The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe with an eye to expansion. Several walls were being built as early as the 7th century BC; these, later joined together and made bigger and stronger, are collectively referred to as the Great Wall. Especially famous is the wall built in 220–206 BC by Qin Shi Huang, the first Emperor of China. Little of that wall remains. The Great Wall has been rebuilt, maintained, and enhanced over various dynasties; the majority of the existing wall is from the Ming Dynasty (1368–1644).
Apart from defense, other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor.
The frontier walls built by different dynasties have multiple courses. Collectively, they stretch from Dandong in the east to Lop Lake in the west, from present-day Sino-Russian border in the north to Qinghai in the south; along an arc that roughly delineates the edge of Mongolian steppe. A comprehensive archaeological survey, using advanced technologies, has concluded that the walls built by the Ming dynasty measure 8,850 km (5,500 mi).This is made up of 6,259 km (3,889 mi) sections of actual wall, 359 km (223 mi) of trenches and 2,232 km (1,387 mi) of natural defensive barriers such as hills and rivers.Another archaeological survey found that the entire wall with all of its branches measures out to be 21,196 km (13,171 mi). Today, the Great Wall is generally recognized as one of the most impressive architectural feats in history.
The collection of fortifications known as the Great Wall of China has historically had a number of different names in both Chinese and English.
In Chinese histories, the term “Long Wall(s)” (長城, changcheng) appears in Sima Qian’s Records of the Grand Historian, where it referred both to the separate great walls built between and north of the Warring States and to the more unified construction of the First Emperor. The Chinese character 城 is a phono-semantic compound of the “place” or “earth” radical 土 and 成, whose Old Chinese pronunciation has been reconstructed as *deŋ. It originally referred to the rampart which surrounded traditional Chinese cities and was used by extension for these walls around their respective states; today, however, it is much more often the Chinese word for “city”.
The longer Chinese name “Ten-Thousand Mile Long Wall” (萬里長城, Wanli Changcheng) came from Sima Qian’s description of it in the Records, though he did not name the walls as such. The ad 493 Book of Song quotes the frontier general Tan Daoji referring to “the long wall of 10,000 miles”, closer to the modern name, but the name rarely features in pre-modern times otherwise. The traditional Chinese mile (里, lǐ) was an often irregular distance that was intended to show the length of a standard village and varied with terrain but was usually standardized at distances around a third of an English mile (540 m). Since China’s metrication in 1930, it has been exactly equivalent to 500 metres or 1,600 feet, which would make the wall’s name describe a distance of 5,000 km (3,100 mi). However, this use of “ten-thousand” (wàn) is figurative in a similar manner to the Greek and English myriad and simply means “innumerable” or “immeasurable”.
Because of the wall’s association with the First Emperor’s supposed tyranny, the Chinese dynasties after Qin usually avoided referring to their own additions to the wall by the name “Long Wall”. Instead, various terms were used in medieval records, including “frontier(s)” (塞, sāi), “rampart(s)” (垣, yuán), “barrier(s)” (障, zhàng), “the outer fortresses” (外堡, wàibǎo), and “the border wall(s)” (t 邊牆, s 边墙, biānqiáng).Poetic and informal names for the wall included “the Purple Frontier” (紫塞, Zǐsāi) and “the Earth Dragon” (t 土龍, s 土龙, Tǔlóng). Only during the Qing period did “Long Wall” become the catch-all term to refer to the many border walls regardless of their location or dynastic origin, equivalent to the English “Great Wall”.
The current English name evolved from accounts of “the Chinese wall” from early modern European travelers. By the 19th century, “The Great Wall of China” had become standard in English, French, and German, although other European languages continue to refer to it as “the Chinese wall.”
