In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic period through European colonization, which began with Christopher Columbus's voyage of 1492. Usually, the era covers the history of Indigenous cultures until significant influence by Europeans. This may have occurred decades or even centuries after Columbus for certain cultures.

Many pre-Columbian civilizations were marked by permanent settlements, cities, agriculture, civic and monumental architecture, major earthworks, and complex societal hierarchies. Some of these civilizations had long faded by the time of the first permanent European colonies (–early 17th centuries), and are known only through archaeological investigations and oral history. Other civilizations were contemporary with the colonial period and were described in European historical accounts of the time. A few, such as the Maya civilization, had their written records. Because many Christian Europeans of the time viewed such texts as pagan, men like Diego de Landa burned them, even while seeking to preserve native histories. Only a few hidden documents have survived in their original languages, while others were transcribed or dictated into Spanish, giving modern historians glimpses of ancient culture and knowledge.

Many Indigenous peoples in the Americas continue traditional practices while evolving and adapting to the modern world.

The alternative terms precontact, pre-colonial, or prehistoric Americas are also used; in Hispanic America, the usual term is pre-Hispanic; in Brazil, the term used is pre-Cabraline.

Historiography
Before the development of archaeology in the 19th century, historians of the pre-Columbian period mainly interpreted the records of the European conquerors and the accounts of early European travelers and antiquaries. It was not until the nineteenth century that the work of people such as John Lloyd Stephens, Eduard Seler, and Alfred Maudslay, and institutions such as the Peabody Museum of Archaeology and Ethnology of Harvard University, led to the reconsideration and criticism of the early European sources. Now, the scholarly study of pre-Columbian cultures is most often based on scientific and multidisciplinary methodologies.

Genetics

The haplogroup most commonly associated with Indigenous genetics is Haplogroup Q1a3a (Y-DNA). Researchers have found genetic evidence that the Q1a3a haplogroup has been in South America since at least 18,000 BC. Y-DNA, like mtDNA, differs from other nuclear chromosomes in that the majority of the Y chromosome is unique and does not recombine during meiosis. This has the effect that the historical pattern of mutations can easily be studied. The pattern indicates Indigenous peoples experienced two very distinctive genetic episodes: first with the initial peopling of the Americas and second with European colonization of the Americas. The former is the determinant factor for the number of gene lineages and founding haplotypes present in today's Indigenous populations.

Human settlement of the Americas occurred in stages from the Bering Sea coastline, with an initial 20,000-year layover on Beringia for the founding population. The micro-satellite diversity and distributions of the Y lineage specific to South America indicate that certain Amerindian populations have been isolated since the initial colonization of the region. The Na-Dené, Inuit, and Indigenous Alaskan populations exhibit haplogroup Q-M242 (Y-DNA) mutations, however, and are distinct from other Indigenous peoples with various mtDNA mutations. This suggests that the earliest migrants into the northern extremes of North America and Greenland derived from later populations.

Settlement of the Americas

Asian nomadic Paleo-Indians are thought to have entered the Americas via the Bering Land Bridge (Beringia), now the Bering Strait, and possibly along the coast. Genetic evidence found in Indigenous peoples' maternally inherited mitochondrial DNA (mtDNA) supports the theory of multiple genetic populations migrating from Asia. After crossing the land bridge, they moved southward along the Pacific coast and through an interior ice-free corridor. Throughout millennia, Paleo-Indians spread throughout the rest of North and South America.

Exactly when the first people migrated into the Americas is the subject of much debate. One of the earliest identifiable cultures was the Clovis culture, with sites dating from some 13,000 years ago. However, older sites dating back to 20,000 years ago have been claimed. Some genetic studies estimate the colonization of the Americas dates from between 40,000 and 13,000 years ago. The chronology of migration models is currently divided into two general approaches. The first is the short chronology theory with the first movement beyond Alaska into the Americas occurring no earlier than 14,000–17,000 years ago, followed by successive waves of immigrants. The second belief is the long chronology theory, which proposes that the first group of people entered the hemisphere at a much earlier date, possibly 50,000–40,000 years ago or earlier.

Artifacts have been found in both North and South America which have been dated to 14,000 years ago, and accordingly humans have been proposed to have reached Cape Horn at the southern tip of South America by this time. In that case, the Inuit would have arrived separately and at a much later date, probably no more than 2,000 years ago, moving across the ice from Siberia into Alaska.

North America

Lithic and Archaic periods

The North American climate was unstable as the ice age receded during the Lithic stage. It finally stabilized about 10,000 years ago; climatic conditions were then very similar to today's. Within this time frame, roughly about the Archaic Period, numerous archaeological cultures have been identified.

Lithic stage and early Archaic period
The unstable climate led to widespread migration, with early Paleo-Indians soon spreading throughout the Americas, diversifying into many hundreds of culturally distinct tribes. The Paleo-Indians were hunter-gatherers, likely characterized by small, mobile bands consisting of approximately 20 to 50 members of an extended family. These groups moved from place to place as preferred resources were depleted and new supplies were sought. During much of the Paleo-Indian period, bands are thought to have subsisted primarily through hunting now-extinct giant land animals such as mastodon and ancient bison. Paleo-Indian groups carried a variety of tools, including distinctive projectile points and knives, as well as less distinctive butchering and hide-scraping implements.

The vastness of the North American continent, and the variety of its climates, ecology, vegetation, fauna, and landforms, led ancient peoples to coalesce into many distinct linguistic and cultural groups. This is reflected in the oral histories of the indigenous peoples, described by a wide range of traditional creation stories which often say that a given people have been living in a certain territory since the creation of the world.

Throughout thousands of years, paleo-Indian people domesticated, bred, and cultivated many plant species, including crops that now constitute 50–60% of worldwide agriculture. In general, Arctic, Subarctic, and coastal peoples continued to live as hunters and gatherers, while agriculture was adopted in more temperate and sheltered regions, permitting a dramatic rise in population.

Middle Archaic period
After the migration or migrations, it was several thousand years before the first complex societies arose, the earliest emerging about seven to eight thousand years ago. As early as 6500 BCE, people in the Lower Mississippi Valley at the Monte Sano site were building complex earthwork mounds, probably for religious purposes. This is the earliest dated of numerous mound complexes found in present-day Louisiana, Mississippi, and Florida. Since the late twentieth century, archeologists have explored and dated these sites. They have found that they were built by hunter-gatherer societies, whose people occupied the sites on a seasonal basis, and who had not yet developed ceramics. Watson Brake, a large complex of eleven platform mounds, was constructed beginning in 3400 BCE and added to over 500 years. This has changed earlier assumptions that complex construction arose only after societies had adopted agriculture, and become sedentary, with stratified hierarchy and usually ceramics. These ancient people had organized to build complex mound projects under a different social structure.

