Legend is a genre of folklore that consists of a narrative featuring human actions perceived or believed both by teller and listeners to have taken place within human history. Narratives in this genre may demonstrate human values, possess certain qualities that give the tale verisimilitude. Legend, for its active and passive participants, includes no happenings that are outside the realm of "possibility," but may include miracles. Legends may be transformed over time, in order to keep them fresh and realistic. Many legends operate within the realm of uncertainty, never being believed by the participants, but never being resolutely doubted; the Brothers Grimm defined legend as folktale grounded. A modern folklorist's professional definition of legend was proposed by Timothy R. Tangherlini in 1990: Legend is a short episodic, traditional ecotypified historicized narrative performed in a conversational mode, reflecting on a psychological level a symbolic representation of folk belief and collective experiences and serving as a reaffirmation of held values of the group to whose tradition it belongs.
Legend is a loanword from Old French that entered English usage circa 1340. The Old French noun legende derives from the Medieval Latin legenda. In its early English-language usage, the word indicated a narrative of an event; the word legendary was a noun meaning a collection or corpus of legends. This word changed to legendry, legendary became the adjectival form. By 1613, English-speaking Protestants began to use the word when they wished to imply that an event was fictitious. Thus, legend gained its modern connotations of "undocumented" and "spurious", which distinguish it from the meaning of chronicle. In 1866, Jacob Grimm described the fairy tale as "poetic, legend historic." Early scholars such as Karl Wehrhan Friedrich Ranke and Will Erich Peuckert followed Grimm's example in focussing on the literary narrative, an approach, enriched after the 1960s, by addressing questions of performance and the anthropological and psychological insights provided in considering legends' social context.
Questions of categorising legends, in hopes of compiling a content-based series of categories on the line of the Aarne–Thompson folktale index, provoked a search for a broader new synthesis. In an early attempt at defining some basic questions operative in examining folk tales, Friedrich Ranke in 1925 characterised the folk legend as "a popular narrative with an objectively untrue imaginary content" a dismissive position, subsequently abandoned. Compared to the structured folktale, legend is comparatively amorphous, Helmut de Boor noted in 1928; the narrative content of legend is in realistic mode, rather than the wry irony of folktale. In Einleitung in der Geschichtswissenschaft, Ernst Bernheim asserted that a legend is a longstanding rumour. Gordon Allport credited the staying-power of some rumours to the persistent cultural state-of-mind that they embody and capsulise; when Willian Jansen suggested that legends that disappear were "short-term legends" and the persistent ones be termed "long-term legends", the distinction between legend and rumour was obliterated, Tangherlini concluded.
In the narrow Christian sense, legenda were hagiographical accounts collected in a legendary. Because saints' lives are included in many miracle stories, legend, in a wider sense, came to refer to any story, set in a historical context but that contains supernatural, divine or fantastic elements. Hippolyte Delehaye distinguished legend from myth: "The legend, on the other hand, has, of necessity, some historical or topographical connection, it refers imaginary events to some real personage, or it localizes romantic stories in some definite spot."From the moment a legend is retold as fiction, its authentic legendary qualities begin to fade and recede: in The Legend of Sleepy Hollow, Washington Irving transformed a local Hudson River Valley legend into a literary anecdote with "Gothic" overtones, which tended to diminish its character as genuine legend. Stories that exceed the boundaries of "realism" are called "fables". For example, the talking animal formula of Aesop identifies his brief stories as fables, not legends.
The parable of the Prodigal Son would be a legend if it were told as having happened to a specific son of a historical father. If it included a donkey that gave sage advice to the Prodigal Son it would be a fable. Legend may be transmitted orally, passed on person-to-person, or, in the original sense, through written text. Jacob de Voragine's Legenda Aurea or "The Golden Legend" comprises a series of vitae or instructive biographical narratives, tied to the liturgical calendar of the Roman Catholic Church, they are presented as lives of the saints, but the profusion of miraculous happenings and above all their uncritical context are characteristics of hagiography. The Legenda was intended to inspire extemporized homilies and sermons appropriate to the saint of the day; the vanishing hitchhiker is the best-known urban legend in America, traceable as far back as 1870, but it is found around the world including in Korea and Russia. In the legend, a young girl in a white dress picked up alongside of the road by a passerby.
