurry. Also see how orbits work
The next thing is, what kinds of manmade things can be seen sixty miles away? Well, you can see a candle at 15 miles in good conditions, so it's not unreasonable to see a 100 watt lightbulb at 60 miles. Don't 100 watt lightbulbs count as manmade? In fact there is a floodlit church tower called Boston Stump which has received complaints from Russian cosmonauts because the lighting is so dazzling and points directly heavenwards.
However, with one notable exception, a type of the thing that is very notably NOT seen when looking at the earth from space is those lines drawn familiarly on maps to denote the boundaries between nations*. Philosophical as it may seem, these sovereignty garden-fences are invisible from space, as they are not part of the physical world, but more part of the political world, which exists in peoples' minds.
* It is true that SOME international borders are visible from space, but this usually happens when a previously existing physical geographical border or change of terrain is later adopted as a nation state border.
Other point of note: Doubt has been cast upon the idea that the Great Wall of China CAN be seen from space. Partly it's a matter of where "space" starts, which is different according to different definitions (for the same reasons as weight/tare), but also there are some larger manmade features such as roads which are broader and longer which are not visible from space.
The problem then is how the legend of "The Great Wall of China is the only man-made object visible from space" came to be thought-up.
My theory is that the Great Wall is visible SOME of the time but not at other times. Presumably it's not visible at night, but it would be at its most visible at dawn when the light of the rising sun casts long shadows.
http://www.snopes.com/science/greatwall.asp - Yes, it's obviously not visible from the MOON!
Also see lighting, space, orbits, optics, etc