Late Archaic period

Until the accurate dating of Watson Brake and similar sites, the oldest mound complex was thought to be Poverty Point, also located in the Lower Mississippi Valley. Built about 1500 BCE, it is the centerpiece of a culture extending over 100 sites on both sides of the Mississippi. The Poverty Point site has earthworks in the form of six concentric half-circles, divided by radial aisles, together with some mounds. The entire complex is nearly a mile across.

Mound building was continued by succeeding cultures, who built numerous sites in the middle Mississippi and Ohio River valleys as well, adding effigy mounds, conical and ridge mounds, and other shapes.

Woodland period

The Woodland period of North American pre-Columbian cultures lasted from roughly 1000 BCE to 1000 CE. The term was coined in the 1930s and refers to prehistoric sites between the Archaic period and the Mississippian cultures. The Adena culture and the ensuing Hopewell tradition during this period built monumental earthwork architecture and established continent-spanning trade and exchange networks.

This period is considered a developmental stage without any massive changes in a short period but instead has a continuous development in stone and bone tools, leatherworking, textile manufacture, tool production, cultivation, and shelter construction. Some Woodland people continued to use spears and atlatls until the end of the period when they were replaced by bows and arrows.

Mississippian culture

The Mississippian culture was spread across the Southeast and Midwest from the Atlantic coast to the edge of the plains, from the Gulf of Mexico to the Upper Midwest, although most intensively in the area along the Mississippi River and Ohio River. One of the distinguishing features of this culture was the construction of complexes of large earthen mounds and grand plazas, continuing the mound-building traditions of earlier cultures. They grew maize and other crops intensively, participated in an extensive trade network, and had a complex stratified society. The Mississippians first appeared around 1000 CE, following and developing out of the less agriculturally intensive and less centralized Woodland period. The largest urban site of these people, Cahokia—located near modern East St. Louis, Illinois—may have reached a population of over 20,000. Other chiefdoms were constructed throughout the Southeast, and its trade networks reached to the Great Lakes and the Gulf of Mexico. At its peak, between the 12th and 13th centuries, Cahokia was the most populous city in North America. (Larger cities did exist in Mesoamerica and South America.) Monks Mound, the major ceremonial center of Cahokia, remains the largest earthen construction of the prehistoric Americas. The culture reached its peak in about 1200–1400 CE, and in most places, it seems to have been in decline before the arrival of Europeans.

Many Mississippian peoples were encountered by the expedition of Hernando de Soto in the 1540s, mostly with disastrous results for both sides. Unlike the Spanish expeditions in Mesoamerica, which conquered vast empires with relatively few men, the de Soto expedition wandered the American Southeast for four years, becoming more bedraggled, losing more men and equipment, and eventually arriving in Mexico as a fraction of its original size. The local people fared much worse though, as the fatalities of diseases introduced by the expedition devastated the populations and produced much social disruption. By the time Europeans returned a hundred years later, nearly all of the Mississippian groups had vanished, and vast swaths of their territory were virtually uninhabited.

Ancestral Puebloans

The Ancestral Puebloans thrived in what is now the Four Corners region in the United States. It is commonly suggested that the culture of the Ancestral Puebloans emerged during the Early Basketmaker II Era during the 12th century BCE. The Ancestral Puebloans were a complex Oasisamerican society that constructed kivas, multi-story houses, and apartment blocks made from stone and adobe, such as the Cliff Palace of Mesa Verde National Park in Colorado and the Great Houses in Chaco Canyon, New Mexico. The Puebloans also constructed a road system that stretched from Chaco Canyon to Kutz Canyon in the San Juan Basin. The Ancestral Puebloans are also known as "Anasazi", though the term is controversial, as the present-day Pueblo peoples consider the term to be derogatory, due to the word tracing its origins to a Navajo word meaning "ancestor enemies".

Hohokam

The Hohokam thrived in the Sonoran desert in what is now the U.S. state of Arizona and the Mexican state of Sonora. The Hohokam were responsible for the construction of a series of irrigation canals that led to the successful establishment of Phoenix, Arizona via the Salt River Project. The Hohokam also established complex settlements such as Snaketown, which served as an important commercial trading center. After 1375 CE, Hohokam society collapsed and the people abandoned their settlements, likely due to drought.

Mogollon

The Mogollon resided in the present-day states of Arizona, New Mexico, and Texas as well as Sonora and Chihuahua. Like most other cultures in Oasisamerica, the Mogollon constructed sophisticated kivas and cliff dwellings. In the village of Paquimé, the Mogollon are revealed to have housed pens for scarlet macaws, which were introduced from Mesoamerica through trade.

Sinagua

The Sinagua were hunter-gatherers and agriculturalists who lived in central Arizona. Like the Hohokam, they constructed kivas and great houses as well as ballcourts. Several of the Sinagua ruins include Montezuma Castle, Wupatki, and Tuzigoot.

Salado

The Salado resided in the Tonto Basin in southeastern Arizona from 1150 CE to the 15th century. Archaeological evidence suggests that they traded with far-away cultures, as evidenced by the presence of seashells from the Gulf of California and macaw feathers from Mexico. Most of the cliff dwellings constructed by the Salado are primarily located in Tonto National Monument.

Iroquois

The Iroquois League of Nations or "People of the Long House" was a politically advanced, democratic society, which is thought by some historians to have influenced the United States Constitution, with the Senate passing a resolution to this effect in 1988. Other historians have contested this interpretation and believe the impact was minimal or did not exist, pointing to numerous differences between the two systems and the ample precedents for the constitution in European political thought.

Calusa

The Calusa were a complex paramountcy/kingdom that resided in southern Florida. Instead of agriculture, the Calusa economy relied on abundant fishing. According to Spanish sources, the "king's house" at Mound Key was large enough to house 2,000 people. The Calusa ultimately collapsed into extinction at around 1750 after succumbing to diseases introduced by the Spanish colonists.

Wichita

The Wichita people were a loose confederation that consisted of sedentary agriculturalists and hunter-gatherers who resided in the eastern Great Plains. They lived in permanent settlements and even established a city called Etzanoa, which had a population of 20,000 people. The city was eventually abandoned around the 18th century after it was encountered by Spanish conquistadors Jusepe Gutierrez and Juan de Oñate.

