Archaeology, or archeology, is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, biofacts or ecofacts and cultural landscapes. Archaeology can be considered a branch of the humanities. In North America archaeology is a sub-field of anthropology, while in Europe it is viewed as either a discipline in its own right or a sub-field of other disciplines. Archaeologists study human prehistory and history, from the development of the first stone tools at Lomekwi in East Africa 3.3 million years ago up until recent decades. Archaeology is distinct from palaeontology, the study of fossil remains, it is important for learning about prehistoric societies, for whom there may be no written records to study. Prehistory includes over 99% of the human past, from the Paleolithic until the advent of literacy in societies across the world. Archaeology has various goals, which range from understanding culture history to reconstructing past lifeways to documenting and explaining changes in human societies through time.
The discipline involves surveying and analysis of data collected to learn more about the past. In broad scope, archaeology relies on cross-disciplinary research, it draws upon anthropology, art history, ethnology, geology, literary history, semiology, textual criticism, information sciences, statistics, paleography, paleontology and paleobotany. Archaeology developed out of antiquarianism in Europe during the 19th century, has since become a discipline practiced across the world. Archaeology has been used by nation-states to create particular visions of the past. Since its early development, various specific sub-disciplines of archaeology have developed, including maritime archaeology, feminist archaeology and archaeoastronomy, numerous different scientific techniques have been developed to aid archaeological investigation. Nonetheless, archaeologists face many problems, such as dealing with pseudoarchaeology, the looting of artifacts, a lack of public interest, opposition to the excavation of human remains.
The science of archaeology grew out of the older multi-disciplinary study known as antiquarianism. Antiquarians studied history with particular attention to ancient artifacts and manuscripts, as well as historical sites. Antiquarianism focused on the empirical evidence that existed for the understanding of the past, encapsulated in the motto of the 18th-century antiquary, Sir Richard Colt Hoare, "We speak from facts not theory". Tentative steps towards the systematization of archaeology as a science took place during the Enlightenment era in Europe in the 17th and 18th centuries. In Europe, philosophical interest in the remains of Greco-Roman civilization and the rediscovery of classical culture began in the late Middle Age. Flavio Biondo, an Italian Renaissance humanist historian, created a systematic guide to the ruins and topography of ancient Rome in the early 15th century, for which he has been called an early founder of archaeology. Antiquarians of the 16th century, including John Leland and William Camden, conducted surveys of the English countryside, drawing and interpreting the monuments that they encountered.
One of the first sites to undergo archaeological excavation was Stonehenge and other megalithic monuments in England. John Aubrey was a pioneer archaeologist who recorded numerous megalithic and other field monuments in southern England, he was ahead of his time in the analysis of his findings. He attempted to chart the chronological stylistic evolution of handwriting, medieval architecture and shield-shapes. Excavations were carried out by the Spanish military engineer Roque Joaquín de Alcubierre in the ancient towns of Pompeii and Herculaneum, both of, covered by ash during the Eruption of Mount Vesuvius in AD 79; these excavations began in 1748 in Pompeii, while in Herculaneum they began in 1738. The discovery of entire towns, complete with utensils and human shapes, as well the unearthing of frescos, had a big impact throughout Europe. However, prior to the development of modern techniques, excavations tended to be haphazard; the father of archaeological excavation was William Cunnington. He undertook excavations in Wiltshire from around 1798.
Cunnington made meticulous recordings of Neolithic and Bronze Age barrows, the terms he used to categorize and describe them are still used by archaeologists today. One of the major achievements of 19th-century archaeology was the development of stratigraphy; the idea of overlapping strata tracing back to successive periods was borrowed from the new geological and paleontological work of scholars like William Smith, James Hutton and Charles Lyell. The application of stratigraphy to archaeology first took place with the excavations of prehistorical and Bronze Age sites. In the third and fourth decades of the 19th-century, archaeologists like Jacques Boucher de Perthes and Christian Jürgensen Thomsen began to put the artifacts they had found in chronological order. A major figure in the development of archaeology into a rigorous science was the army officer and ethnologist, Augustus Pitt Rivers, who began excavations on his land in England in the 1880s, his approach was methodical by the standards of the time, he is regarded as the first scientific archaeologist.
