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
The full moon is the lunar phase when the Moon appears illuminated from Earth's perspective. This occurs when Earth is located between the Moon; this means that the lunar hemisphere facing Earth – the near side – is sunlit and appears as a circular disk, while the far side is dark. The full moon occurs once every month; when the Moon moves into Earth's shadow, a lunar eclipse occurs, during which all or part of the Moon's face may appear reddish due to the Rayleigh scattering of blue wavelengths and the refraction of sunlight through Earth's atmosphere. Lunar eclipses happen only during full moon and around points on its orbit where the satellite may pass through the planet's shadow. A lunar eclipse does not occur every month because the Moon's orbit is inclined 5.14° with respect to the ecliptic plane of Earth. Lunar eclipses happen only. Therefore, a lunar eclipse occurs every 6 months and 2 weeks before or after a solar eclipse, which occurs during new moon around the opposite node; the interval period between a new or full moon and the next same phase, a synodic month, averages about 29.53 days.
Therefore, in those lunar calendars in which each month begins on the day of the new moon, the full moon falls on either the 14th or 15th day of the lunar month. Because a calendar month consists of a whole number of days, a lunar month may be either 29 or 30 days long. A full moon is thought of as an event of a full night's duration; this is somewhat misleading because its phase seen from Earth continuously wanes. Its maximum illumination occurs at the moment waxing. For any given location, about half of these maximum full moons may be visible, while the other half occurs during the day, when the full moon is below the horizon. Many almanacs list full moons not only by date, but by their exact time in Coordinated Universal Time. Typical monthly calendars that include lunar phases may be offset by one day when used in a different time zone. Full moon is a suboptimal time for astronomical observation of the Moon because shadows vanish, it is a poor time for other observations because the bright sunlight reflected by the Moon, amplified by the opposition surge outshines many stars.
On 12 December 2008, the full moon occurred closer to the Earth than it had been at any time for the previous 15 years, called a supermoon. On 19 March 2011, another full supermoon occurred, closer to the Earth than at any time for the previous 18 years. On 14 November 2016, a full supermoon occurred closer to the Earth than at any time for the previous 68 years; the date and approximate time of a specific full moon can be calculated from the following equation: d = 20.362000 + 29.530588861 × N + 102.026 × 10 − 12 × N 2 where d is the number of days since 1 January 2000 00:00:00 in the Terrestrial Time scale used in astronomical ephemerides. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moon's orbit. See New moon for an explanation of the formula and its parameters; the age and apparent size of the full moon vary in a cycle of just under 14 synodic months, referred to as a full moon cycle. Full moons are traditionally associated with temporal insomnia and various "magical phenomena" such as lycanthropy.
Psychologists, have found that there is no strong evidence for effects on human behavior around the time of a full moon. They find that studies are not consistent, with some showing a positive effect and others showing a negative effect. In one instance, the 23 December 2000 issue of the British Medical Journal published two studies on dog bite admission to hospitals in England and Australia; the study of the Bradford Royal Infirmary found that dog bites were twice as common during a full moon, whereas the study conducted by the public hospitals in Australia found that they were less likely. Month names are names of moons in lunisolar calendars. Since the introduction of the solar Julian calendar in the Roman Empire, the Gregorian calendar worldwide, people no longer perceive month names as "moon" names; the traditional Old English month names were equated with the names of the Julian calendar from an early time. Some full moons have developed new names in modern times, e.g. the blue moon, the names "harvest moon" and "hunter's moon" for the full moons of autumn.
Lunar eclipses only happen during a full moon and cast a reddish tint over the face of the moon. This has been called a blood moon in popular culture; the "harvest moon" and "hunter's moon" are traditional terms for the full moons occurri
Archaeoastronomy is the interdisciplinary or multidisciplinary study of how people in the past "have understood the phenomena in the sky, how they used these phenomena and what role the sky played in their cultures". Clive Ruggles argues it is misleading to consider archaeoastronomy to be the study of ancient astronomy, as modern astronomy is a scientific discipline, while archaeoastronomy considers symbolically rich cultural interpretations of phenomena in the sky by other cultures, it is twinned with ethnoastronomy, the anthropological study of skywatching in contemporary societies. Archaeoastronomy is closely associated with historical astronomy, the use of historical records of heavenly events to answer astronomical problems and the history of astronomy, which uses written records to evaluate past astronomical practice. Archaeoastronomy uses a variety of methods to uncover evidence of past practices including archaeology, astronomy and probability, history; because these methods are diverse and use data from such different sources, integrating them into a coherent argument has been a long-term difficulty for archaeoastronomers.
