A nuclear reactor known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid; these either turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating; some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research; as of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world. Just as conventional power-stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms; when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, so on; this is known as a nuclear chain reaction. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions. Used moderators include regular water, solid graphite and heavy water; some experimental types of reactor have used beryllium, hydrocarbons have been suggested as another possibility. The reactor core generates heat in a number of ways: The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms; the reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases three million times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates; the heat is carried away from the reactor and is used to generate steam. Most reactor systems employ a cooling system, physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; the rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events.
Nuclear reactors employ several methods of neutron control to adjust the reactor's power output. Some of these methods arising from the physics of radioactive decay and are accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose; the fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods therefore tend to absorb neutrons; when a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces—often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power; the physics of radioactive decay affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes; these delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder released upon fission.
The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, so considerable time is required to determine when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; this last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, other points in the process interpolated in cents. In some reactors, the coolant acts as a neutron moderator. A moderator increases the power of the reactor by causin
Times Square is a major commercial intersection, tourist destination, entertainment center and neighborhood in the Midtown Manhattan section of New York City at the junction of Broadway and Seventh Avenue. It stretches from West 42nd to West 47th Streets. Brightly adorned with billboards and advertisements, Times Square is sometimes referred to as "The Crossroads of the World", "The Center of the Universe", "the heart of The Great White Way", "the heart of the world". One of the world's busiest pedestrian areas, it is the hub of the Broadway Theater District and a major center of the world's entertainment industry. Times Square is one of the world's most visited tourist attractions, drawing an estimated 50 million visitors annually. 330,000 people pass through Times Square daily, many of them tourists, while over 460,000 pedestrians walk through Times Square on its busiest days. Known as Longacre Square, Times Square was renamed in 1904 after The New York Times moved its headquarters to the newly erected Times Building – now One Times Square – the site of the annual New Year's Eve ball drop which began on December 31, 1907, continues today, attracting over a million visitors to Times Square every year.
Times Square functions as a town square, but is not geometrically a square. Broadway runs diagonally, crossing through the horizontal and vertical street grid of Manhattan laid down by the Commissioners' Plan of 1811, that intersection creates the "bowtie" shape of Times Square; the southern triangle of Times Square has no specific name, but the northern triangle is called Father Duffy Square. It was dedicated in 1937 to Chaplain Francis P. Duffy of New York City's U. S. 69th Infantry Regiment and is the site of a memorial to him, along with a statue of George M. Cohan, as well as the TKTS reduced-price ticket booth run by the Theatre Development Fund. Since 2008, the booth has been backed by a red, triangular set of bleacher-like stairs, used by people to sit, talk and take photographs; when Manhattan Island was first settled by the Dutch, three small streams united near what is now 10th Avenue and 40th Street. These three streams formed the "Great Kill". From there the Great Kill wound through the low-lying Reed Valley, known for fish and waterfowl and emptied into a deep bay in the Hudson River at the present 42nd Street.
The name was retained in a tiny hamlet, Great Kill, that became a center for carriage-making, as the upland to the south and east became known as Longacre. Before and after the American Revolution, the area belonged to John Morin Scott, a general of the New York militia, in which he served under George Washington. Scott's manor house was at what is 43rd Street, surrounded by countryside used for farming and breeding horses. In the first half of the 19th century, it became one of the prized possessions of John Jacob Astor, who made a second fortune selling off lots to hotels and other real estate concerns as the city spread uptown. By 1872, the area had become the center of New York's horse carriage industry; the locality had not been given a name, city authorities called it Longacre Square after Long Acre in London, where the horse and carriage trade was centered in that city. William Henry Vanderbilt ran the American Horse Exchange there. In 1910 it became the Winter Garden Theatre; as more profitable commerce and industrialization of Lower Manhattan pushed homes and prostitution northward from the Tenderloin District, Long Acre Square became nicknamed the Thieves Lair for its rollicking reputation as a low entertainment district.
