Night sky
Overview
 
The term night sky refers to the sky
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...

 as seen at night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

. The term is usually associated with astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, with reference to views of celestial bodies such as star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, and planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s that become visible on a clear night after the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 has set. Natural light sources in a night sky include moonlight
Moonlight
Moonlight is the light that reaches Earth from the Moon. This light does not originate from the Moon, but from sunlight. The Moon does not, however, reflect sunlight like a mirror, but it reflects light from those portions of its surface which the Sun's light strikes. See diffuse reflection.In...

, starlight
Starlight
Starlight refers to the visible radiation emitted by stars other than the Sun.Starlight may also refer to:-Comics:* Starlight , Dr...

, and airglow
Airglow
Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist...

, depending on location and timing. Weather phenomenon such nighttime thunderstorms can dominate the sky, and transient events such as a great comet
Great comet
A Great Comet is a comet that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great...

.

The night sky and studies of it have a historical place in both ancient and modern cultures.
Encyclopedia
The term night sky refers to the sky
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...

 as seen at night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

. The term is usually associated with astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, with reference to views of celestial bodies such as star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, and planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s that become visible on a clear night after the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 has set. Natural light sources in a night sky include moonlight
Moonlight
Moonlight is the light that reaches Earth from the Moon. This light does not originate from the Moon, but from sunlight. The Moon does not, however, reflect sunlight like a mirror, but it reflects light from those portions of its surface which the Sun's light strikes. See diffuse reflection.In...

, starlight
Starlight
Starlight refers to the visible radiation emitted by stars other than the Sun.Starlight may also refer to:-Comics:* Starlight , Dr...

, and airglow
Airglow
Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist...

, depending on location and timing. Weather phenomenon such nighttime thunderstorms can dominate the sky, and transient events such as a great comet
Great comet
A Great Comet is a comet that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great...

.

The night sky and studies of it have a historical place in both ancient and modern cultures. In the past, for instance, farmers have used the state of the night sky as a calendar
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

 to determine when to plant crops
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

. Many cultures have drawn constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

s between star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s in the sky, using them in association with legend
Legend
A legend is a narrative of human actions that are perceived both by teller and listeners to take place within human history and to possess certain qualities that give the tale verisimilitude...

s and mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

 about their deities.

The anciently developed
History of astrology
Astrology, the belief in a connection between the cosmos and terrestrial matters has played an important part in human history.Regional branches of astrology include Western astrology, Indian astrology, and Chinese or East Asian astrology.-Early origins:...

 belief of astrology
Astrology
Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world...

 is generally based on the belief that relationships between heavenly bodies influence or convey information about events on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. The scientific study of the night sky and bodies observed within it, meanwhile, takes place in the science of astronomy.

The visibility of celestial objects in the night sky is affected by light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...

. The presence of the Moon in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best seeing both professional and amateur optical astronomers seek viewing sites located far from major urban areas.

Visual presentation

Stars are, depending on how dark the sky is, hundreds or thousands of white pinpoints of light in an otherwise black sky. To the naked eye, they all appear to be equidistant on a dome above the earth because stars are much too far away for stereopsis
Stereopsis
Stereopsis refers to impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due the the eyes' different positions on the head...

 to offer any depth cues. Visible stars range in color from blue (hot) to red (cold), but with such small points of faint light, most look white because they stimulate the rod cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

s without triggering the cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

s. If it is particularly dark and a particularly faint celestial object is of interest, averted vision
Averted vision
Averted vision is a technique for viewing faint objects which uses peripheral vision. It involves not looking directly at the object, but looking a little off to the side, while continuing to concentrate on the object. This subject is discussed in the popular astronomy literature but only a few...

 may be helpful.

The stars of the night sky cannot be counted unaided because they are so numerous and there is no way to track which have been counted and which have not. Further complicating the count, fainter stars may appear and disappear depending on exactly where the observer is looking. The result is an impression of an extraordinarily vast star field.

Because stargazing is best done from a dark place away from city lights, dark adaptation
Adaptation (eye)
In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.-Efficacy:The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the...

 is important to achieve and maintain. It takes several minutes for eyes to adjust to the darkness necessary for seeing the most stars, and surroundings on the ground are hard to discern. A red flashlight
Flashlight
A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

 (torch) can be used to illuminate star charts, telescope parts, and the like without undoing the dark adaptation. (See Purkinje effect
Purkinje effect
The Purkinje effect is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.This effect introduces a difference in color contrast under different levels of...

).

