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
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate, its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y. Zircon forms in silicate melts with large proportions of high field strength incompatible elements. For example, hafnium is always present in quantities ranging from 1 to 4%; the crystal structure of zircon is tetragonal crystal system. The natural color of zircon varies between colorless, yellow-golden, brown and green. Colorless specimens that show gem quality are a popular substitute for diamond and are known as "Matura diamond"; the name derives from the Persian zargun, meaning "gold-hued". This word is corrupted into "jargoon", a term applied to light-colored zircons; the English word "zircon" is derived from Zirkon, the German adaptation of this word. Yellow and red zircon is known as "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin.
Zircon is ubiquitous in the crust of Earth. It occurs as a common accessory mineral in igneous rocks, in metamorphic rocks and as detrital grains in sedimentary rocks. Large zircon crystals are rare, their average size in granite rocks is about 0.1–0.3 mm, but they can grow to sizes of several centimeters in mafic pegmatites and carbonatites. Zircon is very resistant to heat and corrosion; because of their uranium and thorium content, some zircons undergo metamictization. Connected to internal radiation damage, these processes disrupt the crystal structure and explain the variable properties of zircon; as zircon becomes more and more modified by internal radiation damage, the density decreases, the crystal structure is compromised, the color changes. Zircon occurs in many colors, including reddish brown, green, blue and colorless; the color of zircons can sometimes be changed by heat treatment. Common brown zircons can be transformed into colorless and blue zircons by heating to 800 to 1000 °C. In geological settings, the development of pink and purple zircon occurs after hundreds of millions of years, if the crystal has sufficient trace elements to produce color centers.
Color in this red or pink series is annealed in geological conditions above temperatures of around 400 °C. Zircon is consumed as an opacifier, has been known to be used in the decorative ceramics industry, it is the principal precursor not only to metallic zirconium, although this application is small, but to all compounds of zirconium including zirconium dioxide, one of the most refractory materials known. Other applications include use in refractories and foundry casting and a growing array of specialty applications as zirconia and zirconium chemicals, including in nuclear fuel rods, catalytic fuel converters and in water and air purification systems. Zircon is one of the key minerals used by geologists for geochronology. Zircon is a part of the ZTR index to classify highly-weathered sediments. Zircon is a common accessory to trace mineral constituent of most felsic igneous rocks. Due to its hardness and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands.
Zircon is rare within mafic rocks and rare within ultramafic rocks aside from a group of ultrapotassic intrusive rocks such as kimberlites and lamprophyre, where zircon can be found as a trace mineral owing to the unusual magma genesis of these rocks. Zircon forms economic concentrations within heavy mineral sands ore deposits, within certain pegmatites, within some rare alkaline volcanic rocks, for example the Toongi Trachyte, New South Wales Australia in association with the zirconium-hafnium minerals eudialyte and armstrongite. Australia leads the world in zircon mining, producing 37% of the world total and accounting for 40% of world EDR for the mineral. South Africa is Africa’s main producer, with 30% of world production, second after Australia. Zircon has played an important role during the evolution of radiometric dating. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques; because zircons can survive geologic processes like erosion, transport high-grade metamorphism, they contain a rich and varied record of geological processes.
Zircons are dated by uranium-lead, fission-track, cathodoluminescence, U+Th/He techniques. For instance, imaging the cathodoluminescence emission from fast electrons can be used as a prescreening tool for high-resolution secondary-ion-mass spectrometry to image the zonation pattern and identify regions of interest for isotope analysis; this is done scanning electron microscope. Zircons in sedimentary rock can identify the sediment source. Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth. In addition, the oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.4 billion years ago there was water on the surface of the Earth. This interpretation is supported by additional trace element data, but is the subject of debate. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in the Jack Hills of Western Australia.
According to one of the researchers, "If life arose quickly on Earth... it could be common in the universe
Titanite, or sphene, is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are present. Present are rare earth metals including cerium and yttrium; the International Mineralogical Association Commission on New Minerals and Mineral Names adopted the name titanite and "discredited" the name sphene as of 1982, although papers and books identify the mineral using both names. Sphene was the most used name until the IMA decision, although both were well known; some authorities think it is less confusing as the word is used to describe any chemical or crystal with oxidized titanium such as the rare earth titanate pyrochlores series and many of the minerals with the perovskite structure. The name sphene continues to be publishable in peer-reviewed scientific literature, e.g. a paper by Hayden et al. was published in early 2008 in the journal Contributions to Mineralogy and Petrology. Sphene persists as the informal name for titanite gemstones. Titanite, named for its titanium content, occurs as translucent to transparent, reddish brown, yellow, green, or red monoclinic crystals.