Historic tribes
When the Europeans arrived, Indigenous peoples of North America had a wide range of lifeways from sedentary, agrarian societies to semi-nomadic hunter-gatherer societies. Many formed new tribes or confederations in response to European colonization. These are often classified by cultural regions, loosely based on geography. These can include the following:
 Arctic, including Aleut, Inuit, and Yupik peoples
 Subarctic
 Northeastern Woodlands
 Southeastern Woodlands
 Great Plains
 Great Basin
 Northwest Plateau
 Northwest Coast
 California
 Southwest

Numerous pre-Columbian societies were sedentary, such as the Tlingit, Haida, Chumash, Mandan, Hidatsa, and others, and some established large settlements, even cities, such as Cahokia, in what is now Illinois.

Mesoamerica

Mesoamerica is the region extending from central Mexico south to the northwestern border of Costa Rica that gave rise to a group of stratified, culturally related agrarian civilizations spanning an approximately 3,000-year period before the visits to the Caribbean by Christopher Columbus. Mesoamerican is the adjective generally used to refer to that group of pre-Columbian cultures. This refers to an environmental area occupied by an assortment of ancient cultures that shared religious beliefs, art, architecture, and technology in the Americas for more than three thousand years. Between 2000 and 300 BCE, complex cultures began to form in Mesoamerica. Some matured into advanced pre-Columbian Mesoamerican civilizations such as the Olmec, Teotihuacan, Mayas, Zapotecs, Mixtecs, Huastecs, Purepecha, Toltecs, and Mexica/Aztecs. The Mexica civilization is also known as the Aztec Triple Alliance since they were three smaller kingdoms loosely united together.

These Indigenous civilizations are credited with many inventions: building pyramid temples, mathematics, astronomy, medicine, writing, highly accurate calendars, fine arts, intensive agriculture, engineering, an abacus calculator, and complex theology. They also invented the wheel, but it was used solely as a toy. In addition, they used native copper, silver, and gold for metalworking.

Archaic inscriptions on rocks and rock walls all over northern Mexico (especially in the state of Nuevo León) demonstrate an early propensity for counting. Their number system was base 20 and included zero. These early count markings were associated with astronomical events and underscore the influence that astronomical activities had upon Mesoamerican people before the arrival of Europeans. Many of the later Mesoamerican civilizations carefully built their cities and ceremonial centers according to specific astronomical events.

The biggest Mesoamerican cities, such as Teotihuacan, Tenochtitlan, and Cholula, were among the largest in the world. These cities grew as centers of commerce, ideas, ceremonies, and theology, and they radiated influence outwards onto neighboring cultures in central Mexico.

While many city-states, kingdoms, and empires competed with one another for power and prestige, Mesoamerica can be said to have had five major civilizations: the Olmecs, Teotihuacan, the Toltecs, the Mexica, and the Mayas. These civilizations (except for the politically fragmented Maya) extended their reach across Mesoamerica—and beyond—like no others. They consolidated power and distributed influence in matters of trade, art, politics, technology, and theology. Other regional power players made economic and political alliances with these civilizations over 4,000 years. Many made war with them, but almost all peoples found themselves within one of their spheres of influence.

Regional communications in ancient Mesoamerica have been the subject of considerable research. There is evidence of trade routes starting as far north as the Mexico Central Plateau, and going down to the Pacific coast. These trade routes and cultural contacts then went on as far as Central America. These networks operated with various interruptions from pre-Olmec times and up to the Late Classical Period (600–900 CE).

Olmec civilization

The earliest known civilization in Mesoamerica is the Olmec. This civilization established the cultural blueprint by which all succeeding indigenous civilizations would follow in Mexico. Pre-Olmec civilization began with the production of pottery in abundance, around 2300 BCE in the Grijalva River delta. Between 1600 and 1500 BCE, the Olmec civilization had begun, with the consolidation of power at their capital, a site today known as San Lorenzo Tenochtitlán near the coast in southeast Veracruz. The Olmec influence extended across Mexico, into Central America, and along the Gulf of Mexico. They transformed many peoples' thinking toward a new way of government, pyramid temples, writing, astronomy, art, mathematics, economics, and religion. Their achievements paved the way for the Maya civilization and the civilizations in central Mexico.

Teotihuacan civilization

The decline of the Olmec resulted in a power vacuum in Mexico. Emerging from that vacuum was Teotihuacan, first settled in 300 BCE. By 150 CE, Teotihuacan had risen to become the first true metropolis of what is now called North America. Teotihuacan established a new economic and political order never before seen in Mexico. Its influence stretched across Mexico into Central America, founding new dynasties in the Maya cities of Tikal, Copan, and Kaminaljuyú. Teotihuacan's influence over the Maya civilization cannot be overstated: it transformed political power, artistic depictions, and the nature of economics. Within the city of Teotihuacan was a diverse and cosmopolitan population. Most of the regional ethnicities of Mexico were represented in the city, such as Zapotecs from the Oaxaca region. They lived in apartment communities where they worked their trades and contributed to the city's economic and cultural prowess. Teotihuacan's economic pull impacted areas in northern Mexico as well. It was a city whose monumental architecture reflected a monumental new era in Mexican civilization, declining in political power about 650 CE—but lasting in cultural influence for the better part of a millennium, to around 950 CE.

Maya civilization

Contemporary Teotihuacan's greatness was that of the Maya civilization. The period between 250 CE and 650 CE was a time of intense flourishing of Maya civilized accomplishments. While the many Maya city-states never achieved political unity on the order of the central Mexican civilizations, they exerted tremendous intellectual influence upon Mexico and Central America. The Maya built some of the most elaborate cities on the continent and made innovations in mathematics, astronomy, and calendrics. The Maya also developed the only true writing system native to the Americas using pictographs and syllabic elements in the form of texts and codices inscribed on stone, pottery, wood, or perishable books made from bark paper.

Huastec civilization

The Huastecs were a Maya ethnic group that migrated northwards to the Gulf Coast of Mexico. The Huastecs are considered to be distinct from the Maya civilization, as they separated from the main Maya branch at around 2000 BC and did not possess the Maya script. Other accounts also suggest that the Huastecs migrated as a result of the Classic Maya collapse around the year 900 AD.

Zapotec civilization

The Zapotecs were a civilization that thrived in the Oaxaca Valley from the late 6th century BC until their downfall at the hands of the Spanish conquistadors. The city of Monte Albán was an important religious center for the Zapotecs and served as the capital of the empire from 700 BCE to 700 CE. The Zapotecs resisted the expansion of the Aztecs until they were subjugated in 1502 under Aztec emperor Ahuitzotl. After the Spanish conquest of the Aztec Empire, the Zapotecs resisted Spanish rule until King Cosijopii I surrendered in 1563.

