About Cecil’s Classic Question about the Great Wall of China & the moon. . .
I’ve lived in China & been to the great wall nine times. At parts, it’s barely five feet high & about a foot wide, at least where it’s unreconstructed. I figured no way is this thing visible from the moon.
Then I found a book called “Japan/China” by Nikos Kazantzakis, renowned Greek author, who wrote “The Last Temptation of Christ,” “Zorba the Greek,” and “Oddysey: a Modern Sequel.” “Japan/China” is something of his travelogue from trips to Japan & China; in it, he mentions that the Great Wall is the only man-made object visible from the moon.
“Japan/China” was published for the first time in 1936. Kazantzakis himself died in 1957. Just how old is this Great-Wall-visible-from-the-moon belief, anyhow?
Great Wall Visible? would be the link to Cecil’s column. They really appreciate it when you add this as it allows readers to better understand what you are saying.
There is an earlier discussion. I’ll try to find in and add it here momentarily.
They knew how far away the Moon is, they knew what angular size can be resolved by the human eye, and they knew the sizes of various Earthly objects. The rest is just trig.
I’m more inclined to think that the folks who got the wrong answers didn’t do the math at all. There’s a grand old tradition in journalism of making up numbers out of thin air. What might be interesting, is seeing if there’s a response to any of those early claims, by someone who actually did do the math.
Welcome to the SDMB, and thank you for posting your comment.
Please include a link to Cecil’s column if it’s on the straight dope web site. To include a link, it can be as simple as including the web page location in your post (make sure there is a space before and after the text of the URL).
Cecil’s column can be found on-line at the link provided by our esteemed poster samclem.
Damn, Arnold. You’re gonna lose you Mod license if you don’t keep it straight: Posters suck up to Mods, not the other way around. So, I guess those chocolates I got in the mail today were from you.
I doubt that the Great Wall itself is visible, but I could imagine that the effects of a wall could be seen from space. Isn’t there a several thousand mile long fence in Australia to keep rabbits out of pasture? I imagine the vegetation on the two sides could vary enough to be visible from space.
Think about it. From the Moon, the Earth looks like a big ball of white, with blue marbling. There’s occasionally a hint of brown from a desert peeking out or maybe some dark green from trees. We’re talking the difference between the Sahara and the Congo, not between you and your neighbor’s lawns. No fence line will stand out - even the Great Wall.
What might have inspired people to think it could be visible is the length of the wall is sufficient that it could be discernable. However, that fails to recognize that the width of the wall is tiny, so it would not have enough width to show up as a line. Ergo, the wall should not be visible.
One thing that most posters have ignored is that part of the visibility of an object is its contrast against the background. The smaller the subtense, the more contrast required to see it. For something that’s close to the visual acuity limit, you need white on a black background or vice versa to see anything at all. This is clearly not the case for objects on the Earth. You’re only getting gray on brown or green, which are pretty similar in brightness and therefore provides minimal contrast.
Moreover, your ability to see stuff from Earth orbit takes advantage of color differences, but your color vision is considearbly worse than your monochrome vision. From the moon, you are only able to use your monochrome vision, and at that range everything looks to be at the same contrast.