These crystals are sphenoid in habit and are twinned. Possessing a subadamantine tending to resinous luster, titanite has a hardness of 5.5 and a weak cleavage. Its specific gravity varies between 3.52 and 3.54. Titanite's refractive index is 1.885–1.990 to 1.915–2.050 with a strong birefringence of 0.105 to 0.135. Transparent specimens are noted for their strong trichroism, the three colours presented being dependent on body colour. Owing to the quenching effect of iron, sphene exhibits no fluorescence under ultraviolet light; some titanite has been found to be metamict, in consequence of structural damage due to radioactive decomposition of the significant thorium content. When viewed in thin section with a petrographic microscope, pleochroic halos can be observed in minerals surrounding a titanite crystal. Titanite occurs as a common accessory mineral in intermediate and felsic igneous rocks and associated pegmatites, it occurs in metamorphic rocks such as gneiss and schists and skarns. Source localities include: Pakistan.
Titanite is a source of TiO2, used in pigments. As a gemstone, titanite is some shade of chartreuse, but can be brown or black. Hue depends on Fe content, with low Fe content causing green and yellow colours, high Fe content causing brown or black hues. Zoning is typical in titanite, it is prized for its exceptional dispersive power. Jewelry use of titanite is limited, both because the stone is uncommon in gem quality and is soft. Titanite can be used as a U-Pb geochronometer in metamorphic terranes. Madagascar Sphene Properties and Geological data
Thorium is a weakly radioactive metallic chemical element with symbol Th and atomic number 90. Thorium is tarnishes black when it is exposed to air, forming thorium dioxide. Thorium is an electropositive actinide. All known thorium isotopes are unstable; the most stable isotope, 232Th, has a half-life of 14.05 billion years, or about the age of the universe. In the universe, thorium and uranium are the only three radioactive elements that still occur in large quantities as primordial elements, it is estimated to be over three times as abundant as uranium in the Earth's crust, is chiefly refined from monazite sands as a by-product of extracting rare-earth metals. Thorium was discovered in 1829 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder, its first applications were developed in the late 19th century. Thorium's radioactivity was acknowledged during the first decades of the 20th century.
In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity. Thorium is still being used as an alloying element in TIG welding electrodes but is being replaced in the field with different compositions, it was material in high-end optics and scientific instrumentation, as the light source in gas mantles, but these uses have become marginal. It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, several thorium reactors have been built. Thorium is a moderately soft, bright silvery radioactive actinide metal. In the periodic table, it lies to the right of actinium, to the left of protactinium, below cerium. Pure thorium is ductile and, as normal for metals, can be cold-rolled and drawn. At room temperature, thorium metal has a face-centred cubic crystal structure. Thorium metal has a bulk modulus of about the same as tin's. Aluminium's is 75.2 GPa. Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire.
Thorium is harder than either of them. It becomes superconductive below 1.4 K. Thorium's melting point of 1750 °C is above both those of actinium and protactinium. At the start of period 7, from francium to thorium, the melting points of the elements increase, because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f electrons increases from about 0.4 to about 6: this trend is due to the increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding. Among the actinides up to californium, which can be studied in at least milligram quantities, thorium has the highest melting and boiling points and second-lowest density.
Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points. The properties of thorium vary depending on the degree of impurities in the sample; the major impurity is thorium dioxide. Experimental measurements of its density give values between 11.5 and 11.66 g/cm3: these are lower than the theoretically expected value of 11.7 g/cm3 calculated from thorium's lattice parameters due to microscopic voids forming in the metal when it is cast. These values lie between those of its neighbours actinium and protactinium, part of a trend across the early actinides. Thorium can form alloys with many other metals. Addition of small proportions of thorium improves the mechanical strength of magnesium, thorium-aluminum alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors. Thorium forms eutectic mixtures with chromium and uranium, it is miscible in both solid and liquid states with its lighter congener cerium. All but two elements up to bismuth have an isotope, stable for all purposes, with the exceptions being technetium and promethium.
All elements from polonium onward are measurably radioactive. 232Th is one of the three nuclides beyond bismuth that have half-lives measured in billions of years. Four-fifths of the thorium present at Earth's formation has survived to the present. 232Th is the only isotope of thorium occurring in quantity in nature. Its stability is attributed to its closed nuclear shell with 142 neutrons. Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377. I
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