Archaeoastronomy fills complementary niches in landscape cognitive archaeology. Material evidence and its connection to the sky can reveal how a wider landscape can be integrated into beliefs about the cycles of nature, such as Mayan astronomy and its relationship with agriculture. Other examples which have brought together ideas of cognition and landscape include studies of the cosmic order embedded in the roads of settlements. Archaeoastronomy can be applied to all time periods; the meanings of the sky vary from culture to culture. It is the need to balance the social and scientific aspects of archaeoastronomy which led Clive Ruggles to describe it as: "... field with academic work of high quality at one end but uncontrolled speculation bordering on lunacy at the other". In his short history of'Astro-archaeology' John Michell argued that the status of research into ancient astronomy had improved over the past two centuries, going'from lunacy to heresy to interesting notion and to the gates of orthodoxy.'
Nearly two decades we can still ask the question: Is archaeoastronomy still waiting at the gates of orthodoxy or has it gotten inside the gates? Two hundred years before Michell wrote the above, there were no archaeoastronomers and there were no professional archaeologists, but there were astronomers and antiquarians; some of their works are considered precursors of archaeoastronomy. Late in the nineteenth century astronomers such as Richard Proctor and Charles Piazzi Smyth investigated the astronomical orientations of the pyramids; the term archaeoastronomy was first used by Elizabeth Chesley Baity in 1973, but as a topic of study it may be much older, depending on how archaeoastronomy is defined. Clive Ruggles says that Heinrich Nissen, working in the mid-nineteenth century was arguably the first archaeoastronomer. Rolf Sinclair says that Norman Lockyer, working in the late 19th and early 20th centuries, could be called the'father of archaeoastronomy'. Euan MacKie would place the origin later, stating: "...the genesis and modern flowering of archaeoastronomy must lie in the work of Alexander Thom in Britain between the 1930s and the 1970s".
In the 1960s the work of the engineer Alexander Thom and that of the astronomer Gerald Hawkins, who proposed that Stonehenge was a Neolithic computer, inspired new interest in the astronomical features of ancient sites. The claims of Hawkins were dismissed, but this was not the case for Alexander Thom's work, whose survey results of megalithic sites hypothesized widespread practice of accurate astronomy in the British Isles. Euan MacKie, recognizing that Thom's theories needed to be tested, excavated at the Kintraw standing stone site in Argyllshire in 1970 and 1971 to check whether the latter's prediction of an observation platform on the hill slope above the stone was correct. There was an artificial platform there and this apparent verification of Thom's long alignment hypothesis led him to check Thom's geometrical theories at the Cultoon stone circle in Islay with a positive result. MacKie published new prehistories of Britain. In contrast a re-evaluation of Thom's fieldwork by Clive Ruggles argued that Thom's claims of high accuracy astronomy were not supported by the evidence.
Thom's legacy remains strong, Krupp wrote in 1979, "Almost singlehandedly he has established the standards for archaeo-astronomical fieldwork and interpretation, his amazing results have stirred controversy during the last three decades." His influence endures and practice of statistical testing of data remains one of the methods of archaeoastronomy. The approach in the New World, where anthropologists began to consider more the role of astronomy in Amerindian civilizations, was markedly different, they had access to sources that the prehistory of Europe lacks such as ethnographies and the historical records of the early colonizers. Following the pioneering example of Anthony Aveni, this allowed New World archaeoastronomers to make claims for motives which in the Old World would have been mere speculation; the concentration on h
A tumulus is a mound of earth and stones raised over a grave or graves. Tumuli are known as barrows, burial mounds or kurgans, may be found throughout much of the world. A cairn, a mound of stones built for various purposes, may originally have been a tumulus. Tumuli are categorised according to their external apparent shape. In this respect, a long barrow is a long tumulus constructed on top of several burials, such as passage graves. A round barrow is a round tumulus commonly constructed on top of burials; the internal structure and architecture of both long and round barrows has a broad range, the categorization only refers to the external apparent shape. The method of inhumation may involve a dolmen, a cist, a mortuary enclosure, a mortuary house, or a chamber tomb. Examples of barrows include Duggleby Maeshowe; the word tumulus is Latin for'mound' or'small hill', derived from the Proto-Indo-European root *teuh2- with extended zero grade *tum-,'to bulge, swell' found in tumor, thumb and thousand.