He arranged his artifacts by type or "typologically, within types by date or "chronologically"
Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, stars, nebulae and comets. More all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, the study of the Universe as a whole. Astronomy is one of the oldest of the natural sciences; the early civilizations in recorded history, such as the Babylonians, Indians, Nubians, Chinese and many ancient indigenous peoples of the Americas, performed methodical observations of the night sky. Astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, but professional astronomy is now considered to be synonymous with astrophysics. Professional astronomy is split into theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, analyzed using basic principles of physics.
Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results. Astronomy is one of the few sciences in which amateurs still play an active role in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets. Astronomy means "law of the stars". Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now distinct. Both of the terms "astronomy" and "astrophysics" may be used to refer to the same subject. Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties," while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, dynamic processes of celestial objects and phenomena."
In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could be called astrophysics; some fields, such as astrometry, are purely astronomy rather than astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics" depending on whether the department is affiliated with a physics department, many professional astronomers have physics rather than astronomy degrees; some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, Astronomy and Astrophysics. In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye.
In some locations, early cultures assembled massive artifacts that had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye; as civilizations developed, most notably in Mesopotamia, Persia, China and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, the nature of the Sun and the Earth in the Universe were explored philosophically; the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Ptolemaic system, named after Ptolemy.
A important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the astronomical traditions that developed in many other civilizations. The Babylonians discovered. Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and inven
History is the study of the past as it is described in written documents. Events occurring before written record are considered prehistory, it is an umbrella term that relates to past events as well as the memory, collection, organization and interpretation of information about these events. Scholars who write about history are called historians. History can refer to the academic discipline which uses a narrative to examine and analyse a sequence of past events, objectively determine the patterns of cause and effect that determine them. Historians sometimes debate the nature of history and its usefulness by discussing the study of the discipline as an end in itself and as a way of providing "perspective" on the problems of the present. Stories common to a particular culture, but not supported by external sources, are classified as cultural heritage or legends, because they do not show the "disinterested investigation" required of the discipline of history. Herodotus, a 5th-century BC Greek historian is considered within the Western tradition to be the "father of history", along with his contemporary Thucydides, helped form the foundations for the modern study of human history.
Their works continue to be read today, the gap between the culture-focused Herodotus and the military-focused Thucydides remains a point of contention or approach in modern historical writing. In East Asia, a state chronicle, the Spring and Autumn Annals was known to be compiled from as early as 722 BC although only 2nd-century BC texts have survived. Ancient influences have helped spawn variant interpretations of the nature of history which have evolved over the centuries and continue to change today; the modern study of history is wide-ranging, includes the study of specific regions and the study of certain topical or thematical elements of historical investigation. History is taught as part of primary and secondary education, the academic study of history is a major discipline in university studies; the word history comes from the Ancient Greek ἱστορία, meaning'inquiry','knowledge from inquiry', or'judge'. It was in that sense; the ancestor word ἵστωρ is attested early on in Homeric Hymns, the Athenian ephebes' oath, in Boiotic inscriptions.
The Greek word was borrowed into Classical Latin as historia, meaning "investigation, research, description, written account of past events, writing of history, historical narrative, recorded knowledge of past events, narrative". History was borrowed from Latin into Old English as stær, but this word fell out of use in the late Old English period. Meanwhile, as Latin became Old French, historia developed into forms such as istorie and historie, with new developments in the meaning: "account of the events of a person's life, account of events as relevant to a group of people or people in general, dramatic or pictorial representation of historical events, body of knowledge relative to human evolution, narrative of real or imaginary events, story", it was from Anglo-Norman that history was borrowed into Middle English, this time the loan stuck. It appears in the 13th-century Ancrene Wisse, but seems to have become a common word in the late 14th century, with an early attestation appearing in John Gower's Confessio Amantis of the 1390s: "I finde in a bok compiled | To this matiere an old histoire, | The which comth nou to mi memoire".
