Man-made structures visible from earth orbit without magnification (by mechanical aids such as a camera or binoculars) include highways, dams, and cities.[1][2][3] The most commonly cited example, the Great Wall of China, is barely visible from low earth orbit (under perfect conditions), but not from the moon.[3][4]
Part of the problem of discerning fact from urban legend is defining how far up "space" is, which could be anywhere from the edge of space at an altitude of 100 kilometers (62 mi); to Apollo 12's orbital at 290 km (180 mi); to the moon, which orbits about 381,415 km (237,000 mi) away.[3] A best estimated definition would be in the hundreds of kilometres into space.[3] Another definition is the Kármán line, which is at the altitude of 100 km (62 mi) above the Earth's sea level. That 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.[5]
Cities are easily distinguishable from surrounding countryside by both shuttle astronauts in an Earth orbit 135 miles (217 km) high[1] as from space stations, which orbit much higher, circling the planet at c. 250 miles (400 km). Using binoculars, the astronauts can see roads, airports, dams, harbors, even large vehicles such as ships and planes.[2][6] At an orbit of 160 to 350 miles, many such objects are visible from the Space Station.[3]
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Metropolitan areas are readily visible at night, particularly in industrialized countries, due to a multitude of street lights and other light sources in urban areas (see light pollution).
The Great Wall of China is, proverbially, the man-made object most often cited as being visible from outer space. Claims are made for the factoid that it is visible from the Moon. William Stukeley mentioned this claim in his letter dated 1754.[7] The claim was mentioned by Henry Norman in 1895 as if it was already well-known.[8] 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.[9] The claim the Wall is visible has been debunked many times,[1][2][3] but is still ingrained in popular culture.[10] The viewer would need visual acuity 17,000 times better than normal (20/20) to see the Wall from the Moon, and vision 8 times better than normal to see it from low earth orbit.[11]
"The naked eye can tell the difference between cities and countryside from space." [12]
The "People's House" in Bucharest, Romania is the second largest building in the world, after The Pentagon, and has been cited as being visible from space.[13]
Large-scale pollution is almost invariably described in terms of visibility from space. The BP Spill or Deepwater Horizon oil spill of April 20, 2010 was so described in news stories,[14] on scientific websites,[15][16] as well as jokes about the spill.[17] Algae blooms and sewage, which are indirectly created by man-made pollution, are visible, and have been photographed, from space.[18][19]
The concept has entered popular culture as a factoid ("the Great Wall of China is the only man-made object visible from the moon"), meme ("Many are familiar with the claim") trivia questions,[2] metaphor, urban legend,[1][3] and proverb, that certain constructed objects or effects are so large as to be visible from outer space. For example, a giant beaver dam in Canada was described as "so large it is visible from outer space."[20] Field and Stream asked and answered, "How big? Big enough to be visible ... from outer space."[21]