Etymology.

"Christmas" is a shortened form of "Christ's mass". The word is recorded as "Crīstesmæsse" in 1038 and "Cristes-messe" in 1131. "Crīst" (genitive "Crīstes") is from Greek "Khrīstos" (Χριστός), a translation of Hebrew "Māšîaḥ" (מָשִׁיחַ), "Messiah", meaning "anointed"; and "mæsse" is from Latin "missa", the celebration of the Eucharist.  The form "Christenmas" was also historically used, but is now considered archaic and dialectal. The term derives from Middle English "Cristenmasse", meaning "Christian mass". "Xmas" is an abbreviation of "Christmas" found particularly in print, based on the initial letter chi (Χ) in Greek "Khrīstos" (Χριστός), "Christ", though numerous style guides discourage its use. This abbreviation has precedent in Middle English "Χρ̄es masse" (where "Χρ̄" is an abbreviation for Χριστός).

Other names.

In addition to "Christmas", the holiday has been known by various other names throughout its history. The Anglo-Saxons referred to the feast as "midwinter", or, more rarely, as "Nātiuiteð" (from Latin "nātīvitās" below). "Nativity", meaning "birth",

Nativity.

The gospels of Luke and Matthew describe Jesus as being born in Bethlehem to the Virgin Mary. In Luke, Joseph and Mary travel from Nazareth to Bethlehem for the census, and Jesus is born there and laid in a manger. Angels

History.

The nativity sequences included in the Gospels of Matthew and Luke prompted early Christian writers to suggest various dates for the anniversary. Although no date is indicated in the gospels, early Christians connected Jesus to the Sun through the use of such phrases as "Sun of righteousness." The Romans marked the winter solstice on December 25. The first recorded Christmas celebration was in Rome on December 25, 336. In the 3rd century, the date of the nativity was the subject of great interest. Around AD 200, Clement of Alexandria wrote:  Christmas played a role in the Arian controversy of the fourth century. After this controversy ran its course, the prominence of the holiday declined for a few centuries. The feast regained prominence after 800 when Charlemagne was crowned emperor on Christmas Day. Later during the Protestant Reformation, the Puritans banned Christmas in England, associating it with drunkenness and other misbehavior. It was restored as a legal holiday in England in 1660, but remained disreputable in the minds of many people. In the early 19th century, Christmas was reconceived by Washington Irving, Charles Dickens, and other authors as a holiday emphasizing family, children, kind-heartedness, gift-giving, and Santa Claus.

Introduction of the festival.

Christmas does not appear on the lists of festivals given by the early Christian writers Irenaeus and Tertullian. Origen and Arnobius both fault the pagans for celebrating birthdays, which suggests that Christmas was not celebrated in their time. Arnobius wrote after AD 297. The Chronography of 354 records that a Christmas celebration took place in Rome in 336.  In the East, the birth of Jesus was celebrated in connection with the Epiphany on January 6. This holiday was not primarily about the nativity, but rather the baptism of Jesus. Christmas was promoted in the East as part of the revival of Orthodox Christianity that followed the death of the pro-Arian Emperor Valens at the Battle of Adrianople in 378. The feast was introduced in Constantinople in 379, in Antioch by John Chrysostom towards the end of the fourth century, probably in 388, and in Alexandria in the following century.

Solstice date.

December 25 was the date of the winter solstice on the Roman calendar. A late fourth-century sermon by Saint Augustine explains why this was a fitting day to celebrate Christ's nativity: "Hence it is that He was born on the day which is the shortest in our earthly reckoning and from which subsequent days begin to increase in length. He, therefore, who bent low and lifted us up chose the shortest day, yet the one whence light begins to increase."  Linking Jesus to the Sun was supported by various Biblical passages. Jesus was considered to be the "Sun of righteousness" prophesied by Malachi: "Unto you shall the sun of righteousness arise, and healing is in his wings."

Calculation hypothesis.

The calculation hypothesis suggests that an earlier holiday held on March 25 became associated with the Incarnation. Modern scholars refer to this feast as the Quartodecimal. Christmas was then calculated as nine months later. The calculation hypothesis was proposed by French writer Louis Duchesne in 1889. In modern times, March 25 is celebrated as Annunciation. This holiday was created in the seventh century and was assigned to a date that is nine months before Christmas, in addition to being the traditional date of the equinox. It

History of religions hypothesis.

The rival "History of Religions" hypothesis suggests that the Church selected December 25 date to appropriate festivities held by the Romans in honor of the Sun god Sol Invictus. This cult was established by Aurelian in 274. An explicit expression of this theory appears in an annotation of uncertain date added to a manuscript of a work by 12th-century Syrian bishop Jacob Bar-Salibi. The scribe who added it wrote: In 1743, German Protestant Paul Ernst Jablonski argued Christmas was placed on December 25 to correspond with the Roman solar holiday "Dies Natalis Solis Invicti" and was therefore a "paganization" that debased the true church. It has been argued that, on the contrary, the Emperor Aurelian, who in 274 instituted the holiday of the "Dies Natalis Solis Invicti", did so partly as an attempt to give a pagan significance to a date already important for Christians in Rome.  Hermann Usener and others proposed that the Christians chose this day because it was the Roman feast celebrating the birthday of Sol Invictus. Modern scholar S. E. Hijmans, however, states that "While they were aware that pagans called this day the 'birthday' of Sol Invictus, this did not concern them and it did not play any role in their choice

Post-classical history.

In the Early Middle Ages, Christmas Day was overshadowed by Epiphany, which in western Christianity focused on the visit of the magi. But the medieval calendar was dominated by Christmas-related holidays. The forty days before Christmas became the "forty days of St. Martin" (which began on November 11, the feast of St. Martin of Tours), now known as Advent. In Italy, former Saturnalian traditions were attached to Advent. Around the 12th century, these traditions transferred again to the Twelve Days of Christmas (December 25 – January 5); a time that appears in the liturgical calendars as Christmastide or Twelve Holy Days.  The prominence of Christmas Day increased gradually after Charlemagne was crowned Emperor on Christmas Day in 800. King Edmund the Martyr was anointed on Christmas in 855 and King William I of England was crowned on Christmas Day 1066.  By the High Middle Ages, the holiday had become so prominent that chroniclers routinely noted where

Modern history.

Associating it with drunkenness and other misbehaviour, the Puritans banned Christmas in England in the 17th century. It was restored as a legal holiday in 1660, but remained disreputable. In the early 19th century, the Oxford Movement in the Anglican Church ushered in "the development of richer and more symbolic forms of worship, the building of neo-Gothic churches, and the revival and increasing centrality of the keeping of Christmas itself as a Christian festival" as well as "special charities for the poor" in addition to "special services and musical events". Charles Dickens and other writers helped in this revival of the holiday by "changing consciousness of Christmas and the way in which it was celebrated" as they emphasized family, religion, gift-giving, and social reconciliation as opposed to the historic revelry common in some places.

18th century.

Following the Protestant Reformation, many of the new denominations, including the Anglican Church and Lutheran Church, continued to celebrate Christmas. In 1629, the Anglican poet John Milton penned "On the Morning of Christ's Nativity", a poem that has since been read by many during Christmastide. Donald Heinz, a professor at California State University, states that Martin Luther "inaugurated a period in which Germany would produce a unique culture of Christmas, much copied in North America." Among the congregations of the Dutch Reformed Church, Christmas was celebrated as one of the principal evangelical feasts.  However, in 17th century England, some groups such as the Puritans, strongly condemned the celebration of Christmas, considering

19th century.

In the UK, Christmas Day became a bank holiday in 1834. Boxing Day, the day after Christmas, was added in 1871.  In the early-19th century, writers imagined Tudor Christmas as a time of heartfelt celebration. In 1843, Charles Dickens wrote the novel "A Christmas Carol", which helped revive the "spirit" of Christmas and seasonal merriment. Its instant popularity played a major role in portraying Christmas as a holiday emphasizing family, goodwill, and compassion.  Dickens sought to construct Christmas as a family-centered festival of generosity, linking "worship and feasting, within a context of social reconciliation." Superimposing his humanitarian vision of the holiday, in what has been termed "Carol Philosophy", Dickens influenced many aspects of

20th century.

Up to the 1950s in the UK, many Christmas customs were restricted to the upper classes and better-off families. The mass of the population had not adopted many of the Christmas rituals that later became general. The Christmas tree was rare. Christmas dinner might be beef or goose – certainly not turkey. In their stockings children might get an apple, orange, and sweets. Full celebration of a family Christmas with all the trimmings only became widespread with increased prosperity from the 1950s. National papers were published on Christmas Day until 1912. Post was still delivered on Christmas Day until 1961. League football matches continued in Scotland until the 1970s while in England they ceased at the end of the 1950s.  Under the state atheism of the Soviet Union, after its foundation in 1917, Christmas

Observance and traditions.

Christmas Day is celebrated as a major festival and public holiday in countries around the world, including many whose populations are mostly non-Christian. In some non-Christian areas, periods of former colonial rule introduced the celebration (e.g. Hong Kong); in others, Christian minorities or foreign cultural influences have led populations to observe the holiday. Countries such as Japan, where Christmas is popular despite there being only a small number of Christians, have adopted many of the secular aspects of Christmas, such as gift-giving, decorations, and Christmas trees.  Among countries with a strong Christian tradition, a variety of Christmas celebrations have developed that incorporate regional and local cultures.

