from a small rural town (top) and a
metropolitan area (bottom).
Light pollution dramatically reduces the visibility of
stars. The visibility of astronomical objects is strongly affected by
light 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 an
apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8m might be visible.
Uranus and
Vesta 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 using
technology (a
telescope) rather than being spotted by the naked eye. Theoretically, in a typical dark sky, the
dark adapted human eye would see about 5,600 stars brighter than +6m while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible. In practice, the atmospheric
extinction 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 uses
rods instead of cones to view fainter stars. The visibility of diffuse objects such as
star 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 the
Pleiades,
h/χ Persei, the
Andromeda Galaxy, the
Carina Nebula, the
Orion Nebula,
Omega Centauri,
47 Tucanae, the Ptolemy Cluster
Messier 7 near the tail of
Scorpius and the
globular cluster M13 in
Hercules. The
Triangulum Galaxy (M33) is a difficult
averted vision object and only visible at all if it is higher than 50° in the sky. The globular clusters
M 3 in
Canes Venatici and
M 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 other
Messier objects are also visible under such conditions.
Bode's Galaxy,
Sculptor Galaxy,
Five planets can be recognized as planets from
Earth with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Under typical dark sky conditions
Uranus (magnitude +5.8) can be seen as well with averted vision, as can the asteroid
Vesta at its brighter oppositions. Under perfect dark sky conditions
Neptune 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 the
Solar System—are sometimes added to make seven "planets". During daylight only the
Moon and
Sun are obvious naked eye objects, but in many cases
Venus can be spotted in daylight and in rarer cases
Jupiter. Close to sunset and sunrise, bright stars like
Sirius or even
Canopus 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 of
Tycho 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 a
telescope towards the sky. He immediately discovered the
moons of Jupiter and the
phases of
Venus, among other things. Meteor showers are better observed by naked eye than with binoculars. Such showers include the
Perseids (10–12 August) and the December
Geminids. Some 100
satellites per night, the
International Space Station and the
Milky Way are other popular objects visible to the naked eye. On 19 March 2008, a major
gamma-ray burst (GRB) known as
GRB 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. ==In geodesy and navigation==