The unknown girl in white remains silent for the duration of her ride, thanks the driver, gets
A farmer is a person engaged in agriculture, raising living organisms for food or raw materials. The term applies to people who do some combination of raising field crops, vineyards, poultry, or other livestock. A farmer might own the farmed land or might work as a laborer on land owned by others, but in advanced economies, a farmer is a farm owner, while employees of the farm are known as farm workers, or farmhands. However, in the not so distant past, a farmer was a person who promotes or improves the growth of by labor and attention, land or crops or raises animals. Farming dates back as far as the Neolithic. By the Bronze Age, the Sumerians had an agriculture specialized labor force by 5000–4000 BCE, depended on irrigation to grow crops, they relied on three-person teams. The Ancient Egypt farmers relied and irrigated their water from the Nile. Animal husbandry, the practice of rearing animals for farming purposes, has existed for thousands of years. Dogs were domesticated in East Asia about 15,000 years ago.
Goats and sheep were domesticated around 8000 BCE in Asia. Swine or pigs were domesticated by 7000 BCE in China; the earliest evidence of horse domestication dates to around 4000 BCE. In the U. S. of the 1930s, one farmer could only produce enough food to feed three other consumers. A modern-day farmer produces enough food to feed well over a hundred people. However, some authors consider this estimate to be flawed, as it does not take into account that farming requires energy and many other resources which have to be provided by additional workers, so that the ratio of people fed to farmers is smaller than 100 to 1. More distinct terms are used to denote farmers who raise specific domesticated animals. For example, those who raise grazing livestock, such as cattle, sheep and horses, are known as ranchers, graziers, or stockmen. Sheep and cattle farmers might be referred to as shepherds and cowherds; the term dairy farmer is applied to those engaged in milk production, whether from cattle, sheep, or other milk producing animals.
A poultry farmer is one who concentrates on raising chickens, ducks, or geese, for either meat, egg, or feather production, or all three. A person who raises a variety of vegetables for market may be called a truck farmer or market gardener. Dirt farmer is one who farms his own land. In developed nations, a farmer is defined as someone with an ownership interest in crops or livestock, who provides land or management in their production; those who provide only labor are most called farmhands. Alternatively, growers who manage farmland for an absentee landowner, sharing the harvest are known as sharecroppers or sharefarmers. In the context of agribusiness, a farmer is defined broadly, thus many individuals not engaged in full-time farming can nonetheless qualify under agricultural policy for various subsidies and tax deductions. In the context of developing nations or other pre-industrial cultures, most farmers practice a meager subsistence agriculture—a simple organic farming system employing crop rotation, seed saving and burn, or other techniques to maximize efficiency while meeting the needs of the household or community.
One subsisting in this way may have been known as a peasant. In developed nations, however, a person using such techniques on small patches of land might be called a gardener and be considered a hobbyist. Alternatively, one might be driven into such practices by poverty or, ironically—against the background of large-scale agribusiness—might become an organic farmer growing for discerning consumers in the local food market. Farmers are members of local, regional, or national farmers' unions or agricultural producers' organizations and can exert significant political influence; the Grange movement in the United States was effective in advancing farmers' agendas against railroad and agribusiness interests early in the 20th century. The FNSEA is politically active in France pertaining to genetically modified food. Agricultural producers, both small and large, are represented globally by the International Federation of Agricultural Producers, representing over 600 million farmers through 120 national farmers' unions in 79 countries.