The first theater on the square, the Olympia, was built by cigar manufacturer and impresario Oscar Hammerstein I. According to Gotham: A History of New York City to 1898, "By the early 1890s this once sparsely settled stretch of Broadway was ablaze with electric light and thronged by crowds of middle- and upper-class theatre and cafe patrons." In 1904, New York Times publisher Adolph S. Ochs moved the newspaper's operations to a new skyscraper on 42nd Street at Longacre Square, on the site of the former Pabst Hotel, which had existed on the site for less than a decade since it opened in November 1899. Ochs persuaded Mayor George B. McClellan Jr. to construct a subway station there, the area was renamed "Times Square" on April 8, 1904. Just three weeks the first electrified advertisement appeared on the side of a bank at the corner of 46th Street and Broadway; the north end became Duffy Square, the former Horse Exchange became the Winter Garden Theatre, constructed in 1911. The New York Times moved to more spacious offices one block west of the square in 1913 and sold the building in 1961.
The old Times Building was named the Allied Chemical Building in 1963. Now known as One Times Square, it is famed for the Times Square Ball drop on its roof every New Year's Eve. In 1913, the Lincoln Highway Association, headed by entrepreneur Carl G. Fisher, chose the intersection of 42nd Street and Broadway to be the Eastern Terminus of the Lincoln Highway; this was the first road across the United States, which spanned 3,389 miles coast-to-coast through 13 states to its western terminus in Lincoln Park in San Francisco, California. Times Square grew after World War I, it became a cultural hub full of theatres, music halls, upscale hotels. Times Square became New York's agora, a place to gather to await great tidings and to celebrate them, whether a World Series or a presidential election. Advertising grew in the 1920s, growing
Moses ben Maimon known as Maimonides and referred to by the acronym Rambam, was a medieval Sephardic Jewish philosopher who became one of the most prolific and influential Torah scholars of the Middle Ages. In his time, he was a preeminent astronomer and physician. Born in Córdoba, Almoravid Empire on Passover Eve, 1135 or 1138, he worked as a rabbi and philosopher in Morocco and Egypt, he died in Egypt on December 12, 1204, whence his body was taken to the lower Galilee and buried in Tiberias. During his lifetime, most Jews greeted Maimonides' writings on Jewish law and ethics with acclaim and gratitude as far away as Iraq and Yemen. Yet, while Maimonides rose to become the revered head of the Jewish community in Egypt, his writings had vociferous critics in Spain. Nonetheless, he was posthumously acknowledged as among the foremost rabbinical decisors and philosophers in Jewish history, his copious work comprises a cornerstone of Jewish scholarship, his fourteen-volume Mishneh Torah still carries significant canonical authority as a codification of Talmudic law.
He is sometimes known as "ha Nesher ha Gadol" in recognition of his outstanding status as a bona fide exponent of the Oral Torah. Aside from being revered by Jewish historians, Maimonides figures prominently in the history of Islamic and Arab sciences and is mentioned extensively in studies. Influenced by Al-Farabi and his contemporary Averroes, he in his turn influenced other prominent Arab and Muslim philosophers and scientists, he became a prominent polymath in both the Jewish and Islamic worlds. His full Hebrew name is Rabbi Moshe ben Maimon, whose acronym forms "Rambam", his full Arabic name is Abū ʿImrān Mūsā bin Maimūn bin ʿUbaidallāh al-Qurtabī, or Mūsā bin Maymūn for short. In Latin, the Hebrew ben becomes the Greek-style patronymic suffix -ides, forming "Moses Maimonides". Maimonides was born in Córdoba during what some scholars consider to be the end of the golden age of Jewish culture in the Iberian Peninsula, after the first centuries of the Moorish rule. At an early age, he developed an interest in sciences and philosophy.
