The Great Wall of China

This question appears to be based on what it says on Wikipedia. Nothing wrong with that; as regular readers will know, this website sets great store in Wikipedia and generally uses it as the first port of call for checking facts.

The city of Dandong and Lop Lake are pretty much the eastern and western extremities of the Great Wall of China. But it's important to remember that the Great Wall is not one continuous wall that follows a single line from one end to the other. Rather, it's a network of walls, that were built over the course of two millennia (mainly from the fifth century BC to the 17th century AD).

The first walls were built during the so–called Warring States period – some time between the 5th century BC and 221 BC. This was a time of intensive warfare all around China, as the nobles stopped supporting the Zhou Dynasty (1046–221 BC) and Zhou's vassal states declared themselves independent.

In 221 BC, Qin Shihuang became the first person to declare himself Emperor of China. After consolidating power, he commanded the construction of roughly three thousand miles of walls along the borders of his empire, to protect it from the nomadic Mongol and Turkic tribes that inhabited the plains in the north. The Qin dynasty lasted only 15 years, and was succeeded by the Han – which lasted for over 400 years, until AD 220. A further 3,000 miles of wall were built during the Han dynasty.

Comparatively little building was done over the next 800 years or so. Work started again in earnest in the 11th century AD, during the Liao dynasty, and continued until the end of the Jin dynasty in 1234. About 3,500 miles of new walls were built during this time. There followed a gap of some 130 years, until the start of the Ming dynasty, which lasted from 1368 until 1644. About 4,500 miles of wall were built during this period – most of it new. The Ming walls were built of stone and brick, while most of the earlier walls were made of rammed earth; and it's mainly the Ming walls (extensively restored) that we see today.

Because of the complex nature of the wall(s), estimates of its length vary widely. According to the tour operator China Highlights, the shortest distance between the westernmost and easternmost points of the Ming Great Wall ('The First Strategic Post of the Great Wall' in Jiayuguan, and the village of Hushan on the Korean border) is 2,235 km (1,389 mi)." The First Strategic Post, also known as the Great Wall First Abutment, is one of many forts that were built during the Ming dynasty to guard the Jiayu Pass – about 400 miles east of Lop Lake; Hushan is 12 kilometres (7.5 miles) from the city of Dandong.

In 2012, the Chinese State Administration of Cultural Heritage announced, after a five–year survey, that the official length of the wall was 21,196.18 kilometres (13,170.7 miles). China Highlights notes, however, that "[this is] a misleading figure as some sections of different eras were built on top of or right next to each other. Also isolated sections of fortified wall defending state boundaries are included, not just China's northern border wall(s) that people generally think the Great Wall to be."

China Highlights gives the total length of all the walls that were built over the centuries as approximately 27,000 miles. This is more than enough to stretch round the Equator.

The name by which the wall has been known in China since as far back as the Qin dynasty can be translated as either 'the 5,000 kilometer wall' or simply 'the very long wall'. Five thousand kilometres is pretty close to the actual length of wall that existed at the end of the Qin dynasty (in the 3rd century AD), but a lot more wall has been built since then.

The idea that the Great Wall of China can be seen from space has long since been debunked as a myth. It originated with the English antiquary William Stukeley, who wrote in 1754 that "the Chinese Wall ... makes a considerable figure upon the terrestrial globe, and may be discerned at the Moon." It goes without saying that Stukeley would have had no evidence whatsoever to support this claim. The myth was perpetuated by, among others, the English journalist and politician Sir Henry Norman (1858–1939) and the American sensationalist Robert Ripley (in 1932).

Wikipedia notes that "Unsurprisingly, no lunar astronaut has ever claimed to have seen the Great Wall from the Moon." It goes on to report that several US astronauts have claimed to have seen the wall from low Earth orbit (an altitude of between 100 and 200 miles); but one member of the International Space Station crew responded that "it's less visible than a lot of other objects. And you have to know where to look."