Great Wall of China

 The Great Wall of China is a system of fortifications designed to protect ancient Chinese kingdoms and Imperial China from numerous nomadic populations from the Eurasian Steppe. Several walls were constructed beginning in the 7th century BC, with Qin Shi Huang (220–206 BC), China's first emperor, subsequently connecting selected parts. Only a little portion of the Qin wall has survived. Many consecutive dynasties would later construct and maintain various lengths of boundary walls. The Ming dynasty (1368–1644) built the most well-known parts of the wall.

Aside from defence, the Great Wall has served additional objectives such as border controls, allowing for the application of tariffs on products moved over the Silk Road, trade regulation or promotion, and immigration and emigration control. The building of watchtowers, soldier barracks, garrison stations, signalling capabilities through smoke or fire, and the fact that the Great Wall's course also acted as a transit corridor all contributed to the Great Wall's defensive qualities.

Different dynasties erected different types of boundary walls. They range from Liaodong in the east to Lop Lake in the west, from the present-day Sino–Russian border in the north to the Tao River (Taohe) in the south, roughly delineating the Mongolian steppe's boundary. The Great Wall's defence system is now widely regarded as one of the most spectacular architectural achievements in history.

 

 

 VISIBILITY FROM SPACE :-

 

FROM MOON 

The idea that the wall can be seen from the moon (which has an average orbital radius of 385,000 kilometres or 239,000 miles) is well-known but false. In a letter sent in 1754 by the English antiquary William Stukeley, one of the first documented allusions to the idea that the Great Wall can be viewed from the moon occurs." On the earthly globe, this gigantic wall[Hadrian's wall]of four score miles[130 km]in length is only equalled by the Chinese Wall, which constitutes a significant figure and may be recognised at the Moon," stated Stukeley.

In 1895, Henry Norman made the claim, saying, "Besides its age, it has the repute of being the sole creation of human hands on the planet visible from the Moon. "The question of "canals" on Mars was a hot topic in the late 1800s, and it may have contributed to the assumption that long, thin objects might be seen from space .The notion that the Great Wall can be seen from the moon is also mentioned in a Ripley's Believe It or Not! strip from 1932.The myth that the Great Wall can be seen from the moon has been refuted several times (the apparent breadth of the Great Wall from the Moon would be the same as a human hair viewed from 3 km (2 mi) away), yet it persists in popular culture.

 

FROM LOW EARTH ORBIT

A more contentious subject is whether the wall may be seen from low Earth orbit (as low as 160 km (100 mi)). It is scarcely visible, NASA asserts, and only under near-ideal conditions; it is no more apparent than many other man-made objects.

"At Earth orbits of 100 to 200 miles[160 to 320 km]high, the Great Wall of China is, indeed, visible to the naked eye," said veteran US astronaut Gene Cernan.

The International Space Station's Expedition 7 Science Officer, Ed Lu, adds, "It's not as noticeable as some other things. You must also know where to search."

Chinese astronaut Yang Liwei said in October 2003 that he had not been able to glimpse the Great Wall of China. The European Space Agency (ESA) responded with a news statement claiming that the Great Wall is visible to the naked eye from an orbit between 160 and 320 kilometres (100 and 200 miles).

A image of the wall was taken from the International Space Station by Leroy Chiao, a Chinese-American astronaut. The image was so blurry that the photographer wasn't sure he'd got it. The China Daily then asserted, based on the photos, that the Great Wall can be viewed with the naked eye from'space' under good viewing conditions, assuming one knows just where to look.