In Middle English, the meaning of history was "story" in general. The restriction to the meaning "the branch of knowledge that deals with past events. With the Renaissance, older senses of the word were revived, it was in the Greek sense that Francis Bacon used the term in the late 16th century, when he wrote about "Natural History". For him, historia was "the knowledge of objects determined by space and time", that sort of knowledge provided by memory. In an expression of the linguistic synthetic vs. analytic/isolating dichotomy, English like Chinese now designates separate words for human history and storytelling in general. In modern German and most Germanic and Romance languages, which are solidly synthetic and inflected, the same word is still used to mean both'history' and'story'. Historian in the sense of a "researcher of history" is attested from 1531. In all European languages, the substantive history is still used to mean both "what happened with men", "the scholarly study of the happened", the latter sense sometimes distinguished with a capital letter, or the word historiography.
The adjective historical is attested from 1661, historic from 1669. Historians write in the context of their own time, with due regard to the current dominant ideas of how to interpret the past, sometimes write to provide lessons for their own society. In the words of Benedetto Croce, "All history is contemporary history". History is facilitated by the formation of a "true discourse of past" through the production of narrative and analysis of past events relating to the human race; the modern discipline of history is dedicated to the institutional production of this discourse. All events that are remembered and preserved in some authentic form constitute the historical record; the task of histori
Oxalic acid is an organic compound with the formula C2H2O4. It is a colorless crystalline solid, its condensed formula is HOOCCOOH. Its acid strength is much greater than that of acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate, is a chelating agent for metal cations. Oxalic acid occurs as the dihydrate with the formula C2H2O4·2H2O, it occurs in many foods, but excessive ingestion of oxalic acid or prolonged skin contact can be dangerous. Its name comes from the fact that early investigators isolated oxalic acid from wood-sorrel flowering plants; the preparation of salts of oxalic acid from plants had been known, at the latest, since 1745, when the Dutch botanist and physician Herman Boerhaave isolated a salt from sorrel. By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel. In 1776, Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman produced oxalic acid by reacting sugar with concentrated nitric acid.
By 1784, Scheele had shown that oxalic acid from natural sources were identical. In 1824, the German chemist Friedrich Wöhler obtained oxalic acid by reacting cyanogen with ammonia in aqueous solution; this experiment may represent the first synthesis of a natural product. Oxalic acid is manufactured by the oxidation of carbohydrates or glucose using nitric acid or air in the presence of vanadium pentoxide. A variety of precursors can be used including glycolic ethylene glycol. A newer method entails oxidative carbonylation of alcohols to give the diesters of oxalic acid: 4 ROH + 4 CO + O2 → 2 2 + 2 H2OThese diesters are subsequently hydrolyzed to oxalic acid. 120,000 tonnes are produced annually. Oxalic acid was obtained by using caustics, such as sodium or potassium hydroxide, on sawdust. Although it can be purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst; the hydrated solid can be dehydrated by azeotropic distillation.
Developed in the Netherlands, an electrocatalysis by a copper complex helps reduce carbon dioxide to oxalic acid. Anhydrous oxalic acid exists as two polymorphs; because the anhydrous material is both acidic and hydrophilic, it is used in esterifications. Oxalic acid is a strong acid, despite being a carboxylic acid: Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids, it forms esters such as dimethyl oxalate. It forms. Oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions, e.g. the drug oxaliplatin. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. At least two pathways exist for the enzyme-mediated formation of oxalate. In one pathway, oxaloacetate, a component of the Krebs citric acid cycle, is hydrolyzed to oxalate and acetic acid by the enzyme oxaloacetase: 2− + H2O → C2O2−4 + CH3CO−2 + H+It arises from the dehydrogenation of glycolic acid, produced by the metabolism of ethylene glycol. Calcium oxalate is the most common component of kidney stones.