Visibility from space
From the Moon
One of the earliest known references to the myth that the Great Wall can be seen from the moon appears in a letter written in 1754 by the English antiquary William Stukeley. Stukeley wrote that, “This mighty wall [Hadrian’s wall] of four score miles [130 km] 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.”The claim was also mentioned by Henry Norman in 1895 where he states “besides its age it enjoys the reputation of being the only work of human hands on the globe visible from the Moon.” The issue of “canals” on Mars was prominent in the late 19th century and may have led to the belief that long, thin objects were visible from space. The claim that the Great Wall is visible from the moon also appears in 1932’s Ripley’s Believe It or Not! strip and in Richard Halliburton’s 1938 book Second Book of Marvels.
The claim that the Great Wall is visible from the moon has been debunked many times, but is still ingrained in popular culture. The wall is a maximum 9.1 m (29 ft 10 in) wide, and is about the same color as the soil surrounding it. Based on the optics of resolving power (distance versus the width of the iris: a few millimeters for the human eye, meters for large telescopes) only an object of reasonable contrast to its surroundings which is 110 km (70 mi) or more in diameter (1 arc-minute) would be visible to the unaided eye from the Moon, whose average distance from Earth is 384,393 km (238,851 mi). The apparent width of the Great Wall from the Moon is the same as that of a human hair viewed from 3 km (2 mi) away. To see the wall from the Moon would require spatial resolution 17,000 times better than normal (20/20) vision. Unsurprisingly, no lunar astronaut has ever claimed to have seen the Great Wall from the Moon.
From low Earth orbit
A satellite image of a section of the Great Wall in northern Shanxi, running diagonally from lower left to upper right and not to be confused with the more prominent river running from upper left to lower right. The region pictured is 12 km × 12 km (7 mi × 7 mi).
A more controversial question is whether the Wall is visible from low Earth orbit (an altitude of as little as 160 km (100 mi)). NASA claims that it is barely visible, and only under nearly perfect conditions; it is no more conspicuous than many other man-made objects. Other authors have argued that due to limitations of the optics of the eye and the spacing of photoreceptors on the retina, it is impossible to see the wall with the naked eye, even from low orbit, and would require visual acuity of 20/3 (7.7 times better than normal).
Astronaut William Pogue thought he had seen it from Skylab but discovered he was actually looking at the Grand Canal of China near Beijing. He spotted the Great Wall with binoculars, but said that “it wasn’t visible to the unaided eye.” U.S. Senator Jake Garn claimed to be able to see the Great Wall with the naked eye from a space shuttle orbit in the early 1980s, but his claim has been disputed by several U.S. astronauts. Veteran U.S. astronaut Gene Cernan has stated: “At Earth orbit of 100 to 200 miles [160 to 320 km] high, the Great Wall of China is, indeed, visible to the naked eye.” Ed Lu, Expedition 7 Science Officer aboard the International Space Station, adds that, “it’s less visible than a lot of other objects. And you have to know where to look.”
In 2001, Neil Armstrong stated about the view from Apollo 11: “I do not believe that, at least with my eyes, there would be any man-made object that I could see. I have not yet found somebody who has told me they’ve seen the Wall of China from Earth orbit. … I’ve asked various people, particularly Shuttle guys, that have been many orbits around China in the daytime, and the ones I’ve talked to didn’t see it.”
In October 2003, Chinese astronaut Yang Liwei stated that he had not been able to see the Great Wall of China. In response, the European Space Agency (ESA) issued a press release reporting that from an orbit between 160 and 320 km (100 and 200 mi), the Great Wall is visible to the naked eye. In an attempt to further clarify things, the ESA published a picture of a part of the “Great Wall” photographed from low orbit. However, in a press release a week later, they acknowledged that the “Great Wall” in the picture was actually a river.
Leroy Chiao, a Chinese-American astronaut, took a photograph from the International Space Station that shows the wall. It was so indistinct that the photographer was not certain he had actually captured it. Based on the photograph, the China Daily later reported that the Great Wall can be seen from ‘space’ with the naked eye, under favorable viewing conditions, if one knows exactly where to look. However, the resolution of a camera can be much higher than the human visual system, and the optics much better, rendering photographic evidence irrelevant to the issue of whether it is visible to the naked eye.