He read those Greek philosophers accessible in Arabic translations, was immersed in the sciences and learning of Islamic culture. Though the Gaonic tradition in its North African version, formed the basis of his legal thought, some scholars have argued in the 21st century that Muslim law, including Almohad legal thought had a substantial influence. Maimonides was not known as a supporter of mysticism, although a strong intellectual type of mysticism has been discerned in his philosophy, he expressed disapproval of poetry, the best of which he declared to be false, since it was founded on pure invention. This sage, revered for his personality as well as for his writings, led a busy life, wrote many of his works while travelling or in temporary accommodation. Maimonides studied Torah under his father Maimon, who had in turn studied under Rabbi Joseph ibn Migash, a student of Isaac Alfasi. A Berber dynasty, the Almohads, conquered Córdoba in 1148, abolished dhimmi status in some of their territories.
The loss of this status left the Jewish and Christian communities with conversion to Islam, death, or exile. Many Jews were forced to convert, but due to suspicion by the authorities of fake conversions, the new converts had to wear identifying clothing that set them apart and made them subject to public scrutiny. Maimonides's family, along with most other Jews, chose exile; some say, that it is that Maimonides feigned a conversion to Islam before escaping. This forced conversion was ruled invalid under Islamic law when brought up by a rival in Egypt. For the next ten years, Maimonides moved about in southern Spain settling in Fez in Morocco. During this time, he composed his acclaimed commentary on the Mishnah, during the years 1166–1168. Following this sojourn in Morocco, together with two sons, he sojourned in the Holy Land, before settling in Fustat, Egypt around 1168. While in Cairo, he studied in a yeshiva attached to a small synagogue. In the Holy Land, he prayed at the Temple Mount, he wrote that this day of visiting the Temple Mount was a day of holiness for him and his descendants.
Maimonides shortly thereafter was instrumental in helping rescue Jews taken captive during the Christian King Amalric's siege of the Egyptian town of Bilbays. He sent five letters to the Jewish communities of Lower Egypt asking them to pool money together to pay the ransom; the money was collected and given to two judges sent to Palestine to negotiate with the Crusaders. The captives were released. Following this triumph, the Maimonides family, hoping to increase their wealth, gave their savings to his brother, the youngest son David ben Maimon, a merchant. Maimonides directed his brother to procure goods only at the Sudanese port of ‘Aydhab. After a long arduous trip through the desert, David was unimpressed by the goods on offer there. Against his brother's wishes, David boarded a ship for India, since great wealth was to be found in the East. Before he could reach his destination, David drowned at sea sometime between 1169 and 1177; the death of his brother caused Maimonides to become sick with grief.
In a letter, he wrote: The greatest misfortune that has befallen me during my entire life—worse than anything else—was the demise of the saint, may his memory be blessed, who drowned in the Indian sea
Whitney Museum of American Art
The Whitney Museum of American Art, known informally as the "Whitney", is an art museum in Manhattan. It was founded in 1930 by Gertrude Vanderbilt Whitney, a wealthy and prominent American socialite and art patron after whom it is named; the Whitney focuses on 20th- and 21st-century American art. Its permanent collection comprises more than 23,000 paintings, drawings, photographs, films and artifacts of new media by more than 3,400 artists, it places particular emphasis on exhibiting the work of living artists as well as maintaining an extensive permanent collection of important pieces from the first half of the last century. The museum's Annual and Biennial exhibitions have long been a venue for younger and lesser-known artists whose work is showcased there. From 1966 to 2014, the Whitney was at 945 Madison Avenue on Manhattan's Upper East Side; the museum closed in October 2014 to relocate to a new building designed by Renzo Piano at 99 Gansevoort Street in the West Village/Meatpacking District neighborhoods of Lower Manhattan.