Farmed products might be sold either directly from a farm. In a subsistence economy, farm products might to some extent be either consumed by the farmer's family or pooled by the community. There are several occupational hazards for those in agriculture. Farmers can encounter and be stung or bitten by dangerous insects and other arthropods, including scorpions, fire ants, bees and hornets. Farmers work around heavy machinery which can kill or injure them. Farmers can establish muscle and joints pains from repeated work. Notes Bibliography Media related to Farmers at Wikimedia Commons The dictionary definition of farmer at Wiktionary
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process, it is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Three quarters of the Sun's mass consists of hydrogen; the Sun is a G-type main-sequence star based on its spectral class. As such, it is informally and not accurately referred to as a yellow dwarf, it formed 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System; the central mass became so hot and dense that it initiated nuclear fusion in its core. It is thought that all stars form by this process.
The Sun is middle-aged. It fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result; this energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to become a red giant, it is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, no longer produce energy by fusion, but still glow and give off heat from its previous fusion; the enormous effect of the Sun on Earth has been recognized since prehistoric times, the Sun has been regarded by some cultures as a deity.
The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of, the predominant calendar in use today. The English proper name Sun may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn; the Latin name for the Sun, Sol, is not used in everyday English. Sol is used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars; the related word solar is the usual adjectival term used for the Sun, in terms such as solar day, solar eclipse, Solar System. A mean Earth solar day is 24 hours, whereas a mean Martian'sol' is 24 hours, 39 minutes, 35.244 seconds. The English weekday name Sunday stems from Old English and is a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου.
The Sun is a G-type main-sequence star. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is heavy-element-rich, star; the formation of the Sun may have been triggered by shockwaves from more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars; the heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is 1 astronomical unit, though the distance varies as Earth moves from perihelion in January to aphelion in July.
At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports all life on Earth by photosynthesis, drives Earth's climate and weather; the Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres; the tidal effect of the planets is weak and does not affect the shape of the Sun. The Sun rotates faster at its equator than at its poles; this differential rotation is caused by convective motion
Night or nighttime is the period from sunset to sunrise in each twenty-four hours, when the Sun is below the horizon. However it is subjective. Night can be defined as the time between morning. There is no exact time for when night ends; the start of night begins when evening ends, subjective, but is believed to end at astronomical sunset, when night may begin. There can be no precise definition in terms of clock time, but it is considered to start around 9 pm and to last to about 5 am. Since sunset and sunrise vary throughout the year there can be no precise definition in terms of clock time. Night and morning overlap when one considers morning to start past 12 am, which can be described as'morning-night duality'. Night is used as a farewell,'good night', and sometimes shortened to'night'. Unlike'good morning','good afternoon', and'good evening','good night' is not used a greeting. Complete darkness or astronomical night is the period between astronomical dusk and astronomical dawn when the Sun is between 18 and 90 degrees below the horizon and does not illuminate the sky.
As seen from latitudes between 48.5° and 66.5° north or south of the Equator, complete darkness does not occur around the summer solstice because although the Sun sets, it is never more than 18° below the horizon at lower culmination. The opposite of night is day; the start and end points of time for a night vary, based on factors such as latitude. Twilight is the period of night after sunset or before sunrise when the Sun still illuminates the sky when it is below the horizon. At any given time, one side of Earth is bathed in sunlight while the other side is in the shadow caused by Earth blocking the sunlight; the central part of the shadow is called the umbra. Natural illumination at night is still provided by a combination of moonlight, planetary light, zodiacal light and airglow. In some circumstances, aurorae and bioluminescence can provide some illumination; the glow provided by artificial lighting is sometimes referred to as light pollution because it can interfere with observational astronomy and ecosystems.