Early investigators isolated oxalic acid from wood-sorrel. Members of the spinach family and the brassicas are high in oxalates, as are sorrel and umbellifers like parsley. Rhubarb leaves contain about 0.5% oxalic acid, jack-in-the-pulpit contains calcium oxalate crystals. The Virginia creeper, a common decorative vine, produces oxalic acid in its berries as well as oxalate crystals in the sap, in the form of raphides. Bacteria produce oxalates from oxidation of carbohydrates. Plants of the genus Fenestraria produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites. Carambola known as starfruit contains oxalic acid along with caramboxin; the formation of occurring calcium oxalate patinas on certain limestone and marble statues and monuments has been proposed to be caused by the chemical reaction of the carbonate stone with oxalic acid secreted by lichen or other microorganisms. Oxidized bitumen or bitumen exposed to gamma rays contains oxalic acid among its degradation products.
Oxalic acid may increase the leaching of radionuclides conditioned in bitumen for radioactive waste disposal. The conjugate base of oxalic acid is the hydrogenoxalate anion, its conjugate base is a competitive inhibitor of the lactate dehydrogenase enzyme. LDH catalyses the conversion of pyruvate to lactic acid oxidising the coenzyme NADH to NAD+ and H+ concurrently. Restoring NAD+ levels is essential to the continuation of anaerobic energy metabolism through glycolysis; as cancer cells preferentially use anaerobic metabolism inhibition of LDH has been shown to inhibit tumor formation and growth, thus is an interesting potential course of cancer treatment. About 25% of produced oxalic acid will be used as a mordant in dyeing processes, it is used in bleaches for pulpwood. It is used in baking powder and as a third reagent in silica analysis instruments. Oxalic acid's main applications include cleani
The terms anno Domini and before Christ are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ"; this calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years from the start of this epoch, BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 follows the year 1 BC; this dating system was devised in 525 by Dionysius Exiguus of Scythia Minor, but was not used until after 800. The Gregorian calendar is the most used calendar in the world today. For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication and commercial integration, recognized by international institutions such as the United Nations.
Traditionally, English followed Latin usage by placing the "AD" abbreviation before the year number. However, BC is placed after the year number, which preserves syntactic order; the abbreviation is widely used after the number of a century or millennium, as in "fourth century AD" or "second millennium AD". Because BC is the English abbreviation for Before Christ, it is sometimes incorrectly concluded that AD means After Death, i.e. after the death of Jesus. However, this would mean that the approximate 33 years associated with the life of Jesus would neither be included in the BC nor the AD time scales. Terminology, viewed by some as being more neutral and inclusive of non-Christian people is to call this the Current or Common Era, with the preceding years referred to as Before the Common or Current Era. Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years; the Anno Domini dating system was devised in 525 by Dionysius Exiguus to enumerate the years in his Easter table.
His system was to replace the Diocletian era, used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians. The last year of the old table, Diocletian 247, was followed by the first year of his table, AD 532; when he devised his table, Julian calendar years were identified by naming the consuls who held office that year—he himself stated that the "present year" was "the consulship of Probus Junior", 525 years "since the incarnation of our Lord Jesus Christ". Thus Dionysius implied that Jesus' incarnation occurred 525 years earlier, without stating the specific year during which his birth or conception occurred. "However, nowhere in his exposition of his table does Dionysius relate his epoch to any other dating system, whether consulate, year of the world, or regnal year of Augustus. Among the sources of confusion are: In modern times, incarnation is synonymous with the conception, but some ancient writers, such as Bede, considered incarnation to be synonymous with the Nativity.
The civil or consular year began on 1 January but the Diocletian year began on 29 August. There were inaccuracies in the lists of consuls. There were confused summations of emperors' regnal years, it is not known. Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", hence subtracted thirty years from that date, or that Dionysius counted back 532 years from the first year of his new table, it has been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world. At the time, it was believed by some that the resurrection of the dead and end of the world would occur 500 years after the birth of Jesus; the old Anno Mundi calendar theoretically commenced with the creation of the world based on information in the Old Testament.
