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Naked eye
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.
In astronomy, the naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions, passing comets, meteor showers, and the brightest asteroids, including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye.
Some basic properties of the human eye are:
Visual perception allows a person to gain much information about their surroundings:
The visibility of astronomical objects is strongly affected by light pollution. Even a few hundred kilometers away from a metropolitan area where the sky can appear to be very dark, it is still the residual light pollution that sets the limit on the visibility of faint objects. For most people, these are likely to be the best observing conditions within their reach. Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8m might be visible.
The angular resolution of the naked eye is about 1′; however, some people have sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented. Uranus and Vesta had most probably been seen but could not be recognized as planets because they appear so faint even at maximum brightness; Uranus's magnitude varies from +5.3m to +5.9m, and Vesta's from +5.2m to +8.5m (so that it is only visible near its opposition dates). Uranus, when discovered in 1781, was the first planet discovered using technology (a telescope) rather than being spotted by the naked eye.
Theoretically, in a typical dark sky, the dark adapted human eye would see about 5,600 stars brighter than +6m while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible. In practice, the atmospheric extinction and dust reduces this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution can be as low as 2m, as few as 50 stars are visible. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars.
The visibility of diffuse objects such as star clusters and galaxies is much more strongly affected by light pollution than is that of planets and stars. Under typical dark conditions only a few such objects are visible. These include the Pleiades, h/χ Persei, the Andromeda Galaxy, the Carina Nebula, the Orion Nebula, Omega Centauri, 47 Tucanae, the Ptolemy Cluster Messier 7 near the tail of Scorpius and the globular cluster M13 in Hercules. The Triangulum Galaxy (M33) is a difficult averted vision object and only visible at all if it is higher than 50° in the sky. The globular clusters M 3 in Canes Venatici and M 92 in Hercules are also visible with the naked eye under such conditions. Under really dark sky conditions, however, M33 is easy to see, even in direct vision. Many other Messier objects are also visible under such conditions. The most distant objects that have been seen by the naked eye are nearby bright galaxies such as Centaurus A, Bode's Galaxy, Sculptor Galaxy, and Messier 83.
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Naked eye
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.
In astronomy, the naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions, passing comets, meteor showers, and the brightest asteroids, including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye.
Some basic properties of the human eye are:
Visual perception allows a person to gain much information about their surroundings:
The visibility of astronomical objects is strongly affected by light pollution. Even a few hundred kilometers away from a metropolitan area where the sky can appear to be very dark, it is still the residual light pollution that sets the limit on the visibility of faint objects. For most people, these are likely to be the best observing conditions within their reach. Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8m might be visible.
The angular resolution of the naked eye is about 1′; however, some people have sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented. Uranus and Vesta had most probably been seen but could not be recognized as planets because they appear so faint even at maximum brightness; Uranus's magnitude varies from +5.3m to +5.9m, and Vesta's from +5.2m to +8.5m (so that it is only visible near its opposition dates). Uranus, when discovered in 1781, was the first planet discovered using technology (a telescope) rather than being spotted by the naked eye.
Theoretically, in a typical dark sky, the dark adapted human eye would see about 5,600 stars brighter than +6m while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible. In practice, the atmospheric extinction and dust reduces this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution can be as low as 2m, as few as 50 stars are visible. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars.
The visibility of diffuse objects such as star clusters and galaxies is much more strongly affected by light pollution than is that of planets and stars. Under typical dark conditions only a few such objects are visible. These include the Pleiades, h/χ Persei, the Andromeda Galaxy, the Carina Nebula, the Orion Nebula, Omega Centauri, 47 Tucanae, the Ptolemy Cluster Messier 7 near the tail of Scorpius and the globular cluster M13 in Hercules. The Triangulum Galaxy (M33) is a difficult averted vision object and only visible at all if it is higher than 50° in the sky. The globular clusters M 3 in Canes Venatici and M 92 in Hercules are also visible with the naked eye under such conditions. Under really dark sky conditions, however, M33 is easy to see, even in direct vision. Many other Messier objects are also visible under such conditions. The most distant objects that have been seen by the naked eye are nearby bright galaxies such as Centaurus A, Bode's Galaxy, Sculptor Galaxy, and Messier 83.
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