The funeral of Patroclus is described in book 23 of the Iliad. Patroclus is burned on a pyre, his bones are collected into a golden urn in two layers of fat; the barrow is built on the location of the pyre. Achilles sponsors funeral games, consisting of a chariot race, wrestling, running, a duel between two champions to the first blood, discus throwing and spear throwing. Beowulf's body is taken to Hronesness. During cremation, the Geats lament the death of their lord, a widow's lament being mentioned in particular, singing dirges as they circumambulate the barrow. Afterwards, a mound is built on top of a hill, overlooking the sea, filled with treasure. A band of twelve of the best warriors ride around the barrow, singing dirges in praise of their lord. Parallels have been drawn to the account of Attila's burial in Jordanes' Getica. Jordanes tells that as Attila's body was lying in state, the best horsemen of the Huns circled it, as in circus games. An Old Irish Life of Columcille reports that every funeral procession "halted at a mound called Eala, whereupon the corpse was laid, the mourners marched thrice solemnly round the spot."
Archaeologists classify tumuli according to their location and date of construction. Some British types are listed below: Bank barrow Bell barrow Bowl barrow D-shaped barrow – round barrow with a purposely flat edge at one side defined by stone slabs. Disc barrow Fancy barrow – generic term for any Bronze Age barrows more elaborate than a simple hemispherical shape. Long barrow Oval barrow – a Neolithic long barrow consisting of an elliptical, rather than rectangular or trapezoidal mound. Platform barrow – The least common of the recognised types of round barrow, consisting of a flat, wide circular mound that may be surrounded by a ditch, they occur across southern England with a marked concentration in East and West Sussex. Pond barrow – a barrow consisting of a shallow circular depression, surrounded by a bank running around the rim of the depression, from the Bronze Age. Ring barrow – a bank that encircles a number of burials. Round barrow – a circular feature created by the Bronze Age peoples of Britain and the Romans and Saxons.
Divided into subclasses such as saucer and bell barrow—the Six Hills are a rare Roman example. Saucer barrow – a circular Bronze Age barrow that features a low, wide mound surrounded by a ditch that may have an external bank. Square barrow – burial site of Iron Age date, consisting of a small, ditched enclosure surrounding a central burial, which may have been covered by a mound. In 2015, the first long barrow in thousands of years, inspired by those built in the Neolithic Period, was built near All Cannings in England; the project was steward of Stonehenge. The barrow was designed to have a large number of private niches within the stone and earth structure to receive cremation urns; the structure received significant media attention, with national press writing extensively about the revival of the structures, various episodes of filming, for example by BBC Countryfile as it was being built. It was subscribed within eighteen months; this was followed soon after by a new barrow near St Neots. Further plans to revive barrows are at Soulton in Shropshire.
The word kurgan is of Turkic origin, derives from Proto-Turkic *Kur-. In Ukraine and Russia, there are royal kurgans of Varangian chieftains, such as the Black Grave in Ukrainian Chernihiv, Oleg's Grave in Russian Staraya Ladoga, vast, intricate Rurik's Hill near Russian Novgorod. Other important kurgans are found in Ukraine and South Russia and are associated with much more ancient steppe peoples, notably the Scythians and early Indo-Europeans The steppe cultures found in Ukraine and South Russia continue into Central Asia, in particular Kazakhstan. Salweyn in northern Somalia contains a large field of cairns, which stretches for a distance of around 8 km. An excavation of one of these tumuli by Georges Révoil in 1881 uncovered a tomb, beside which were artefacts pointing to an ancient, advanced civilization; the interred objects included pottery shards from Samos, some well-crafted enamels, a mask of Ancient Greek design. Tumuli are one of the most prominent types of prehistoric monuments spread throughout northern and southern Albania.
