Great Wall of China
The Great Wall of China is any of several extended frontier fortifications in northern China. These were constructed in ancient times as barriers against invading armies. The original wall was built in 220–206 BC by Qin Shi Huang, the emperor who unified China. In modern times, this wall is no longer recognizable from the the ground, but it has been mapped using satellite photography. The modern form of the Great Wall was built during the Ming dynasty (1368 - 1644). The claim that the wall can be seen from space is an urban legend.
The Great Wall winds some 2,400 km (1,500 mi) along the edge of the Mongolian plateau from Gansu Province in the west to the Yellow Sea in the east. Its width ranges from 4 to 12 m (12 to 40 ft) and its height from 6 to 15 m (20 to 50 ft). It makes possible much more effective military defense of China from invaders. It is 4,000 miles long and more than 20 feet tall.
The Mongols got passed the wall simply by bribing the guards. In 1644, the Ming were overthrown by peasant rebels and a Ming general opened the Great Wall to the Manchu, who conquered China and founded the Qing dynasty.
Work started in 220 BC under Qin Shi Huang, connecting older walls which were constructed by individual states. Unlike the wall we see today, it was originally an earthen and wooden rampart structure, and had earlier precedents, walls built by the various states of the Warring States period to keep out nomads in the north. There was a huge human cost involved; it is believed over a million people died in the construction.
In traditional history, Qin Shi Huang is depicted as a ruthless tyrant who let thousand die building the wall, including the husband of Lady Meng. Lady Meng traveled to the wall to weep for her husband. The gods finally took pity on her and the wall cracked open. This allowed her to retrieve the body and give her husband proper burial. In the People's Republic, the traditional view is reversed and Qin Shi Huang is portrayed as a hero who united China. Communist leader Mao Zedong liked to compare himself to Qin Shi Huang, so those who opposed Mao could use the story of Lady Meng to criticize him indirectly.
The Great Wall that tourists visit today is a different structure that was built during the Ming dynasty, far to the south of Qin Shi Huang's wall. Towers were built; originally as watchtowers, but later cannons were added.
Visibility from space
The Great Wall is not the only man-made structure visible from space, or the moon. This claim was popularized by a May 1932 Ripley's Believe it or not cartoon which stated that the wall is "the mightiest work of man, the only one that would be visible to the human eye from the moon." The claim also appears in Richard Halliburton's Second Book of Marvels (1938).
It has been disputed whether it is even visible from low earth orbit. A 2008 article in Scientific American stated: "The unglamorous truth is that the wall is only visible from low orbit under a specific set of weather and lighting conditions. And many other structures that are less spectacular from an earthly vantage point—desert roads, for example—appear more prominent from an orbital perspective."
There is no truth to the notion that there are few human structures visible from space; Cecil Adams comments:
Any number of man-made structures can be seen from space, provided we mean "structure" to mean "anything built." Many of these are things that look like long, straight lines when seen from far off, such as highways, railroads, canals, and of course walls. If the orbit is low enough you can see even more, I have here a photo of Cape Canaveral taken during the Gemini V flight in which the big Launch Complex 39, used for the Apollo missions, is clearly visible. Another photo of the Nile delta, taken from a height of 100 miles, shows an extensive road network. Gemini V astronauts Gordon Cooper and Charles Conrad were able to spot, among other things, a special checkerboard pattern that had been laid out in Texas, a rocket-sled test in New Mexico, and the aircraft carrier that would later pick them up in the Atlantic, along with a destroyer trailing in its wake.
As to whether the Wall can be seen from space, Apollo 12 astronaut Alan Bean says, "The only thing you can see from the Moon is a beautiful sphere, mostly white, some blue and patches of yellow, and every once in a while some green vegetation. No man-made object is visible at this scale." 
According to NASA director Adnan Lulla, "A lot has been said and written about how visible the wall is. In fact, it is very, very difficult to distinguish the Great Wall of China in astronaut photography, because the materials that were used in the wall are similar in color and texture to the materials of the land surrounding the wall -- the dirt." 
The Great Wall was listed as World Heritage by the UN in 1987. Although long sections of the wall are now in ruins or have disappeared, it is still one of the more remarkable structures on earth and China's best-known monument.
- ↑ Hucker, Charles O. China's Imperial Past: An Introduction to Chinese History and Culture. Stanford UP, 1997.
- ↑ Susan Wise Bauer, The History of the Ancient World
- ↑ http://factsanddetails.com/china.php?itemid=27&catid=2&subcatid=1
- ↑ 4.0 4.1 Hvistendahl, Mara, Is China's Great Wall Visible from Space? Though it stretches for some 4,500 miles, the ancient Chinese fortification is not as visible from orbit as modern desert roads", Scientific American, February 21, 2008.
- ↑ BBC News, Shanghai Great Wall visible in space photo, by Francis Markus, 19 April, 2005 BBC News report of "China Daily" article 4/19/2005
- ↑ Is the Great Wall of China the only man-made object you can see from space? Cecil Adams, The Straight Dope
- ↑ NASA, China's Wall Less Great in View from Space, 05.09.05
- ↑ http://www.nasa.gov/vision/space/workinginspace/great_wall.html
- ↑ http://www.thegreatwall.com.cn/en
- Great Wall of China Encyclopaedia Britannica.