Church attendance.

Christmas Day (inclusive of its vigil, Christmas Eve), is a Festival in the Lutheran Churches, a holy day of obligation in the Roman Catholic Church, and a Principal Feast of the Anglican Communion. Other Christian denominations do not rank their feast days but nevertheless place importance on Christmas Eve/Christmas Day, as with other Christian

Decorations.

Nativity scenes are known from 10th-century Rome. They were popularised by Saint Francis of Assisi from 1223, quickly spreading across Europe. Different types of decorations developed across the Christian world, dependent on local tradition and available resources, and can vary from simple representations of the crib to far more elaborate sets – renowned manger scene traditions include the colourful "Kraków szopka" in Poland, which imitate Kraków's historical buildings as settings, the elaborate Italian "presepi" (, and ), or the Provençal crèches in southern France, using hand-painted terracotta figurines called "santons". In certain parts of the world, notably Sicily, living nativity scenes following the tradition of Saint Francis are a

Nativity play.

For the Christian celebration of Christmas, the viewing of the Nativity play is one of the oldest Christmastime traditions, with the first reenactment of the Nativity of Jesus taking place in A.D. 1223. In that year, Francis of Assisi assembled a Nativity scene outside of his church in Italy and children sung

Music and carols.

The earliest extant specifically Christmas hymns appear in fourth-century Rome. Latin hymns such as "Veni redemptor gentium", written by Ambrose, Archbishop of Milan, were austere statements of the theological doctrine of the Incarnation in opposition to Arianism. "Corde natus ex Parentis" ("Of the Father's love begotten") by the Spanish poet Prudentius (d. 413) is still sung in some churches today. In the 9th and 10th centuries, the Christmas "Sequence" or "Prose" was introduced in North European monasteries, developing under Bernard of Clairvaux into a sequence of rhymed stanzas. In the 12th century the Parisian monk Adam of St. Victor began to derive music from popular songs, introducing something closer to the traditional Christmas carol.  The songs now known specifically as carols were originally communal folk songs sung during celebrations such as "harvest tide" as well as Christmas. It was only later that carols began to be sung in church. Traditionally, carols have often been based on medieval chord patterns, and it is this that gives them their uniquely characteristic musical sound. Some carols like "Personent hodie", "Good King Wenceslas", and "The Holly and the

Traditional cuisine.

A special Christmas family meal is traditionally an important part of the holiday's celebration, and the food that is served varies greatly from country to country. Some regions have special meals for Christmas Eve, such as Sicily, where 12 kinds of

Cards.

Christmas cards are illustrated messages of greeting exchanged between friends and family members during the weeks preceding Christmas Day. The traditional greeting reads "wishing you a Merry Christmas and a Happy New Year", much like that of the first commercial Christmas card, produced by Sir Henry Cole in London in 1843. The custom of sending them has become popular among a wide cross-section of people with the emergence of the modern trend towards exchanging E-cards.  Christmas cards are purchased in considerable quantities and feature artwork, commercially designed and relevant to the

Commemorative stamps.

A number of nations have issued commemorative stamps at Christmastide. Postal customers will often use these stamps to mail Christmas cards, and they are

Gift giving.

The exchanging of gifts is one of the core aspects of the modern Christmas celebration, making it the most profitable time of year for retailers and businesses throughout the world. On Christmas, people exchange gifts based on the Christian tradition associated with Saint Nicholas, and the gifts of gold, frankincense, and myrrh which were given to the baby Jesus by the Magi. The practice of gift giving in the Roman celebration of Saturnalia may have influenced Christian Christian customs, but on the other hand the Christian "core dogma of the Incarnation, however, solidly established the giving and receiving of gifts as the structural principle of that recurrent yet unique event", because it was the Biblical Magi, "together with all their fellow men, who received the gift of God through man's renewed participation in the divine life."

Gift-bearing figures.

A number of figures are associated with Christmas and the seasonal giving of gifts. Among these are Father Christmas, also known as Santa Claus (derived from the Dutch for Saint Nicholas), Père Noël, and the Weihnachtsmann; Saint Nicholas or Sinterklaas; the Christkind; Kris Kringle; Joulupukki; tomte/nisse; Babbo Natale; Saint Basil; and Ded Moroz. The Scandinavian tomte (also called nisse) is sometimes depicted as a gnome instead of Santa Claus.  The best known of these figures today is red-dressed Santa Claus, of diverse origins. The name Santa Claus can be traced back to the Dutch "Sinterklaas", which means simply Saint Nicholas. Nicholas was a 4th-century Greek bishop of Myra, a city in the Roman province of Lycia, whose ruins are from modern Demre in southwest Turkey. Among other saintly attributes, he was noted for the care of children, generosity, and the giving of gifts. His feast day, December 6, came to be celebrated in many countries with the giving of gifts.  Saint Nicholas traditionally appeared in bishop's attire, accompanied by helpers, inquiring about the behaviour of children during the past year before deciding whether they deserved a gift or not. By the 13th century, Saint Nicholas was well known in the Netherlands, and the practice of gift-giving in his name spread to other parts of central and southern Europe. At the Reformation in 16th–17th-century Europe, many Protestants changed the gift bringer to the Christ Child or "Christkindl", corrupted in English to Kris Kringle, and the

Date according to Julian calendar.

Some jurisdictions of the Eastern Orthodox Church, including those of Russia, Georgia, Ukraine, Macedonia, Montenegro, Serbia, and Jerusalem, mark feasts using the older Julian calendar. As of, there is a difference of 13 days between the Julian calendar and the modern Gregorian calendar, which is used internationally for most secular purposes. As a result, December 25 on the Julian calendar currently corresponds to January 7 on the calendar used by most governments and people in everyday life. Therefore, the aforementioned Orthodox Christians mark December 25 (and thus Christmas) on the day that is internationally considered to be January 7.  However, other Orthodox Christians, such as those belonging to the jurisdictions of Constantinople, Bulgaria, Greece, Romania, Antioch, Alexandria, Albania, Cyprus, Finland, and the Orthodox Church in America, among others, began using the Revised Julian calendar in the early 20th century, which at present corresponds

Economy.

Christmas is typically a peak selling season for retailers in many nations around the world. Sales increase dramatically as people purchase gifts, decorations, and supplies to celebrate. In the United States, the "Christmas shopping season" starts as early as October. In Canada, merchants begin advertising campaigns just before Halloween (October 31), and step up their marketing following Remembrance Day on November 11. In the UK and Ireland, the Christmas shopping season starts from mid-November, around the time when high street Christmas lights are turned on. In the United States, it has been calculated that a quarter of all personal spending takes place during the Christmas/holiday shopping season. Figures from the U.S. Census Bureau reveal that expenditure in department stores nationwide rose from $20.8 billion in November 2004 to $31.9 billion in December 2004, an increase of 54 percent. In other sectors, the pre-Christmas increase in spending was even greater, there being a November–December buying surge of 100 percent in bookstores and 170 percent in jewelry stores. In the same year employment in American retail stores rose from 1.6 million to 1.8 million in the two months leading up to Christmas. Industries completely dependent on Christmas include

Controversies.

Christmas has at times been the subject of controversy and attacks from various sources. Historically it was prohibited by Puritans when they briefly held power in England (1647–1660), and in Colonial America where the Puritans outlawed the celebration of Christmas in 1659. The Parliament of Scotland, which was dominated by Presbyterians, passed a series of acts outlawing the observance of Christmas between 1637 and 1690; Christmas Day did not become a public holiday in Scotland until 1958. Christmas celebrations have also been prohibited by atheist states such as the Soviet Union and more recently majority Muslim states such as Somalia, Tajikistan and Brunei.  Some Christians and organizations such as Pat Robertson's American Center for Law and Justice cite alleged attacks on Christmas (dubbing them a "war on Christmas"). Such groups claim that any specific mention of the term "Christmas" or its religious aspects is being increasingly censored, avoided, or discouraged by a number of advertisers, retailers, government (prominently schools), and other public and private organizations. One controversy is the occurrence of Christmas trees being renamed Holiday trees. In the U.S. there has been a tendency to replace the greeting "Merry Christmas" with "Happy Holidays", which is considered inclusive at the time of the Jewish celebration of Hanukkah, Kwanzaa, and Humanlight. In the U.S. and Canada, where the use of the term "Holidays" is most prevalent, opponents have denounced its usage and avoidance of using the term "Christmas" as being politically correct. In 1984, the U.S. Supreme Court ruled in "Lynch v. Donnelly" that a Christmas display (which included a Nativity scene) owned and displayed by the city of Pawtucket, Rhode Island, did not violate the First Amendment. American Muslim scholar Abdul Malik Mujahid has said that Muslims must treat Christmas with respect, even if they disagree with it.  The government of the People's Republic of China officially espouses state atheism, and has conducted antireligious campaigns to this end. In December 2018, officials raided Christian churches just prior to Christmastide and coerced them to close; Christmas trees and Santa Clauses were also forcibly removed.

