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Author Topic: Great Wall of China Debacle  (Read 13569 times)

Offline NakedScientist

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Great Wall of China Debacle
« on: 13/05/2003 12:59:11 »
I received an email regarding the following 'ask the naked scientists' query which we featured in one of our shows last year. The visitor's comments are below the quote from the show at the top. What do you think of this ? I (currently) stand by our answer.

TNS

Q: Geoff, Cottenham : Is it true that the Great Wall of China is the
only man-made object visible from space ?
A: This is a myth ! Although the Great Wall is extremely long it is
only about 5 metres or so wide. At distances greater than about 20 km
up (about twice the height of Mt. Everest), a person even with
perfect eyesight, but without the aid of a telescope or binoculars,
cannot physically resolve something that small on the ground.
Allowing for the shadow cast by the Great Wall (making it wider) or
sand accumulating on the windward side you might be able to see it
from 60 km up, in places, but since this is still too low for a
spacecraft to orbit, it's safe to say that you can't see it from
space. Indeed, Neil Armstrong and other astronauts from the Apollo
missions have all declared that it cannot be seen from space with the
Naked Eye."

Response from J.W. Senders :

A visual angle of one degree at a distance of 20km would be
20000/57.3 = 349 meters.  A one minute angle would be 349/60 meters
or about 5.8 meters.   A one second angle would be very nearly 10 cm
in breadth.  A 5 meter "line" would have a visual angle of 0.86
minutes and be readily visible on some observations.  When one brings
into play the recorded visual acuit of the astronauts as a group (far
better than 20/20) detection is much more likely.

  I suggest that you read Hecht and Mintz (1939) who found a threshold
for an extended dark line against a bright background to be about 0.5
seconds of arc.  That is well below the 5 meter wall at a distance
of (as you said) 20 km.    Choose a good day and observe linearly
extended power or other cables from the right side (NOT into the
sun!!) and discover what you can see.   A 1cm cable will subtend a
second or so arc (at a distance of about 2km
 
 From a readily available referfence I take the following:
"4.2.1 Minimum Visible.  The patt
There is another more common myth at work.  A threshold of, let us
say, one minute does not mean, as commonly believed, that anything
smaller could not be seen but rather that such a stimulus would be
seen less than 50%  (or whatever was specified as the threshold
probability) of the time.  Some very clever people were thus misled
into attributing the astronauts' claims, of having seen a variety of
linearly extended objects from orbit, to some mental disturbance
induced by weightlessness or whatever.

Many years ago I made some straightforward calculations of the
visibility from orbit of aggregates of 1cm power lines (claimed to
have been seen and denied by almost all critics).  They would have
been readily visible, depending on the contrast, quite a
(relativeley) large perent of the time.

J.W. Senders/A. Crichton-Harris


 

Offline Exodus

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Re: Great Wall of China Debacle
« Reply #1 on: 14/05/2003 12:29:05 »
I'm sure one of my lecturers said that it isn't possible to see the great wall from space! Its only a number of meters wide! I propose it is thought to be visible as it follows topography?

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Offline Donnah

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Re: Great Wall of China Debacle
« Reply #2 on: 15/05/2003 03:06:46 »
That was one of the questions an audience member asked Roberta Bondar when she spoke in Calgary.  Her answer was that you cannot see it from space.
 

Offline chris

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Re: Great Wall of China Debacle
« Reply #3 on: 15/05/2003 22:41:07 »
Forgive my ignorance but who is Roberta Bondar, an astronaut presumably ?

Chris
 

Offline george

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Re: Great Wall of China Debacle
« Reply #4 on: 15/05/2003 22:44:18 »
No offense to the people who wrote the query about the great wall of china and sparked this thread off; in fact we should probably believe them if they say it is visible from space because that is clearly where they are from.

I don't know about everyone else but I could follow the reasoning behind the naked science answer, but I can't actually understand a word of that they've said...

George
 

Offline Quantumcat

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Re: Great Wall of China Debacle
« Reply #5 on: 15/05/2003 22:53:19 »
All it means is that it doesn't matter how long something is, if it's too thin you can't see it! Compare it to trying to see a stretched-out hair at 20 metres :p
 

Offline chris

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Re: Great Wall of China Debacle
« Reply #6 on: 15/05/2003 23:48:21 »
That's not true - they are arguing the opposite, that you CAN see it.
 

Offline Quantumcat

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Re: Great Wall of China Debacle
« Reply #7 on: 16/05/2003 01:10:01 »
...... hmm .... I didn't read all of it before I guess. I can't really follow it too well either [xx(]
 

Offline Donnah

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Re: Great Wall of China Debacle
« Reply #8 on: 16/05/2003 02:02:47 »
Chris, you got it, Roberta Bondar is a Canadian astronaut.  After seeing our beautiful planet from space, she has devoted much of her time and energy to environmental issues such as preserving parklands.

George, hahaha!  I agree that the comprehensibility of the question could be dramatically improved with some professional editing.
 

