Dating back over two thousand three hundred years, the Great Wall of China winds its way from east to west across the northern part of the country. The current remains were built during Ming Dynasty and have a length of 8 851.8 km according to 2009 work by the Chinese State Administration of Cultural Heritage and National Bureau of Surveying and Mapping Agency. However, if you take into account the different parts of the wall built by other dynasties, its length is almost twenty two thousand kilometres.
The average height of the wall is between six and seven metres, and its width is between four to five metres. This width would allow five horses, or ten men, to walk side by side. The sheer size of the structure has led people to believe that it could be seen from space. This was first described by William Stukeley in 1754, when he wrote in reference to Hadrianâ€™s Wall that â€˜This mighty wall of four score miles in length is only exceeded by the Chinese Wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the Moon.â€™
Despite Stukeleyâ€™s personal opinion not having any scientific basis, it has been repeated many times since. By the time humans began to go into space, it was considered a fact. Unfortunately, astronauts such as Buzz Aldrin, Chris Hatfield and even Chinaâ€™s first astronaut, Yang Liwei, have all confirmed that the Great Wall is not visible from space by the naked eye. Even Pixalytics has got a little involved in this debate. Two years ago we wrote a blog saying that we couldnâ€™t see the wall on Landsat imagery as the spatial resolution was not small enough to be able to distinguish it from its surroundings.
Anyone who is familiar with the QI television series on the BBC will know that they occasionally ask the same question in different shows and give different answers when new information comes to light. This time itâ€™s our turn!
Last week Sam was a speaker at the TEDx One Step Beyond event at the National Space Centre in Leicester â€“ youâ€™ll hear more of that in a week or two. However, in exploring some imagery for the event we looked for the Great Wall of China within Sentinel-2 imagery. And guess what? We found it! In the image at the top, the Great Wall can be seen cutting down the centre from the top left.
It was difficult to spot. The first challenge was getting a cloud free image of northern China, and we only found one covering our area of interest north of Beijing! Despite Sentinel-2 having 10 m spatial resolution for its visible wavelengths, as noted above, the wall is generally narrower. This means it is difficult to see the actual wall itself, but it is possible to see its path on the image. This ability to see very small things from space by their influence on their surroundings is similar to how we are able to spot microscopic phytoplankton blooms. The image on the right is a screenshot from Sentinel Application Platform tool (SNAP) which shows the original Sentinel-2 image of China on the top left and the zoomed section identifying the wall.
So whilst the Great Wall of China might not be visible from space with the naked eye, it is visible from our artificial eyes in the skies, like Sentinel-2.