Naked eye, also calledbare eye orunaided eye, is the practice of engaging invisual perception unaided by amagnifying,light-collectingoptical instrument, such as atelescope ormicroscope, oreye protection.
Inastronomy, the naked eye may be used to observe celestial events andobjects visible without equipment, such asconjunctions, passingcomets,meteor showers, and the brightestasteroids, including4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye.
Some basic properties of thehuman eye are:
Visual perception allows a person to gain much information about their surroundings:
The visibility of astronomical objects is strongly affected bylight pollution. Even a few hundred kilometers away from a metropolitan area where the sky can appear to be very dark, it is still the residual light pollution that sets the limit on the visibility of faint objects. For most people, these are likely to be the best observing conditions within their reach. Under such "typical" dark sky conditions, the naked eye can see stars with anapparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8m might be visible.[4]
The angular resolution of the naked eye is about 1′; however, some people have sharper vision than that. There is anecdotal evidence that people hadseen the Galileanmoons of Jupiter before telescopes were invented.[5]Uranus andVesta had most probably been seen but could not be recognized as planets because they appear so faint even at maximum brightness; Uranus's magnitude varies from +5.3m to +5.9m, and Vesta's from +5.2m to +8.5m (so that it is only visible near its opposition dates). Uranus, when discovered in 1781, was the first planet discovered usingtechnology (atelescope) rather than being spotted by the naked eye.
Theoretically, in a typical dark sky, thedark adapted human eye would see about 5,600 stars brighter than +6m[6] while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible.[4] In practice, the atmosphericextinction and dust reduces this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution can be as low as 2m, as few as 50 stars are visible. Colors can be seen but this is limited by the fact that the eye usesrods instead of cones to view fainter stars.
The visibility of diffuse objects such asstar clusters and galaxies is much more strongly affected by light pollution than is that of planets and stars. Under typical dark conditions only a few such objects are visible. These include thePleiades,h/χ Persei, theAndromeda Galaxy, theCarina Nebula, theOrion Nebula,Omega Centauri,47 Tucanae, the Ptolemy ClusterMessier 7 near the tail ofScorpius and theglobular cluster M13 inHercules. TheTriangulum Galaxy (M33) is a difficultaverted vision object and only visible at all if it is higher than 50° in the sky. The globular clustersM 3 inCanes Venatici andM 92 in Hercules are also visible with the naked eye under such conditions. Under really dark sky conditions, however, M33 is easy to see, even in direct vision. Many otherMessier objects are also visible under such conditions.[4] The most distant objects that have been seen by the naked eye are nearby bright galaxies such asCentaurus A,[7]Bode's Galaxy,[8][9][10]Sculptor Galaxy,[10] andMessier 83.[11]
Five planets can be recognized as planets fromEarth with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Under typical dark sky conditionsUranus (magnitude +5.8) can be seen as well with averted vision, as can the asteroidVesta at its brighter oppositions. Under perfect dark sky conditionsNeptune may be visible to the naked eye only if Neptune is at its maximum brightness (magnitude +7.8). The Sun and the Moon—the remaining noticeable naked-eye objects of theSolar System—are sometimes added to make seven "planets". During daylight only theMoon andSun are obvious naked eye objects, but in many casesVenus can be spotted in daylight and in rarer casesJupiter. Close to sunset and sunrise, bright stars likeSirius or evenCanopus can be spotted with the naked eye as long as one knows the exact position in which to look.Historically, the zenith of naked-eye astronomy was the work ofTycho Brahe (1546–1601). He built an extensive observatory to make precise measurements of the heavens without any instruments for magnification. In 1610,Galileo Galilei pointed atelescope towards the sky. He immediately discovered themoons of Jupiter and thephases ofVenus, among other things.
Meteor showers are better observed by naked eye than with binoculars. Such showers include thePerseids (10–12 August) and the DecemberGeminids. Some 100satellites per night, theInternational Space Station and theMilky Way are other popular objects visible to the naked eye.[12]
On 19 March 2008, a majorgamma-ray burst (GRB) known asGRB 080319B, set a new record as the farthest object that can be seen from Earth with the naked eye. It occurred about 7.5 billion years ago, the light taking that long to reach Earth.
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 to 20°. The distance of a person, just covered up by the outstretched thumbnail, is about 100 meters. The vertical can be estimated to about 2° and, in the northern hemisphere, observing thePole Star and using a protractor can give the observer's geographiclatitude, up to 1 degree of accuracy.
TheBabylonians,Mayans,ancient Egyptians,ancient Indians, andChinese measured all the basics of their respective time and calendar systems by naked eye:
In a similar manner staroccultations by the moon can be observed. By using a digitalclock an accuracy of 0.2 second is possible. This represents only 200 meters at the moon's distance of 385,000 km.
Observing a nearby small object without amagnifying glass or amicroscope, the size of the object depends on the viewing distance. Under normal lighting conditions (light source ~ 1000 lumens at height 600–700 mm, viewing angle ~ 35 degrees) the angular size recognized by naked eye will be round 1 arc minute = 1/60 degrees = 0.0003 radians.[1] At a viewing distance of 16" = ~ 400 mm, which is considered a normal reading distance in the US, the smallest object resolution will be ~ 0.116 mm. For inspection purposes laboratories use a viewing distance of 200–250 mm,[citation needed] which gives the smallest size of the object recognizable to the naked eye of ~0.058–0.072 mm (58–72 micrometers). Theaccuracy of a measurement ranges from 0.1 to 0.3 mm and depends on the experience of the observer. The latter figure is the usual positional accuracy of faint details in maps and technical plans.
.jpg)
A cleanatmosphere is indicated by the fact that the Milky Way is visible. Comparing the zenith with thehorizon shows how the "blue quality" is degraded depending on the amount of air pollution and dust. The twinkling of a star is an indication of theturbulence of the air. This is of importance inmeteorology and for the "seeing" of astronomy.
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 from a city by its "light dome".