No, You Can’t See the Great Wall of China from Space | Space

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…