No, you can’t see the Great Wall from space

When I was a kid, maybe 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 would almost instinctively know that this claim was bullshit. Of course, I wasn’t as skeptical back then (I knew nothing about optics or physics or eye biology) and my response was “Wow!”

But I do Now that you know about these topics and have some experience with seeing objects from space, let’s take a look at why the claim is not true.

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The fundamental problem (as with many “you can see X from space” type statements) is resolution, which is an astronomer’s term for the ability to distinguish between two objects as opposed to them being so close that they blend into a single point. You’ve probably experienced this: When you’re driving on the highway at night, you see the headlights of an oncoming car as a single source of light, but when the car gets close enough you can see that there are actually two.

Resolution is usually expressed in degrees, which depends on the size of the object and its distance from the observer. As a concrete example, the Moon is about 3,500 kilometers wide and 380,000 kilometers away from the Earth. If you do the math, you’ll find that the Moon appears to be about 0.5 degrees in diameter (as you know, a circle, i.e. the surrounding horizon, has a circumference of 360 degrees).

The average human eye has a visual resolution of about 1 arcminute (1 degree is 60 arcminutes). Someone with very keen eyesight might be able to distinguish objects half that size. The Moon is much larger in the sky, so it appears as a disk with easily visible features.

But what about the Great Wall of China? Although it is very long, this structure is actually quite narrow. One of the widest parts may be 10 meters wide. Can something that big be seen from the moon?

At that distance, the Great Wall would appear as a line 1/10,000th the thickness of an angle. Resolving that feature would be roughly the same as seeing a human hair with the naked eye from a kilometer away. I think we’d all agree that that’s just not possible.

But let’s modify this statement a bit. Imagine that we are now in low Earth orbit, watching the Earth move from the cupola of the International Space Station (ISS), which is about 400 km above the Earth’s surface, so from there the Great Wall would appear to be about 0.1 arcminutes wide. Still, it would be too small to see with the naked eye.

But there’s still a chance: our eyes can see very thin objects a little better if they’re very long and have a strong contrast with the surrounding environment. Astronauts in orbit can see, for example, a road across the desert or the wake of a ship as it sails at sea. Could this help here?

Unfortunately, that’s not the case: the Great Wall is generally made of stones that don’t contrast well with the terrain, and tend to curve to follow it, especially near steep cliffs or hills.

And we know this from experience: astronauts have tried to see it, but have never been able to see it reliably. (Detailed orbital snapshots are taken using telephoto lenses with a resolution far superior to the human eye.) Even China’s first astronaut, Yang Liwei, said he couldn’t see it from orbit, and national pride no doubt motivated him to try.

Depending on certain conditions, teeth It’s undetectable from orbit — for example, when the sun is low in the sky at sunrise or sunset, the wall casts a long shadow, making its presence obvious — but that’s not the same in reality. look Is it the wall itself?

So what about other man-made structures? The Great Wall is long, but we’ve built much larger structures.

The Egyptian pyramids are obvious candidates. The Great Pyramid of Giza is about 230 meters on each side at its base, large enough that it can be easily seen by eye even from low orbit. The dusty stone doesn’t contrast well with the surrounding sand, but the light falling on the structure itself adds to the contrast; at a low sun angle, half of the pyramid is illuminated and half is in shadow, distinguishing it from the sandy landscape. Former NASA astronaut Leroy Chao claims to have seen two pyramids, while others have reportedly tried and failed.

NASA claims that China’s Three Gorges Dam is visible from space. Although I have not seen any reports of astronauts actually seeing it. Should Become familiar YesThe dam is huge — more than 100 meters wide at its base and 2.3 kilometers long. Its tan color contrasts with the blue water of the Yangtze River it spans, and is easily seen in photos taken by astronauts with long lens.

It’s worth noting that an object doesn’t necessarily need to be resolved to be seen. Stars are a good example. While they are huge, millions of kilometers in diameter, they are also very far away. The closest ones (besides the Sun) are over 40 trillion kilometers away. The thing about stars is that they are all too difficult to resolve with the naked eye, but are easily visible (at least at night). But in this case, brightEven unresolved objects can be seen if they emit or reflect enough light.

So city lights would be easily visible from orbit, and possibly even from the Moon. One drawback is that bright lights on the Moon would ruin the view. Astronauts on the Moon had a hard time seeing the stars in the bright light, even though the sky itself was pitch black. Also, the Earth on the Moon is much brighter than the Moon seen from Earth, making city lights even harder to spot. Perhaps if an astronaut on the Moon were to hide in the shadow of a large rock, and the Earth were a thin crescent, the city could be faintly seen on the night side of the Earth. This would be a fun experiment to try, and one I hope future lunar explorers will give it a try.

While we’re thinking about all this, let’s put ourselves in a different position: Can you resolve the ISS from the ground? Its light is certainly visible, and at times it can be brighter than Venus.

The answer is yes (Sort of). The ISS has a diameter of about 100 meters, and when passing directly overhead and closest to the observer, its size is just under 1 arcminute, so if you have sharp eyesight, you may see the ISS as slightly larger than a dot. I should add that even with modest binoculars you can easily see the ISS as a short line, as I did myself.

And finally, while it’s hard to see individual man-made objects from space, our impact on Earth is not. An astronaut’s guide to life on EarthCanadian astronaut Chris Hadfield writes that from the ISS, the effects of deforestation in Madagascar have been seen, with soil being dumped into the ocean. Smoke from wildfires exacerbated by climate change is easy to spot. And, of course, our cities burn fiercely from just a few hundred kilometers above.

Although we are small from the perspective of the universe, our influence is not small.