Mixtec civilization

Like the Zapotecs, the Mixtecs thrived in the Oaxaca Valley. The Mixtecs consisted of separate independent kingdoms and city-states, rather than a single unified empire. The Mixtecs would eventually be conquered by the Aztecs until the Spanish conquest. The Mixtecs saw the Spanish conquest as an opportunity for liberation and established agreements with the conquistadors that allowed them to preserve their cultural traditions, though relatively few sections resisted Spanish rule.

Totonac civilization

The Totonac civilization was concentrated in the present-day states of Veracruz and Puebla. The Totonacs were responsible for the establishment of cities, such as El Tajín as important commercial trading centers. The Totonacs would later assist in the Spanish conquest of the Aztec Empire as an opportunity to liberate themselves from Aztec military imperialism.

Toltec civilization

The Toltec civilization was established in the 8th century CE. The Toltec Empire expanded its political borders to as far south as the Yucatán peninsula, including the Maya city of Chichen Itza. The Toltecs established vast trading relations with other Mesoamerican civilizations in Central America and the Puebloans in present-day New Mexico. During the Post-Classic era, the Toltecs suffered a subsequent collapse in the early 12th century, due to famine and civil war. The Toltec civilization was so influential to the point where many groups such as the Aztecs claimed to be descended from.

Aztec/Mexica/Triple Alliance civilization

With the decline of the Toltec civilization came political fragmentation in the Valley of Mexico. Into this new political game of contenders to the Toltec throne stepped outsiders: the Mexica. They were also a desert people, one of seven groups who formerly called themselves "Azteca", in memory of Aztlán, but they changed their name after years of migrating. Since they were not from the Valley of Mexico, they were initially seen as crude and unrefined in the ways of the Nahua civilization. Through political maneuvers and ferocious martial skills, they managed to rule Mexico as the head of the 'Triple Alliance' which included two other Aztec cities, Tetxcoco and Tlacopan.

Latecomers to Mexico's central plateau, the Mexica thought of themselves, nevertheless, as heirs of the civilizations that had preceded them. For them, arts, sculpture, architecture, engraving, feather-mosaic work, and the calendar, were bequest from the former inhabitants of Tula, the Toltecs.

The Mexica-Aztecs were the rulers of much of central Mexico by about 1400 (while Yaquis, Coras, and Apaches commanded sizable regions of northern desert), having subjugated most of the other regional states by the 1470s. At their peak, the Valley of Mexico where the Aztec Empire presided, saw a population growth that included nearly 1 million people during the late Aztec period (1350–1519).

Their capital, Tenochtitlan, is the site of modern-day Mexico City. At its peak, it was one of the largest cities in the world with population estimates of 200,000–300,000. The market established there was the largest ever seen by the conquistadores on arrival.

Tarascan/Purepecha civilization

Initially, the lands that would someday comprise the lands of the powerful Tarascan Empire were inhabited by several independent communities. Around 1300, however, the first Cazonci, Tariacuri, united these communities and built them into one of the most advanced civilizations in Mesoamerica. Their capital at Tzintzuntzan was just one of the many cities—there were ninety more under its control. The Tarascan Empire was among the largest in Central America, so it is no surprise that they routinely came into conflict with the neighboring Aztec Empire. Out of all the civilizations in its area, the Tarascan Empire was the most prominent in metallurgy, harnessing copper, silver, and gold to create items such as tools, decorations, and even weapons and armor. Bronze was also used. The great victories over the Aztecs by the Tarascans cannot be understated. Nearly every war they fought in resulted in a Tarascan victory. Because the Tarascan Empire had little links to the former Toltec Empire, they were also quite independent in culture from their neighbors. The Aztecs, Tlaxcaltec, Olmec, Mixtec, Maya, and others were very similar to each other, however. This is because they were all directly preceded by the Toltecs, and they therefore shared almost identical cultures. The Tarascans, however, possessed a unique religion, as well as other things.

Tlaxcala republic

Tlaxcala was a Nahua republic and confederation in central Mexico. The Tlaxcalans fiercely resisted Aztec expansion during the Flower Wars ever since the Aztecs expelled them from Lake Texcoco. The Tlaxcalans would later ally with the Spanish conquistadors under Hernán Cortés as an opportunity to liberate them from the Aztecs and managed to successfully conquer the Aztecs with the help of the conquistadors. The Spaniards would reward the Tlaxcalans for preserving their culture and for their assistance in defeating the Aztecs. The Tlaxcalans would once again assist to the Spaniards during the Mixtón War and the conquest of Guatemala.

Cuzcatlan

Cuzcatlan was a Pipil confederacy of kingdoms and city-states located in present-day El Salvador. According to legend, Cuzcatlan was established by Toltec migrants during the Classic Maya collapse in approximately 1200 AD. During the Spanish conquest of El Salvador, Cuzcatlan was forced to surrender to conquistador Pedro de Alvarado in 1528.

Lenca

The Lenca people were composed of several distinct multilingual confederations and city-states in present-day El Salvador and Honduras. Cities such as Yarumela were important commercial centers for the Lenca. During the Spanish conquest, several Lenca leaders such as Lempira resisted conversion to Christianity, while others converted peacefully.

Nicarao

An offshoot of the Pipil people from El Salvador, the Nicarao people were a tribal confederation that flourished in present-day Nicaragua. The migration of the Nicarao is theorized to have led to the fall of the city of Teotihuacan and the Toltec city of Tula. The Nicarao civilization was disestablished during the Spanish conquest of Nicaragua in 1522.

Nicoya kingdom

The Nicoya kingdom was an elective monarchy that thrived in the Nicoya peninsula in Costa Rica. It existed from 800 CE until THE Spanish arrival in the 16th century.

South America

By the first millennium, South America's vast rainforests, mountains, plains, and coasts were the home of millions of people. Estimates vary, but 30–50 million are often given, and 100 million by some estimates. Some groups formed permanent settlements. Among those groups were Chibcha-speaking peoples ("Muisca" or "Muysca"), Valdivia, Quimbaya, Calima, Marajoara culture, and the Tairona. The Muisca of Colombia, postdating the Herrera Period, Valdivia of Ecuador, the Quechuas, and the Aymara of Peru and Bolivia were the four most important sedentary Amerindian groups in South America. Since the 1970s, numerous geoglyphs have been discovered on deforested land in the Amazon rainforest, Brazil, supporting Spanish accounts of complex and ancient Amazonian civilizations, such as Kuhikugu.

The theory of pre-Columbian contact across the South Pacific Ocean between South America and Polynesia has received support from several lines of evidence, although solid confirmation remains elusive. A diffusion by human agents has been put forward to explain the pre-Columbian presence in Oceania of several cultivated plant species native to South America, such as the bottle gourd (Lagenaria siceraria) or sweet potato (Ipomoea batatas). Direct archaeological evidence for such pre-Columbian contacts and transport has not emerged. Similarities noted in the names of edible roots in Maori and Ecuadorian languages ("kumari") and Melanesian and Chilean ("gaddu") have been inconclusive.