Some well-known local tumuli are: Kamenica Tumulus Lofkënd Tumulus Pazhok Tumulus More than 50 burial mounds were found in Kupres. Man from Kupres- the skeleton found
Patina is a thin layer that variously forms on the surface of copper and similar metals, or certain stones, wooden furniture, or any similar acquired change of a surface through age and exposure. Additionally, leather aficionados use the term to describe the ageing of high quality leather; the patina on leather goods are unique to the type of leather, frequency of use, exposure. Patinas can provide a protective covering to materials that would otherwise be damaged by corrosion or weathering, they may be aesthetically appealing. On metal, patina is a coating of various chemical compounds such as oxides, sulfides, or sulfates formed on the surface during exposure to atmospheric elements, a common example of, rust which forms on iron or steel when exposed to oxygen. Patina refers to accumulated changes in surface texture and colour that result from normal use of an object such as a coin or a piece of furniture over time. Archaeologists use the term "patina" to refer to a corticated layer that develops over time, due to a range of complex factors on flint tools and ancient stone monuments./ This has led stone tool analysts in recent times to prefer the term "cortification" as a better term to describe the process than "patination".
In geology and geomorphology, the term "patina" is used to refer to discolored film or thin outer layer produced either on or within the surface of a rock or other material by either the development of a weathering rind within the surface of a rock, the formation of desert varnish on the surface of a rock, or combination of both. It refers to development as the result of weathering of a case-hardened layer, called "cortex" by geologists, within the surface of either a flint or chert nodule; the word "patina" comes from the Italian patina derived from the Latin patĭna. Figuratively, patina can refer to any fading, darkening or other signs of age, which are felt to be natural or unavoidable; the chemical process by which a patina forms or is deliberately induced is called patination, a work of art coated by a patina is said to be patinated. The green patina that forms on copper and bronze, sometimes called verdigris consists of varying mixtures of copper chlorides, sulfides and carbonates, depending upon environmental conditions such as sulfur-containing acid rain.
In clean air rural environments, the patina is created by the slow chemical reaction of copper with carbon dioxide and water, producing a basic copper carbonate. In industrial and urban air environments containing sulfurous acid rain from coal-fired power plants or industrial processes, the final patina is composed of sulphide or sulphate compounds. A patina layer takes many years to develop under natural weathering. Buildings in damp coastal/marine environments will develop patina layers faster than ones in dry inland areas. Facade cladding with alloys of copper, e.g. brass or bronze, will weather differently from "pure" copper cladding. A lasting gold colour is possible with copper-alloy cladding, for example Colston Hall in Bristol, or the Novotel at Paddington Central, London. Antique and well-used firearms will develop a patina on the steel after the bluing, parkerizing, or other finish has worn. Firearms in this state are considered more valuable than ones that have been re-blued or parkerized.
The patina protects the firearm from the more damaging rust that would occur were the patina to be polished off. Artists and metalworkers deliberately add patinas as a part of the original design and decoration of art and furniture, or to simulate antiquity in newly made objects; the process is called distressing. A wide range of chemicals, both household and commercial, can give a variety of patinas, they are used by artists as surface embellishments either for color, texture, or both. Patination composition varies with the reacted elements and these will determine the color of the patina. For copper alloys, such as bronze, exposure to chlorides leads to green, while sulfur compounds tend to brown; the basic palette for patinas on copper alloys includes chemicals like ammonium sulfide, liver of sulfur, cupric nitrate and ferric nitrate. For artworks, patination is deliberately accelerated by applying chemicals with heat. Colors range from matte sandstone yellow to deep blues, whites and various blacks.
