naked eye

The naked eye

For the song by Luscious Jackson, See Naked Eye (song).

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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.

Basic Accuracies

The basic accuracies of human eyes are:

• Quick autofocus from distances of 10 cm (young people) ... 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
• 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 excellent 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 colours —and its changes by time and direction
• ... and other qualities of objects.

Naked eye in astronomy

Normally the naked eye can see stars with an apparent magnitude up to +6^m; 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 recognised 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, at up to +6^m the human eye would see about 2,500 stars in the clear sky but, in practice, the atmospheric extinction and dust reduce the number to 1500 to 2000 (in the center of a city as few as 200 to 500, due to light pollution). Colours 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. Additionally, some star clusters can be detected, such as Pleiades, h/χ Persei, M13 in Hercules, and the Orion Nebula. Two of the 18 objects listed in the SEDS Messier catalog as being brighter than +6^m are true galaxies: the Andromeda Galaxy (M31) and the Triangulum Galaxy (M33). Five planets can be recognised as planets from earth with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Under very dark 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.

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.

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.

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.

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.

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