A 2007 paper published in PNAS put forward DNA and archaeological evidence that domesticated chickens had been introduced into South America via Polynesia by late pre-Columbian times. These findings were challenged by a later study published in the same journal, that cast doubt on the dating calibration used and presented alternative mtDNA analyses that disagreed with a Polynesian genetic origin. The origin and dating remain an open issue. Whether or not early Polynesian–American exchanges occurred, no compelling human-genetic, archaeological, cultural, or linguistic legacy of such contact has turned up.

Norte Chico civilization

On the north-central coast of present-day Peru, Norte Chico or Caral (as known in Peru) was a civilization that emerged around 3000 BCE (contemporary with urbanism's rise in Mesopotamia). It had a cluster of large-scale urban settlements of which the Sacred City of Caral, in the Supe Valley, is one of the largest and best-studied sites. The civilization did not know machinery or pottery but still managed to develop trade, especially cotton and dehydrated fish. It was a hierarchical society that managed its ecosystems and had intercultural exchange. Its economy was heavily dependent on agriculture and fishing on the nearby coast. It is considered one of the cradles of civilization in the world and Caral is the oldest known civilization in the Americas.

Valdivia culture

The Valdivia culture was concentrated on the coast of Ecuador. Their existence was recently discovered by archeological findings. Their culture is among the oldest found in the Americas, spanning from 3500 to 1800 BCE. The Valdivia lived in a community of houses built in a circle or oval around a central plaza. They were sedentary people who lived off farming and fishing, though occasionally they hunted for deer. From the remains that have been found, scholars have determined that Valdivians cultivated maize, kidney beans, squash, cassava, chili peppers, and cotton plants, the last of which was used to make clothing. Valdivian pottery initially was rough and practical, but it became showy, delicate, and big over time. They generally used red and gray colors, and the polished dark red pottery is characteristic of the Valdivia period. In its ceramics and stone works, the Valdivia culture shows a progression from the most simple to much more complicated works.

Cañari people

The Cañari were the indigenous natives of today's Ecuadorian provinces of Cañar and Azuay. They were an elaborate civilization with advanced architecture and complex religious beliefs. The Inca destroyed and burned most of their remains. The Cañari's old city was replaced twice, first by the Incan city of Tumebamba and later on the same site by the colonial city of Cuenca. The city was also believed to be the site of El Dorado, the city of gold from the mythology of Colombia.

The Cañari were most notable for having repelled the Incan invasion with fierce resistance for many years until they fell to Tupac Yupanqui. Many of their descendants are still present in Cañar. The majority did not mix with the colonists or become Mestizos.

Chavín civilization

The Chavín, a Peruvian preliterate civilization, established a trade network and developed agriculture by 900 BCE, according to some estimates and archeological finds. Artifacts were found at a site called Chavín in modern Peru at an elevation of . The Chavín civilization spanned from 900 to 300 BCE.

Muisca confederation

The Chibcha-speaking communities were the most numerous, the most territorially extended and the most socio-economically developed of the pre-Hispanic Colombians. By the 8th century, the indigenous people had established their civilization in the northern Andes. At one point, the Chibchas occupied part of what is now Panama, and the high plains of the Eastern Sierra of Colombia.

The areas that they occupied in Colombia were the present-day Departments of Santander (North and South), Boyacá, and Cundinamarca. This is where the first farms and industries were developed. It is also where the independence movement originated. They are currently the richest areas in Colombia. The Chibcha developed the most populous zone between the Maya region and the Inca Empire. Next to the Quechua of Peru and the Aymara in Bolivia, the Chibcha of the eastern and north-eastern Highlands of Colombia developed the most notable culture among the sedentary Indigenous peoples in South America.

In the Colombian Andes, the Chibcha comprised several tribes who spoke similar languages (Chibcha). They included the following: the Muisca, Guane, Lache, Cofán, and Chitareros.

Tairona confederation

The Tairona civilization thrived in the Sierra Nevada de Santa Marta mountain range in northern Colombia. Studies suggest that the civilization thrived from the 1st century CE until the Spanish arrival in the 16th century. The descendants of the Tairona, such as the Kogi were one of the few indigenous groups in the Americas to have escaped full colonial conquest and retain a majority of their indigenous cultures.

Moche civilization

The Moche thrived on the north coast of Peru from about 100 to 800 CE. The heritage of the Moche is seen in their elaborate burials. Some were recently excavated by UCLA's Christopher B. Donnan in association with the National Geographic Society.

As skilled artisans, the Moche were a technologically advanced people. They traded with distant peoples such as the Maya. What has been learned about the Moche is based on the study of their ceramic pottery; the carvings reveal details of their daily lives. The Larco Museum of Lima, Peru, has an extensive collection of such ceramics. They show that the people practiced human sacrifice, had blood-drinking rituals and that their religion incorporated non-procreative sexual practices (such as fellatio).

Wari Empire

The Wari Empire was located in the western portion of Peru and existed from the 6th century to the 11th century. Wari, as the former capital city was called, is located 11 km (6.8 mi) northeast of the city of Ayacucho. This city was the center of a civilization that covered much of the highlands and coast of Peru. The best-preserved remnants, besides the Wari Ruins, are the recently discovered Northern Wari ruins near the city of Chiclayo, and Cerro Baul in Moquegua. Also well-known are the Wari ruins of Pikillaqta ("Flea Town"), a short distance southeast of the Cusco en route to Lake Titicaca.

Tiwanaku Empire

The Tiwanaku empire was based in western Bolivia and extended into present-day Peru and Chile from 300 to 1000. Tiwanaku is recognized by Andean scholars as one of the most important South American civilizations before the birth of the Inca Empire in Peru; it was the ritual and administrative capital of a major state power for approximately five hundred years. The ruins of the ancient city state are near the south-eastern shore of Lake Titicaca in Tiwanaku Municipality, Ingavi Province, La Paz Department, about  west of La Paz.

Inca Empire

Holding their capital at the great cougar-shaped city of Cusco, Peru, the Inca civilization dominated the Andes region from 1438 to 1533. Known as Tawantinsuyu, or "the land of the four regions", in Quechua, the Inca civilization was highly distinct and developed. Inca rule extended to nearly a hundred linguistic or ethnic communities, some 9 to 14 million people connected by a 40,000-kilometer road system. Cities were built with precise stonework, constructed over many levels of mountain terrain. Terrace farming was a useful form of agriculture. There is evidence of excellent metalwork and even successful brain surgery in the Inca civilization.