Christmas (or Feast of the Nativity) is an annual festival commemorating the birth of Jesus Christ, observed primarily on December 25 as a religious and cultural celebration among billions of people around the world. A feast central to the Christian liturgical year, it is preceded by the season of Advent or the Nativity Fast and initiates the season of Christmastide, which historically in the West lasts twelve days and culminates on Twelfth Night; in some traditions, Christmastide includes an octave. Christmas Day is a public holiday in many of the world's nations, is celebrated religiously by a majority of Christians, as well as culturally by many non-Christians, and forms an integral part of the holiday season centered around it.

圣诞节,又称耶诞节,东正教称基督降生节,是基督教纪念耶稣降生的节日,教会通常将此节日定于12月25日。其为基督教礼仪年历的重要节日,部分教派会透过将临期及圣诞夜来准备,并以八日庆典与圣诞期延续庆祝。圣诞节也是许多国家和地区、尤其是西方国家等以基督教文化为主流之地区的公共假日;在教会以外的场合,圣诞节已转化成一种民俗节日,并常与日期相近的公历新年合称「圣诞及新年季」。


Discovery and naming.

Oberon was discovered by William Herschel on January 11, 1787; on the same day he discovered Uranus's largest moon, Titania. He later reported the discoveries of four more satellites, although they were subsequently revealed as spurious. For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's, although the moon can be seen from Earth with a present-day high-end amateur telescope.  All of the moons of Uranus are named after characters created by William Shakespeare or Alexander Pope. The name Oberon was derived from Oberon, the King of the Fairies in "A Midsummer Night's Dream". The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lassell, who had discovered the other two moons, Ariel and Umbriel, the year before. The adjectival form of the name is "Oberonian,".  Oberon was initially referred to as "the second satellite of Uranus", and in 1848 was given the designation'by William Lassell, although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV). In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet by Roman numerals, and since then Oberon has been designated '.

Orbit.

Oberon orbits Uranus at a distance of about 584,000 km, being the farthest from the planet among its five major moons. Oberon's orbit has a small orbital eccentricity and inclination relative to the equator of Uranus. Its orbital period is around 13.5 days, coincident with its rotational period. In other words, Oberon is a synchronous satellite, tidally locked, with one face always pointing toward the planet. Oberon spends a significant part of its orbit outside the Uranian magnetosphere. As a result, its surface is directly struck by the solar wind. This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by the magnetospheric plasma, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).  Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Oberon) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith over one of the poles at each solstice. The "Voyager 2" flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was in darkness. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. One such event, which lasted for about six minutes, was observed on May 4, 2007, when Oberon occulted Umbriel.

Composition and internal structure.

Oberon is the second largest and most massive of the Uranian moons after Titania, and the ninth most massive moon in the Solar System. Oberon's density of 1.63 g/cm3, which is higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and a dense non-ice component. The latter could be made of rock and carbonaceous material including heavy organic compounds. The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are stronger on Oberon's trailing hemisphere than on the leading hemisphere. This is the opposite of what is observed on other Uranian moons, where the leading hemisphere exhibits stronger water ice signatures. The cause of this asymmetry is not known, but it may be related to impact gardening (the creation of soil via impacts) of the surface, which is stronger on the leading hemisphere. Meteorite impacts tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind. The dark material itself may have formed as a result of radiation processing of methane clathrates or radiation darkening of other organic compounds.  Oberon may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core (480 km) is about 63% of the radius of the moon, and its mass is around 54% of the moon's mass—the proportions are dictated by the moon's composition. The pressure in the center of Oberon is about 0.5 GPa (5 kbar). The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Oberon may possess a liquid ocean layer at the core–mantle boundary. The thickness of this ocean, if it exists, is up to 40 km and its temperature is around 180 K. However, the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present.

Surface features and geology.

Oberon is the second-darkest large moon of Uranus after Umbriel. Its surface shows a strong opposition surge: its reflectivity decreases from 31% at a phase angle of 0° (geometrical albedo) to 22% at an angle of about 1°. Oberon has a low Bond albedo of about 14%. Its surface is generally red in color, except for fresh impact deposits, which are neutral or slightly blue. Oberon is, in fact, the reddest among the major Uranian moons. Its trailing and leading hemispheres are asymmetrical: the latter is much redder than the former, because it contains more dark red material. The reddening of the surfaces is often a result of space weathering caused by bombardment of the surface by charged particles and micrometeorites over the age of the Solar System. However, the color asymmetry of Oberon is more likely caused by accretion of a reddish material spiraling in from outer parts of the Uranian system, possibly from irregular satellites, which would occur predominately on the leading hemisphere.  Scientists have recognized two classes of geological feature on Oberon: craters and chasmata ('canyons'—deep, elongated, steep-sided depressions which would probably be described as rift valleys or escarpments if on Earth). Oberon's surface is the most heavily cratered of all the Uranian moons, with a crater density approaching saturation—when the formation of new craters is balanced by destruction of old ones. This high number of craters indicates that Oberon has the most ancient surface among Uranus's moons. The crater diameters range up to 206 kilometers for the largest known crater, Hamlet. Many large craters are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice. The largest craters, Hamlet, Othello and Macbeth, have floors made of a very dark material deposited after their formation. A peak with a height of about 11 km was observed in some "Voyager" images near the south-eastern limb of Oberon, which may be the central peak of a large impact basin with a diameter of about 375 km. Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania. The canyons' sides are probably scarps produced by normal faults which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later. The most prominent Oberonian canyon is Mommur Chasma.  The geology of Oberon was influenced by two competing forces: impact crater formation and endogenic resurfacing. The former acted over the moon's entire history and is primarily responsible for its present-day appearance. The latter processes were active for a period following the moon's formation. The endogenic processes were mainly tectonic in nature and led to the formation of the canyons, which are actually giant cracks in the ice crust. The canyons obliterated parts of the older surface. The cracking of the crust was caused by the expansion of Oberon by about 0.5%, which occurred in two phases corresponding to the old and young canyons.  The nature of the dark patches, which mainly occur on the leading hemisphere and inside craters, is not known. Some scientists hypothesized that they are of cryovolcanic origin (analogs of lunar maria), while others think that the impacts excavated dark material buried beneath the pure ice (crust). In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior.

Origin and evolution.

Oberon is thought to have formed from an accretion disc or subnebula: a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity. The precise composition of the subnebula is not known; however, the relatively high density of Oberon and other Uranian moons compared to the moons of Saturn indicates that it may have been relatively water-poor. Significant amounts of carbon and nitrogen may have been present in the form of carbon monoxide and N instead of methane and ammonia. The moons that formed in such a subnebula would contain less water ice (with CO and N trapped as clathrate) and more rock, explaining the higher density.  Oberon's accretion probably lasted for several thousand years. The impacts that accompanied accretion caused heating of the moon's outer layer. The maximum temperature of around 230 K was reached at the depth of about 60 km. After the end of formation, the subsurface layer cooled, while the interior of Oberon heated due to decay of radioactive elements present in its rocks. The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust leading to cracking. The present-day system of canyons may be a result of this process, which lasted for about 200 million years, implying that any endogenous activity from this cause ceased billions of years ago.  The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice if some antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present. Further melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle. A layer of liquid water ('ocean') rich in dissolved ammonia may have formed at the core–mantle boundary. The eutectic temperature of this mixture is 176 K. If the temperature dropped below this value the ocean would have frozen by now. Freezing of the water would have led to expansion of the interior, which may have also contributed to the formation of canyon-like graben. Still, present knowledge of the evolution of Oberon is very limited.

Exploration.

So far the only close-up images of Oberon have been from the "Voyager 2" probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest approach of "Voyager 2" to Oberon was 470,600 km, the best images of this moon have spatial resolution of about 6 km. The images cover about 40% of the surface, but only 25% of the surface was imaged with a resolution that allows geological mapping. At the time of the flyby the southern hemisphere of Oberon was pointed towards the Sun, so the dark northern hemisphere could not be studied. No other spacecraft has ever visited the Uranian system.

Oberon, also designated, is the outermost major moon of the planet Uranus. It is the second-largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's "A Midsummer Night's Dream". Its orbit lies partially outside Uranus's magnetosphere.

。 天卫四又称为奥伯龙(英语:Oberon)是距离天王星最远的大卫星,其体积和质量在天王星所有卫星中均位列次席,同时也是太阳系质量第九大的卫星。英国天文学家威廉·赫歇尔在1787年首次观测到该卫星。天卫四的名称奥伯龙来自于莎士比亚戏剧《仲夏夜之梦》当中的一个角色。天卫四的公转轨道有一部分位于天王星磁圈之外。


Old World Bronze Age.

Please see the article on Chronology of the ancient Near East for a discussion regarding the accuracy and resolution of dates for events of the 2nd millennium BC in the Near East (Babylon, etc.).

Middle Bronze Age.

Spending much of their energies in trying to recuperate from the chaotic situation that existed at the turn of the millennium, the most powerful civilizations of the time, Egypt and Mesopotamia, turned their attention to more modest goals. The Pharaohs of the Middle Kingdom of Egypt and their contemporary Kings of Babylon, of Amorite origin, brought good governance without much tyranny, and favoured elegant art and architecture. Farther east, the Indus Valley civilization was in a period of decline, possibly as a result of intense, ruinous flooding.  Egypt and Babylonia's military tactics were still based on foot soldiers transporting their equipment on donkeys. Combined with a weak economy and difficulty in maintaining order, this was a fragile situation that crumbled under the pressure of external forces they could not oppose.

