The claim the Great Wall of China is visible from outer space has been debunked many times, but is still ingrained in popular culture. A viewer would need visual acuity 8 times better than normal to see it from low earth orbit.

Claims are made for the factoid that the Wall is visible from the Moon. William Stukeley mentioned this claim in his letter dated 1754. The claim was mentioned by Henry Norman in 1895 as if it was already well-known. The issue of "canals" on Mars was prominent in the late 19th century and may have led to the belief that long, thin objects were visible from space. A viewer would need visual acuity 17,000 times better than normal to see the Wall from the Moon.

Man-made structures visible from earth orbit without magnification (by mechanical aids such as a camera or binoculars) include highways, dams, and cities. The most commonly cited example, the Great Wall of China, is barely visible from Low Earth Orbit (LEO; under perfect conditions), but not from the moon.

Part of the problem in authoritatively answering questions about visibility from space is that such questions are often very vaguely put: Askers frequently do not precisely define how far above sea level their hypothetical or actual observer in space is located. When just speaking of "space," an asker might mean anywhere from the edge of space (Kármán line) at an altitude of 100 kilometers (62 mi) to Low Earth Orbit (160-2,000 km) to Apollo 12's orbit at 290 km (180 mi) to the Moon, which orbits about 381,415 km (237,000 mi) away. .

The Kármán line is accepted by the Fédération Aéronautique Internationale (FAI), which is an international standard setting and record-keeping body for aeronautics and astronautics, as the boundary between the Earth's atmosphere and outer space. However, a practical definition for the purpose of answering questions on what astronauts generally can see from space would reflect altitudes at which actual manned spacecraft typically orbit the Earth - often several hundreds of kilometres into space.