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Concept
Naked eye
Summary
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. Vision corrected to normal acuity using corrective lenses is still considered "naked".
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:
Quick autofocus from distances of 25 cm (young people) to 50 cm (most people 50 years and older) to infinity.
Angular resolution: about 1 arcminute, approximately 0.017° or 0.0003 radians, which corresponds to 0.3 m at a 1 km distance.
Field of view (FOV): simultaneous visual perception in an area of about 160° × 175°.
Ability to see faint stars up to +8 magnitude under a perfectly dark sky.
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' (3–6 m per 1 km), see the measurements of Tycho Brahe.
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 their surroundings:
the distances and 3-dimensional position of things and persons
the vertical (plumb line) and the slope of plain objects
luminosities and colors and their changes by time and direction
Naked-eye planets
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.
Official source
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