The Great Wall, we hear, is so vast that it stands out from space. Few people stop to ask who first said so.
The question can be tested against the reports of astronauts and the photographs they took. Here is how the belief began, what the eye can pick out from far away, and what the evidence says.
The Claim: Where the Great Wall Idea Comes From
Richard Halliburton's 1938 book Second Book of Marvels, the Orient said that astronomers claim the Great Wall is the only human-made thing visible to the human eye from the Moon.
The exact source of the claim is unknown, and Halliburton helped to spread it. Notably, the idea predates spaceflight, so it arose before anyone had gone up to check.
It also helps to know what the "Wall" is. The Great Wall is a series of walls built in different periods, not one continuous wall.
The Science You Need: Why Size and Contrast Matter
Whether you can see something from far away depends on more than its length. Width and contrast with the surroundings matter a great deal.
The Great Wall is only a few metres wide, similar to the width of highways and airport runways. It is also about the same colour as the soil around it, so there is little contrast between the Wall and the land.
The Wall is long but thin, like a thread. A thread a foot long cannot be seen from 100 yards away, though a human head can. Lighting plays a part too: oblique (low-angle) lighting widens the Wall's shadow, so visibility changes with conditions.
What the Evidence Shows From the Moon and From Orbit
From the Moon, Earth appears as a sphere that is mostly white (clouds) and blue (ocean), with patches of yellow (deserts) and some green vegetation. No human-made object is visible at that scale.
Astronaut Gene Cernan has said the Wall can be seen with the naked eye from 160 to 320 km up, a view other astronauts and the photographic record don't support. Ed Lu, science officer of Expedition 7 on the International Space Station, said the Wall is less visible than many other objects.
On 24 November 2004, ISS commander Leroy Chiao photographed central Inner Mongolia from about 348 km up with a 180 mm lens, in a photo that supposedly shows a section of the Wall. Sections of the Wall also show up clearly in radar imagery from the Space Shuttle Endeavour, but it is very difficult or impossible to see from low Earth orbit with the naked eye.
Verdict: Myth, With a Caveat for Low Orbit
Verdict: Myth. Snopes rates the claim that the Great Wall is visible from the Moon as False.
There is a simple test of the claim. If the Wall were visible from the Moon, it would be easy to see from near-Earth orbit. Instead, from orbit it is barely visible, and only in nearly perfect conditions.
The Wall is no more conspicuous from low Earth orbit than many other human-made objects, and is very difficult or impossible to see unaided. The caveat is that radar imagery from the Space Shuttle Endeavour shows sections of it clearly.
What It Means in Everyday Life
The claim probably began as a way to convey the Wall's grand scale to people who had only seen small black-and-white photographs.
The same ideas help with similar claims: width, colour contrast and lighting decide what can be seen from orbit. Length alone does not.
Halliburton, who helped spread the claim, was known for tall tales, which shows why such claims need checking.