Gertrude Vanderbilt Whitney, the museum's namesake and founder, was a well-regarded sculptor as well as a serious art collector. As a patron of the arts, she had achieved some success with the Whitney Studio Club, a New York–based exhibition space she created in 1918 to promote the works of avant-garde and unrecognized American artists. Whitney favored the radical art of the American artists of the Ashcan School such as John French Sloan, George Luks and Everett Shinn, as well as others such as Edward Hopper, Stuart Davis, Charles Demuth, Charles Sheeler, Max Weber. With the aid of her assistant, Juliana R. Force, Whitney collected nearly 700 works of American art. In 1929, she offered to donate over 500 to the Metropolitan Museum of Art, but the museum declined the gift. This, along with the apparent preference for European modernism at the opened Museum of Modern Art, led Whitney to start her own museum for American art, in 1929. Whitney Library archives from 1928 reveal that during this time the Studio Club used the gallery space of Wilhelmina Weber Furlong of the Art Students League to exhibit traveling shows featuring modernist work.
In 1931, architect Noel L. Miller converted three row houses on West 8th Street in Greenwich Village—one of which, 8 West 8th Street had been the location of the Studio Club—to be the museum's home as well as a residence for Whitney. Force became the museum's first director, under her guidance it concentrated on displaying the works of new and contemporary American artists. In 1954, the museum left its original location and moved to a small structure on 54th Street connected to and behind the Museum of Modern Art on 53rd Street. On April 15, 1958, a fire on MOMA's second floor that killed one person forced the evacuation of paintings and staff on MOMA's upper floors to the Whitney. Among the paintings evacuated was A Sunday Afternoon on the Island of La Grande Jatte, on loan from the Art Institute of Chicago. In 1961, the Whitney began seeking a site for a larger building. In 1966 it settled at the southeast corner of Madison Avenue and 75th Street on Manhattan's Upper East Side; the building and built 1963–1966 by Marcel Breuer and Hamilton P. Smith in a distinctively modern style, is distinguished from the neighboring townhouses by its staircase façade made of granite stones and its external upside-down windows.
In 1967, Mauricio Lasansky showed The Nazi Drawings. The exhibition traveled to the Whitney Museum of American Art in New York, where it appeared with shows by Louise Nevelson and Andrew Wyeth as the first exhibits in the new museum; the institution grappled with space problems for decades. From 1973 to 1983 the Whitney operated its first branch at 55 Water Street, a building owned by Harold Uris, who gave the museum a lease for $1 a year. In 1983 Philip Morris International installed a Whitney branch in the lobby of its Park Avenue headquarters. In 1981 the museum opened an exhibition space in Stamford, housed at Champion International. In the late 1980s, the Whitney entered into arrangements with Park Tower Realty, I. B. M. and The Equitable Life Assurance Society of the United States, setting up satellite museums with rotating exhibitions in their buildings' lobbies. Each museum had its own director, with all plans approved by a Whitney committee; the institution attempted to expand its landmark building in 1978, commissioning UK architects Derek Walker and Norman Foster to design a tall tower alongside it, the first of several proposals from leading architects.
But each time the effort was abandoned, because of either both. To secure additional space for the museum's collections, then-director Thomas N. Armstrong III developed plans for a 10-story, $37.5 million addition to the main building. The proposed addition, designed by Michael Graves and announced in 1985, drew immediate opposition. Graves had proposed demolishing the flanking brownstones down to the East 74th Street corner for a complementary addition; the project lost the support of the museum's trustees, the plans were dropped in 1989. Between 1995 and 1998, the building underwent a expansion by Richard Gluckman. In 2001, Rem Koolhaas was commissioned to submit two designs for a $200 million expansion; those plans were dropped in 2003. New York restaurateur Danny Meyer opened Untitled, a restaurant in the museum, in March 2011; the space was designed by the Rockwell Group. The Whitney developed a new main building, designed by Renzo Piano, in the West Village and Meatpacking District in lower Manhattan.
The new museum, at the intersection of Gansevoort and Washington Streets, was bu
Shlomo Yitzchaki, today known by the acronym Rashi, was a medieval French rabbi and author of a comprehensive commentary on the Talmud and commentary on the Tanakh. Acclaimed for his ability to present the basic meaning of the text in a concise and lucid fashion, Rashi appeals to both learned scholars and beginner students, his works remain a centerpiece of contemporary Jewish study, his commentary on the Talmud, which covers nearly all of the Babylonian Talmud, has been included in every edition of the Talmud since its first printing by Daniel Bomberg in the 1520s. His commentary on Tanakh—especially on the Chumash — serves as the basis for more than 300 "supercommentaries" which analyze Rashi's choice of language and citations, penned by some of the greatest names in rabbinic literature. Rashi's surname, derives from his father's name, Yitzhak; the acronym is sometimes fancifully expanded as Rabban Shel YIsrael which means the rabbi of Israel, or as Rabbenu SheYichyeh. He may be cited in Hebrew and Aramaic texts as "Shlomo son of Rabbi Yitzhak", "Shlomo son of Yitzhak", "Shlomo Yitzhaki", myriad similar respectful derivatives.
In older literature, Rashi is sometimes referred to as Jarchi or Yarhi, his abbreviated name being interpreted as Rabbi Shlomo Yarhi. This was understood to refer to the Hebrew name of Lunel in Provence, popularly derived from the French lune "moon", in Hebrew ירח, in which Rashi was assumed to have lived at some time or to have been born, or where his ancestors were supposed to have originated. Simon and Wolf claimed that only Christian scholars referred to Rashi as Jarchi, that this epithet was unknown to the Jews. Bernardo de Rossi, demonstrated that Hebrew scholars referred to Rashi as Yarhi. In 1839, Leopold Zunz showed that the Hebrew usage of Jarchi was an erroneous propagation of the error by Christian writers, instead interpreting the abbreviation as it is understood today: Rabbi Shlomo Yitzhaki; the evolution of this term has been traced. Rashi was an only child born at Champagne, in northern France, his mother's brother was Simon the Rabbi of Mainz. Simon was a disciple of Rabbeinu Gershom Meor HaGolah.
On his father's side, Rashi has been claimed to be a 33rd-generation descendant of Yochanan Hasandlar, a fourth-generation descendant of Gamaliel the Elder, reputedly descended from the royal line of King David. In his voluminous writings, Rashi himself made no such claim at all; the main early rabbinical source about his ancestry, Responsum No. 29 by Solomon Luria, makes no such claim either. His fame made him the subject of many legends. One tradition contends. Rashi's father, Yitzhak, a poor winemaker, once found a precious jewel and was approached by non-Jews who wished to buy it to adorn their idol. Yitzhak agreed to travel with them to their land, but en route, he cast the gem into the sea. Afterwards he was visited by either the Voice of God or the prophet Elijah, who told him that he would be rewarded with the birth of a noble son "who would illuminate the world with his Torah knowledge."Another legend states that Rashi's parents moved to Worms, Germany while Rashi's mother was pregnant.
As she walked down one of the narrow streets in the Jewish quarter, she was imperiled by two oncoming carriages. She pressed herself against a wall, which opened to receive her; this miraculous niche is still visible in the wall of the Worms Synagogue. According to tradition, Rashi was first brought to learn Torah by his father on Shavuot day at the age of five, his father was his main Torah teacher until his death. At the age of 17 he married and soon after went to learn in the yeshiva of Rabbi Yaakov ben Yakar in Worms, returning to his wife three times yearly, for the Days of Awe and Shavuot; when Rabbi Yaakov died in 1064, Rashi continued learning in Worms for another year in the yeshiva of his relative, Rabbi Isaac ben Eliezer Halevi, chief rabbi of Worms. He moved to Mainz, where he studied under another of his relatives, Rabbi Isaac ben Judah, the rabbinic head of Mainz and one of the leading sages of the Lorraine region straddling France and Germany. Rashi's teachers were students of Rabbeinu Gershom and Rabbi Eliezer Hagadol, leading Talmudists of the previous generation.
From his teachers, Rashi imbibed the oral traditions pertaining to the Talmud as they had been passed down for centuries, as well as an understanding of the Talmud's unique logic and form of argument. Rashi took concise, copious notes from what he learned in yeshiva, incorporating this material in his commentaries, he returned to Troyes at the age of 25, after which time his mother died, he was asked to join the Troyes Beth din. He began answering halakhic questions. Upon the death of the head of the Bet din, Rabbi Zerach ben Abraham, Rashi assumed the court's leadership and answered hundreds of halakhic queries. In around 1070 he founded a yeshiva, it is thought by some that Rashi earned his living as a vintner since Rashi shows an extensive knowledge of its utensils and process, but there is no evidence for this. Most scholars and a Jewish oral tradition contend; the only reason given for the centuries-old tradition that he was a vintner being not true is that the soil in all of Troyes is not optimal for wine growing grapes, claimed by the research of Rabbi Haym Soloveitchik.
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or from a combination of fission and fusion reactions. Both bomb types release large quantities of energy from small amounts of matter; the first test of a fission bomb released an amount of energy equal to 20,000 tons of TNT. The first thermonuclear bomb test released energy equal to 10 million tons of TNT. A thermonuclear weapon weighing little more than 2,400 pounds can release energy equal to more than 1.2 million tons of TNT. A nuclear device no larger than traditional bombs can devastate an entire city by blast and radiation. Since they are weapons of mass destruction, the proliferation of nuclear weapons is a focus of international relations policy. Nuclear weapons have been used twice in war, both times by the United States against Japan near the end of World War II. On August 6, 1945, the U. S. Army Air Forces detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima.
S. Army Air Forces detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki; these bombings caused injuries that resulted in the deaths of 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are subjects of debate. Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over two thousand times for testing and demonstration. Only a few nations are suspected of seeking them; the only countries known to have detonated nuclear weapons—and acknowledge possessing them—are the United States, the Soviet Union, the United Kingdom, China, India and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Turkey and the Netherlands are nuclear weapons sharing states. South Africa is the only country to have independently developed and renounced and dismantled its nuclear weapons.
The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons, but its effectiveness has been questioned, political tensions remained high in the 1970s and 1980s. Modernisation of weapons continues to this day. There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, those that use fission reactions to begin nuclear fusion reactions that produce a large amount of the total energy output. All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is from fission reactions are referred to as atomic bombs or atom bombs; this has long been noted as something of a misnomer, as their energy comes from the nucleus of the atom, just as it does with fusion weapons. In fission weapons, a mass of fissile material is forced into supercriticality—allowing an exponential growth of nuclear chain reactions—either by shooting one piece of sub-critical material into another or by compression of a sub-critical sphere or cylinder of fissile material using chemically-fueled explosive lenses.
The latter approach, the "implosion" method, is more sophisticated than the former. A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself; the amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons of TNT. All fission reactions generate the remains of the split atomic nuclei. Many fission products are either radioactive or moderately radioactive, as such, they are a serious form of radioactive contamination. Fission products are the principal radioactive component of nuclear fallout. Another source of radioactivity is the burst of free neutrons produced by the weapon; when they collide with other nuclei in surrounding material, the neutrons transmute those nuclei into other isotopes, altering their stability and making them radioactive. The most used fissile materials for nuclear weapons applications have been uranium-235 and plutonium-239.
Less used has been uranium-233. Neptunium-237 and some isotopes of americium may be usable for nuclear explosives as well, but it is not clear that this has been implemented, their plausible use in nuclear weapons is a matter of dispute; the other basic type of nuclear weapon produces a large proportion of its energy in nuclear fusion reactions. Such fusion weapons are referred to as thermonuclear weapons or more colloquially as hydrogen bombs, as they rely on fusion reactions between isotopes of hydrogen. All such weapons derive a significant portion of their energy from fission reactions used to "trigger" fusion reactions, fusion reactions can themselves trigger additional fission reactions. Only six countries—United States, United Kingdom, China and India—have conducted thermonuclear weapon tests. North Korea claims to have tested a fusion weapon as of January 2016. Thermonuclear weapons a
Book of Leviticus
The Book of Leviticus is the third book of the Torah and of the Old Testament. Most of its chapters consist of God's speeches to Moses, which God commands Moses to repeat to the Israelites; this takes place within the story of the Israelites' Exodus after they escaped Egypt and reached Mt. Sinai; the Book of Exodus narrates how Moses led the Israelites in building the Tabernacle with God's instructions. In Leviticus, God tells the Israelites and their priests how to make offerings in the Tabernacle and how to conduct themselves while camped around the holy tent sanctuary. Leviticus takes place during the month or month-and-a-half between the completion of the Tabernacle and the Israelites' departure from Sinai; the instructions of Leviticus emphasize ritual and moral practices rather than beliefs. They reflect the world view of the creation story in Genesis 1 that God wishes to live with humans; the book teaches that faithful performance of the sanctuary rituals can make that possible, so long as the people avoid sin and impurity whenever possible.
The rituals the sin and guilt offerings, provide the means to gain forgiveness for sins and purification from impurities so that God can continue to live in the Tabernacle in the midst of the people. Scholars agree that Leviticus developed over a long time and that it reached its present form in the Persian period; the English name Leviticus comes from the Latin Leviticus, in turn from the Greek Greek Λευιτικόν, referring to the priestly tribe of the Israelites, “Levi.” The Greek expression is in turn a variant of the rabbinic Hebrew torat kohanim, "law of priests", as many of its laws relate to priests. In Hebrew the book is called Vayikra, from the opening of the book, va-yikra "And He called." I. Laws on sacrifice A. Instructions for the laity on bringing offerings 1–5; the types of offering: burnt, peace, reparation offerings B. Instructions for the priests 1–6; the various offerings, with the addition of the priests' cereal offering 7. Summary II. Institution of the priesthood A. Ordination of Aaron and his sons B.
Aaron makes the first sacrifices C. Judgement on Nadab and Abihu III. Uncleanliness and its treatment A. Unclean animals B. Childbirth as a source of uncleanliness C. Unclean diseases D. Cleansing of diseases E. Unclean discharges IV. Day of Atonement: purification of the tabernacle from the effects of uncleanliness and sin V. Prescriptions for practical holiness A. Sacrifice and food B. Sexual behaviour C. Neighbourliness D. Grave crimes E. Rules for priests F. Rules for eating sacrifices G. Festivals H. Rules for the tabernacle I. Blasphemy J. Sabbatical and Jubilee years K. Exhortation to obey the law: blessing and curse VI. Redemption of votive gifts Chapters 1–5 describe the various sacrifices from the sacrificers' point of view, although the priests are essential for handling the blood. Chapters 6–7 go over much the same ground, but from the point of view of the priest, who, as the one carrying out the sacrifice and dividing the "portions", needs to know how do this. Sacrifices are between God, the priest, the offerers, although in some cases the entire sacrifice is a single portion to God—i.e.
Burnt to ashes. Chapters 8–10 describe how Moses consecrates Aaron and his sons as the first priests, the first sacrifices, God's destruction of two of Aaron's sons for ritual offenses; the purpose is to underline the character of altar priesthood as an Aaronite privilege, the responsibilities and dangers of their position. With sacrifice and priesthood established, chapters 11–15 instruct the lay people on purity. Eating certain animals produces uncleanliness; the reasoning behind the food rules are obscure. Leviticus 16 concerns the Day of Atonement; this is the only day on which the High Priest is to enter the holiest part of the sanctuary, the holy of holies. He is to sacrifice a bull for the sins of the priests, a goat for the sins of the laypeople; the priest is to send a second goat into the desert to "Azazel", bearing the sins of the whole people. Azazel may be a wilderness-demon. Chapters 17–26 are the Holiness code, it begins with a prohibition on all slaughter of animals outside the Temple for food, prohibits a long list of sexual contacts and child sacrifice.
The "holiness" injunctions which give the code its name begin with the next section: there is are penalties for the worship of Molech, consulting mediums and wizards, cursing one's parents and engaging in unlawful sex. Priests receive instruction on acceptable bodily defects; the punishment for blasphemy is death, there is the setting of rules for eating sacrifices.