Unrest of the 16th century.

About a century before the middle of the millennium, bands of Indo-European invaders came from the Central Asian plains and swept through Western Asia and Northeast Africa. They were riding fast two-wheeled chariots powered by horses, a system of weaponry developed earlier in the context of plains warfare. This tool of war was unknown among the classical civilizations. Egypt and Babylonia's foot soldiers were unable to defend against the invaders: in 1630 BC, the Hyksos swept into the Nile Delta, and in 1595 BC, the Hittites swept into Mesopotamia.

Late Bronze Age.

The people in place were quick to adapt to the new tactics, and a new international situation resulted from the change. Though during most of the second half of the 2nd millennium BC several regional powers competed relentlessly for hegemony, many developments occurred: there was new emphasis on grandiose architecture, new clothing fashions, vivid diplomatic correspondence on clay tablets, renewed economic exchanges, and the New Kingdom of Egypt played the role of the main superpower. Among the great states of the time, only Babylon refrained from taking part in battles, mainly due to its new position as the world's religious and intellectual capital.  The Bronze Age civilization at its final period of time, displayed all its characteristic social traits: low level of urbanization, small cities centered on temples or royal palaces, strict separation of classes between an illiterate mass of peasants and craftsmen, and a powerful military elite, knowledge of writing and education reserved to a tiny minority of scribes, and pronounced aristocratic life.  Near the end of the 2nd millennium BC, new waves of barbarians, this time riding on horseback, wholly destroyed the Bronze Age world, and were to be followed by waves of social changes that marked the beginning of different times. Also contributing to the changes were the Sea Peoples, ship-faring raiders of the Mediterranean.

Significant people.

Most people known by name from this period are kings or emperors:  An exception is may be Sinuhe, protagonist of an Egyptian tale set in the 20th century BC, although the general consensus considers him a fictional character.  Europe is still entirely within the prehistoric era; much of Europe enters the Bronze Age early in the 2nd millennium.  The desiccation of the Sahara is complete. Neolithisation of Sub-Saharan Africa is initiated via expansion from the dried Sahara, reaching West and East Africa. Later in the 2nd millennium, pastoralism and iron metallurgy spread to Central Africa via the Bantu migration.

Languages.

In the history of the Egyptian language, the early 2nd millennium saw a transition from Old Egyptian to Middle Egyptian. As the most used written form of the Ancient Egyptian language, it is frequently (incorrectly) referred to simply as "Hieroglyphics".  The earliest attested Indo-European language, the Hittite language, first appears in cuneiform in the 16th century BC (Anitta text), before disappearing from records in the 13th century BC. Hittite is the best known and the most studied language of the extinct Anatolian branch of Indo-European languages.  The first Northwest Semitic language, Ugaritic, is attested in the 14th century BC. The first fully phonemic script Proto-Canaanite developed from Egyptian hieroglyphs, becoming the Phoenician alphabet by 1200 BC. The Phoenician alphabet was spread throughout the mediterranean by Phoenician maritime traders and become one of the most widely used writing systems in the world, and the parent of virtually all alphabetic writing systems. The Phoenician language is also the first Canaanite language, the Northwest Semitic languages spoken by the ancient peoples of the Canaan region: the Israelites, Phoenicians, Amorites, Ammonites, Moabites and Edomites.  Mycenaean Greek, the most ancient attested form of the Greek language, was used on the Greek mainland, Crete and Cyprus in the Mycenaean period.

The 2nd millennium BC spanned the years 2000 through 1001 BC. In the Ancient Near East, it marks the transition from the Middle to the Late Bronze Age. The Ancient Near Eastern cultures are well within the historical era: The first half of the millennium is dominated by the Middle Kingdom of Egypt and Babylonia. The alphabet develops. At the center of the millennium, a new order emerges with Minoan Greek dominance of the Aegean and the rise of the Hittite Empire. The end of the millennium sees the Bronze Age collapse and the transition to the Iron Age.

前2千纪,或称前第2个千年,标示着人类从青铜时代中期向后期转变。


States.

The mass of positronium is 1.022 MeV, which is twice the electron mass minus the binding energy of a few eV. The ground state of positronium, like that of hydrogen, has two possible configurations depending on the relative orientations of the spins of the electron and the positron.  The "singlet" state,, with antiparallel spins ("S" = 0, "M" = 0) is known as "para"-positronium ("p"-Ps). It has a mean lifetime of 0.125 ns and decays preferentially into two gamma rays with energy of each (in the center-of-mass frame). By detecting these photons the position at which the decay occurred can be determined. This process is used in positron-emission tomography. "Para"-positronium can decay into any even number of photons (2, 4, 6,...), but the probability quickly decreases with the number: the branching ratio for decay into 4 photons is.  "Para-"positronium lifetime in vacuum is approximately  The "triplet" state, S, with parallel spins ("S" = 1, "M" = −1, 0, 1) is known as "ortho"-positronium ("o"-Ps). It has a mean lifetime of, and the leading decay is three gammas. Other modes of decay are negligible; for instance, the five-photons mode has branching ratio of ≈.  "Ortho"-positronium lifetime in vacuum can be calculated approximately as:  However more accurate calculations with corrections to O(α2) yield a value of 7.040 μs for the decay rate, corresponding to a lifetime of.  Positronium in the 2S state is metastable having a lifetime of against annihilation. The positronium created in such an excited state will quickly cascade down to the ground state, where annihilation will occur more quickly.

Measurements.

Measurements of these lifetimes and energy levels have been used in precision tests of quantum electrodynamics, confirming quantum electrodynamics (QED) predictions to high precision.  Annihilation can proceed via a number of channels, each producing gamma rays with total energy of (sum of the electron and positron mass-energy), usually 2 or 3, with up to 5 gamma ray photons recorded from a single annihilation.  The annihilation into a neutrino–antineutrino pair is also possible, but the probability is predicted to be negligible. The branching ratio for "o"-Ps decay for this channel is (electron neutrino–antineutrino pair) and (for other flavour) in predictions based on the Standard Model, but it can be increased by non-standard neutrino properties, like relatively high magnetic moment. The experimental upper limits on branching ratio for this decay (as well as for a decay into any "invisible" particles) are < for "p"-Ps and < for "o"-Ps.

Energy levels.

While precise calculation of positronium energy levels uses the Bethe–Salpeter equation or the Breit equation, the similarity between positronium and hydrogen allows a rough estimate. In this approximation, the energy levels are different because of a different effective mass, "m"*, in the energy equation (see electron energy levels for a derivation):  where:  Thus, for positronium, its reduced mass only differs from the electron by a factor of 2. This causes the energy levels to also roughly be half of what they are for the hydrogen atom.  So finally, the energy levels of positronium are given by  The lowest energy level of positronium () is −6.8 electronvolts (eV). The next level is. The negative sign is a convention that implies a bound state. Positronium can also be considered by a particular form of the two-body Dirac equation; Two particles with a Coulomb interaction can be exactly separated in the (relativistic) center-of-momentum frame and the resulting ground-state energy has been obtained very accurately using finite element methods of J. Shertzer and confirmed more recently. The Dirac equation whose Hamiltonian comprises two Dirac particles and a static Coulomb potential is not relativistically invariant. But if one adds the (or, where is the fine-structure constant) terms, where, then the result is relativistically invariant. Only the leading term is included. The contribution is the Breit term; workers rarely go to because at one has the Lamb shift, which requires quantum electrodynamics.

History.

Stjepan Mohorovičić predicted the existence of positronium in a 1934 article published in "Astronomische Nachrichten", in which he called it the "electrum". Other sources credit Carl Anderson as having predicted its existence in 1932 while at Caltech. It was experimentally discovered by Martin Deutsch at MIT in 1951 and became known as positronium. Many subsequent experiments have precisely measured its properties and verified predictions of quantum electrodynamics. There was a discrepancy known as the ortho-positronium lifetime puzzle that persisted for some time, but was eventually resolved with further calculations and measurements. Measurements were in error because of the lifetime measurement of unthermalised positronium, which was only produced at a small rate. This had yielded lifetimes that were too long. Also calculations using relativistic quantum electrodynamics are difficult to perform, so they had been done to only the first order. Corrections that involved higher orders were then calculated in a non-relativistic quantum electrodynamics.

Exotic compounds.

Molecular bonding was predicted for positronium. Molecules of positronium hydride (PsH) can be made. Positronium can also form a cyanide and can form bonds with halogens or lithium.  The first observation of di-positronium molecules—molecules consisting of two positronium atoms—was reported on 12 September 2007 by David Cassidy and Allen Mills from University of California, Riverside.

Natural occurrence.

Positronium in high energy states has been predicted to be the dominant form of atomic matter in the universe in the far future if proton decay occurs. Natural formation of positronium atoms is predicted to begin in about years. These atoms are postulated to be far larger than the present observable universe, with estimated radii of 1 quintillion parsecs (about 3.1 · meters). Due to their immense sizes, natural positronium atoms would have very long lifetimes, estimated at years.

Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states. The energy levels of the two particles are similar to that of the hydrogen atom (which is a bound state of a proton and an electron). However, because of the reduced mass, the frequencies of the spectral lines are less than half of those for the corresponding hydrogen lines.

