Why you cannot see the Great Wall from the Moon

Not even the best of human eyes at a simple glace could see the Great Wall of China from Space. The impossibility is due to the limitation of the human eye when it comes to seeing small diffusing objects. The relevant parameter is not the Wall’s length (about 7300 km), but its width, which is usually less than 6 m.

[...] To illustrate this with a simple example, looking at the Great Wall from a distance of 160 km would be the same as looking at a 2 cm diameter cable from more than half a kilometre away! No matter how good the atmospheric conditions, lighting and contrast are — unless the object was self-illuminated or it reflected the sun as a small mirror — it would be totally impossible to see this cable (or, for similar reasons, the Great Wall) at a simple glance, because the eye would need a visual acuity greater than approximately 20/3, which is 7.7 times the normal visual acuity, and more than three times the maximum acuity reached by a falcon, an eagle, or a human eye.
[...] [I]t would be even less likely to see the Great Wall from the moon, situated at a minimum distance of 350,000 km, because the visual acuity would have to be 17,000 times (!) better than that of the normal human eye (in this case it would amount to seeing the cable from a distance of more than 1000 km).
[...] As a simple exercise, Google Earth© can be used to see the Wall at lat.=40.48234, lon.=116.180592 if one is close enough to the ground. However, once you are more than 40 miles away, it cannot be seen. This simple experiment does not really answer the question since the visualization of the Wall will depends not only on our vision, but also on the satelite image resolution, our computer screen, etc. Despite this, it can be observed that, at a height of 40 miles, the Wall is not visible but the landing runway of the Yongning Airport, located about 4 miles WNW to the Wall, is.
Moreover, if the Great Wall was visible from Space, then, contrary to common claims, it would not be the only visible manmade object since astronauts would also enjoy the view of the Pyramids of Egypt, the Golden Gate Bridge, the Eiffel Tower, and probably their own house in case it is more than 6 m wide and long.

Norberto López-Gil
Ciencias de la Visión, Universidad de Murcia

from 'Is it Really Possible to See the Great Wall of China from Space with a Naked Eye?', Journal of Optometry 2008;1:3-4