A naked eye is a figure of speech, referring to human visual perception unaided by enhancing equipment such as a telescope or binoculars. The term is often used in astronomy when referring to events that can be viewed by the general public, such as 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.
Eye characteristics and everyday life
The basic accuracies of human eyes are:
- 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 +6.5m magnitude
- 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 old Egyptians
- Interval estimations (e. g. at a plan on paper) to 3–5 %.
- Unconscious recognizing of movement (i.e. "alarm system" & reflexes)
- Speed estimation to 5–10 %.
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 speed of the cars on the street
- 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 in 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. The five planets that can be seen from earth with the naked eye, plus the Sun and the Moon, are called the Seven Lights of the Earth.
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 International Space Station or the Milky Way are other popular objects.
Many other things can be estimated without an instrument. If an arm is stretched, the span of the hand corresponds to an angle of 18-20°. The distance of a person is about 600 meters, if just covered up by the 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 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 better than 1 minute (0.001 %)
- 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, star occultations by the moon can be observed. By a digital clock, one gets ca. 0.2 s - which are only 200 meters 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 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 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.
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