电子偶素或称正电子素(英语:Positronium)是一个电子与一个正电子组成的亚稳态的束缚态(ee),化学符号是Ps。最早由麻省理工学院物理学家英语:Martin Deutsch在1951年发现。


Limits.

The frequency boundaries of the radio spectrum are a matter of convention in physics and are somewhat arbitrary. Since radio waves are the lowest frequency category of electromagnetic waves, there is no lower limit to the frequency of radio waves. At the high frequency end the radio spectrum is bounded by the infrared band. The boundary between radio waves and infrared waves is defined at different frequencies in different scientific fields. The terahertz band, from 300 gigahertz to 3 terahertz, can be considered either as microwaves or infrared. It is the highest band categorized as radio waves by the International Telecommunications Union, but spectroscopic scientists consider these frequencies part of the far infrared band.  The "practical" limits of the radio spectrum, the frequencies which are useful practically for radio communication, are determined by technological limitations which are unlikely to be overcome. So although the radio spectrum is becoming increasingly congested, there is little prospect of additional frequency bandwidth outside that currently in use becoming available.  The lowest frequencies used for radio communication are limited by the increasing size of transmitting antennas required. The size of antenna required to radiate radio power efficiently increases in proportion to wavelength or inversely with frequency. Below about 10 kHz (a wavelength of 30 km) elevated wire antennas kilometers in diameter are required, so very few radio systems use frequencies below this. A second limit is the decreasing bandwidth available at low frequencies, which limits the data rate that can be transmitted. Below about 30 kHz audio modulation is impractical and only slow baud rate data communication is used. The lowest frequencies that have been used for radio communication are around 80 Hz, in ELF submarine communications systems built by a few nations' navies to communicate with their submerged submarines hundreds of meters underwater. These employ huge ground dipole antennas 20–60 km long excited by megawatts of transmitter power.  The highest frequencies useful for radio communication are limited by the absorption of microwave energy by the atmosphere. As frequency increases above 30 GHz (the beginning of the millimeter wave band), atmospheric gases absorb increasing amounts of power, so the power in a beam of radio waves decreases exponentially with distance from the transmitting antenna. At 30 GHz useful communication is limited to about 1 km, but as frequency increases the range at which the waves can be received decreases. At 300 GHz the radio waves are attenuated to zero within a few meters, so the atmosphere is essentially opaque.

Bands.

A radio band is a small contiguous section of the radio spectrum frequencies, in which channels are usually used or set aside for the same purpose. To prevent interference and allow for efficient use of the radio spectrum, similar services are allocated in bands. For example, broadcasting, mobile radio, or navigation devices, will be allocated in non-overlapping ranges of frequencies.  For each of these bands the ITU has a bandplan which dictates how it is to be used and shared, to avoid interference and to set protocol for the compatibility of transmitters and receivers.

ITU.

As a matter of convention, the ITU divides the radio spectrum into 12 bands, each beginning at a wavelength which is a power of ten (10) metres, with corresponding frequency of 3×10 hertz, and each covering a decade of frequency or wavelength. Each of these bands has a traditional name. For example, the term "high frequency" (HF) designates the wavelength range from 100 to 10 metres, corresponding to a frequency range of 3 MHz to 30 MHz. This is just a naming convention and is not related to allocation; the ITU further divides each band into subbands allocated to different uses. Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque, until it becomes transparent again in the near-infrared and optical window frequency ranges.  These "ITU radio bands" are defined in the ITU Radio Regulations. Article 2, provision No. 2.1 states that "the radio spectrum shall be subdivided into nine frequency bands, which shall be designated by progressive whole numbers in accordance with the following table."  The table originated with a recommendation of the IVth CCIR meeting, held in Bucharest in 1937, and was approved by the International Radio Conference held at Atlantic City, NJ in 1947. The idea to give each band a number, in which the number is the logarithm of the approximate geometric mean of the upper and lower band limits in Hz, originated with B.C. Fleming-Williams, who suggested it in a letter to the editor of "Wireless Engineer" in 1942. (For example, the approximate geometric mean of Band 7 is 10 MHz, or 10 Hz.)

IEEE radar bands.

Frequency bands in the microwave range are designated by letters. This convention began around World War 2 with military designations for frequencies used in radar, which was the first application of microwaves. Unfortunately there are several incompatible naming systems for microwave bands, and even within a given system the exact frequency range designated by a letter may vary somewhat between different application areas. One widely used standard is the "IEEE radar bands" established by the US Institute of Electrical and Electronics Engineers.

Applications.

Broadcasting.

Broadcast frequencies:  Designations for television and FM radio broadcast frequencies vary between countries, see Television channel frequencies and FM broadcast band. Since VHF and UHF frequencies are desirable for many uses in urban areas, in North America some parts of the former television broadcasting band have been reassigned to cellular phone and various land mobile communications systems. Even within the allocation still dedicated to television, TV-band devices use channels without local broadcasters.  The Apex band in the United States was a pre-WWII allocation for VHF audio broadcasting; it was made obsolete after the introduction of FM broadcasting.

Air band.

Airband refers to VHF frequencies 118 to 137 MHz, used for navigation and voice communication with aircraft. Trans-oceanic aircraft also carry HF radio and satellite transceivers.

Marine band.

The greatest incentive for development of radio was the need to communicate with ships out of visual range of shore. From the very early days of radio, large oceangoing vessels carried powerful long-wave and medium-wave transmitters. High-frequency allocations are still designated for ships, although satellite systems have taken over some of the safety applications previously served by 500 kHz and other frequencies. 2182 kHz is a medium-wave frequency still used for marine emergency communication.  Marine VHF radio is used in coastal waters and relatively short-range communication between vessels and to shore stations. Radios are channelized, with different channels used for different purposes; marine Channel 16 is used for calling and emergencies.

Amateur radio frequencies.

Amateur radio frequency allocations vary around the world. Several bands are common for amateurs worldwide, usually in the HF part of the spectrum. Other bands are national or regional allocations only due to differing allocations for other services, especially in the VHF and UHF parts of the radio spectrum.

Citizens' band and personal radio services.

Citizens' band radio is allocated in many countries, using channelized radios in the upper HF part of the spectrum (around 27 MHz). It is used for personal, small business and hobby purposes. Other frequency allocations are used for similar services in different jurisdictions, for example UHF CB is allocated in Australia. A wide range of personal radio services exist around the world, usually emphasizing short-range communication between individuals or for small businesses, simplified license requirements or in some countries covered by a class license, and usually FM transceivers using around 1 watt or less.

Industrial, scientific, medical.

The ISM bands were initially reserved for non-communications uses of RF energy, such as microwave ovens, radio-frequency heating, and similar purposes. However, in recent years the largest use of these bands has been by short-range low-power communications systems, since users do not have to hold a radio operator's license. Cordless telephones, wireless computer networks, Bluetooth devices, and garage door openers all use the ISM bands. ISM devices do not have regulatory protection against interference from other users of the band.

Land mobile bands.

Bands of frequencies, especially in the VHF and UHF parts of the spectrum, are allocated for communication between fixed base stations and land mobile vehicle-mounted or portable transceivers. In the United States these services are informally known as business band radio. See also Professional mobile radio.  Police radio and other public safety services such as fire departments and ambulances are generally found in the VHF and UHF parts of the spectrum. Trunking systems are often used to make most efficient use of the limited number of frequencies available. The demand for mobile telephone service has led to large blocks of radio spectrum allocated to cellular frequencies.

Radio control.

Reliable radio control uses bands dedicated to the purpose. Radio-controlled toys may use portions of unlicensed spectrum in the 27 MHz or 49 MHz bands, but more costly aircraft, boat, or land vehicle models use dedicated radio control frequencies near 72 MHz to avoid interference by unlicensed uses. The 21st century has seen a move to 2.4 gigahertz spread spectrum RC control systems.  Licensed amateur radio operators use portions of the 6-meter band in North America. Industrial remote control of cranes or railway locomotives use assigned frequencies that vary by area.

Radar.

Radar applications use relatively high power pulse transmitters and sensitive receivers, so radar is operated on bands not used for other purposes. Most radar bands are in the microwave part of the spectrum, although certain important applications for meteorology make use of powerful transmitters in the UHF band.

The radio spectrum is the part of the electromagnetic spectrum with frequencies from 30 Hz to 300 GHz. Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particularly in telecommunication. To prevent interference between different users, the generation and transmission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU).

无线电波是频率介于3赫兹和约300吉赫之间的电磁波,也作射频电波,或简称射频、射电。


Acoustics.

Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound, and infrasound. A scientist who works in the field of acoustics is an "acoustician", while someone working in the field of acoustical engineering may be called an "acoustical engineer". An audio engineer, on the other hand, is concerned with the recording, manipulation, mixing, and reproduction of sound.  Applications of acoustics are found in almost all aspects of modern society, subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, bioacoustics, electro-acoustics, environmental noise, musical acoustics, noise control, psychoacoustics, speech, ultrasound, underwater acoustics, and vibration.

Definition.

Sound is defined as "(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a)." Sound can be viewed as a wave motion in air or other elastic media. In this case, sound is a stimulus. Sound can also be viewed as an excitation of the hearing mechanism that results in the perception of sound. In this case, sound is a sensation.

Physics of sound.

