[Seany (OP)]

OK... Everyone probably knows that you can see the Great wall of China from the moon..

The wall isn't too tall, it's just a long thing, which makes it probably an easy view, as it is a wide.

What else would we be able to see from the moon? Man-made things..

Is the great wall of china the only thing we can see, that is man-made? Considering all the houses, buildings, cars, people?

[dentstudent]

Hi Seany - do you mean "you can see the Great Wall of China from SPACE?". I would think that it's quite easy to see the wall from the wall ;-)

BUT, I don't think you can see the wall from space - it's another of those urban myths.

[Seany (OP)]

LOL sorry, I meant from the MOON. [] Never said space, as it is too broad and big. See the title.. Heehee.. Modificed now. []

[dentstudent]

OK, but even so...."seen from space" normally means by orbiting astronauts who are rather closer than the moon, and therefore you'd not be able to see the wall from the moon. I haven't tried it yet, but you could try looking for it on Google Earth, and seeing how close you are before you can see the wall.

[Seany (OP)]

Mehh.. So it's just a myth?

[dentstudent]

I believe so...I've just checked on Google Earth, and they have a similar thread. As they say, it's only a few metres wide and so difficult to resolve with the human eye. Look at it the other way - what can you see on the moon?

[Seany (OP)]

Yup that's my other question.. Seeing that there are over 6bn people living.. With lots of buildings, houses, cars and everything. Can we not atleast spot where there are lots of buildings.. Or cities or anything?

[dentstudent]

So I messed around with Google Earth and it seems that nothing man-made is really clear beyong about 60km altitude. I have no idea about how accurate this is and relevant to an astronauts view, but it appears that you can make out where cities are up to about this point. You need to be a lot closer before you can begin to determine where the streets and buildings are. As far as cars go, I have trouble seeing them from a plane at 5km....Really big things like Lake Geneva I could make out until 1000km out, but the moon is 250,000 miles away!

[Seany (OP)]

How about the wall?

[dentstudent]

Well, I couldn't make it out at all! Have a look at this link too....

http://en.wikipedia.org/wiki/Great_Wall_of_China

[another_someone]

I believe so...I've just checked on Google Earth, and they have a similar thread. As they say, it's only a few metres wide and so difficult to resolve with the human eye. Look at it the other way - what can you see on the moon?

The issue of what you can see from the Earth towards the Moon is not exactly the same as what you can see from the Moon towards the Earth, because of the effect of the Earth's atmosphere.

There are two different limitations of what can be seen from one looking towards the other - the first is simple the angle which the object covers within one's vision (and the limits of diffraction), and the sendon is the effect of the distortions of air currents and heat diffraction within the Earth's atmosphere.

The limits of the angle an object takes within one's field of view is the same whether one looks from the Moon to the Earth, or the Earth to the moon, but a good telescope, or even a good pair of binoculars, can overcome that limit (and in fact the eye is capable of resolving far more just on its own than people often give it credit for).

The limitations of the atmosphere will be different when looking towards the Earth (where the atmospheric cover is close to the target you are viewing, and so have less effect on your view of the target) than when looking up from the Earth, where the atmosphere is closer to the viewer than it is to the target, so the disturbances therein have a greater effect upon your view.

The other factor to bear in mind is the angle of view at which you are viewing an object (side on, the height will be what matters, whereas from above, it is the width, and at a 45 degree angle, it will be a cross sectional view that is larger than either the height or the width), and it also depends upon the angle of the Sun (and hence the effect of any shadows you might see).

Google Earth is not really demonstrating the limit of what can be resolved from space.  I have seen satellite images of cities, such as Hamburg and Venice, where individual motor cars can be resolved, so I have no doubt that satellite images can resolve the great wall of china.

I have no doubt that one could image the Great Wall of China from the Moon - the real unknown is whether it can be seen with the naked eye, or whether it would need a telescope to give adequate resolving power.

[dentstudent]

yup quite right - and of course extra occular aids (sounds nasty) can resolve rather more, but I think that it's clear that with the unaided eye, it is not possible to see the Wall from the moon, nor any other man-made object on earth. Err, I think.

[Seany (OP)]

I still think you would be able to see it.. It is a very well-known rumour/myth/fact..

[another_someone]

http://www.space.com/imageoftheday/image_of_day_040512.html

"In Earth's orbit at a height of 160 to 320 kilometers [100-200 miles], the Great Wall of China is indeed visible to the naked eye," says astronaut Eugene Cernan.

A low angle of sunlight casting long shadows can help.

"You can see the Great Wall," confirms astronaut Ed Lu, who was the science officer of Expedition Seven on the International Space Station. The station circles Earth higher than Yang Liwei's orbit.

The misconception is wrapped up in broader myths about what is and what is not visible from space. For the record: No manmade structures on Earth can be seen with the unaided astronaut's eye from the Moon. But many things -- highways, dams and even large vehicles -- are easily spotted from Earth-orbit with no optical aids.

http://geography.about.com/od/specificplacesofinterest/a/greatwall.htm

For some reason, some urban legends tend to get stated and never disappear. This legend even appears as a erroneous Trivial Pursuit question. The legend? Many are familiar with the claim that the Great Wall of China is the only man-made object visible from space or from the moon with the naked eye. This is simply not true.

The myth of being able to see the Great Wall from space originated in Richard Halliburton's 1938 (long before humans saw the earth from space) book Second Book of Marvels said that the Great Wall of China is the only man-made object visible from the moon.

From a low orbit of the earth, many artificial objects are visible on the earth, such as highways, ships in the sea, railroads, cities, fields of crops, and even some individual buildings. While at a low orbit, the Great Wall of China can certainly be seen from space but it is not unique in that regard.

However, when leaving the earth's orbit and acquiring an altitude of more than a few thousand miles, no man-made objects are visible at all. NASA says, "The Great Wall can barely be seen from the Shuttle, so it would not be possible to see it from the Moon with the naked eye." Thus, it'd be tough to spot the Great Wall of China or any other object from the moon. Furthermore, from the moon, even the continents are barely visible.

[syhprum]

The resolution of the unaided human eye is normally quoted as one arc minute, from the orbit of the Moon this corresponds to 112 Km it would need a very wide wall!

[Bored chemist]

I don't think you need to get to the moon to answer the question about the great wall. It's a few metres wide; how far away from something a few metres wide do you need to be before you can't see it?
If the eye can resolve 1 second then a 3 metre object should be resolved (with good lighting and contrast) at about 10 Km. That's well inside the range that you could look at things on earth.

[Seany (OP)]

Even if it is a couple of metres wide.. It's so long, that it is like a trail, ..

[syhprum]

Bored Chemist has credited the unaided eye with a far higher resolution than it in fact has, the normally quoted figure is one minute not one second!

[Bored chemist]

Fortunately, I did the maths with the correct value (I'm better at maths than typing) but thanks for pointing it out.
Seany, a typical DNA molecule form a person is about a metre long but it's too narrow to see.
In principle, because it sticks up, its shadow might be big enough to see from a greater distance (than 10Km) but not from the moon.