Offline Quantumcat

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Re: Great Wall of China Debacle
« Reply #9 on: 16/05/2003 13:16:06 »
Definitely .... it's quite hard to read
 

Offline Exodus

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Re: Great Wall of China Debacle
« Reply #10 on: 22/05/2003 15:16:28 »
One thing you CAN see from space are algal blooms when they reproduce!

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Offline chris

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Re: Great Wall of China Debacle
« Reply #11 on: 22/05/2003 15:47:50 »
quote:
Originally posted by Exodus

One thing you CAN see from space are algal blooms when they reproduce!

Resident Tour Operator - The Naked Scientists



What do they look like and where do they tend to occur ? How big are they ? Presumably larger than the Great Wall of China ;)

I never forget a face, but in your case I'll make an exception - Groucho Marx
 

Offline Pappy

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Re: Great Wall of China Debacle
« Reply #12 on: 13/11/2003 19:47:27 »
Didn't the Chinese astronaut who just returnd from orbit indicate that the Great Wall was NOT visible from space?
 

Offline chris

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Re: Great Wall of China Debacle
« Reply #13 on: 13/11/2003 21:49:02 »
Pappy - where did you see that - it would be very useful to have that piece of information because the person who wrote the original post refuses to believe me !

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
 

Offline tweener

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Re: Great Wall of China Debacle
« Reply #14 on: 14/11/2003 03:28:17 »
I also have trouble following the argument about seeing the great wall from space.  And I have never been to space to see for myself (unfortunately!).  But I also know that the human eye can see very small extended objects better than square or round objects the same width.  From very high up in an airplane, you can see roads a lot better than houses and silos.  Contrast is also a big factor, in that something that stands out from the background is much easier to percieve than something that sort of blends.  I don't know where the great wall fares in this respect, but I would guess it is fairly low contrast to the surrounding terrain.

In the end, we will have to believe the astronauts that have been there.  It's still not very scientific because atmospheric conditions and the angle of the sun would play a big role, but until I can go there myself I'll just have to take their word for it that it can't be seen.


----
John
 

Offline roberth

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Re: Great Wall of China Debacle
« Reply #15 on: 14/11/2003 04:19:07 »
http://geography.about.com/library/weekly/aa090100a.htm
I'm still trying to find the Chinese astronaut's version, but I did read that he stated that the wall was not visible from space.
 

Offline Pappy

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Re: Great Wall of China Debacle
« Reply #16 on: 14/11/2003 14:22:41 »
Chris - I heard it as an aside on the news. I don't know where you would go to find a quote from the astronaut - sorry.
 

Offline CosmicAudioChic

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Great Wall of China Debacle
« Reply #17 on: 29/01/2009 05:26:27 »
The Great Wall myth has been around for years...before we even traveled into space. The earliest citing we have comes from Richard Halliburton's Second Book of Marvels, the Orient -published in 1938, which states that "Astronomers say that the Great Wall is the only man-made thing on our planet visible to the human eye from the moon." Halliburton was an adventurer-lecturer whose travel writings were extremely popular and sold quite well during the first half of the twentieth century.
 

Offline Bored chemist

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Great Wall of China Debacle
« Reply #18 on: 29/01/2009 07:00:44 »
Visible from the moon is a different matter from visible from low earth orbit.
Can anyone be bothered to get a scale picture of the wall and see how far away they can see it from?
 

Offline dentstudent

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Great Wall of China Debacle
« Reply #19 on: 29/01/2009 07:31:01 »
Visible from the moon is a different matter from visible from low earth orbit.
Can anyone be bothered to get a scale picture of the wall and see how far away they can see it from?

Do it with Google earth. You can soon see that it is not visible at all.
 

Offline Chemistry4me

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Great Wall of China Debacle
« Reply #20 on: 29/01/2009 07:44:03 »
Is that thing accurate? I mean Google Earth?
 

Offline Vern

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Great Wall of China Debacle
« Reply #21 on: 29/01/2009 13:37:59 »
Is that thing accurate? I mean Google Earth?
Sure; it's like a clock that's stopped. That clock is exactly correct twice each day. You can Google just about any answer you want; then you have to decide which is correct. You can use Google Earth and see actual pictures of earth from any altitude you choose. Then you will be the expert.

BTW you can't see the great wall even from the ISS altitude.
 

Offline demadone

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Great Wall of China Debacle
« Reply #22 on: 30/01/2009 10:18:41 »
You can see the great wall from space..........on google earth.
 

Offline dentstudent

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Great Wall of China Debacle
« Reply #23 on: 30/01/2009 10:24:28 »
You can see the great wall from space..........on google earth.

How? Is there a good internet connection up there?
 

Offline Vern

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Great Wall of China Debacle
« Reply #24 on: 30/01/2009 11:15:57 »
You can see the great wall from space..........on google earth.
I think what you are seeing is the valley that the wall is in. That is a light area a few miles wide and I did notice that you can see that. The wall itself though, there's not a chance that you can see it.
 

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Great Wall of China Debacle
« Reply #24 on: 30/01/2009 11:15:57 »

 

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