Sound can propagate through a medium such as air, water and solids as longitudinal waves and also as a transverse wave in solids (see Longitudinal and transverse waves, below). The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel with the sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the "average" position of the particles over time does not change). During propagation, waves can be reflected, refracted, or attenuated by the medium.  The behavior of sound propagation is generally affected by three things:  When sound is moving through a medium that does not have constant physical properties, it may be refracted (either dispersed or focused).  The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.

Longitudinal and transverse waves.

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. It requires a medium to propagate. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation.  Sound waves may be "viewed" using parabolic mirrors and objects that produce sound.  The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium.

Sound wave properties and characteristics.

Although there are many complexities relating to the transmission of sounds, at the point of reception (i.e. the ears), sound is readily dividable into two simple elements: pressure and time. These fundamental elements form the basis of all sound waves. They can be used to describe, in absolute terms, every sound we hear.  In order to understand the sound more fully, a complex wave such as the one shown in a blue background on the right of this text, is usually separated into its component parts, which are a combination of various sound wave frequencies (and noise).  Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:  Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from to. Sometimes speed and direction are combined as a velocity vector; wave number and direction are combined as a wave vector.  Transverse waves, also known as shear waves, have the additional property, "polarization", and are not a characteristic of sound waves.

Speed of sound.

The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. The first significant effort towards measurement of the speed of sound was made by Isaac Newton. He believed the speed of sound in a particular substance was equal to the square root of the pressure acting on it divided by its density:  This was later proven wrong when found to incorrectly derive the speed. The French mathematician Laplace corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but adiabatic. He added another factor to the equation—"gamma"—and multiplied formula_2 by formula_3, thus coming up with the equation formula_4. Since formula_5, the final equation came up to be formula_6, which is also known as the Newton–Laplace equation. In this equation, "K" is the elastic bulk modulus, "c" is the velocity of sound, and formula_7 is the density. Thus, the speed of sound is proportional to the square root of the ratio of the bulk modulus of the medium to its density.  Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In air at sea level, the speed of sound is approximately using the formula. In fresh water, also at 20 °C, the speed of sound is approximately. In steel, the speed of sound is about. The speed of sound is also slightly sensitive, being subject to a second-order anharmonic effect, to the sound amplitude, which means there are non-linear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array).  If relativistic effects are important, the speed of sound is calculated from the relativistic Euler equations.

Perception of sound.

A distinct use of the term "sound" from its use in physics is that in physiology and psychology, where the term refers to the subject of "perception" by the brain. The field of psychoacoustics is dedicated to such studies. Webster's 1936 dictionary defined sound as: "1. The sensation of hearing, that which is heard; specif.: a. Psychophysics. Sensation due to stimulation of the auditory nerves and auditory centers of the brain, usually by vibrations transmitted in a material medium, commonly air, affecting the organ of hearing. b. Physics. Vibrational energy which occasions such a sensation. Sound is propagated by progressive longitudinal vibratory disturbances (sound waves)." This means that the correct response to the question: "if a tree falls in the forest with no one to hear it fall, does it make a sound?" is "yes", and "no", dependent on whether being answered using the physical, or the psychophysical definition, respectively.  The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound frequencies between approximately 20 Hz and 20,000 Hz (20 kHz), The upper limit decreases with age. Sometimes "sound" refers to only those vibrations with frequencies that are within the hearing range for humans or sometimes it relates to a particular animal. Other species have different ranges of hearing. For example, dogs can perceive vibrations higher than 20 kHz.  As a signal perceived by one of the major senses, sound is used by many species for detecting danger, navigation, predation, and communication. Earth's atmosphere, water, and virtually any physical phenomenon, such as fire, rain, wind, surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, marine and terrestrial mammals, have also developed special organs to produce sound. In some species, these produce song and speech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.  Noise is a term often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal. However, in sound perception it can often be used to identify the source of a sound and is an important component of timbre perception (see above).  Soundscape is the component of the acoustic environment that can be perceived by humans. The acoustic environment is the combination of all sounds (whether audible to humans or not) within a given area as modified by the environment and understood by people, in context of the surrounding environment.  There are, historically, six experimentally separable ways in which sound waves are analysed. They are: pitch, duration, loudness, timbre, sonic texture and spatial location. Some of these terms have a standardised definition (for instance in the ANSI Acoustical Terminology ANSI/ASA S1.1-2013). More recent approaches have also considered temporal envelope and temporal fine structure as perceptually relevant analyses.

Pitch.

Pitch is perceived as how "low" or "high" a sound is and represents the cyclic, repetitive nature of the vibrations that make up sound. For simple sounds, pitch relates to the frequency of the slowest vibration in the sound (called the fundamental harmonic). In the case of complex sounds, pitch perception can vary. Sometimes individuals identify different pitches for the same sound, based on their personal experience of particular sound patterns. Selection of a particular pitch is determined by pre-conscious examination of vibrations, including their frequencies and the balance between them. Specific attention is given to recognising potential harmonics. Every sound is placed on a pitch continuum from low to high. For example: white noise (random noise spread evenly across all frequencies) sounds higher in pitch than pink noise (random noise spread evenly across octaves) as white noise has more high frequency content. Figure 1 shows an example of pitch recognition. During the listening process, each sound is analysed for a repeating pattern (See Figure 1: orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name).

Duration.

Duration is perceived as how "long" or "short" a sound is and relates to onset and offset signals created by nerve responses to sounds. The duration of a sound usually lasts from the time the sound is first noticed until the sound is identified as having changed or ceased. Sometimes this is not directly related to the physical duration of a sound. For example; in a noisy environment, gapped sounds (sounds that stop and start) can sound as if they are continuous because the offset messages are missed owing to disruptions from noises in the same general bandwidth. This can be of great benefit in understanding distorted messages such as radio signals that suffer from interference, as (owing to this effect) the message is heard as if it was continuous. Figure 2 gives an example of duration identification. When a new sound is noticed (see Figure 2, Green arrows), a sound onset message is sent to the auditory cortex. When the repeating pattern is missed, a sound offset messages is sent.

Loudness.

Loudness is perceived as how "loud" or "soft" a sound is and relates to the totalled number of auditory nerve stimulations over short cyclic time periods, most likely over the duration of theta wave cycles. This means that at short durations, a very short sound can sound softer than a longer sound even though they are presented at the same intensity level. Past around 200 ms this is no longer the case and the duration of the sound no longer affects the apparent loudness of the sound. Figure 3 gives an impression of how loudness information is summed over a period of about 200 ms before being sent to the auditory cortex. Louder signals create a greater 'push' on the Basilar membrane and thus stimulate more nerves, creating a stronger loudness signal. A more complex signal also creates more nerve firings and so sounds louder (for the same wave amplitude) than a simpler sound, such as a sine wave.

Timbre.

Timbre is perceived as the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice) and represents the pre-conscious allocation of a sonic identity to a sound (e.g. “it's an oboe!"). This identity is based on information gained from frequency transients, noisiness, unsteadiness, perceived pitch and the spread and intensity of overtones in the sound over an extended time frame. The way a sound changes over time (see figure 4) provides most of the information for timbre identification. Even though a small section of the wave form from each instrument looks very similar (see the expanded sections indicated by the orange arrows in figure 4), differences in changes over time between the clarinet and the piano are evident in both loudness and harmonic content. Less noticeable are the different noises heard, such as air hisses for the clarinet and hammer strikes for the piano.

Sonic texture.

Sonic texture relates to the number of sound sources and the interaction between them. The word 'texture', in this context, relates to the cognitive separation of auditory objects. In music, texture is often referred to as the difference between unison, polyphony and homophony, but it can also relate (for example) to a busy cafe; a sound which might be referred to as 'cacophony'. However texture refers to more than this. The texture of an orchestral piece is very different from the texture of a brass quintet because of the different numbers of players. The texture of a market place is very different from a school hall because of the differences in the various sound sources.

Spatial location.

Spatial location (see: Sound localization) represents the cognitive placement of a sound in an environmental context; including the placement of a sound on both the horizontal and vertical plane, the distance from the sound source and the characteristics of the sonic environment. In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification. This is the main reason why we can pick the sound of an oboe in an orchestra and the words of a single person at a cocktail party.

Sound pressure level.

Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. A square of this difference (i.e., a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of this average provides a root mean square (RMS) value. For example, 1 Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm formula_8 Pa) and (1 atm formula_9 Pa), that is between 101323.6 and 101326.4 Pa. As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or "L" is defined as  Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level matches perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.

Ultrasound.

Ultrasound is sound waves with frequencies higher than 20,000 Hz (or 20 kHz). Ultrasound is not different from "normal" (audible) sound in its physical properties, except in that humans cannot hear it. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.  Ultrasound is commonly used for medical diagnostics such as sonograms.

Infrasound.

Infrasound is sound waves with frequencies lower than 20 Hz. Although sounds of such low frequency are too low for humans to hear, whales, elephants and other animals can detect infrasound and use it to communicate. It can be used to detect volcanic eruptions and is used in some types of music.

In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.

声音是振动产生的声波,通过介质(气体、固体、液体)传播并能被人或动物听觉器官所感知的波动现象。


Letters.