Some patina colors are achieved by the mixing of colors from the reaction with the metal surface with pigments added to the chemicals. Sometimes the surface is enhanced by oiling, or other types of lacquers or clear-coats. More the French sculptor Auguste Rodin used to instruct assistants at his studio to urinate over bronzes stored in the outside yard. A patina can be produced on copper by the application of vinegar; this patina will not last on the outside of a building like a "true" patina. It is used as pigment. Patina is found on slip rings and commutators; this type of patina is formed by corrosion, what elements the air might hold, residue from the wear of the carbon brush and moisture. Patinas can be found in woks or other metal baking dishes, which form when properly seasoned; the patina on a wok is a dark coating of oils that have been burned onto it to prevent food sticking and to enhance the flavor of the foods cooked in it
Cornwall is a county in South West England in the United Kingdom. The county is bordered to the north and west by the Celtic Sea, to the south by the English Channel, to the east by the county of Devon, over the River Tamar which forms most of the border between them. Cornwall forms the westernmost part of the South West Peninsula of the island of Great Britain; the furthest southwestern point of Great Britain is Land's End. Cornwall has a population of 563,600 and covers an area of 3,563 km2; the county has been administered since 2009 by Cornwall Council. The ceremonial county of Cornwall includes the Isles of Scilly, which are administered separately; the administrative centre of Cornwall, its only city, is Truro. Cornwall is the homeland of the Cornish people and the cultural and ethnic origin of the Cornish diaspora, it retains a distinct cultural identity that reflects its history, is recognised as one of the Celtic nations. It was a Brythonic kingdom and subsequently a royal duchy; the Cornish nationalist movement contests the present constitutional status of Cornwall and seeks greater autonomy within the United Kingdom in the form of a devolved legislative Cornish Assembly with powers similar to those in Wales and Scotland.
In 2014, Cornish people were granted minority status under the European Framework Convention for the Protection of National Minorities, giving them recognition as a distinct ethnic group. First inhabited in the Palaeolithic and Mesolithic periods, Cornwall continued to be occupied by Neolithic and Bronze Age peoples, by Brythons with strong ethnic, linguistic and cultural links to Wales and Brittany the latter of, settled by Britons from the region. Mining in Cornwall and Devon in the south-west of England began in the early Bronze Age. Few Roman remains have been found in Cornwall, there is little evidence that the Romans settled or had much military presence there. After the collapse of the Roman Empire, Cornwall was a part of the Brittonic kingdom of Dumnonia, ruled by chieftains of the Cornovii who may have included figures regarded as semi-historical or legendary, such as King Mark of Cornwall and King Arthur, evidenced by folklore traditions derived from the Historia Regum Britanniae.
The Cornovii division of the Dumnonii tribe were separated from their fellow Brythons of Wales after the Battle of Deorham in 577 AD, came into conflict with the expanding English kingdom of Wessex. The regions of Dumnonia outside of Cornwall had been annexed by the English by 838 AD. King Athelstan in 936 AD set the boundary between the English and Cornish at the high water mark of the eastern bank of the River Tamar. From the early Middle Ages and culture were shared by Brythons trading across both sides of the Channel, resulting in the corresponding high medieval Breton kingdoms of Domnonée and Cornouaille and the Celtic Christianity common to both areas. Tin mining was important in the Cornish economy. In the mid-19th century, the tin and copper mines entered a period of decline. Subsequently, china clay extraction became more important, metal mining had ended by the 1990s. Traditionally and agriculture were the other important sectors of the economy. Railways led to a growth of tourism in the 20th century.
Cornwall is noted for coastal scenery. A large part of the Cornubian batholith is within Cornwall; the north coast has many cliffs. The area is noted for its wild moorland landscapes, its long and varied coastline, its attractive villages, its many place-names derived from the Cornish language, its mild climate. Extensive stretches of Cornwall's coastline, Bodmin Moor, are protected as an Area of Outstanding Natural Beauty; the modern English name Cornwall is a compound of two ancient demonyms coming from two different language groups: Corn- originates from the Brythonic tribe, the Cornovii. The Celtic word "kernou" is cognate with the English word "horn". -wall derives from the Old English exonym walh, meaning "foreigner" or "Roman". In the Cornish language, Cornwall is known as Kernow which stems from a similar linguistic background; the present human history of Cornwall begins with the reoccupation of Britain after the last Ice Age. The area now known as Cornwall was first inhabited in the Mesolithic periods.