It was believed that, based on the Anno Mundi calendar, Jesus was born in the year 5500 with the year 6000 of the Anno Mundi calendar marking the end of the world. Anno Mundi 6000 was thus equated with the resurrection and the end of the world but this date had passed in the time of Dionysius; the Anglo-Saxon historian the Venerable Bede, familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, completed in 731. In this same history, he used another Latin term, ante vero incarnationis dominicae tempus anno sexagesimo, equivalent to the English "before Christ", to identify years before the first year of this era. Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e. the Annunciation on March 25". On the continent of Europe, Anno
An ice core is a core sample, removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper, an ice core contains ice formed over a range of years. Cores are powered drills; the physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core. The proportions of different oxygen and hydrogen isotopes provide information about ancient temperatures, the air trapped in tiny bubbles can be analysed to determine the level of atmospheric gases such as carbon dioxide. Since heat flow in a large ice sheet is slow, the borehole temperature is another indicator of temperature in the past; these data can be combined to find the climate model. Impurities in ice cores may depend on location. Coastal areas are more to include material of marine origin, such as sea salt ions. Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past, when cold deserts were scoured by wind.
Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice. Some volcanic events that were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronise their time scales. Ice cores have been studied since the early 20th century, several cores were drilled as a result of the International Geophysical Year. Depths of over 400 m were reached, a record, extended in the 1960s to 2164 m at Byrd Station in Antarctica. Soviet ice drilling projects in Antarctica include decades of work at Vostok Station, with the deepest core reaching 3769 m. Numerous other deep cores in the Antarctic have been completed over the years, including the West Antarctic Ice Sheet project, cores managed by the British Antarctic Survey and the International Trans-Antarctic Scientific Expedition. In Greenland, a sequence of collaborative projects began in the 1970s with the Greenland Ice Sheet Project.
An ice core is a vertical column through a glacier, sampling the layers that formed through an annual cycle of snowfall and melt. As snow accumulates, each layer presses on lower layers, making them denser until they turn into firn. Firn is not dense enough to prevent air from escaping; the depth at which this occurs varies with location, but in Greenland and the Antarctic it ranges from 64 m to 115 m. Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal. At Summit Camp in Greenland, the depth is 77 m and the ice is 230 years old; as further layers build up, the pressure increases, at about 1500 m the crystal structure of the ice changes from hexagonal to cubic, allowing air molecules to move into the cubic crystals and form a clathrate. The bubbles disappear and the ice becomes more transparent. Two or three feet of snow may turn into less than a foot of ice; the weight above makes deeper layers of ice flow outwards. Ice is lost at the edges of the glacier to icebergs, or to summer melting, the overall shape of the glacier does not change much with time.
The outward flow can distort the layers, so it is desirable to drill deep ice cores at places where there is little flow. These can be located using maps of the flow lines. Impurities in the ice provide information on the environment from; these include soot and other types of particle from forest fires and volcanoes. The lowest layer of a glacier, called basal ice, is formed of subglacial meltwater that has refrozen, it can be up to about 20 m thick, though it has scientific value, it does not retain stratigraphic information. Cores are drilled in areas such as Antarctica and central Greenland where the temperature is never warm enough to cause melting, but the summer sun can still alter the snow. In polar areas, the sun night during the local summer and invisible all winter, it can make some snow sublimate, leaving so less dense. When the sun approaches its lowest point in the sky, the temperature drops and hoar frost forms on the top layer. Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow.
