Great Wall Myth Revisted
Posted on Monday 17 May 2004 to Frontiers
An
image acquired by Proba's High Resolution Camera on 25 March 2004 shows
a short stretch of the 7240-km-long Great Wall of China snaking along
hilltops northeast of Beijing, running from the top middle of the image
down to bottom right. The white watercourse that meanders from the
middle of the left side down to the bottom of the image is the initial
part of the 1500-km-long Da Yunhe or Grand Canal, a linked series of
natural and man-made waterways that represents an engineering
achievement on a par with the Great Wall. Credit: The European Space Agency
Much crowing went on in the world media last year after Chinese
Astronaut, Yang Liwei, admitted that he failed to see the Great Wall of
China during his 21 hour mission in low earth orbit. As I noted at the time,
it seemed that the hoary old myth that "the Great Wall of China was the
only manmade
structure visible from space" was in the process of being replaced by
another myth that "the Great Wall of China was not visible from
space".
The truth is that it is possible to see the Great Wall from space but that it is very hard to do so and only under the very best weather conditions and probably only with the aid of binoculars.
The difficulty with the Great Wall is that much of it was made from
materials which are very similar in color to the surrounding
countryside and that much of the wall (those bits not
walked on by tourists i.e. nearly all of it) is in quite poor condition,
looking in places more like a long pile of rubble rather than an actual wall.
The other problem is that it's quite narrow with an average
width of only 5 metres. From Yang Liwei's perspective, flying at an
altitude of more than 200 kilometres, the Great Wall's apparent width would be
only about 5 seconds of arc. The moon by way of comparison is 30 minutes
of arc (that's 360 times bigger) and much much brighter. Compounding
Yang's problem is the fact that his space capsule was moving incredibly
quickly,
orbiting the earth 14 times in his 21 hours in space and for at least
half of that time China would have been in complete darkness.
Update: Ahem... a little correction appears to be
in order here.
In the comments section of this post Jason commented that he
thought that the river in the shot looked like a ridge and we discussed
the weirdness that happens to shadows in aerial photgraphs
which leads to things appearing reversed (like, for example, the craters of the moon appearing like domes in this shot). Aaron then mentioned that the European Space Agency has retracted their claim of being able to see the Great Wall from 600 km up.
Apparently, they mistook a river for a ridge...