The most comprehensive primary source on Van Gogh is the correspondence between him and his younger brother, Theo. Their lifelong friendship, and most of what is known of Vincent's thoughts and theories of art, are recorded in the hundreds of letters they exchanged from 1872 until 1890. Theo van Gogh was an art dealer and provided his brother with financial and emotional support, and access to influential people on the contemporary art scene.  Theo kept all of Vincent's letters to him; Vincent kept few of the letters he received. After both had died, Theo's widow Johanna arranged for the publication of some of their letters. A few appeared in 1906 and 1913; the majority

Life.

Early years.

Vincent Willem van Gogh was born on 30 March 1853 into a Dutch Reformed Church family in Groot-Zundert, in the predominantly Catholic province of North Brabant in the Netherlands. He was the oldest surviving child of Theodorus van Gogh, a minister of the Dutch Reformed Church, and Anna Cornelia Carbentus. Van Gogh was given the name of his grandfather, and of a brother stillborn exactly a year before his birth. Vincent was a common name in the Van Gogh family: his grandfather, Vincent (1789–1874), who received a degree in theology at the University of Leiden in 1811, had six sons, three of whom became art dealers. This Vincent may have been named after his own great-uncle, a sculptor (1729–1802).  Van Gogh's mother came from a prosperous family in The Hague, and his father was the youngest son of a minister. The two met when Anna's younger sister, Cornelia, married Theodorus's older brother Vincent (Cent). Van Gogh's parents married in May 1851

Etten, Drenthe and The Hague.

Van Gogh returned to Etten in April 1881 for an extended stay with his parents. He continued to draw, often using his neighbours as subjects. In August 1881, his recently widowed cousin, Cornelia "Kee" Vos-Stricker, daughter of his mother's older sister Willemina and Johannes Stricker, arrived for a visit. He was thrilled and took long walks with her. Kee was seven years older than he was, and had an eight-year-old son. Van Gogh surprised everyone by declaring his love to her and proposing marriage. She refused with the words "No, nay, never" (""nooit, neen, nimmer""). After Kee returned to Amsterdam, Van Gogh went to The Hague to try to sell

Emerging artist.

Nuenen and Antwerp (1883–1886).

In Nuenen, Van Gogh focused on painting and drawing. Working outside and very quickly, he completed sketches and paintings of weavers and their cottages. Van Gogh also completed "The Parsonage Garden at Nuenen", which was stolen from the Singer Laren in March 2020. From August 1884, Margot Begemann, a neighbour's daughter ten years his senior, joined him on his forays; she fell in love and he reciprocated, though less enthusiastically. They wanted to marry, but neither side of their families were in favour. Margot was distraught and took an overdose of strychnine, but survived after Van Gogh rushed her to a nearby hospital. On 26 March 1885, his father died of a heart attack.  Van Gogh painted several groups of still

Paris (1886–1888).

Van Gogh moved to Paris in March 1886 where he shared Theo's rue Laval apartment in Montmartre, and studied at Fernand Cormon's studio. In June the brothers took a larger flat at 54 rue Lepic. In Paris, Vincent painted portraits of friends and acquaintances, still life paintings, views of Le Moulin de la Galette, scenes in Montmartre, Asnières and along the Seine. In 1885 in Antwerp he had become interested in Japanese ukiyo-e woodblock prints, and had used them to decorate the walls of his studio; while in Paris he collected hundreds of them. He tried his hand at Japonaiserie, tracing a figure from a reproduction on the cover of the magazine "Paris Illustre", "The Courtesan or Oiran" (1887), after Keisai Eisen, which he then graphically enlarged in a painting.  After seeing the portrait of Adolphe Monticelli at the Galerie Delareybarette, Van Gogh adopted a brighter palette and a bolder attack, particularly in paintings such as his "Seascape at Saintes-Maries" (1888). Two years later, Vincent and Theo paid for the publication of a book on Monticelli paintings, and Vincent bought some of Monticelli's works to add to his collection.  Van Gogh learned about Fernand Cormon's atelier from Theo. He worked at the studio in April and May 1886, where he frequented the circle of the Australian artist John Peter Russell, who painted his portrait in 1886. Van Gogh also met fellow students

Artistic breakthrough.

Arles (1888–89).

Ill from drink and suffering from smoker's cough, in February 1888 Van Gogh sought refuge in Arles. He seems to have moved with thoughts of founding an art colony. The Danish artist Christian Mourier-Petersen became his companion for two months, and at first Arles appeared exotic. In a letter, he described it as a foreign country: "The Zouaves, the brothels, the adorable little Arlésienne going to her First Communion, the priest in his surplice, who looks like a dangerous rhinoceros, the people drinking absinthe, all seem to me creatures from another world."  The time in Arles became one of Van Gogh's more prolific periods: he completed 200 paintings, and more than 100 drawings and watercolours. He was enchanted by the local countryside and light; his works from this period are rich in yellow, ultramarine and mauve. His paintings include harvests, wheat fields and general rural landmarks from the area, including "The Old Mill" (1888), a picturesque structure bordering the wheat fields. This was one of seven canvases sent to Pont-Aven on 4 October 1888 in an exchange of works with Paul Gauguin, Émile Bernard, Charles Laval and others.  The portrayals of Arles are informed by Van

Gauguin's visit (1888).

When Gauguin agreed to visit Arles in 1888, Van Gogh hoped for friendship, and the realisation of his idea of an artists' collective. While waiting, in August he painted "Sunflowers". When Boch visited again, Van Gogh painted a portrait of him, as well as the study "The Poet Against a Starry Sky."  In preparation for Gauguin's visit, Van Gogh bought two beds on advice from the station's postal supervisor Joseph Roulin, whose portrait he painted. On 17 September, he spent his first night in the still sparsely furnished Yellow House. When Gauguin consented to work and live in Arles with him, Van Gogh started to work on the "Décoration for the Yellow House," probably the most

Hospital in Arles (December 1888).

The exact sequence of events which led to Van Gogh's mutilation of his ear is not known. Gauguin stated, 15 years later, that the night followed several instances of physically threatening behaviour. Their relationship was complex, and Theo may have owed money to Gauguin, who was suspicious that the brothers were exploiting him financially. It seems likely that Van Gogh had realised that Gauguin was planning to leave. The following days saw heavy rain, leading to the two men being shut in the Yellow House. Gauguin reported that Van Gogh followed when Gauguin left the house for a walk,

Saint-Rémy (May 1889 – May 1890).

Van Gogh entered the Saint-Paul-de-Mausole asylum on 8 May 1889, accompanied by his caregiver, Frédéric Salles, a Protestant clergyman. Saint-Paul was a former monastery in Saint-Rémy, located less than from Arles, and was run by a former naval doctor, Théophile Peyron. Van Gogh had two cells with barred windows, one of which he used as a studio. The clinic and its garden became the main subjects of his paintings. He made several studies of the hospital's interiors, such as "Vestibule of the Asylum" and "Saint-Rémy (September 1889)". Some of his works from this time are characterised by swirls, such as "The

Auvers-sur-Oise (May–July 1890).

In May 1890, Van Gogh left the clinic in Saint-Rémy to move nearer to both Dr Paul Gachet in the Paris suburb of Auvers-sur-Oise and to Theo. Gachet was an amateur painter and had treated several other artists – Camille Pissarro had recommended him. Van Gogh's first impression was that Gachet was "iller than I am, it seemed to me, or let's say just as much." The painter Charles Daubigny moved to Auvers in 1861, and in turn drew other artists there, including Camille Corot and Honoré Daumier. In July 1890, Van Gogh completed two paintings of "Daubigny's Garden", one of which is likely his final work. During his last weeks at Saint-Rémy, his thoughts returned to "memories of the North", and several of

Death.

On 27 July 1890, aged 37, Van Gogh shot himself in the chest with a 7mm Lefaucheux à broche revolver. There were no witnesses and he died 30 hours after the incident. The shooting may have taken place in the wheat field in which he had been painting, or a local barn. The bullet was deflected by a rib and passed through his chest without doing apparent damage to internal organs – probably stopped by his spine. He was able to walk back to the Auberge Ravoux, where he was attended to by two doctors, but without a surgeon present the bullet could not be removed. The doctors tended to him as best they could, then left him alone in his room, smoking his pipe. The following morning Theo rushed to his brother's side, finding him in good spirits. But within hours Vincent began to fail, suffering from an untreated infection resulting from the wound. He died in the early hours of 29 July. According to Theo, Vincent's last words were: "The sadness will

Style and works.

Artistic development.

Van Gogh drew, and painted with watercolours while at school, but only a few examples survive and the authorship of some has been challenged. When he took up art as an adult, he began at an elementary level. In early 1882, his uncle, Cornelis Marinus, owner of a well-known gallery of contemporary art in Amsterdam, asked for drawings of The Hague. Van Gogh's work did not live up to expectations. Marinus offered a second commission, specifying the subject matter in detail, but was again disappointed with the result. Van Gogh persevered; he experimented with lighting in his studio using variable shutters, and with different drawing materials. For more than a year he worked on single figures – highly elaborate studies in black and white, which at the time gained him only criticism. Later, they were recognised as early masterpieces.  In

Major series.

Van Gogh's stylistic developments are usually linked to the periods he spent living in different places across Europe. He was inclined to immerse himself in local cultures and lighting conditions, although he maintained a highly individual visual outlook throughout. His evolution as an artist was slow, and he was aware of his painterly limitations. He moved home often, perhaps to expose himself to new visual stimuli, and through exposure develop his technical skill. Art historian Melissa McQuillan believes the moves also reflect later stylistic changes, and that Van Gogh used the moves to avoid conflict, and as a coping mechanism for when the idealistic artist was faced with the realities of his then current situation.

