Great Wall of China
The Great Wall of China (Simplified Chinese: 万里长城; Traditional Chinese: 萬里長城; Pinyin: Wànlĭ Chángchéng; literally "10,000 Li¹ long wall") is a Chinese fortification built from 3rd century BC until the beginning of the 17th century, in order to protect the various dynasties from raids by Mongol, Turkic, and other nomadic tribes coming from areas in modern-day Mongolia and Manchuria. Several walls were built since the 3rd century BC, the most famous being the Great Wall built between 220 BC and 200 BC by the First Emperor, Qin Shi Huangdi, which was located much further north than the current Great Wall of China built during the Ming Dynasty, and little of it remains. The Wall stretches over a formidable 6,352 km (3,948 miles).
From outer space
There is a long standing disagreement about how visible the wall is in space.
Richard Halliburton's 1938 book Second Book of Marvels said the Great Wall is the only man-made object visible from the moon, and a "Ripley's Believe It or Not" cartoon from the same decade makes a similar claim.
The distance from Earth to the moon is about a thousand times greater than the distance from the earth to a spacecraft in near-earth orbit. If the Great Wall were visible from the moon, it would be easy to see from near-earth orbit. In fact, from near-Earth orbit, it is barely visible, and only under nearly perfect conditions. And it is no more conspicuous than many other manmade objects.
Astronaut William Pogue thought he had seen it from Skylab but discovered he was actually looking at the Grand Canal near Beijing. He spotted the Great Wall with binoculars, but said that "it wasn't visible to the unaided eye." An Apollo astronaut said no human structures were visible at a distance of a few thousand miles. 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 professional U.S. astronauts. Chinese astronaut Yang Liwei said he couldn't see it at all.
These inconsistent results suggest the visibility of the Great Wall depends greatly on the seeing conditions, and also the direction of the light (oblique lighting increasing the angular size of the Wall through the addition of a shadow to the physical width of the Wall). Features on the moon that are dramatically visible at times can be undetectable on others, due to changes in lighting direction. The Great Wall is only a few meters wide — sized similar to highways and airport runways — and is about the same color as the soil surrounding it.
(courtesy wikipedia.com)
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