Constellations

There are no markings on the night sky, though there exist many sky maps
Star chart
A star chart is a map of the night sky. Astronomers divide these into grids to use them more easily. They are used to identify and locate astronomical objects such as stars, constellations and galaxies. They have been used for human navigation since time immemorial...

 to aid stargazers in identifying constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

s and other celestial objects. Constellations are prominent because their stars tend to be brighter than other nearby stars in the sky. Different cultures have created different groupings of constellations based on differing interpretations of the more-or-less random patterns of dots in the sky. Constellations were identified without regard to distance to each star, but instead as if they were all dots on a dome.

Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...

 is among the most prominent and recognizable constellations. The Big Dipper
Big Dipper
The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...

 (which has a wide variety of other names) is helpful for navigation in the northern hemisphere because it points to Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

, the north star.

The pole star
Pole star
The term "Pole Star" usually refers to Polaris, which is the current northern pole star, also known as the North Star.In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent...

s are special because they are approximately in line with the Earth's axis of rotation so they appear to stay in one place while the other stars rotate around them through the course of a night (or a year).

Planets

Planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, named for the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 word for "wanderer," process through the star field a little each day. Planets, to the naked eye, look like additional points of light in the sky. The disc is not apparent without binoculars or a telescope. Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 is the most prominent planet, often called the "morning star" or "evening star" because it is brighter than the stars and visible near sunrise or sunset depending on its location in its orbit. Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 are also visible to the naked eye.

The Moon

Earth's Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 is a gray disc in the sky with cratering visible to the naked eye. It spans, depending on its exact location, 29-43 arcminutes
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

 - which is about the size of a thumbnail at arm's length, and is readily identified. Over 28 days, the moon goes through a full cycle of lunar phase
Lunar phase
A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...

s. People can generally identify phases within a few days by looking at the moon. Unlike stars and planets, the light reflected from the moon is bright enough to be seen during the day.

Some of the most spectacular moons come during the full moon phase near sunset or sunrise. The moon on the horizon benefits from the moon illusion
Moon illusion
The Moon illusion is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the sun and star constellations. It has been known since ancient times, and recorded by numerous different cultures...

 which makes it appear larger. The light reflected from the moon traveling through the atmosphere also colors the moon orange and/or red.

Comets

Comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s come to the night sky only rarely. Comets are illuminated by the sun, and their tails extend away from the sun. A comet with visible tail is quite unusual - a great comet
Great comet
A Great Comet is a comet that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great...

 appears about once a decade. They tend to be visible only shortly before sunrise or after sunset because those are the times they are close enough to the sun to show a tail.

Clouds

Cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s obscure the view of other objects in the sky, though varying thicknesses of cloudcover have differing effects. A very thin cirrus cloud
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

 in front of the moon might produce a rainbow-colored ring around the moon. Stars and planets are too small or dim to take on this effect, and are instead only dimmed (often to the point of invisibility). Thicker cloudcover obscures celestial objects entirely, making the sky black or reflecting city lights back down. Clouds are often close enough to afford some depth perception, though they are hard to see without moonlight or light pollution.

Other objects

On clear deep nights in unpolluted areas, when the moon is thin or below the horizon, a band of what looks like white dust, the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, can be seen.

Shortly after sunset and before sunrise, artificial satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s often look like stars - similar in brightness and size, but different because they move relatively quickly; those that fly in near-earth orbit cross the sky in a couple of minutes. Some satellites (including space junk) appear to blink or have a periodic fluctuation in brightness because they are rotating.

Meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

s (commonly known as shooting stars) streak across the sky very infrequently. During a meteor shower, they may average one a minute not at regular intervals, but otherwise their appearance is a random surprise. The occasional meteor will make a bright, fleeting streak across the sky, and they can be very bright in comparison to the night sky.

Aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 are also visible at night, distinguishable at a distance from other objects because their lights blink.

See also

  • Amateur astronomy
    Amateur astronomy
    Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

  • Asterism (astronomy)
    Asterism (astronomy)
    In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...

  • Astrology
    Astrology
    Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world...

  • Astronomical object
    Astronomical object
    Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...

  • Constellation
    Constellation
    In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

  • Earth's shadow
    Earth's shadow
    The Earth's shadow or Earth shadow are names for the shadow that the Earth itself casts on its atmosphere. This shadow is often visible from the surface of the Earth, as a dark band in the sky near the horizon...

  • Planetarium
    Planetarium
    A planetarium is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation...


External links

  • A virtual panorama of winter night. Pokljuka
    Pokljuka
    Pokljuka is a foresty plateau at the altitude of around 1300 metres, situated partly in the municipality of Bled and partly in municipality of Bohinj in northwestern Slovenia. It is part of Triglav national park...

    , Slovenia. Burger.si. Accessed 28 February 2011.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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