Aymara kingdoms

The Aymara kingdoms consisted of a confederation of separate diarchies that lasted from 1151 after the fall of Tiwanaku until 1477 when they were conquered by the Inca Empire. The Aymara kingdoms were primarily located in the Altiplano in Bolivia as well as some parts of Peru and Chile.

Arawaks and Caribs
Archeologists have discovered evidence of the earliest known inhabitants of the Venezuelan area in the form of leaf-shaped flake tools, together with chopping and plano–convex scraping implements exposed on the high riverine terraces of the Pedregal River in western Venezuela. Late Pleistocene hunting artifacts, including spear tips, come from a similar site in northwestern Venezuela known as El Jobo. According to radiocarbon dating, these date from 13,000 to 7000 BCE.

Taima-Taima, yellow Muaco, and El Jobo in Falcón are some of the sites that have yielded archeological material from these times. These groups co-existed with megafauna like megatherium, glyptodonts, and toxodonts.

Archaeologists identify a Meso-Indian period from 7000–5000 CE to 1000 CE In this period, hunters and gatherers of megafauna started to turn to other food sources and established the first tribal structures.

Beginning around 1000 CE archaeologists speak of the Neo-Indian period, which ends with the European Conquest and Colony period.

It is not known how many people lived in Venezuela before the Spanish Conquest; it may have been around a million people, in addition to today's peoples included groups such as the Arawaks, Caribs, and Timoto-cuicas. The number was much reduced after the Conquest, mainly through the spread of new diseases from Europe. There were two main north-south axes of the pre-Columbian population, producing maize in the west and manioc in the east. Large parts of the Llanos plains were cultivated through a combination of slash-and-burn and permanent settled agriculture. The indigenous peoples of Venezuela had already encountered crude oils and asphalts that seeped up through the ground to the surface.  Known to the locals as mene, the thick, black liquid was primarily used for medicinal purposes, as an illumination source, and for the caulking of canoes.

In the 16th century when Spanish colonization began in Venezuelan territory, the population of several indigenous peoples such as the Mariches (descendants of the Caribes) declined.

Diaguita confederation

The Diaguita consisted of several distinct chiefdoms across the Argentine Northwest. The Diaguita culture emerged around 1000 CE after the replacement of the Las Ánimas complex. The Diaguita resisted Spanish colonialism during the Calchaquí Wars until they were forced to surrender and submit to Spanish rule in 1667.

Taíno

The Taíno people were fragmented into numerous chiefdoms across the Greater Antilles, the Lucayan Archipelago, and the northern Lesser Antilles. The Taíno were the first pre-Columbian people to encounter Christopher Columbus during his voyage in 1492. The Taíno would later be subject to slavery by the Spanish colonists under the encomienda system until they were deemed virtually extinct in 1565.

Huetar kingdoms

The Huetar people were a major ethnic group that lived in Costa Rica. The Huetar were composed of several independent kingdoms, such as the western kingdom ruled by Garabito and the eastern kingdom ruled by El Guarco and Correque. After their annexation into Spanish administration, the descendants of the Huetar currently reside in the Quitirrisí reserve.

Marajoara culture

The Marajoara culture flourished on Marajó Island at the mouth of the Amazon River in northern Brazil between 800 to 1400 CE. The Marajoara consisted of a complex society that built mounds and constructed sophisticated settlements. The indigenous people of the area adopted methods of large-scale agriculture through the use of terra preta, which would support complex chiefdoms. Studies suggest that the civilization housed around 100,000 inhabitants.

Kuhikugu

Located in the Xingu Indigenous Park in Brazil, Kuhikugu consisted of an urban complex that housed around 50,000 inhabitants and 20 settlements. The civilization was likely established by the ancestors of the Kuikuro people. The people also constructed roads, bridges, and trenches for defensive purposes and were purported to be farmers, as evidenced by the fields of cassava and the use of terra preta. Like most other Amazonian civilizations, the disappearance of Kuhikugu was largely attributed to Old World diseases introduced by European colonists.

Cambeba

Also known as the Omagua, Umana, and Kambeba, the Cambeba are an indigenous people in Brazil's Amazon basin. The Cambeba were a populous, organized society in the late pre-Columbian era whose population suffered a steep decline in the early years of the Columbian Exchange. The Spanish explorer Francisco de Orellana traversed the Amazon River during the 16th century and reported densely populated regions running hundreds of kilometers along the river. These populations left no lasting monuments, possibly because they used local wood as their construction material as stone was not locally available. While it is possible Orellana may have exaggerated the level of development among the Amazonians, their semi-nomadic descendants have the odd distinction among tribal indigenous societies of a hereditary, yet landless, aristocracy. Archaeological evidence has revealed the continued presence of semi-domesticated orchards, as well as vast areas of land enriched with terra preta. Both of these discoveries, along with Cambeba ceramics discovered within the same archaeological levels suggest that a large and organized civilization existed in the area.

Agricultural development

Early inhabitants of the Americas developed agriculture, developing and breeding maize (corn) from ears  in length to the current size that is familiar today. Potatoes, tomatoes, tomatillos (a husked green tomato), pumpkins, chili peppers, squash, beans, pineapple, sweet potatoes, the grains quinoa and amaranth, cocoa beans, vanilla, onion, peanuts, strawberries, raspberries, blueberries, blackberries, papaya, and avocados were among other plants grown by natives. Over two-thirds of all types of food crops grown worldwide are native to the Americas.

Early Indigenous peoples began using fire in a widespread manner. Intentional burning of vegetation was taken up to mimic the effects of natural fires that tended to clear forest understories, thereby making travel easier and facilitating the growth of herbs and berry-producing plants that were important for both food and medicines. This created the pre-Columbian savannas of North America.

While not as widespread as in Afro-Eurasia, indigenous Americans did have livestock. Domesticated turkeys were common in Mesoamerica and some regions of North America; they were valued for their meat, feathers, and, possibly, eggs. There is documentation of Mesoamericans utilizing hairless dogs, especially the Xoloitzcuintle breed, for their meat. Andean societies had llamas and alpacas for meat and wool, as well as for beasts of burden. Guinea pigs were raised for meat in the Andes. Iguanas and a range of wild animals, such as deer and pecari, were another source of meat in Mexico, Central, and northern South America.

By the 15th century, maize had been transmitted from Mexico and was being farmed in the Mississippi embayment, as far as the East Coast of the United States, and as far north as southern Canada. Potatoes were used by the Inca, and chocolate was used by the Aztecs.

