The Naked eye reference article from the English Wikipedia on 24-Jul-2004
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Naked eye

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Figure of speech, meaning to directly observe an object with no 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.

Normally 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 Jupiter's natural satellites.