Naked eye
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A naked eye is a figure of speech, referring to human eyes unaided by enhancing equipment such as a telescope or binoculars. Often used in astronomy when referring to events that can be viewed by the general public, for example astronomical conjunctions, comets or meteor showers.
Basic sky lore and various tests show an impressive wealth of phenomena that can be seen with the unaided eye.
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Eye characteristics and everyday life
The basic accuracies of human eyes are:
- Angular resolution 1–2′ (about 0,02°), 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 +6,5m magnitude
- Photometry (brightness) to ±10 percent 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 old Egyptians
- Interval estimations (e. g. at a plan on paper) to 3–5 per cent.
- Unconscious recognizing of movement ==> "alarm system" & reflexes
- Speed estimation to 5–10 percent.
Every day with this "instrument" we estimate a lot of things and their dimensions, without being conscious of this:
- the distances and 3-dimensional position of things and persons,
- the speed of the cars at the street we are driving
- the vertical (plumb line) and the slope of plain things
- luminosities and colours — and its changes by time and direction
- ... and other qualities of objects.
Naked Eye Astronomy
Normally the naked eye can see stars with apparent magnitude up to +6m; angular resolution of the naked eye is about 1′. However, some people have a sharper vision than that; there is anecdotal evidence that they have seen Uranus and the Galilean moons of Jupiter.
Theoretically, up to +6m the human eye would see about 2.500 stars in the starlit sky, but in practice the atmospheric extinction and dust reduce the number to 1500 - 2000 (in the center of a city only 200-500). Colours can be seen just at bright stars and the planets. Additionally some star clusters can be detected (Pleiades), h/χ Persei, M13 in Hercules), and the Andromeda and Orion Nebula.
Meteor showers are better observed by naked eye than with binoculars - eg. the Perseids (August 10 - 12) or the December Geminids. Some 100 satellites per night, the ISS or the Milky Way are other popular objects. If a satellite disappeares in the in Earth’s shadow - or at a Lunar eclipse - we can imagine the depths of space.
Space, Geodesy and Navigation
We can estimate a lot of other things without any instrument. If we stretch the arm, our hand span corresponds to an angle of 18-20°. The distance of a person is about 600 Meters, if just covered up by our thumb. The vertical can be estimated to about 2°, and the Pole Star and a protractor gives our geographic latitude up to 1 degree.
The ancient Egyptians, Babylonians or Mayas have measured all the basics of our time and calendar system by naked eye:
- the length of a year and a month to ±0,1 hour or better than 1 minute (0,001 percent)
- the 24 hours of a day, and the equinoxes
- the periods of the Planets - e.g. by Mayan astronomers to 5-10 minutes for Venus and Mars.
In a similar matter we can observe star occultations by the moon. By a digital clock we get ca. 0,2 sec - which are only 200meters in 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 the Astronomers.
Light pollution is a heavy problem for hobby astronomers but becomes less late at night when many lights are shut off. Air dust can be seen even far away of 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, £14.99), ISBN 086315168X, 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.
Web Links
- Naked Eye Observing (http://www.rocketroberts.com/astro/nakedeye.htm) in Astronomy
- Naked-Eye Stargazing: (http://www.kstrom.net/isk/stars/stareye.html) Learning the Sky and its constellations
- Naked Eye Navigation (http://www.pvs-hawaii.com/voyages/rapanui_naked_eye.htm), Polynesia Voyages
- Naked Eye (http://www.hyperdictionary.com/dictionary/naked+eye), Index of different aspects
- Naked Eye (http://www.golem.de/9911/5080.html) Internet summerde:Freiäugig