Astronomy You can’t see the Great Wall of China from space

You can’t see the Great Wall of China from space

Francesco Busiello •February 10, 2014, ,

Photo by Severin.stalder

It’s an often quoted myth but it is not true at all that the Great Wall of China can be seen from space. From up high, above the atmosphere, it’s possible to see city lights and also, according to this Popular Mechanics article, the vast greenhouses of Almeria, in Spain.

The Great Wall of China is really rather long (over 6000km) but it is also very narrow, on average about 6 metres wide (ranges from 5m to 9m) and doesn’t stand out much from its surroundings. To be able to see it from near-earth orbit (160km up) you’d need to have triple the maximum visual acuity of a falcon.

But let’s pretend that the government of China decided to expand the width of the wall so that it could be seen from space. How wide would it have to be?

In a letter published in the Journal of Optometry, Dr Lopez-Gil argues that being able to view the Great wall from the ISS at its closest approach to earth (160km) would be equivalent to being able to view a 2cm cable half a kilometre away!

Which clearly is not going to happen.

You’d need a visual acuity of 20/3, at least 7.7 times better than what you have now (assuming you have 20/20 vision to start with). Even if the eye was optically perfect you’d still not be able to see the wall. The size of the visual cones in the central fovea is the limiting factor. The image of the Great Wall would be a third of the size of a cone and hence undetectable.

Greenhouses in Almeria – Source:NASA

Even from the edge of space (80 km) it would be impossible to see as you’d need a visual acuity of 20/5 and obviously it would be impossible to see it from the moon as that would require a vision 17 THOUSAND times better than what we have.

I mean, it’s kind of obvious. If the Great Wall of China could be seen from space and it’s only 6 metres wide then you’d be able to see most other human-made objects that are at least as wide like the pyramids, St.Peter’s basilica, roads and probably the house you live in (you can see roads from space especially ones that starkly stand out from their surroundings, like ones in the desert).

A Snellen chart is used to estimate visual acuity. – Author: Wikipedia user Jeff Dahl

So how big would it have to be? To see it from ISS orbit it would have to be at least 7.7 times bigger (and even then it would still be difficult to see) so about 45-50 metres wide. From the moon, to barely see it, it would have to be about 100 km wide!

Of course, there is an easier way. Human eyes are very good at picking up sudden changes of luminance (intensity of light). It’s how we can see stars in the night sky even though they’re really quite tiny. You could put lights on the Great Wall, or completely cover it with sunlight-reflecting mirrors, and it would be much much easier to see from space.

-Francesco

References and further reading