Whether an object is visible depends significantly on the height above sea level from where it is observed. The Kármán line, at 100 kilometres (62mi), is accepted by the World Air Sports Federation, an international standard-setting and record-keeping body for aeronautics and astronautics, as the boundary between the Earth's atmosphere and outer space.[3] However, astronauts typically orbit the Earth at several hundreds of kilometres;[4] the ISS, for example, orbits at about 420km (260mi) above the Earth,[5] and the Moon orbits at about 380,000km (240,000mi) away.[4]
Examples
From US Space Shuttles, which typically orbited at around 135mi (217km), cities were easily distinguishable from surrounding countryside.[1] Using binoculars, astronauts could even see roads, dams, harbors, even large vehicles such as ships and planes.[2][6] At night, cities are also easily visible from the higher orbit of the ISS.
Metropolitan areas are clearly visible at night, particularly in industrialized countries, due to a multitude of street lights and other light sources in urban areas (see light pollution).
Cooling pond of Chernobyl
The region around the Chernobyl nuclear power plant, as seen from the Russian space station Mir in 1997
The 10-kilometre (6.2mi) long cooling pond of the Chernobyl Nuclear Power Plant is visible from space. In April 1997 it was photographed from the Mir space station, which was in orbit somewhere between 296km (184mi) and 421km (262mi).
The greenhouse complex that covers about 26thousand hectares (64thousand acres; 100 square miles) in the province of Almería, Andalucía, Spain[7] is visible from space.[8] It is sometimes referred to as the "Plastic sea" ("Mar de plástico" in Spanish) due to the high concentration of these greenhouse structures.
This area produces much of the fruit and vegetables that are sold in the rest of Spain and Europe. Apart from the area depicted in the photo, other zones of the province of Almería (and also the south of Spain) have large concentrations of white-plastic greenhouses too.
The two 8km x 2km rectangular east-west aligned Baray Lakes east and west of Angkor Wat have been created in the 11th century. They are visible with the bare eye from the 410km altitude of the ISS.
Baray Lakes visible from ISS, 28mm lens, 417km altitude
The claim that the Great Wall of China is the only man-made object visible from the Moon or outer space has been debunked many times, but remains a common misconception in popular culture.[1][2][4] According to astronauts Eugene Cernan and Ed Lu, the Great Wall is visible from the lower part of low Earth orbit, but only under very favorable conditions.[11]
Different claims were historically made for the factoid that the Great Wall is visible from the Moon. William Stukeley mentioned this claim in his letter dated 1754,[12] and Henry Norman made the same claim in 1895.[13] 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.[14] A viewer would need visual acuity 17 000 times better than the norm to see the Great Wall from the Moon.[15]
Theoretical calculation of visibility from the ISS
The human naked eye has an angular resolution of approximately 280 microradians[17] (μrad) (approx 0.016° or 1 minute of arc), and the ISS targets an altitude of 400km.[18] Using basic trigonometric relations, this means that an astronaut on the ISS with 20/20 vision could potentially detect objects that are 112m or greater in all dimensions. However, since this would be at the absolute limit of the resolution, objects on the order of 100m would appear as unidentifiable specks, if not rendered invisible due to other factors, such as atmospheric conditions or poor contrast. For readability of text from the ISS, using the same trigonometric principles and a recommended character size of about 18 arcminutes,[19] or about 5,000 μrad, each letter would need to be about 2km (1.3mi) in size for clear legibility in good conditions.
↑ Hvistendahl, Mara (February 21, 2008). "Is China's Great Wall Visible from Space?". Scientific American. Retrieved 2024-08-14. [T]he wall is only visible from low orbit under a specific set of weather and lighting conditions. And many other structures that are less spectacular from an earthly vantage point—desert roads, for example—appear more prominent from an orbital perspective.
↑ The Family Memoirs of the Rev. William Stukeley (1887) Vol. 3, p. 142. (1754) "Chinese wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the moon."
↑ Norman, Henry, The Peoples and Politics of the Far East, p. 215. (1895) "Besides its age it enjoys the reputation of being the only work of human hands on the globe visible from the moon."
↑ Miller, David; Schor, Paulo; Peter Magnante. "Optics of the Normal Eye", pg. 54 of Ophthalmology by Yanoff, Myron; Duker, Jay S. ISBN978-0-323-04332-8
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Spaceflight is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such as satellites in orbit around Earth, but also includes space probes for flights beyond Earth orbit. Such spaceflights operate either by telerobotic or autonomous control. The first spaceflights began in the 1950s with the launches of the Soviet Sputnik satellites and American Explorer and Vanguard missions. Human spaceflight programs include the Soyuz, Shenzhou, the past Apollo Moon landing and the Space Shuttle programs. Other current spaceflight are conducted to the International Space Station and to China's Tiangong Space Station.
Human spaceflight programs have been conducted, started, or planned by multiple countries and companies. Until the 21st century, human spaceflight programs were sponsored exclusively by governments, through either the military or civilian space agencies. With the launch of the privately funded SpaceShipOne in 2004, a new category of human spaceflight programs – commercial human spaceflight – arrived. By the end of 2022, three countries and one private company (SpaceX) had successfully launched humans to Earth orbit, and two private companies had launched humans on a suborbital trajectory.
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Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection.
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The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
The Great Wall of China is a series of fortifications that were built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic groups from the Eurasian Steppe. Several walls were built from as early as the 7th century BC, with selective stretches later joined by Qin Shi Huang (220–206 BC), the first emperor of China. Little of the Qin wall remains. Later on, many successive dynasties built and maintained multiple stretches of border walls. The best-known sections of the wall were built by the Ming dynasty (1368–1644).
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