It continued to be occupied by Neolithic and Bronze Age people. According to John T. Koch and others, Cornwall in the Late Bronze Age was part of a maritime trading-networked culture called the Atlantic Bronze Age, in modern-day Ireland, Wales, France and Portugal. During the British Iron Age, like all of Britain, was inhabited by a Celtic people known as the Britons with distinctive cultural relations to neighbouring Brittany; the Common Brittonic spoken at the time developed into several distinct tongues, including Cornish, Breton and Pictish. The first account of Cornwall comes from the 1st-century BC Sicilian Greek historian Diodorus Siculus quoting or paraphrasing the 4th-century BCE geographer P
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material, excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is used for elemental analysis and chemical analysis in the investigation of metals, glass and building materials, for research in geochemistry, forensic science and art objects such as paintings and murals; when materials are exposed to short-wavelength X-rays or to gamma rays, ionization of their component atoms may take place. Ionization consists of the ejection of one or more electrons from the atom, may occur if the atom is exposed to radiation with an energy greater than its ionization energy. X-rays and gamma rays can be energetic enough to expel held electrons from the inner orbitals of the atom; the removal of an electron in this way makes the electronic structure of the atom unstable, electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of, equal to the energy difference of the two orbitals involved.
Thus, the material emits radiation. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy; each element has electronic orbitals of characteristic energy. Following removal of an inner electron by an energetic photon provided by a primary radiation source, an electron from an outer shell drops into its place. There are a limited number of ways in which this can happen, as shown in Figure 1; the main transitions are given names: an L→K transition is traditionally called Kα, an M→K transition is called Kβ, an M→L transition is called Lα, so on. Each of these transitions yields a fluorescent photon with a characteristic energy equal to the difference in energy of the initial and final orbital; the wavelength of this fluorescent radiation can be calculated from Planck's Law: λ = h c E The fluorescent radiation can be analysed either by sorting the energies of the photons or by separating the wavelengths of the radiation.
Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material. This is the basis of a powerful technique in analytical chemistry. Figure 2 shows the typical form of the sharp fluorescent spectral lines obtained in the wavelength-dispersive method. In order to excite the atoms, a source of radiation is required, with sufficient energy to expel held inner electrons. Conventional X-ray generators are most used, because their output can be "tuned" for the application, because higher power can be deployed relative to other techniques. However, gamma ray sources can be used without the need for an elaborate power supply, allowing an easier use in small portable instruments; when the energy source is a synchrotron or the X-rays are focused by an optic like a polycapillary, the X-ray beam can be small and intense. As a result, atomic information on the sub-micrometre scale can be obtained. X-ray generators in the range 20–60 kV are used, which allow excitation of a broad range of atoms.
The continuous spectrum consists of "bremsstrahlung" radiation: radiation produced when high-energy electrons passing through the tube are progressively decelerated by the material of the tube anode. A typical tube output spectrum is shown in Figure 3. In energy dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a solid-state detector which produces a "continuous" distribution of pulses, the voltages of which are proportional to the incoming photon energies; this signal is processed by a multichannel analyser which produces an accumulating digital spectrum that can be processed to obtain analytical data. In wavelength dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a diffraction grating monochromator; the diffraction grating used is a single crystal. By varying the angle of incidence and take-off on the crystal, a single X-ray wavelength can be selected; the wavelength obtained is given by Bragg's law: n ⋅ λ = 2 d ⋅ sin where d is the spacing of atomic layers parallel to the crystal surface.
In energy dispersive analysis and detection are a single operation, as mentioned above. Proportional counters or various types of solid-state detectors are used, they all share the same detection principle: An incoming X-ray photon ionises a large number of detector atoms with the amount of charge produced being proportional to the energy of the incoming photon. The charge is collected and the process repeats itself for the next photon. Detector speed is critical, as all charge carriers measured have to come from the same photon to measure the photon energy correctly; the spectrum is built up by dividing the energy spectrum into discrete bins and counting the number of pulses registered within each energy bin. EDXRF detector types vary in resolution and the means of cooling