See also

 1491: New Revelations of the Americas Before Columbus by Charles C. Mann
 Before the Revolution, 2013 book by Daniel K. Richter
 List of pre-Columbian cultures
 Metallurgy in pre-Columbian America
 Periodization of pre-Columbian Peru
 Population history of indigenous peoples of the Americas
 Pre-Columbian trans-oceanic contact theories
 Pre-Columbian history of Brazil

References

Bibliography

External links
 published on July 5, 2019, Quartz
Collection: "Pre-Columbian Central and South America" from the University of Michigan Museum of Art
Ancient American art at the Denver Art Museum
Art of the Americas at the Cleveland Museum of Art

Historical eras

Pre-Columbian era


Freedom of speech is the right to state one's opinions and ideas without being stopped or punished. Sometimes this is also called Freedom of expression. Freedom of speech is thought to also include freedom of the press and Freedom of information. However, new laws are usually needed to allow information to be used easily.
Most people think freedom of speech is necessary for a democratic government. In countries without free speech, people might be afraid to say what they think. Then, the government does not know what the people want. If the government does not know what they want, it cannot respond to their wants. Without free speech, the government does not have to worry as much about doing what the people want. Some people say this is why some governments do not allow free speech: they do not want to be criticised, or they fear there would be revolution if everyone knew everything that was happening in the country.

A well-known liberal thinker, John Stuart Mill, believed that freedom of speech is important because the society that people live in has a right to hear people's ideas. It's not just important because everyone should have a right to express him or herself.

Few countries with "free speech" let everything be said. For example, the United States Supreme Court said that it was against the law to shout "fire" in a crowded theater if there is no fire, because this might cause people to panic. The International Covenant on Civil and Political Rights also says that it is not okay to cause national, racial or religious hatred. Also, some countries have laws against hate speech.

As Tocqueville pointed out, people may be hesitant to speak freely not because of fear of government punishment but because of social pressures.  When an individual announces an unpopular opinion, he or she may face the disdain of their community or even be subjected to violent reactions.  While this type of suppression of speech is even more difficult to prevent than government suppression is, there are questions about whether it truly falls within the ambit of freedom of speech, which is typically regarded as a legal right to be exercised against the government, or immunity from governmental action.

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 Free Speech Flag
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References

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Freedom of speech


A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric telescope.

From the time of Newton to the 1800s, the mirror itself was made of metalusually speculum metal. This type included Newton's first designs and the largest telescope of the 19th century, the Leviathan of Parsonstown with a  wide metal mirror. In the 19th century a new method using a block of glass coated with very thin layer of silver began to become more popular by the turn of the century. Common telescopes which led to the Crossley and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as the metal mirror designs were noted for their drawbacks. Chiefly the metal mirrors only reflected about  of the light and the metal would tarnish. After multiple polishings and tarnishings, the mirror could lose its precise figuring needed.

Reflecting telescopes became extraordinarily popular for astronomy and many famous telescopes, such as the Hubble Space Telescope, and popular amateur models use this design. In addition, the reflection telescope principle was applied to other electromagnetic wavelengths, and for example, X-ray telescopes also use the reflection principle to make image-forming optics.

History

The idea that curved mirrors behave like lenses dates back at least to Alhazen's 11th century treatise on optics, works that had been widely disseminated in Latin translations in early modern Europe. Soon after the invention of the refracting telescope, Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge of the principles of curved mirrors, discussed the idea of building a telescope using a mirror as the image forming objective. There were reports that the Bolognese Cesare Caravaggi had constructed one around 1626 and the Italian professor Niccolò Zucchi, in a later work, wrote that he had experimented with a concave bronze mirror in 1616, but said it did not produce a satisfactory image. The potential advantages of using parabolic mirrors, primarily reduction of spherical aberration with no chromatic aberration, led to many proposed designs for reflecting telescopes. The most notable being James Gregory, who published an innovative design for a 'reflecting' telescope in 1663. It would be ten years (1673), before the experimental scientist Robert Hooke was able to build this type of telescope, which became known as the Gregorian telescope.

Five years after Gregory designed his telescope and five years before Hooke built the first such Gregorian telescope, Isaac Newton in 1668 built his own reflecting telescope, which is generally acknowledged as the first reflecting telescope. It used a spherically ground metal primary mirror and a small diagonal mirror in an optical configuration that has come to be known as the Newtonian telescope.

Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time meant it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century (built by Léon Foucault in 1858), long-lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which use both a spherical mirror and a lens (called a corrector plate) as primary optical elements, mainly used for wide-field imaging without spherical aberration.

The late 20th century has seen the development of adaptive optics and lucky imaging to overcome the problems of seeing, and reflecting telescopes are ubiquitous on space telescopes and many types of spacecraft imaging devices.

Technical considerations

A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or a secondary mirror may be added to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation.

The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror.

Some telescopes use primary mirrors which are made differently. Molten glass is rotated to make its surface paraboloidal, and is kept rotating while it cools and solidifies. (See Rotating furnace.) The resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach the exact figure needed.

Optical errors
Reflecting telescopes, just like any other optical system, do not produce "perfect" images. The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design.

Because the primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a secondary mirror, film holder, or detector near that focal point partially obstructing the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to diffraction effects of the obstruction as well as diffraction spikes caused by most secondary support structures.

The use of mirrors avoids chromatic aberration but they produce other types of aberrations. A simple spherical mirror cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror near its edge do not converge with those that reflect from nearer the center of the mirror, a defect called spherical aberration. To avoid this problem most reflecting telescopes use parabolic shaped mirrors, a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects near the center of the image they produce, (light traveling parallel to the mirror's optical axis), but towards the edge of that same field of view they suffer from off axis aberrations:

 Coma – an aberration where point sources (stars) at the center of the image are focused to a point but typically appears as "comet-like" radial smudges that get worse towards the edges of the image.
 Field curvature – The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field. It is sometimes corrected by a field flattening lens.
 Astigmatism – an azimuthal variation of focus around the aperture causing point source images off-axis to appear elliptical. Astigmatism is not usually a problem in a narrow field of view, but in a wide field image it gets rapidly worse and varies quadratically with field angle.
 Distortion – Distortion does not affect image quality (sharpness) but does affect object shapes. It is sometimes corrected by image processing.

There are reflecting telescope designs that use modified mirror surfaces (such as the Ritchey–Chrétien telescope) or some form of correcting lens (such as catadioptric telescopes) that correct some of these aberrations.

Use in astronomical research

Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:
 Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or in a catadioptric telescope.
 In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
 Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration. Reducing this to acceptable levels usually involves a combination of two or three aperture sized lenses (see achromat and apochromat for more details). The cost of such systems therefore scales significantly with aperture size. An image obtained from a mirror does not suffer from chromatic aberration to begin with, and the cost of the mirror scales much more modestly with its size.
 There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter. In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.

