The Great Wall of China is not a continuous wall but is a collection of short walls that often follow the crest of hills on the southern edge of the Mongolian plain. The Great Wall of China, known as "long Wall of 10,000 Li" in China, extends about 8,850 kilometers (5,500 miles).
Building the Great Wall of China
A first set of walls, designed to keep Mongol nomads out of China, were built of earth and stones in wood frames during the Qin Dynasty (221 to 206 BCE).
Some additions and modifications were made to these simple walls over the next millennium but the major construction of the "modern" walls began in the Ming Dynasty (1388 to 1644 CE).
The Ming fortifications were established in new areas from the Qin walls. They were up to 25 feet (7.6 meters) high, 15 to 30 feet (4.6 to 9.1 meters) wide at the base, and from 9 to 12 feet (2.7 to 3.7 meters) wide at the top (wide enough for marching troops or wagons). At regular intervals, guard stations and watch towers were established.
Since the Great Wall was discontinuous, Mongol invaders had no trouble breaching the wall by going around it, so the wall proved unsuccessful and was eventually abandoned. Additionally, a policy of mollification during the subsequent Ch'ing Dynasty that sought to pacify the Mongol leaders through religious conversion also helped to limit the need for the Great Wall.
Through Western contact with China from the 17th through 20th centuries, the legend of the Great Wall of China grew along with tourism to the wall. Restoration and rebuilding took place in the 20th century and in 1987 the Great Wall of China was made a World Heritage Site. Today, a portion of the Great Wall of China, about 50 miles (80 km) from Beijing, receives thousands of tourists each day.
Can You See It From Outer Space or the Moon?
For some reason, some urban legends tend to get started and never disappear. Many are familiar with the claim that the Great Wall of China is the only man-made object visible from space or from the moon with the naked eye. This is simply not true.
The myth of being able to see the Great Wall from space originated in Richard Halliburton's 1938 (long before humans saw the Earth from space) book Second Book of Marvels said that the Great Wall of China is the only man-made object visible from the moon.
From a low orbit of the Earth, many artificial objects are visible, such as highways, ships in the sea, railroads, cities, fields of crops, and even some individual buildings. While at a low orbit, the Great Wall of China can certainly be seen from space, it is not unique in that regard.
However, when leaving the Earth's orbit and acquiring an altitude of more than a few thousand miles, no man-made objects are visible at all. NASA says, "The Great Wall can barely be seen from the Shuttle, so it would not be possible to see it from the Moon with the naked eye." Thus, it would be tough to spot the Great Wall of China or any other object from the moon. Furthermore, from the moon, even the continents are barely visible.
Regarding the origination of the story, Straight Dope's pundit Cecil Adams says, "Nobody knows exactly where the story got started, although some think it was speculation by some bigshot during an after-dinner speech in the early days of the space program."
NASA astronaut Alan Bean is quoted in Tom Burnam's book More Misinformation…
"The only thing you can see from the moon is a beautiful sphere, mostly white (clouds), some blue (ocean), patches of yellow (deserts), and every once in a while some green vegetation. No man-made object is visible on this scale. In fact, when first leaving earth's orbit and only a few thousand miles away, no man-made object is visible at that point either."