A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Is the Great Wall of China the only manmade object you can see from space?

January 31, 1986

Dear Cecil:

I can hardly believe my Genus Edition ears. My boyfriend has the audacity to claim that his geography professor knows more than I do. In a game of Trivial Pursuit not long ago I correctly answered that the only man-made structure on earth visible from outer space was the Great Wall of China. He has the nerve to tell me that his teacher said this is not true, and worse, he is taking the prof's word over mine. Who's right, the geek from college or the trivia buff?

Cecil replies:

Prepare to eat crow, babycakes — those wankers at Trivial Pursuit have screwed up again. Any number of man-made structures can be seen from space, provided we construe "structure" to mean "anything built." Many of these are things that look like long, straight lines when seen from afar, such as highways, railroads, canals, and of course walls. If the orbit is low enough you can see even more. I have here a photo of Cape Canaveral taken during the Gemini V flight in which the big Launch Complex 39, used for the Apollo missions, is clearly visible. Another photo of the Nile delta, taken from a height of 100 miles, shows an extensive road network. Gemini V astronauts Gordon Cooper and Charles Conrad were able to spot, among other things, a special checkerboard pattern that had been laid out in Texas, a rocket-sled test in New Mexico, and the aircraft carrier that would later pick them up in the Atlantic, along with a destroyer trailing in its wake. Take it from me, honey — Trivial Pursuit is one game I never lose.

The Teeming Millions hit the wall, Part One

Dear Cecil:

Recently a reader incorrectly stated that the only man-made object visible from Earth orbit was the Great Wall of China. You set him straight — but you may be interested to know the Wall is the only man-made object visible with the unaided human eye from the surface of the moon. The quarter-million miles does make a perceptible difference.

Cecil replies:

Nice try, Ken, but you whiffed too. According to NASA, the earth as seen from the moon takes up less than one degree of arc in the sky. Basically it looks like a big blue marble. No man-made detail can be seen at all; sometimes even the continents are barely distinguishable.

The NASA folks, I gather, are getting a little tired of hearing about the Great Wall of China. Nobody knows exactly where the story got started, although some think it was speculation by some bigshot during an after-dinner speech in the early days of the space program. The Teeming Millions are humbly requested to give it a rest.

The Teeming Millions hit the wall, Part Two

Dear Cecil:

I was greatly disturbed by your blindly taking the word of some NASA goofball who is unable to perform even simple math correctly. Given an earth-moon distance of 239,000 miles and the diameter of the earth as 7,920 miles, the angle subtended by the earth from the surface of the moon is almost two degrees, not "less than one degree." Furthermore, how can you even consider that the earth looks "basically … like a big blue marble"? The earth is 3.7 times as large in the lunar sky as the moon is from earth, and I can easily see a large amount of detail on the lunar surface, even through earth's polluted atmosphere. The moon, of course, has no atmosphere or city lights to obscure the view.

In fact, the data on visual acuity do not seem to indicate that "no man-made detail can be seen at all." Hugh Davson, in Physiology of the Eye, 4th edition, states that in the common eyechart-type measure, a monocular resolving power of approximately 20 seconds of arc is observed, and when visual acuity is measured by the power to detect a single line on a uniform background, the normal eye can resolve 0.5 seconds of arc. Davson gives an increase in acuity of the square root of two for binocular vision, thus indicating a potential resolution from the moon of objects 0.4 miles across. While the earth may not be a perfect test surface, neither are all eyes "normal." And I can name several man-made details many times larger than 0.4 miles across — the average city, for one. Admittedly, the Great Wall at 12-40 feet in width is much smaller than 0.4 miles, but I think in magnitude of error, it was you, not Mr. L., who really "whiffed on this one."

Cecil replies:

Think so? Tom Burnam, author of More Misinformation (1980), quotes a letter from astronaut Alan Bean on the subject:

The only thing you can see from the moon is a beautiful sphere, mostly white (clouds), some blue (ocean), patches of yellow (deserts), and every once in a while some green vegetation. No man-made object is visible on this scale. In fact, when first leaving earth's orbit and only a few thousand miles away, no man-made object is visible at that point either.

You're right about one thing, though. The earth takes up two degrees of arc in the lunar sky, not one.

Late news

The November 20, 2010 edition of the Canadian Broadcasting Corporation's science radio show "Quirks and Quarks" featured an interview with Canadian astronaut Chris Hadfield, a veteran of two Space Shuttle missions. Hadfield said the idea you could see the Great Wall of China from space was science fiction:

It's eight or nine thousand kilometers long, but it's very narrow. At its widest it's only nine meters wide and in a lot of places it's down to five meters or less. It's also made of the local materials — local dirt, local mud, local bricks — so it's kind of the [same] color as the surrounding area, and it follows the contours of the ground, so it's almost perfectly camouflaged. It'd be hard to see from an airplane in a lot of places. Even from the [International] Space Station, which is only 400 kilometers up, we can't see it — it's just too well hidden … it's like they were trying to hide it from astronauts.

Discernible manmade objects, according to Hadfield, are those whose color or shape makes them stand out from their surroundings — highways, harbors, sports stadiums or, interestingly, the shadows of the pyramids in Egypt.

Sadly, Hadfield reports, the myth about the Great Wall shows no signs of abating. When visiting the wall as a tourist he was confidently told by the tour guide it was one of just two manmade objects visible from the moon. (What the other one was she didn't know.) He gamely but no doubt futilely attempted to inform her that from the moon you could barely see China. To listen to the interview with Hadfield, click here.

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