Is the Great Wall of China one wall, built by one emperor and visible from space?

The Great Wall is not one wall, and it was not built by one emperor. It is a collection of fortifications built across the northern borders of ancient Chinese states and Imperial China, as protection against nomadic groups from the Eurasian Steppe. The first walls date to the 7th century BC, and they were joined together in the Qin dynasty. Successive dynasties then expanded the system, and the best-known sections were built by the Ming dynasty, between 1368 and 1644.

The first emperor's role is more surprising than the legend. When Qin unified China in 221 BC, he ordered the destruction of the sections of the walls that had divided his empire among the former states. Then, to face the Xiongnu people in the north, he ordered new walls to connect the fortifications along the northern frontier. Almost none of that work survives, and there are no historical records of the exact length and course of the Qin walls.

What visitors photograph today is mostly Ming work. After the Ming were defeated by the Oirats in the Battle of Tumu, they rebuilt the defences on a grander scale, using bricks and stone instead of rammed earth. A 2012 survey found that the Ming wall measures 8,850 km, while the Qin, Han and earlier sites add up to 3,080 km.

The space claims shrink too. The wall cannot be seen by the naked eye from the Moon, a myth found as early as a 1754 letter by William Stukeley, and NASA says that from low Earth orbit it is barely visible, and only under nearly perfect conditions. And, contrary to urban legend, no human bones or body parts were ever put into its mortar, which was made of sticky rice soup mixed with slaked lime.