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Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, 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 , , and the brightest , including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye.
Visual perception allows a person to gain much information about their surroundings:
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.Zezong, Xi, "The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo", Chinese Physics 2 (3) (1982): 664–67. 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 the 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 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, Double cluster, 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,SEDS, Messier 81
Sculptor Galaxy, and Messier 83.Classical planet can be recognized as planets from Earth with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Under typical dark sky conditions Uranus (magnitude +5.8) can be seen as well with averted vision, as can the asteroid Vesta at its brighter oppositions. Under perfect dark sky conditions Neptune may be visible to the naked eye only if Neptune is at its maximum brightness (magnitude +7.8). The Sun and the Moon—the remaining noticeable naked-eye objects of the Solar System—are sometimes added to make seven "planets". During daylight only the Moon and Sun are obvious naked eye objects, but in many cases Venus can be spotted in daylight and in rarer cases Jupiter. Close to sunset and sunrise, bright stars like Sirius or even Canopus can be spotted with the naked eye as long as one knows the exact position in which to look. Historically, the zenith of naked-eye astronomy was the work of Tycho Brahe (1546–1601). He built an extensive observatory to make precise measurements of the heavens without any instruments for magnification. In 1610, Galileo Galilei pointed a telescope towards the sky. He immediately discovered the Galilean moons and the Planetary phase of Venus, among other things.
Meteor showers are better observed by naked eye than with binoculars. Such showers include the Perseids (10–12 August) and the December Geminids. Some 100 per night, the International Space Station and the Milky Way are other popular objects visible to the naked eye.
On 19 March 2008, a major gamma-ray burst (GRB) known as GRB 080319B, set a new record as the farthest object that can be seen from Earth with the naked eye. It occurred about 7.5 billion years ago, the light taking that long to reach Earth.
The , Mayans, , Indian Astronomy, and Chinese measured all the basics of their respective time and calendar systems by naked eye:
In a similar manner star by the moon can be observed. By using a digital clock an accuracy of 0.2 second is possible. This represents only 200 meters at the moon's distance of 385,000 km.
Light pollution is a significant problem for amateur astronomers but becomes less late at night when many lights are shut off. Air dust can be seen even far away from a city by its "light dome".
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