Great Wall of China is a series
of stone and earthen fortifications in northern China, built
originally to protect the northern borders of the Chinese Empire
against intrusions by various nomadic groups. Several walls have
been built since the 5th century BC that are referred to
collectively as the Great Wall,
which has been rebuilt and maintained from the 5th century BC
through the 16th century. One of the most famous is the wall
built between 220–206 BC by the first Emperor of China, Qin Shi
Huang. Little of that wall remains; the majority of the existing
wall was built during the Ming Dynasty.
Great Wall stretches from
Shanhaiguan in the east, to Lop Nur in the west, along an arc
that roughly delineates the southern edge of Inner Mongolia. The
most comprehensive archaeological survey, using advanced
technologies, has concluded that the entire
Great Wall, with all of its
branches, stretches for 8,851.8 km (5,500.3 mi). This is made up
of 6,259.6 km (3,889.5 mi) sections of actual wall, 359.7 km
(223.5 mi) of trenches and 2,232.5 km (1,387.2 mi) of natural
defensive barriers such as hills and rivers.
use of bricks, the Great Wall
was mainly built from rammed earth, stones, and wood. During the
Ming Dynasty, 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.
portions north of Beijing and near tourist centers have been
preserved and even extensively renovated, in many locations the
Wall is in disrepair. Those parts might serve as a village
playground or a source of stones to rebuild houses and roads.
Sections of the Wall are also prone to graffiti and vandalism.
Parts have been destroyed because the Wall is in the way of
60 km (37 mi) of the wall in Gansu province may disappear in the
next 20 years, due to erosion from sandstorms. In places, the
height of the wall has been reduced from more than five meters
(16.4 ft) to less than two meters. The square lookout towers
that characterize the most famous images of the wall have
disappeared completely. Many western sections of the wall are
constructed from mud, rather than brick and stone, and thus are
more susceptible to erosion.
Watchtowers and barracks
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 from space
Visibility from the moon
One of the
earliest known references to this myth appears in a letter
written in 1754 by the English antiquary William Stukeley.
Stukeley wrote that, "This mighty wall of four score miles in
length (Hadrian's Wall) is only exceeded by the Chinese Wall,
which makes a considerable figure upon the terrestrial globe,
and may be discerned at the moon." The claim was also mentioned
by Henry Norman in 1895 where he states "besides its age it
enjoys the reputation of being the only work of human hands on
the globe visible from the moon." The issue of "canals" on Mars
was prominent in the late 19th century and may have led to the
belief that long, thin objects were visible from space. The
claim that the Great Wall is
visible also appears in a 1932 Ripley's Believe it or Not
cartoon and in Richard Halliburton's 1938 book Second Book of
the Great Wall is visible
has been debunked many times, but is still ingrained in popular
culture. The wall is a maximum 9.1 m (30 ft) wide, and is about
the same color as the soil surrounding it. Based on the optics
of resolving power (distance versus the width of the iris: a few
millimeters for the human eye, meters for large telescopes) only
an object of reasonable contrast to its surroundings which is 70
mi (110 km) or more in diameter (1 arc-minute) would be visible
to the unaided eye from the moon, whose average distance from
Earth is 384,393 km (238,851 mi). The apparent width of the
Great Wall from the moon is
the same as that of a human hair viewed from 2 miles away. 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.
Visibility from low earth
controversial question is whether the Wall is visible from low
earth orbit (an altitude of as little as 100 miles (160 km)).
NASA claims that it is barely visible, and only under nearly
perfect conditions; it is no more conspicuous than many other
man-made objects. Other authors have argued that due to
limitations of the optics of the eye and the spacing of
photoreceptors on the retina, it is impossible to see the wall
with the naked eye, even from low orbit, and would require
visual acuity of 20/3 (7.7 times better than normal).
William Pogue thought he had seen it from Skylab but discovered
he was actually looking at the Grand Canal of China near
Beijing. He spotted the Great Wall
with binoculars, but said that "it wasn't visible to the unaided
eye." 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 U.S. astronauts. Veteran U.S.
astronaut Gene Cernan has stated: "At Earth orbit of 100 miles
(160 km) to 200 miles (320 km) high, the
Great Wall of China is, indeed,
visible to the naked eye." Ed Lu, Expedition 7 Science Officer
aboard the International Space Station, adds that, "it's less
visible than a lot of other objects. And you have to know where
Neil Armstrong stated about the view from Apollo 11: "I do not
believe that, at least with my eyes, there would be any man-made
object that I could see. I have not yet found somebody who has
told me they've seen the Wall of China from Earth orbit. ...I've
asked various people, particularly Shuttle guys, that have been
many orbits around China in the daytime, and the ones I've
talked to didn't see it."
2003, Chinese astronaut Yang Liwei stated that he had not been
able to see the Great Wall of China.
In response, the European Space Agency (ESA) issued a press
release reporting that from an orbit between 160 and 320 km, the
Great Wall is visible to the
naked eye. In an attempt to further clarify things, the ESA
published a picture of a part of the “Great
Wall” photographed from Space. However, in a press
release a week later (no longer available in the ESA’s website),
they acknowledged that the "Great Wall"
in the picture was actually a river.
Chiao, a Chinese-American astronaut, took a photograph from the
International Space Station that shows the wall. It was so
indistinct that the photographer was not certain he had actually
captured it. Based on the photograph, the China Daily later
reported that the Great Wall
can be seen from space with the naked eye, under favorable
viewing conditions, if one knows exactly where to look. However,
the resolution of a camera can be much higher than the human
visual system, and the optics much better, rendering
photographic evidence irrelevant to the issue of whether it is
visible to the naked eye.
Great Wall of China -