Great Wall of China

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 BCE; 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 BCE 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.

Great Wall (Boom Crash Opera song)

Great Wall is a song by Australian rock band Boom Crash Opera. It was the first single from their self-titled 1987 album, and reached number five on the Australian music charts.

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CfA2 Great Wall

The Great Wall (also called Coma Wall), sometimes specifically referred to as the CfA2 Great Wall, is an immense galaxy filament. It is one of the largest known superstructures in the observable universe.

This structure was discovered c. 1989 by a team of American astronomers led by Margaret J. Geller and John Huchra while analyzing data gathered by the Second CfA Redshift Survey of the Harvard-Smithsonian Center for Astrophysics (CfA).

Characteristics

The CfA2 Great Wall is a galaxy wall, one of the three subtypes of galaxy filaments, the other two being galaxy sheets and supercluster complexes. The term "Great" has been added to distinguish it as an even larger type compared to standard galaxy walls. Great walls are so rare that only five and possibly six (if the disputed Hercules-Corona Borealis Great Wall was counted) of them have been discovered to date.

The CfA2 Great Wall has the maximum dimensions of either 500 million or 750 million light years; depending upon the figure and the reference used. It is 200 million light years in width and about 16 million light years in thickness. Its nearest point is about 300 million light years from Earth, while its farthest point is 550 million light years away, respectively. It consists of three massive galaxy superclusters, described below: