The gray is a derived unit of ionizing radiation dose in the International System of Units. It is defined as the absorption of one joule of radiation energy per kilogram of matter, it is used as a unit of the radiation quantity absorbed dose which measures the energy deposited by ionizing radiation in a unit mass of matter being irradiated, is used for measuring the delivered dose of ionising radiation in applications such as radiotherapy, food irradiation and radiation sterilization. As a measure of low levels of absorbed dose, it forms the basis for the calculation of the radiation protection unit the sievert, a measure of the health effect of low levels of ionizing radiation on the human body; the gray is used in radiation metrology as a unit of the radiation quantity kerma. The gray is an important unit in ionising radiation measurement and was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue; the gray was adopted as part of the International System of Units in 1975.
The corresponding cgs unit to the gray is the rad, which remains common in the United States, though "strongly discouraged" in the style guide for U. S. National Institute of Standards and Technology authors; the gray has a number of fields of application in measuring dose: The measurement of absorbed dose in tissue is of fundamental importance in radiobiology and radiation therapy as it is the measure of the amount of energy the incident radiation deposits in the target tissue. The measurement of absorbed dose is a complex problem due to scattering and absoption, many specialist dosimeters are available for these measurements, can cover applications in 1-D, 2-D and 3-D. In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, while lymphomas are treated with 20 to 40 Gy. Preventive doses are around 45–60 Gy in 1.8–2 Gy fractions. The average radiation dose from an abdominal X-ray is 0.7 milli-Grays, that from an abdominal CT scan is 8 mGy, that from a pelvic CT scan is 6 mGy, that from a selective CT scan of the abdomen and the pelvis is 14 mGy.
The absorbed dose plays an important role in radiation protection, as it is the starting point for calculating the stochastic health risk of low levels of radiation, defined as the probability of cancer induction and genetic damage. The gray measures the total absorbed energy of radiation, but the probability of stochastic damage depends on the type and energy of the radiation and the types of tissues involved; this probability is related to the equivalent dose in sieverts, which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles on equivalent dose and effective dose; the International Committee for Weights and Measures states: "In order to avoid any risk of confusion between the absorbed dose D and the dose equivalent H, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose D and the name sievert instead of joules per kilogram for the unit of dose equivalent H." 1 G y = 1 J k g = 1 m 2 s 2 The accompanying diagrams show how absorbed dose is first obtained by computational techniques, from this value the equivalent doses are derived.
For X-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for alpha particles one gray is equivalent to 20 sieverts, a radiation weighting factor is applied accordingly. Radiation poisoning - The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation; these are effects which are certain to happen, as opposed to the uncertain effects of low levels of radiation which have a probability of causing damage. A whole-body acute exposure to 5 grays or more of high-energy radiation leads to death within 14 days; this dose represents 375 joules for a 75 kg adult. The gray is used to measure absorbed dose rates in non-tissue materials for processes such as radiation hardening, food irradiation and electron irradiation. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes. Kerma is used in radiation metrology as a measure of the liberated energy of ionisation due to irradiation, is expressed in grays.
Kerma dose is different from absorbed dose, depending on the radiation energies involved because ionization energy is not accounted for. Whilst equal at low energies, kerma is much higher than absorbed dose at higher energies, because some energy escapes from the absorbing volume in the form of bremsstrahlung or fast-moving electrons. Kerma, when applied to air, is equivalent to the legacy roentgen unit of radiation exposure, but there is a difference in the definition of these two units; the gray is defined independently of any target material, the roengten was defined by the ionisation effect in dry air, w
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan
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
Long and short scales
The long and short scales are two of several large-number naming systems for integer powers of ten that use the same words with different meanings. The long scale is based on powers of one million, whereas the short scale is based on powers of one thousand. For whole numbers less than a thousand million the two scales are identical. From a thousand million up the two scales diverge, using the same words for different numbers, which can cause misunderstanding; every new term greater than million is one thousand times as large as the previous term. Thus, billion means a thousand millions, trillion means a thousand billions, so on. Thus, an n-illion equals 103n + 3; every new term greater than million is one million times as large as the previous term. Thus, billion means a million millions, trillion means a million billions, so on. Thus, an n-illion equals 106n. Countries where the long scale is used include most countries in continental Europe and most that are French-speaking, Spanish-speaking and Portuguese-speaking countries, except Brazil.
The short scale is now used in most English-speaking and Arabic-speaking countries, in Brazil, in the former Soviet Union and several other countries. Number names are rendered in the language of the country, but are similar everywhere due to shared etymology; some languages in East Asia and South Asia, have large number naming systems that are different from both the long and short scales, for example the Indian numbering system. For most of the 19th and 20th centuries, the United Kingdom used the long scale, whereas the United States used the short scale, so that the two systems were referred to as British and American in the English language. After several decades of increasing informal British usage of the short scale, in 1974 the government of the UK adopted it, it is used for all official purposes. With few exceptions, the British usage and American usage are now identical; the first recorded use of the terms short scale and long scale was by the French mathematician Geneviève Guitel in 1975.
To avoid confusion resulting from the coexistence of short and long term in any language, the SI recommends using the Metric prefix, which keeps the same meaning regardless of the country and the language. Long and short scales remain in de facto use for counting money; the relationship between the numeric values and the corresponding names in the two scales can be described as: The relationship between the names and the corresponding numeric values in the two scales can be described as: The root mil in million does not refer to the numeral, 1. The word, derives from the Old French, from the earlier Old Italian, milione, an intensification of the Latin word, mille, a thousand; that is, a million is a big thousand, much as a great gross is a dozen gross or 12×144 = 1728. The word milliard, or its translation, is found in many European languages and is used in those languages for 109. However, it is unknown in American English, which uses billion, not used in British English, which preferred to use thousand million before the current usage of billion.
The financial term, which derives from milliard, is used on financial markets, as, unlike the term, billion, it is internationally unambiguous and phonetically distinct from million. Many long scale countries use the word billiard for one thousand long scale billions, the word trilliard for one thousand long scale trillions, etc; the existence of the different scales means that care must be taken when comparing large numbers between languages or countries, or when interpreting old documents in countries where the dominant scale has changed over time. For example, British English and Italian historical documents can refer to either the short or long scale, depending on the date of the document, since each of the three countries has used both systems at various times in its history. Today, the United Kingdom uses the short scale, but France and Italy use the long scale; the pre-1974 former British English word billion, post-1961 current French word billion, post-1994 current Italian word bilione, German Billion.
Therefore, each of these words translates to the American English or post-1974 British English word: trillion, not billion. On the other hand, the pre-1961 former French word billion, pre-1994 former Italian word bilione, Brazilian Portuguese word bilhão and the Welsh word biliwn all refer to 109, being short scale terms; each of these words translates to post-1974 British English word billion. The term billion meant 1012 when introduced. In long scale countries, milliard was defined to its current value of 109, leaving billion at its original 1012 value and so on for the larger numbers; some of these countries, but not all, introduced new words billiard, etc. as intermediate terms. In some short scale countries, milliard was defined to 109 and billion dropped altogether, with trillion redefined down to 1012 and so on for the larger numbers. In many short scale countries, milliard was dropped altogether and billion was redefined down to 109, adjusting downwards the value of trillion and all
Paleontology or palaeontology is the scientific study of life that existed prior to, sometimes including, the start of the Holocene Epoch. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments. Paleontological observations have been documented as far back as the 5th century BC; the science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, developed in the 19th century. The term itself originates from Greek παλαιός, palaios, "old, ancient", ὄν, on, "being, creature" and λόγος, logos, "speech, study". Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of anatomically modern humans, it now uses techniques drawn from a wide range of sciences, including biochemistry and engineering. Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life all the way back to when Earth became capable of supporting life, about 3.8 billion years ago.
As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates. Body fossils and trace fossils are the principal types of evidence about ancient life, geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more paleontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is difficult, as many do not fit well into the Linnaean taxonomy classifying living organisms, paleontologists more use cladistics to draw up evolutionary "family trees"; the final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how organisms are related by measuring the similarity of the DNA in their genomes.
Molecular phylogenetics has been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend. The simplest definition of paleontology is "the study of ancient life"; the field seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, what they can tell us about the Earth's organic and inorganic past". Paleontology is one of the historical sciences, along with archaeology, astronomy, cosmology and history itself: it aims to describe phenomena of the past and reconstruct their causes. Hence it has three main elements: description of past phenomena; when trying to explain the past and other historical scientists construct a set of hypotheses about the causes and look for a smoking gun, a piece of evidence that accords with one hypothesis over the others. Sometimes the smoking gun is discovered by a fortunate accident during other research. For example, the discovery by Luis and Walter Alvarez of iridium, a extra-terrestrial metal, in the Cretaceous–Tertiary boundary layer made asteroid impact the most favored explanation for the Cretaceous–Paleogene extinction event, although the contribution of volcanism continues to be debated.
The other main type of science is experimental science, said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena. This approach cannot prove a hypothesis, since some experiment may disprove it, but the accumulation of failures to disprove is compelling evidence in favor. However, when confronted with unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists use the same approach as historical scientists: construct a set of hypotheses about the causes and look for a "smoking gun". Paleontology lies between biology and geology since it focuses on the record of past life, but its main source of evidence is fossils in rocks. For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest. Paleontology has some overlap with archaeology, which works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as a species.
When dealing with evidence about humans and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate. In addition, paleontology borrows techniques from other sciences, including biology, ecology, chemistry and mathematics. For example, geochemical signatures from rocks may help to discover when life first arose on Earth, analyses of carbon isotope ratios may help to identify climate changes and to explain major transitions such as the Permian–Triassic extinction event. A recent discipline, molecular phylogenetics, compares the DNA and RNA of modern organisms to re-construct the "family trees" of their
Harvard University Press
Harvard University Press is a publishing house established on January 13, 1913, as a division of Harvard University, focused on academic publishing. It is a member of the Association of American University Presses. After the retirement of William P. Sisler in 2017, the university appointed as Director George Andreou; the press maintains offices in Cambridge, Massachusetts near Harvard Square, in London, England. The press co-founded the distributor TriLiteral LLC with Yale University Press. TriLiteral was sold to LSC Communications in 2018. Notable authors published by HUP include Eudora Welty, Walter Benjamin, E. O. Wilson, John Rawls, Emily Dickinson, Stephen Jay Gould, Helen Vendler, Carol Gilligan, Amartya Sen, David Blight, Martha Nussbaum, Thomas Piketty; the Display Room in Harvard Square, dedicated to selling HUP publications, closed on June 17, 2009. HUP owns the Belknap Press imprint, which it inaugurated in May 1954 with the publication of the Harvard Guide to American History; the John Harvard Library book series is published under the Belknap imprint.
Harvard University Press distributes the Loeb Classical Library and is the publisher of the I Tatti Renaissance Library, the Dumbarton Oaks Medieval Library, the Murty Classical Library of India. It is distinct from Harvard Business Press, part of Harvard Business Publishing, the independent Harvard Common Press, its 2011 publication Listed: Dispatches from America's Endangered Species Act by Joe Roman received the 2012 Rachel Carson Environment Book Award from the Society of Environmental Journalists. Hall, Max. Harvard University Press: A History. Cambridge, MA: Harvard University Press. ISBN 978-0-674-38080-6. Official website Blog of Harvard University Press
Astronomy & Astrophysics
Astronomy & Astrophysics is a peer-reviewed scientific journal covering theoretical and instrumental astronomy and astrophysics. It is one of the premier journals for astronomy in the world; the journal is published by EDP Sciences in 16 issues per year. The editor-in-chief is Thierry Forveille. Previous editors in chief include Claude Bertout, James Lequeux, Michael Grewing, Catherine Cesarsky and George Contopoulos. Astronomy & Astrophysics was formed in 1969 by the merging of several national journals of individual European countries into one comprehensive publication; these journals, with their ISSN and date of first publication are as follows: Annales d'Astrophysique ISSN 0365-0499, established in 1938 Arkiv för Astronomi ISSN 0004-2048, established in 1948 Bulletin of the Astronomical Institutes of the Netherlands ISSN 0365-8910, established in 1921 Bulletin Astronomique ISSN 0245-9787, established in 1884 Journal des Observateurs ISSN 0368-3389, established in 1915 Zeitschrift für Astrophysik ISSN 0372-8331, established in 1930The publishing of Astronomy & Astrophysics was further extended in 1992 by the incorporation of Bulletin of the Astronomical Institutes of Czechoslovakia, established in 1947.
Astronomy & Astrophysics published articles in either English, French, or German, but articles in French and German were always few. They were discontinued, in part due to difficulties in finding adequately specialized independent referees who were fluent in those languages; the original sponsoring countries were the four countries whose journals merged to form Astronomy & Astrophysics, together with Belgium, Denmark and Norway. The European Southern Observatory participated as a "member country". Norway withdrew, but Austria, Italy and Switzerland all joined; the Czech Republic, Hungary and Slovakia all joined as new members in the 1990s. In 2001 the words "A European Journal" were removed from the front cover in recognition of the fact that the journal was becoming global in scope, in 2002 Argentina was admitted as an "observer". In 2004 the Board of Directors decided that the journal "will henceforth consider applications for sponsoring membership from any country in the world with well-documented active and excellent astronomical research".
Argentina became the first non-European country to gain full membership in 2005. Brazil and Portugal all gained "observer" status at this time and have since progressed to full membership; this journal is listed in the following databases: All letters to the editor and all articles published in the online sections of the journal are open access upon publication. Articles in the other sections of the journal are made available 12 months after publication, through the publisher's site and via the Astrophysics Data System. Authors have the option to pay for immediate open access; the Astrophysical Journal The Astronomical Journal Monthly Notices of the Royal Astronomical Society History and purpose of Astronomy & Astrophysics journal. S. R. Pottasch. EDP Sciences. 2012