Portraits.

The portraits gave Van Gogh his best opportunity to earn. He believed they were "the only thing in painting that moves me deeply and that gives me a sense of the infinite." He wrote to his sister that he wished to paint portraits that would endure, and that he would use colour to capture their emotions and character rather than aiming for photographic realism. Those closest to Van Gogh are mostly

Self-portraits.

Van Gogh created more than 43 self-portraits between 1885 and 1889. They were usually completed in series, such as those painted in Paris in mid-1887, and continued until shortly before his death. Generally the portraits were studies, created during introspective periods when he was reluctant to mix with others, or when he lacked models, and so painted himself.  The self-portraits reflect an unusually high degree of self-scrutiny. Often they were intended to mark important periods in his life; for example, the mid-1887 Paris series were painted at the point where he became aware of Claude Monet, Paul Cezanne and Signac. In "Self-Portrait with Grey Felt Hat", heavy strains of paint spread outwards across the canvas. It is one of his most renowned self-portraits of

Flowers.

Van Gogh painted several landscapes with flowers, including roses, lilacs, irises, and sunflowers. Some reflect his interests in the language of colour, and also in Japanese ukiyo-e. There are two series of dying sunflowers. The first was painted in Paris in 1887 and shows flowers lying on the ground. The second set was completed a year later in Arles and is of bouquets in a vase positioned in early morning light. Both are built from thickly layered paintwork, which, according to the London National Gallery, evoke the "texture of the seed-heads".  In these series, Van Gogh was not preoccupied by his usual interest in filling his paintings with subjectivity and emotion; rather, the two series are intended to display his technical skill and working methods to Gauguin, who was about to visit. The 1888

Cypresses and olives.

Fifteen canvases depict cypresses, a tree he became fascinated with in Arles. He brought life to the trees, which were traditionally seen as emblematic of death. The series of cypresses he began in Arles featured the trees in the distance, as windbreaks in fields; when he was at Saint-Rémy he brought them to the foreground. Vincent wrote to Theo in May 1889: "Cypresses still preoccupy me, I should like to do something with them like my canvases of sunflowers"; he went on to say, "They are beautiful in line and proportion like an Egyptian obelisk."  In mid-1889, and at his sister Wil's request, Van Gogh painted several smaller versions of

Orchards.

The "Flowering Orchards" (also the "Orchards in Blossom") are among the first groups of work completed after Van Gogh's arrival in Arles in February 1888. The 14 paintings are optimistic, joyous and visually expressive of the burgeoning spring. They are delicately sensitive and unpopulated. He painted swiftly, and although he brought to this series a version of Impressionism, a strong sense of personal style began to emerge during this period. The transience of the blossoming trees, and the passing of the season, seemed to align with his sense of impermanence

Wheat fields.

Van Gogh made several painting excursions during visits to the landscape around Arles. He made paintings of harvests, wheat fields and other rural landmarks of the area, including "The Old Mill" (1888); a good example of a picturesque structure bordering the wheat fields beyond. At various points, Van Gogh painted the view from his window – at The Hague, Antwerp, and Paris. These works culminated in "The Wheat Field" series, which depicted the view from his cells in the asylum at Saint-Rémy.  Many of the late paintings are sombre but essentially optimistic and, right up to the

Reputation and legacy.

After Van Gogh's first exhibitions in the late 1880s, his reputation grew steadily among artists, art critics, dealers and collectors. In 1887, André Antoine hung Van Gogh's alongside works of Georges Seurat and Paul Signac, at the Théâtre Libre in Paris; some were acquired by Julien Tanguy. In 1889, his work was described in the journal "Le Moderniste Illustré" by Albert Aurier as characterised by "fire, intensity, sunshine". Ten paintings were shown at the Société des Artistes Indépendants, in Brussels in January 1890. French president Marie François Sadi Carnot was said to have been impressed by Van Gogh's work.  After Van Gogh's death, memorial exhibitions were held in Brussels, Paris, The Hague and Antwerp. His work was shown in several high-profile exhibitions, including six works at "Les XX"; in 1891 there was a retrospective exhibition in Brussels. In 1892, Octave Mirbeau wrote that Van Gogh's suicide was an "infinitely sadder loss for art... even though the populace has not crowded to a magnificent funeral, and poor Vincent van Gogh, whose demise means the extinction of a beautiful flame of genius, has gone to his death as obscure and neglected as he lived."  Theo died in January 1891, removing Vincent's most vocal and well-connected champion. Theo's widow Johanna van Gogh-Bonger was a Dutchwoman in her twenties who had not known either her husband or her brother-in-law very long and who suddenly had to take care of several hundreds of paintings, letters and drawings, as well as her infant son, Vincent Willem van Gogh. Gauguin was not inclined to offer assistance in promoting Van Gogh's reputation, and Johanna's brother Andries Bonger also seemed lukewarm about his work. Aurier, one of Van Gogh's earliest supporters among the critics, died of typhoid fever in 1892 at the age of twenty-seven. In 1892, Émile Bernard organised a small solo show of Van Gogh's paintings in Paris, and Julien Tanguy exhibited his Van Gogh paintings with several consigned from Johanna van Gogh-Bonger. In April 1894, the Durand-Ruel Gallery in Paris agreed to take 10 paintings on consignment from Van Gogh's estate. In 1896, the Fauvist painter Henri Matisse, then an unknown art student, visited John Russell on Belle Île off Brittany. Russell had been a close friend of Van Gogh; he introduced Matisse to the Dutchman's work, and gave him a Van Gogh drawing. Influenced by Van Gogh, Matisse abandoned his earth-coloured palette for bright colours.  In Paris in 1901, a large Van Gogh retrospective was held at the Bernheim-Jeune Gallery, which excited André Derain and Maurice de Vlaminck, and contributed to the emergence of Fauvism. Important group exhibitions took place with the Sonderbund artists in Cologne in 1912, the Armory Show, New York in 1913, and Berlin in 1914. Henk Bremmer was instrumental in teaching and talking about Van Gogh, and introduced Helene Kröller-Müller to Van Gogh's art; she became an avid collector of his work. The early figures in German Expressionism such as Emil Nolde acknowledged a debt to Van Gogh's work. Bremmer assisted Jacob Baart de la Faille, whose "catalogue raisonné" "L'Oeuvre de Vincent van Gogh" appeared in 1928.  Van Gogh's fame reached its first peak in Austria and Germany before World War I, helped by the publication of his letters in three volumes in 1914. His letters are expressive and literate, and have been described as among the foremost 19th-century writings of their kind. These began a compelling mythology of Van Gogh as an intense and dedicated painter who suffered for his art and died young. In 1934, the novelist Irving Stone wrote a biographical novel of Van Gogh's life titled "Lust for Life", based on Van Gogh's letters to Theo. This novel and the 1956 film further enhanced his fame, especially in the United States where Stone surmised only a few hundred people had heard of van Gogh prior to his surprise best-selling book.  In 1957, Francis Bacon based a series of paintings on reproductions of Van Gogh's "The Painter on the Road to Tarascon", the original of which was destroyed during the Second World War. Bacon was inspired by an image he described as "haunting", and regarded Van Gogh as an alienated outsider, a position which resonated with him. Bacon identified with Van Gogh's theories of art and quoted lines written to Theo: "[R]eal painters do not paint things as they are... [T]hey paint them as "they themselves" feel them to be."  Van Gogh's works are among the world's most expensive paintings. Those sold for over US$100 million (today's equivalent) include "Portrait of Dr Gachet"," " and "Irises". The Metropolitan Museum of Art acquired a copy of "Wheat Field with Cypresses" in 1993 for US$57 million. In 2015, "L'Allée des Alyscamps" sold for US$66.3 million at Sotheby's, New York, exceeding its reserve of US$40 million.

Van Gogh Museum.

Van Gogh's nephew and namesake, Vincent Willem van Gogh (1890–1978), inherited the estate after his mother's death in 1925. During the early 1950s he arranged for the publication of a complete edition of the letters presented in four volumes and several languages. He then began negotiations with the Dutch government to subsidise a foundation to purchase and house the entire collection. Theo's son participated in planning the project

Vincent Willem van Gogh (; 30 March 185329 July 1890) was a Dutch post-impressionist painter who is among the most famous and influential figures in the history of Western art. In just over a decade, he created about 2,100 artworks, including around 860 oil paintings, most of which date from the last two years of his life. They include landscapes, still lifes, portraits and self-portraits, and are characterised by bold colours and dramatic, impulsive and expressive brushwork that contributed to the foundations of modern art. He was not commercially successful, and his suicide at 37 came after years of mental illness, depression and poverty.

文森·威廉·梵高( ;,1853年-3月30日-1890年-7月29日),早年曾译梵高柯,荷兰后印象派画家。他是表现主义的先驱,并深深影响了二十世纪艺术,尤其是野兽派与德国表现主义。梵高的作品,如《星夜》、《向日葵》、《有乌鸦的麦田》等,现已跻身于全球最知名的艺术作品的行列。他在2004年票选最伟大的荷兰人当中,排名第十,次于第九伟大的17世纪画家林布兰。