Many people believe that you can see the Great Wall of China from space. But in 2003, China's first astronaut came back to Earth and said, "I could not see it."

The wall is very long, but it is only a few meters wide. From the space station, about 400 kilometers up, it is too thin to see with your eyes. Its color is also like the land around it.

In 2003, Yang Liwei became China's first astronaut. Back on Earth, he said he had not seen the Great Wall of China from space. People had long said it was the only human-made structure visible from space with the naked eye.

The wall is very long, but length is not what matters. Width is. In many places the wall is less than six meters wide. From the International Space Station, a person with normal eyesight can only make out objects about 100 meters wide or more.

The wall is also built from materials with colors like the land around it, so it blends in. City lights at night are much easier to see. And people were telling the story long before anyone went into space to check.

When Yang Liwei, China's first astronaut, returned in 2003, he admitted that he had not seen the Great Wall. The admission led even Chinese textbooks to reconsider a famous claim, that the wall is the only human-made structure visible from space with the naked eye. The wall's great length makes the idea sound plausible.

But whether something can be seen depends on its width. Most sections are less than six meters wide, about the width of a two-lane road. The human eye can resolve details of about one minute of arc. From the International Space Station, some 400 kilometers up, an object would therefore need to be about 100 meters wide to be distinguished. On this basis, the vision scientist López-Gil concluded in a 2008 paper that spotting the wall unaided from orbit is practically impossible.

Contrast makes things harder still, because the wall was largely built from materials that resemble the surrounding terrain. Width alone does not settle every case, though. Some astronauts have reported seeing narrow features with high contrast, such as roads, in favorable conditions. The legend itself appeared in print decades before anyone flew in space, so it was never based on observation.