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In natural science and technology those observations and measurements are called free- eyed or free-sighted that are obtained without optical aids (except for corrections of ametropia of the eyes), i.e. H. without using a telescope , magnifying glass , microscope or camera, for example .

The term is mainly used in astronomy and geodesy , as well as in physics and biology. It is also described in other words, such as "with the naked eye", "with the naked eye", "directly" or "visible without magnification". The colloquial language uses the term less often, because almost all daily activities are free-eyed and do not need to be mentioned. Only with more demanding tasks such as recognizing the finest details, with precise measurements or unusual lighting conditions, the performance limit of the eye is given closer attention.

Interesting phenomena

Interesting phenomena and interesting natural phenomena can be detected with the naked (naked) eye , but also the reachability of amazing accuracies. Here are a few examples:

Estimation of geometric sizes

  • Symmetries can be estimated to an accuracy of 1 to 2 percent if you adopt a favorable position and have some experience.
  • Intervals over a short distance better than 10 percent, on white paper even to 3 to 5 percent accurate, and details on a map about 0.2 mm
  • Linearity of a meter rod mm to about 0.5, a straight border line at about 1 cm per 50 m (see also Alignment )
  • Usual distances and speeds: with reference to a few percent, without reference to 10 to 20 percent
  • Brightness , color tones: similar to above (about 3 to 20 percent), see also Weber-Fechner law

Recognizability of fine details

  • Resolving power : depending on the contrast 0.01 to 0.02 ° or an average of 3 cm at 100 m, with double stars about 200 "
  • Detectability of thin lines: with good contrast, 5 to 10 percent of the resolution, i.e. 1 to 3 mm at 100 m (simple self-test e.g. with a wire fence)
  • Brightness differences to 0.2 to 0.5 percent (if directly comparable), otherwise around 10 percent

Astronomical phenomena for freedom of view

  • In the highest category of nocturnal darkness, stars with an apparent brightness (magnitude) above 6.8 can be seen with the naked eye when the view is clear . That is about 3000 stars per hemisphere , which are considered to be free-sighted in the astronomical sense . This good visibility requires a distance of up to 100 km to the next larger city, and is therefore crucial for the location of optical observatories. In Central Europe, even in more remote areas with good conditions, only about 2000 can be seen due to atmospheric refraction, in a large city a few hundred to a few dozen ( see also light pollution , light protection area ).
  • the five bright planets (Mercury, Venus, Mars, Jupiter, Saturn). Mercury only at certain times, when the ecliptic is steep. Under the best conditions, Uranus and the minor planet (4) Vesta can also be seen. During a passage through Venus also the “slice” of the planet in front of the sun.
  • some double stars , especially the little rider in the big chariot, with very good eyesight also Epsilon Lyrae .
  • Color differences on very bright stars - like on the white Sirius and the red Betelgeuse .
  • some star clusters (even if the individual stars were invisible) and nebulae (e.g. Pleiades, Orion, Andromeda and Triangle Nebulae . The latter is the most distant clear-sighted object at 2.8 million light years)
  • The Milky Way (whose single stars stay below the visibility limit)
  • Sometimes northern lights . In Northern Europe they can get brighter than the Milky Way.
  • Dozens of shooting stars per night, although it only the size of dust grains have
  • regular and irregular comets , an average of one per year, see comet visibility

Accuracy of open- eye astrometry ( visibility of apparent movement):

Observations in Biology

  • Detectability of fine structures: e.g. B. insect feelers up to 0.01 mm
  • Size and color differences
  • Movement patterns, flight behavior
  • Determine the vertical ( plumb direction ) at 1 to 2 °
  • Unconscious recognition of movements (warning reflex )
  • Estimation of speeds at 5 to 10 percent


  • N. Davidson: Sky Phenomena: A Guide to Naked Eye Observation of the Heavens. FlorisBooks (208p, £ 14.99), Edinburgh, 1993, ISBN 0-86315-168-X
  • H. Mucke : Astronomy in the open-air planetarium Vienna . Accompanying volume to free-eyed observations in the Sterngarten Georgenberg, Vienna 1995.
  • H. Kahmen (Ed.): Geodesy for Geotechnical and Structural Engineering. Proceedings, Eisenstadt, 1999.

Web links

Individual evidence

  1. Gottfried Gerstbach : Eye and Seeing - the long road to digital recognition . Sternenbote vol. 42, issue 8, pp. 160–180, Vienna 2000
  2. International Dark-Sky Reserves ( Memento of April 9, 2014 in the Internet Archive ),, accessed online on October 24, 2013 (link no longer available)
  3. How many stars can be seen with the naked eye? PM Magazine, archived from the original on August 31, 2014 ; accessed on October 31, 2014 .