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The naked eye is a figure of speech referring to human visual perception that is unaided by enhancing equipment, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked". The term is often used in astronomy when referring to events that can be viewed without equipment, such as astronomical conjunctions, passage of comets, or meteor showers. Sky lore and various tests demonstrate an impressive wealth of phenomena that can be seen with the unaided eye.

Contents 1 Basic Accuracies 2 Naked eye in astronomy 3 Space, geodesy, and navigation 4 Small objects and maps 5 Environmental and light pollution 6 Literature 7 References 8 External links

[edit] Basic Accuracies

The basic accuracies of human eyes are:

• Quick autofocus from distances of 10 cm (young people) to 50 cm (most people 50 years and older) to infinity
• Angular resolution: 1-2' (about 0.02°-0.03°), which corresponds to 30–60 cm at a 1 km distance
• Field of view (FOV): simultaneous visual perception in an area of about 130° × 160°
• Faint stars up to +6.5 magnitude under a modern dark sky
• Distance (Factor of Light intensity and light interference) the Andromeda Galaxy is visible at 2.5 million light years or 10 million trillion km.
• Photometry (brightness) to ±10% or 1% of intensity - in a range between night and day of 1:10,000,000,000
• Symmetries of 10-20' (5–10 cm per 1 km) see the measurements of Tycho Brahe and the Egyptians
• Interval estimations (for example at a plan on paper) to 3-5%
• Unconscious recognizing of movement (that is "alarm system" and reflexes)

Visual perception allows a person to gain much information about his or her environment:

• the distances and 3-dimensional position of things and persons,
• the vertical (plumb line) and the slope of plain objects
• luminosities and colors - and its changes by time and direction
• ... and other qualities of objects.

[edit] Naked eye in astronomy

The visibility of astronomical objects is strongly affected by light pollution. Even a few hundred kilometers away from metropolitan areas 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. Such conditions are, however, the best observing conditions in reach of most people. Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6^m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8^m might be visible.^[1]

The angular resolution of the naked eye is about 1′ however, some people have a sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented. Of similar magnitude, Uranus and Vesta had most probably been seen but could not be recognized as planets because they appear so small even at maximum brightness that their motion could not be detected. 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 +6^m while in perfect dark sky conditions the about 45,000 stars brighter than +8^m might be visible.^[1] In practice, the atmospheric extinction and dust reduce this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution ca be +4^m or less, as few as 200 to 500 stars are visible. Colors can be seen but just at bright stars and the planets, due to the eye's use of rods instead of cones for fainter stars.

The visibility of diffuse objects such as star clusters and galaxies is much stronger affected by light pollution than the visibility of planets and stars. Under typical dark conditions, only a few such objects are visible, such as the Pleiades, h/? Persei, Andromeda galaxy, the Orion Nebula, 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 about 50° in the sky. Under real dark sky conditions, however, M33 is easy to see, even in direct vision. Many other Messier objects are also visible under such conditions.^[1] The farthest objects that have been seen by the naked eye are nearby bright galaxies such as Centaurus A ^[2], Bode's Galaxy^[3][4][5], Sculptor Galaxy ^[5], and Messier 83 ^[6].

Five planets 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. The Sun and the Moon—the remaining noticeable naked-eye objects of the solar system—are sometimes added to make seven "planets."

Historically, the zenith of naked-eye astronomy was the work of Tycho Brahe (1546–1601), who built an extensive observatory to make precise measurements of the heavens without any instruments for magnification. In 1610, Galileo Galilei became the first person known to point a telescope towards the sky. He immediately discovered the moons of Jupiter and the phases of Venus, among other things.

Meteor showers are better observed by naked eye than with binoculars - the Perseids (10–12 August) or the December Geminids. Some 100 satellites per night, the International Space Station or the Milky Way are other popular visible objects.

[edit] Space, geodesy, and navigation

Many other things can be estimated without an instrument. If an arm is stretched the span of the hand corresponds to an angle of 18 to 20°. The distance of a person, just covered up by the outstretched thumbnail, is about 100 meters. The vertical can be estimated to about 2° and, in the northern hemisphere, the Pole Star and a protractor gives the observer's geographic latitude, up to 1 degree.

The Babylonians, Mayans and ancient Egyptians measured all the basics of their respective time and calendar systems by naked eye:

• the length of a year and a month to ±0.1 hour or to better than 1 minute (0.001%)
• the 24 hours of a day, and the equinoxes
• the periods of the Planets - by Mayan astronomers within 5 to 10 minutes for Venus and Mars.

In a similar manner star occultations by the moon can be observed. By using a digital clock one gets around 0.2 s - which represents only 200 meters at the moon's distance of 385,000 km.

[edit] Small objects and maps

Observing a nearby small object without a magnifying glass or a microscope, the usual distance is 20–25 cm. At this close range, 0.05 mm can be seen clearly. The accuracy of a measurement depends on the experience (0.1 to 0.3 mm). The latter figure is the usual position accuracy of faint details in maps, and also of technical plans.

[edit] Environmental and light pollution

If the Milky Way is visible, this is an indicator for a clean atmosphere. If we look both at the zenith and toward the horizon, the "blue quality" will degrade depending on the air pollution and dust. The star twinkling is a measure for the air turbulences — e.g. in meteorology and for the "seeing" of astronomy.

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". In winter the snow cover in the city can be estimated without going there.

[edit] Literature

• Adobe Comp.: ”Photoshop”, program handbook, versions 5/6, Adobe 2002.
• Davidson, N.: Sky Phenomena: A Guide to Naked Eye Observation of the Heavens. FlorisBooks (208p), ISBN 0-86315-168-X, Edinburgh 1993.
• Gerstbach G.: Auge und Sehen — der lange Weg zu digitalem Erkennen. Astro Journal Sternenbote, 20p., Vol.2000/8, Vienna 2000.
• Kahmen H. (Ed.): Geodesy for Geotechnical and Structural Engineering. Proceedings, Eisenstadt 1999.

[edit] References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (October 2009)

1. ^ ^{a b c} John E. Bortle (February 2001). "The Bortle Dark-Sky Scale". Sky & Telescope. http://www.skyandtelescope.com/resources/darksky/3304011.html. Retrieved 2009-11-18. 
2. ^ http://astronomy-mall.com/Adventures.In.Deep.Space/aintno.htm
3. ^ SEDS, Messier 81
4. ^ S. J. O'Meara (1998). The Messier Objects. Cambridge: Cambridge University. ISBN 0-521-55332-6. 
5. ^ ^{a b} http://messier.obspm.fr/xtra/supp/m81naked.txt
6. ^ http://www.springerlink.com/content/q1696565458u3286/

[edit] External links

• Naked Eye Observing in Astronomy
• Naked-Eye Stargazing: Learning the Sky and its constellations
• Naked Eye Navigation, Polynesia Voyages
• Detection of weak optical signals by the human visual system: Perspectives in Neuroscience and in Quantum Physics