Great Wall of China
The Great Wall of China is an ancient wall in China. The wall is made of cement, rocks and powdered dirt. It was built to protect the north of the empire of China from enemy attacks. It is the longest structure humans have ever built. It is about 21,196 kilometres (13,171 miles) long, 9.1 metres (30 feet) wide and 15 metres (50 feet) high. The earlier sections on the wall are made of compacted dirt and stone. Later in the Ming Dynasty they used bricks. There are 7,000 watch towers, block houses for soldiers and beacons to send smoke signals.
Nineteen walls have been built that were called the Great Wall of China. The first was built in the 7th century BC. The most famous wall was built between 226–200 BC by the first Emperor of Imperial China, Qin Shai Hong. Not much of this wall will remain as people have been stealing from it. It was much farther north than the current wall. The current wall was built during the Ming Dynasty.
The First Emperor of China started the Qin Dynasty. The Xiongnu tribes in the north of China (who are also called the Huns) were his enemies. The land in some parts of China is easy to cross, so Qin Shi Huang started building the Great Wall to make it more difficult for the Xiongnu to invade China.
Other dynasties in China worked more on the wall and made it longer. The Han, Sui, Northern and Jin Dynasties all repaired, rebuilt or expanded the Great Wall. During the Ming Dynasty, major rebuilding work took place. Sections of the wall were built with bricks and stone instead of earth.
Construction and rebuilding of the Great Wall
Builders used materials that were nearby. Some parts of the wall were made of mud, straw, and twigs. Thousands of workers died from giant falling stones, exhaustion, disease, animal attacks, and starvation. The idea that workers who died were buried in and under the Great Wall is a myth.
Before the use of bricks, the Great Wall was mainly built from rammed earth, stones, and wood. During the Ming, however, bricks were heavily used in many areas of the wall, as were materials such as tiles, lime, and stone. The size and weight of the bricks made them easier to work with than earth and stone, so construction quickened. Additionally, bricks could bear more weight and endure better than rammed earth. Stone can hold under its own weight better than brick, but is more difficult to use. Consequently, stones cut in rectangular shapes were used for the foundation, inner and outer brims, and gateways of the wall. Battlements line the uppermost portion of the vast majority of the wall, with defensive gaps a little over 30 cm (12 in) tall, and about 23 cm (9.1 in) wide. From the parapets, guards could survey the surrounding land. Communication between the army units along the length of the Great Wall, including the ability to call reinforcements and warn garrisons of enemy movements, was of high importance. Signal towers were built upon hill tops or other high points along the wall for their visibility. Wooden gates could be used as a trap against those going through. Barracks, stables, and armories were built near the wall's inner surface.
Visibility from space
Rumours about astronauts being able to see the Great Wall from the moon are scientifically not proven. The Great Wall has shown up in radar images taken from space, but scientists are sure it is not possible for astronauts to see the wall with a naked eye. One astronaut who spoke about being able to see the Great Wall from space was Neil Armstrong. He said that on the moon it was very clear that the wall was not visible. However, the astronaut William Pogue was able to see the wall from a Low Earth Orbit distance (300-530 km height), but only with binoculars and with lots of practice.
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