On Earth, an average night lasts shorter than daytime due to two factors. Firstly, the Sun's apparent disk has an angular diameter of about 32 arcminutes. Secondly, the atmosphere refracts sunlight so that some of it reaches the ground when the Sun is below the horizon by about 34'; the combination of these two factors means that light reaches the ground when the center of the solar disk is below the horizon by about 50'. Without these effects and night would be the same length on both equinoxes, the moments when the Sun appears to contact the celestial equator. On the equinoxes, daytime lasts 14 minutes longer than night does at the Equator, longer towards the poles; the summer and winter solstices mark the longest nights, respectively. The closer a location is to either the North Pole or the South Pole, the wider the range of variation in the night's duration. Although daytime and night nearly equalize in length on the equinoxes, the ratio of night to day changes more at high latitudes than at low latitudes before and after an equinox.
In the Northern Hemisphere, Denmark experiences shorter nights in June than India. In the Southern Hemisphere, Antarctica sees longer nights in June than Chile. Both hemispheres experience the same patterns of night length at the same latitudes, but the cycles are 6 months apart so that one hemisphere experiences long nights while the other is experiencing short nights. In the region within either polar circle, the variation in daylight hours is so extreme that part of summer sees a period without night intervening between consecutive days, while part of winter sees a period without daytime intervening between consecutive nights; the phenomenon of day and night is due to the rotation of a celestial body about its axis, creating an illusion of the sun rising and setting. Different bodies spin at different rates, however; some may spin much faster than Earth, while others spin slowly, leading to long days and nights. The planet Venus rotates once every 224.7 days – by far the slowest rotation period of any of the major planets.
In contrast, the gas giant Jupiter's sidereal day is 56 minutes. However, it is not just the sidereal rotation period which determines the length of a planet's day-night cycle but the length of its orbital period as well - Venus has a rotation period of 224.7 days, but a day-night cycle just 116.75 days long due to its retrograde rotation and orbital motion around the Sun. Mercury has the longest day-night cycle as a result of its 3:2 resonance between its orbital period and rotation period - this resonance gives it a day-night cycle 176 days long. A planet may experience large temperature variations between day and night, such as Mercury, the planet closest to the sun; this is one consideration in terms of planetary habitability or the possibility of extraterrestrial life. The disappearance of sunlight, the primary energy source for life on Earth, has dramatic impacts on the morphology and behavior of every organism; some animals sleep during the night, whilst other nocturnal animals including moths and crickets are active during this time.
The effects of day and night are not seen in the animal kingdom alone. For example, crassu
History of astrology
Astrological beliefs in correspondences between celestial observations and terrestrial events have influenced various aspects of human history, including world-views and many elements of social culture. Among Indo-European peoples, astrology has been dated to the 3rd millennium BC, with roots in calendrical systems used to predict seasonal shifts and to interpret celestial cycles as signs of divine communications; until the 17th century, astrology was considered a scholarly tradition, it helped drive the development of astronomy. It was accepted in political and cultural circles, some of its concepts were used in other traditional studies, such as alchemy and medicine. By the end of the 17th century, emerging scientific concepts in astronomy, such as heliocentrism, undermined the theoretical basis of astrology, which subsequently lost its academic standing and became regarded as a pseudoscience. Empirical scientific investigation has shown that predictions and recommendations based on these systems are not accurate.
In the 20th century, astrology gained broader consumer popularity through the influence of regular mass media products, such as newspaper horoscopes. Astrology, in its broadest sense, is the search for human meaning in the sky, it has been argued that astrology began as a study as soon as human beings made conscious attempts to measure and predict seasonal changes by reference to astronomical cycles. Early evidence of such practices appears as markings on bones and cave walls, which show that lunar cycles were being noted as early as 25,000 years ago. With the Neolithic agricultural revolution new needs were met by increasing knowledge of constellations, whose appearances in the night-time sky change with the seasons, allowing the rising of particular star-groups to herald annual floods or seasonal activities. By the 3rd millennium BC, widespread civilisations had developed sophisticated awareness of celestial cycles, are believed to have consciously oriented their temples to create alignment with the heliacal risings of the stars.
