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What is the only man made object you can see with the naked eye looking from space to earth?
Question
#17900. Asked by Bishop. (Apr 04 02 2:27 AM)
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eliasen
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The usual answer that you hear is 'The Great Wall of China' but if you think about it logically you'll see that it's false.
The great wall of China is no larger dimensionally than an interstate highway, probably much less wide. If you could see one, you could clearly see the other.
Many astronauts have said that they can locate moving ships from space because of the v-shaped wake they leave.
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Senior Moments
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If we take 'space' to mean a low Earth orbit such as the one travelled by the Space Shuttle (roughly 160 to 350 miles above Earth), the Great Wall claim fails twice. First of all, it's not the only object visible from that distance: NASA's Earth from Space photographic archive (particularly the Human Interactions section) shows that pictures taken from low orbit reveal human-built structures such as highways, airports, bridges, dams, and components of the Kennedy Space Center. Secondly, even though other objects are visible at this distance, according to Shuttle astronaut Jay Apt, the Great Wall is barely discernable, if not invisible.
 http://www.snopes2.com/science/greatwal.htm
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Son of The Household Cavalry
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Most of Holland has been reclaimed from the Zeider Zee by man so surely that must be visible from space
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Stevo
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There seems to be a lot of scientific evidence against the Great Wall of China, for the reasons given above. Also, Armstrong and co stated they could not see it from the moon.
It is claimed that Kennecott Copper Mine, which is the largest man-made excavation in the world, is visible from space, so it could be the answer.
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Gnomon
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At night-time, you can see many man-made lights. The brightest are the oil-wells of northern Africa and Saudi-Arabia, where gas which sits on top of the oil is burnt off 24 hours a day, and the Japanese squid-fishing fleet, which uses bright lights to lure the squid to the surface to be caught.
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eliasen
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An order-of-magnitude estimate.
The human eye has an angular resolution of about 1 arcminute.
The Space Shuttle flies at varying altitudes, but we'll use 180 miles as a lower limit.
For small angles, the angle subtended by an object of width w at a distance d is given by:
angle = w/d
(Where angle is in radians.)
Plugging in the numbers, the human eye can ideally detect objects about 84 meters or 276 feet in size or larger from low earth orbit. You'd theoretically be able to see things that are larger than this size in both dimensions.
Sensitive, trained, keen-eyed observers might discern smaller items. Plug in your own numbers for vision acuity and orbital height.
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eliasen
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Just so I don't get flamed by optics experts, and to prove to my college optics teacher that I did actually learn something, you may have to increase this size by the factor where the Airy disks of two points on the earth clearly don't overlap.
And, yes, you could easily detect a bright light that's smaller than this; after all, we can easily see stars that are fractions of an arcsecond in diameter.
Another off-the-cuff with the same assumptions:
To discern something on the earth from the moon, it would have to be at least 70 miles or 111 kilometers. In BOTH dimensions.
Apr 04 02, 9:37 AM
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Socrates
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What about atmospheric pollution?
I'm also sure you could see a nuclear explosion.
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eliasen
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If you can see fishing lights, then certainly you could see a nuclear explosion.
About atmospheric pollution, astronauts in the International Space Station filmed the dust and smoke plume from the World Trade Center disaster.
Before 9/11, you could have quite possibly make out the Trade Center from its shadow. The shadows could be quite long, and the World Trade Center buildings were close to 64 meters on a side, or, if you measure diagonally, 89 meters, so they're large enough in both dimensions.
You could have even seen the 'Five Acre Plaza' in the WTC complex. A 5 acre square would be about 142 meters on a side, clearly larger than the minimum size I calculated above.
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Jac
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Just to add re Great Wall of China, one of the Apollo astronauts (I think it may have been Jim Lovell) is quoted by Arthur C Clarke as saying that although you cannot see the Wall you can in fact easily trace its line by the effect it has on local weather patterns. What that means visually I can't say, because it's a while since I read the essay, but I did make a note of it at the time because this question comes up so often!
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eliasen
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I wonder if Clarke's comments are somewhat mistaken. I haven't read his comments, but the Great Wall was usually built along the top of the highest ridge in the area. I would think that if there is a weather effect, it's certainly because of the 1000-foot mountain ridge that the wall is built atop of, NOT the 10-foot wall.
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