The visibility of astronomical objects is strongly affected by light pollution.
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.
Under really dark sky conditions, however, M33 is easy to see, even in direct vision.
Many other Messier objects are also visible under such conditions.
By using a digital clock an accuracy of 0.2 second is possible.
This represents only 200 meters at the moon's distance of 385,000 km.Meteor showers are better observed by naked eye than with binoculars.Such showers include the Perseids (10–12 August) and the December Geminids.In astronomy, the naked eye may be used to observe events that can be viewed without equipment, such as an astronomical conjunction, the passage of a comet, or a meteor shower.Sky lore and various tests demonstrate an impressive wealth of phenomena that can be seen with the unaided eye.Historically, the zenith of naked-eye astronomy was the work of Tycho Brahe (1546–1601).