As a result, alternating bands of lighter and darker ice can be seen in an ice core. Ice cores are collected by cutting around a cylinder of ice in a way that enables it to be brought to the surface. Early cores were collected with hand augers and they are still used for short holes. A design for ice core augers was patented in 1932 and they have changed little since. An auger is a cylinder with helical metal ribs wrapped around the outside, at the lower end of which are cutting blades. Hand augers can be rotated by a T handle or a brace handle, some can be attached to handheld electric drills to power the rotation. With the aid of a tripod for lowering and raising the auger, cores up to 50 m deep can be retri
Chronology is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events, it is "the determination of the actual temporal sequence of past events". Chronology is a part of periodization, it is a part of the discipline of history including earth history, the earth sciences, study of the geologic time scale. Chronology is the science of locating historical events in time, it relies upon chronometry, known as timekeeping, historiography, which examines the writing of history and the use of historical methods. Radiocarbon dating estimates the age of living things by measuring the proportion of carbon-14 isotope in their carbon content. Dendrochronology estimates the age of trees by correlation of the various growth rings in their wood to known year-by-year reference sequences in the region to reflect year-to-year climatic variation. Dendrochronology is used in turn as a calibration reference for radiocarbon dating curves.
The familiar terms calendar and era concern two complementary fundamental concepts of chronology. For example, during eight centuries the calendar belonging to the Christian era, which era was taken in use in the 8th century by Bede, was the Julian calendar, but after the year 1582 it was the Gregorian calendar. Dionysius Exiguus was the founder of that era, nowadays the most widespread dating system on earth. An epoch is the date. Ab Urbe condita is Latin for "from the founding of the City", traditionally set in 753 BC, it was used to identify the Roman year by a few Roman historians. Modern historians use it much more than the Romans themselves did. Before the advent of the modern critical edition of historical Roman works, AUC was indiscriminately added to them by earlier editors, making it appear more used than it was, it was used systematically for the first time only about the year 400, by the Iberian historian Orosius. Pope Boniface IV, in about the year 600, seems to have been the first who made a connection between these this era and Anno Domini.
Dionysius Exiguus’ Anno Domini era was extended by Bede to the complete Christian era. Ten centuries after Bede, the French astronomers Philippe de la Hire and Jacques Cassini, purely to simplify certain calculations, put the Julian Dating System and with it an astronomical era into use, which contains a leap year zero, which precedes the year 1. While of critical importance to the historian, methods of determining chronology are used in most disciplines of science astronomy, geology and archaeology. In the absence of written history, with its chronicles and king lists, late 19th century archaeologists found that they could develop relative chronologies based on pottery techniques and styles. In the field of Egyptology, William Flinders Petrie pioneered sequence dating to penetrate pre-dynastic Neolithic times, using groups of contemporary artefacts deposited together at a single time in graves and working backwards methodically from the earliest historical phases of Egypt; this method of dating is known as seriation.
Known wares discovered at strata in sometimes quite distant sites, the product of trade, helped extend the network of chronologies. Some cultures have retained the name applied to them in reference to characteristic forms, for lack of an idea of what they called themselves: "The Beaker People" in northern Europe during the 3rd millennium BCE, for example; the study of the means of placing pottery and other cultural artifacts into some kind of order proceeds in two phases and typology: Classification creates categories for the purposes of description, typology seeks to identify and analyse changes that allow artifacts to be placed into sequences. Laboratory techniques developed after mid-20th century helped revise and refine the chronologies developed for specific cultural areas. Unrelated dating methods help reinforce a chronology, an axiom of corroborative evidence. Ideally, archaeological materials used for dating a site should complement each other and provide a means of cross-checking. Conclusions drawn from just one unsupported technique are regarded as unreliable.
The fundamental problem of chronology is to synchronize events. By synchronizing an event it becomes possible to relate it to the current time and to compare the event to other events. Among historians, a typical need to is to synchronize the reigns of kings and leaders in order to relate the history of one country or region to that of another. For example, the Chronicon of Eusebius is one of the major works of historical synchronism; this work has two sections. The first contains narrative chronicles of nine different kingdoms: Chaldean, Median, Persian, Greek, Peloponnesian and Roman; the second part is a long table synchronizing the events from each of the nine kingdoms in parallel columns. The adjacent image shows two pages from the second section. By comparing the parallel columns, the reader can determine which events were contemporaneous, or how many years separated two different events. To place all the events on the same time scale, Eusebius used an Anno Mundi era, meaning that events were dated from the supposed beginning of t