When I was a mere lad, probably in middle school, I remember a friend telling me that the Apollo astronauts could see the Great Wall of China from the moon.
If someone told me that now, I’d know almost instinctively that this claim is baloney. Of course, back then when I was a lot less skeptical (and didn’t know anything about optics or physics or ocular biology), my reaction was, “Cool!”
But I do know about those topics now and have some experience dealing with seeing objects from space. So let’s look at just why that claim can’t be true.
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The basic problem (as with so many “you can see X from space”-type statements) is resolution. This is a term astronomers use for the ability to distinguish two closely spaced objects, as opposed to seeing them so close together that they merge into a single dot. You likely have experience with this: driving on a highway at night, you’ll see an oncoming car’s headlights as one single light source until the car gets close enough to reveal there are, in fact, two.
Resolution is usually expressed in terms of an angle that is determined by the size of the object and its distance from the observer. For a concrete example, the moon is about 3,500 kilometers wide and 380,000 km away from Earth; do the math and you’ll find that the moon appears to be about 0.5 degrees across (where, as you may recall, a circle—say, the horizon around you—is 360 degrees in circumference).
The average human eye offers a visual resolution of about one arcminute (there are 60 arcminutes in a degree); someone with very keen vision might be able to resolve an object half that size. The moon is much larger than this in the sky, so it looks like a disk with features that are easily seen.
What about the Great Wall of China, though? While it is very long, this structure is actually quite narrow. One of the wider parts might be 10 meters. Can something that size be seen from the moon?
At that distance, the Great Wall would appear as a line one ten-thousandth of an arcminute thick. Resolving that feature would be roughly the equivalent of seeing a human hair from a kilometer away with your unaided eyes. I think you’ll agree that that’s flatly impossible!
But let’s modify the claim somewhat: Say now we’re in low-Earth orbit, watching our planet rolling along from the cupola of the International Space Station (ISS). That’s a perch roughly 400 km above Earth’s surface, so from there the Great Wall would appear about 0.1 arcminute in width. So even then, it’s still too small to see by eye!
There’s still a chance, however. Our eyes can be a little better at seeing objects that are very narrow if they’re very long and have high contrast with the surrounding environment. Orbiting astronauts can see roads across deserts, for example, and the wakes of seafaring ships. Does that help here?
Sadly, no. The Great Wall is made of stone that generally doesn’t contrast well against the terrain, and it tends to curve as it follows the landscape, especially near steep cliffs and hills.
And we know this empirically, too: astronauts have tried to see it but have never reliably done so. (Orbital snapshots that show any details are captured using a telephoto lens, which has a much higher resolution than the human eye.) Even China’s first taikonaut, Yang Liwei, said he couldn’t see it from orbit, and certainly national pride would have motivated him to try.
I’ll note that there may be certain conditions when it really is detectable from orbit. For example, when the sun is low in the sky at sunrise or sunset, the wall can cast a long shadow, revealing its presence. But that’s not really the same as seeing the wall itself, is it?
Well then, what about other human-made objects? The Great Wall is long, but we’ve certainly built structures that are bulkier.
The pyramids in Egypt are obvious candidates. The Great Pyramid of Giza is about 230 meters across on each side of its base, making it easily big enough to resolve by eye from low-Earth orbit. The dusty stone doesn’t provide a lot of contrast with the surrounding sand, but more contrast can come from the play of light across the structure itself: at low sun angles, half the pyramid is lit and half is in shadow, distinguishing it against the sandy landscape. One former NASA astronaut, Leroy Chiao, claims to have seen two of the pyramids, while others have reportedly tried but failed.
NASA claims the Three Gorges Dam in China can be seen from space. I haven’t seen any reports of astronauts actually spotting it, but it should be possible. The dam is immense: it’s more than 100 meters across at its base and 2.3 km long. Its tan color contrasts with the blue waters of the Yangtze River it spans and shows up easily in photographs taken by astronauts using a telephoto lens.
It’s worth noting that you don’t necessarily need to resolve an object to see it. Stars are great examples. While they’re huge, some millions of kilometers in diameter, they are also fantastically distant; even the closest (besides the sun) is more than 40 trillion km away. They are all unresolved dots by eye yet easily visible (at least at night). In this case, though, they’re bright. Even something unresolved can be seen if it’s emitting or reflecting enough light.
That’s why city lights can be easily seen from orbit and are likely even visible from the moon under some circ*mstances. One difficulty is that the brightly lit lunar landscape can ruin the view—astronauts on the moon had difficulty seeing stars through the glare, even though the sky itself was black. Also, Earth in the moon’s sky is far brighter than the moon is from Earth, which can make city lights more difficult to tease out. Perhaps if a lunar astronaut hid in the shadow of a big boulder and Earth was in a thin crescent phase, cities might be faintly visible on our planet’s night side. That would be a fun experiment to try, and I hope future lunar explorers give it a shot.
And while we’re thinking about all this, let’s turn the tables: Can you resolve the ISS from the ground? Its light is certainly visible and can be brighter than Venus at times.
It turns out that the answer is yes (kind of). The ISS is about 100 meters across, which means that it can be slightly less than one arcminute in size when it passes directly overhead and is therefore closest to an observer. So someone with keen vision could see it as just barely more than a dot. I’ll note that with even modest binoculars, you can easily see it as a stubby line; I’ve done so myself.
And finally, while individual human-made objects are difficult to see from space, our effects on the planet are not. In his book An Astronaut’s Guide to Life on Earth, Canadian astronaut Chris Hadfield wrote that from the ISS, he could see the effects of deforestation in Madagascar as soil was dumped into the ocean. Wildfires, exacerbated by climate change, have plumes that can easily be spotted. And, of course, our cities glow fiercely from just a few hundred kilometers above.
From space we are tiny—but our impact is not.