There is scattered evidence to suggest that the oldest known astrological references are copies of texts made during this period in Mesopotamia. Two, from the Venus tablet of Ammisaduqa are reported to have been made during the reign of king Sargon of Akkad. Another, showing an early use of electional astrology, is ascribed to the reign of the Sumerian ruler Gudea of Lagash; this describes how the gods revealed to him in a dream the constellations that would be most favourable for the planned construction of a temple. However, controversy attends the question of whether they were genuinely recorded at the time or ascribed to ancient rulers by posterity; the oldest undisputed evidence of the use of astrology as an integrated system of knowledge is therefore attributed to the records that emerge from the first dynasty of Mesopotamia. Babylonian astrology was the first organized system of astrology, arising in the 2nd millennium BC. There is speculation that astrology of some form appeared in the Sumerian period in the 3rd millennium BC, but the isolated references to ancient celestial omens dated to this period are not considered sufficient evidence to demonstrate an integrated theory of astrology.
The history of scholarly celestial divination is therefore reported to begin with late Old Babylonian texts, continuing through the Middle Babylonian and Middle Assyrian periods. By the 16th century BC the extensive employment of omen-based astrology can be evidenced in the compilation of a comprehensive reference work known as Enuma Anu Enlil, its contents consisted of 70 cuneiform tablets comprising 7,000 celestial omens. Texts from this time refer to an oral tradition - the origin and content of which can only be speculated upon. At this time Babylonian astrology was mundane, concerned with the prediction of weather and political matters, prior to the 7th century BC the practitioners' understanding of astronomy was rudimentary. Astrological symbols represented seasonal tasks, were used as a yearly almanac of listed activities to remind a community to do things appropriate to the season or weather. By the 4th century, their mathematical methods had progressed enough to calculate future planetary positions with reasonable accuracy, at which point extensive ephemerides began to appear.
Babylonian astrology developed within the context of divination. A collection of 32 tablets with inscribed liver models, dating from about 1875 BC, are the oldest known detailed texts of Babylonian divination, these demonstrate the same interpretational format as that employed in celestial omen analysis. Blemishes and marks found on the liver of the sacrificial animal were interpreted as symbolic signs which presented messages from the gods to the king; the gods were believed to present themselves in the celestial images of the planets or stars with whom they were associated. Evil celestial omens attached to any particular planet were therefore seen as indications of dissatisfaction or disturbance of the god that planet represented; such indications were met with attempts to appease the god and find
The Milky Way is the galaxy that contains our Solar System. The name describes the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye; the term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος. From Earth, the Milky Way appears as a band. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610; until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies; the Milky Way is a barred spiral galaxy with a diameter between 200,000 light-years. It is estimated to contain 100 -- more than 100 billion planets; the Solar System is located at a radius of 26,490 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust.
The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, assumed to be a supermassive black hole of 4.100 million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at 220 kilometers per second; the constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter"; the rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of 600 km per second with respect to extragalactic frames of reference; the oldest stars in the Milky Way are nearly as old as the Universe itself and thus formed shortly after the Dark Ages of the Big Bang. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, itself a component of the Laniakea Supercluster.
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, the term “Milky Way” is limited to this band of light; the light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars; the area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a low surface brightness, its visibility can be reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible. It should be visible if the limiting magnitude is +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but prominent when viewed from rural areas when the Moon is below the horizon.
Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution. As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations; the Galactic Center lies in the direction of Sagittarius. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga; the band continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two equal hemispheres. The galactic plane is inclined by about 60° to the ecliptic. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic, relative to the galactic plane; the north galactic pole is situated at right ascension 12h 49m, declination +27.4° near β Comae Berenices, the south galactic pole is near α Sculptoris.
Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear low or high in the sky. For observers from latitudes 65° north to 65° south, the Milky Way passes directly overhead twice a day; the Milky Way is the second-largest galaxy in the Local Group, with its stellar disk 100,000 ly in diameter and, on average 1,000 ly thick. The Milky Way is 1.5 trillion times the mass of the Sun. To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter, the Milky Way would be the size of the contiguous United States. There is a ring-like filament of stars rippling above and below the flat galactic plane, wrapping around the Milky Way at a diameter of 150,000–180,000 light-years, which may be part of the Milky Way itself. Estimates of the mass of the Milky Way vary, depending upon the method and data used; the low end of the estimate range is 5.8×1011 solar masses, somewhat less than that of the Andromeda Galaxy.
Measurements using the Very Long Baseline Array in 2009 found
Airglow is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be dark after the effects of starlight and diffused sunlight from the far side are removed; the airglow phenomenon was first identified in 1868 by Swedish physicist Anders Ångström. Since it has been studied in the laboratory, various chemical reactions have been observed to emit electromagnetic energy as part of the process. Scientists have identified some of those processes that would be present in Earth's atmosphere, astronomers have verified that such emissions are present. Airglow is caused by various processes in the upper atmosphere of Earth, such as the recombination of atoms which were photoionized by the Sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, chemiluminescence caused by oxygen and nitrogen reacting with hydroxyl free radicals at heights of a few hundred kilometres, it is not noticeable during the daytime due to the scattering of sunlight.
At the best ground-based observatories, airglow limits the photosensitivity of optical telescopes. For this reason, space telescopes like Hubble can observe much fainter objects than current ground-based telescopes at visible wavelengths. Airglow at night may be bright enough for a ground observer to notice and appears bluish. Although airglow emission is uniform across the atmosphere, it appears brightest at about 10° above the observer's horizon, since the lower one looks, the greater the depth of atmosphere one is looking through. Low down, atmospheric extinction reduces the apparent brightness of the airglow. One airglow mechanism is when an atom of nitrogen combines with an atom of oxygen to form a molecule of nitric oxide. In the process, a photon is emitted; this photon may have any of several different wavelengths characteristic of nitric oxide molecules. The free atoms are available for this process, because molecules of nitrogen and oxygen are dissociated by solar energy in the upper reaches of the atmosphere and may encounter each other to form NO.
Other species that can create air glow in the atmosphere are hydroxyl, atomic oxygen and lithium. The sky brightness is measured in units of apparent magnitude per square arcsecond of sky. In order to calculate the relative intensity of airglow, we need to convert apparent magnitudes into fluxes of photons. At visible wavelengths, we need the parameter S0, the power per square centimetre of aperture and per micrometre of wavelength produced by a zeroth-magnitude star, to convert apparent magnitudes into fluxes — S0 = 4.0×10−12 W cm−2 µm−1. If we take the example of a V=28 star observed through a normal V band filter, the number of photons we receive per square centimeter of telescope aperture per second from the source is Ns: N s = 10 − 28 / 2.5 × S 0 × B h ν. At V band, the emission from airglow is V = 22 per square arc-second at a high-altitude observatory on a moonless night; this gives the number of photons from airglow, Na: N a = 10 − 23 / 2.5 × S 0 × B h ν The signal-to-noise for an ideal ground-based observation with a telescope of area A, arising from Poisson statistics, is only: S / N = A × N s N s + N a If we assume a 10 m diameter ideal ground-based telescope and an unresolved star: every second, over a patch the size of the seeing-enlarged image of the star, 35 photons arrive from the star and 3500 from air-glow.
So, over an hour 1.3×107 arrive from the air-glow, 1.3×105 arrive from the source. We can compare this with "real" answers from exposure time calculators. For an 8 m unit Very Large Telescope telescope, according to the FORS exposure time calculator you need 40 hours of observing time to reach V = 28, while the 2.4 m Hubble only takes 4 hours according to the ACS exposure time calculator. A hypothetical 8 m Hubble telescope would take about 30 minutes, it should be clear from this calculation that reducing the view field size can make fainter objects more detectable against the airglow.