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. Several walls were being built as early as the 7th
century BC; these, later joined together and made bigger and
stronger, are now collectively referred to as the Great Wall. Especially
famous is the wall built 220–206 BC by Qin Shi Huang, the first Emperor of China. Little of that wall remains.
Since then, the Great Wall has on and off been rebuilt, maintained, and
enhanced; the majority of the existing wall is from the Ming Dynasty.
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.
A comprehensive archaeological survey, using advanced
technologies, has concluded that the Ming walls 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.The claim the Great Wall is visible from the moon has been
debunked many times, but is still ingrained in popular culture. 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.
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