A lot of people have heard that the Great Wall that stretches across China can be seen from space. The idea started in Europe. A writer named Stukeley wrote in 1754 that it can be seen from the moon.
Although it stretches 21,196 km (13,170 mi), because it is thin and a lot of it is crumbling and its color matches the land, maybe no astronaut has ever really seen it.
This idea of seeing the wall from space became popular in the 1930s. People believed they saw canals on Mars. It was a scientific idea then, and people thought that the Great Wall should be able to be seen from outer space too. A popular book called Ripley's Believe it or Not popularized the idea in 1932.
However, after manned spaceflight started in the 1960s, though several astronauts said that they saw it, people aren't sure that they did. They may have seen something that they thought was the wall such as a road or canal.
China's first astronaut didn't see it. He went to space on the Shenzhou 5 (神舟五号, shénzhōu wǔ hào) in 2003. Other Chinese astronauts haven't seen it with their own eyes either.
Much of it is crumbling, and much of it matches the color of the land because it was made of mud bricks or baked bricks. Roads look like it, and so do natural features because it typically follows the ridges.
People think that only someone with sharp eyes in low earth orbit can pick out sections of the wall in excellent viewing conditions, if they know exactly where it is. However, it is much easier to see other construction such as highways or canals, such as the Grand Canal, since they stand out more clearly.
In 2004, an American astronaut named Leroy Chiao took a photograph of a region on China's northern border about 200 miles from Beijing. The sun was at the right angle to cast a long shadow, and experts claimed the wall was visible in the picture.
The wall has also been photographed from space when there was snow on the ground that made the dark wall stand out.
The northern areas of China are increasingly smoggy and dusty, so it is getting harder to see the wall every year.
The Great Wall was last extensively built up and reinforced by the Ming Dynasty, but nowadays most of it is rapidly eroding and some of it is being torn down. Some people have been taking bricks and cut stones to use for construction, and in some places it has been torn down to make way for modern construction.
However, a few places such as the sections near Beijing that tourists go to have been rebuilt or protected. More remote sections of original wall you can get to on foot, but without protection they may disappear completely!
The Great Wall is a national symbol and China's most popular tourist attraction. See it for yourself on our Great Wall tours!