Reflecting telescope designs

Gregorian

The Gregorian telescope, described by Scottish astronomer and mathematician James Gregory in his 1663 book Optica Promota, employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the Vatican Advanced Technology Telescope, the Magellan telescopes, the Large Binocular Telescope, and the Giant Magellan Telescope.

Newtonian

The Newtonian telescope was the first successful reflecting telescope, completed by Isaac Newton in 1668. It usually has a paraboloid primary mirror but at focal ratios of about f/10 or longer a spherical primary mirror can be sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project.

The Cassegrain design and its variations

The Cassegrain telescope (sometimes called the "Classic Cassegrain") was first published in a 1672 design attributed to Laurent Cassegrain. It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. The folding and diverging effect of the secondary mirror creates a telescope with a long focal length while having a short tube length.

Ritchey–Chrétien

The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.

Three-mirror anastigmat

Including a third curved mirror allows correction of the remaining distortion, astigmatism, from the Ritchey–Chrétien design. This allows much larger fields of view.

Dall–Kirkham

The Dall–Kirkham Cassegrain telescope's design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey–Chrétien system, it does not correct for off-axis coma. Field curvature is actually less than a classical Cassegrain. Because this is less noticeable at longer focal ratios, Dall–Kirkhams are seldom faster than f/15.

Off-axis designs

There are several designs that try to avoid obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror's optical axis, commonly called off-axis optical systems.

Herschelian

The Herschelian reflector is named after William Herschel, who used this design to build very large telescopes including the 40-foot telescope in 1789. In the Herschelian reflector the primary mirror is tilted so the observer's head does not block the incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a Newtonian secondary mirror since the speculum metal mirrors of that time tarnished quickly and could only achieve 60% reflectivity.

Schiefspiegler

A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector") uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios — most Schiefspieglers use f/15 or longer, which tends to restrict useful observation to the Moon and planets.
A number of variations are common, with varying numbers of mirrors of different types. The Kutter (named after its inventor Anton Kutter) style uses a single concave primary, a convex secondary and a plano-convex lens between the secondary mirror and the focal plane, when needed (this is the case of the catadioptric Schiefspiegler).
One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice — each light path reflects along a different meridional path.

Stevick-Paul

Stevick-Paul telescopes are off-axis versions of Paul 3-mirror systems with an added flat diagonal mirror.
A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary.
The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus.
The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal.
The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the Petzval surface which is gently curved.

Yolo

The Yolo was developed by Arthur S. Leonard in the mid-1960s. Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. Most Yolos use toroidal reflectors. The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary.
Like Schiefspieglers, many Yolo variations have been pursued. The needed amount of toroidal shape
can be transferred entirely or partially to the primary mirror. In large focal ratios optical assemblies, both primary and secondary mirror can be left spherical and a spectacle correcting lens is added between the secondary mirror and the focal plane (catadioptric Yolo). The addition of a convex, long focus tertiary mirror leads to Leonard's Solano configuration. The Solano telescope doesn't contain any toric surfaces.

Liquid-mirror telescopes

One design of telescope uses a rotating mirror consisting of a liquid metal in a tray that is spun at constant speed. As the tray spins, the liquid forms a paraboloidal surface of essentially unlimited size. This allows making very big telescope mirrors (over 6 metres), but they are limited to use by zenith telescopes.

Focal planes

Prime focus

In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary mirror. At the focal point is some type of structure for holding a film plate or electronic detector. In the past, in very large telescopes, an observer would sit inside the telescope in an "observing cage" to directly view the image or operate a camera. Nowadays CCD cameras allow for remote operation of the telescope from almost anywhere in the world. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.

Radio telescopes often have a prime focus design. The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna.

Cassegrain focus

For telescopes built to the Cassegrain design or other related designs, the image is formed behind the primary mirror, at the focal point of the secondary mirror. An observer views through the rear of the telescope, or a camera or other instrument is mounted on the rear. Cassegrain focus is commonly used for amateur telescopes or smaller research telescopes. However, for large telescopes with correspondingly large instruments, an instrument at Cassegrain focus must move with the telescope as it slews; this places additional requirements on the strength of the instrument support structure, and potentially limits the movement of the telescope in order to avoid collision with obstacles such as walls or equipment inside the observatory.

Nasmyth and coudé focus

Nasmyth

The Nasmyth design is similar to the Cassegrain except the light is not directed through a hole in the primary mirror; instead, a third mirror reflects the light to the side of the telescope to allow for the mounting of heavy instruments. This is a very common design in large research telescopes.

Coudé

Adding further optics to a Nasmyth-style telescope to deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives a coudé focus (from the French word for elbow). The coudé focus gives a narrower field of view than a Nasmyth focus and is used with very heavy instruments that do not need a wide field of view. One such application is high-resolution spectrographs that have large collimating mirrors (ideally with the same diameter as the telescope's primary mirror) and very long focal lengths. Such instruments could not withstand being moved, and adding mirrors to the light path to form a coudé train, diverting the light to a fixed position to such an instrument housed on or below the observing floor (and usually built as an unmoving integral part of the observatory building) was the only option. The 60-inch Hale telescope (1.5 m), Hooker Telescope, 200-inch Hale Telescope, Shane Telescope, and Harlan J. Smith Telescope all were built with coudé foci instrumentation. The development of echelle spectrometers allowed high-resolution spectroscopy with a much more compact instrument, one which can sometimes be successfully mounted on the Cassegrain focus. Since inexpensive and adequately stable computer-controlled alt-az telescope mounts were developed in the 1980s, the Nasmyth design has generally supplanted the coudé focus for large telescopes.

Fibre-fed spectrographs

For instruments requiring very high stability, or that are very large and cumbersome, it is desirable to mount the instrument on a rigid structure, rather than moving it with the telescope. Whilst transmission of the full field of view would require a standard coudé focus, spectroscopy typically involves the measurement of only a few discrete objects, such as stars or galaxies. It is therefore feasible to collect light from these objects with optical fibers at the telescope, placing the instrument at an arbitrary distance from the telescope. Examples of fiber-fed spectrographs include the planet-hunting spectrographs HARPS or ESPRESSO.

Additionally, the flexibility of optical fibers allow light to be collected from any focal plane; for example, the HARPS spectrograph utilises the Cassegrain focus of the ESO 3.6 m Telescope, whilst the Prime Focus Spectrograph is connected to the prime focus of the Subaru telescope.

See also
Catadioptric telescopes
Honeycomb mirror
List of largest optical reflecting telescopes
List of largest optical telescopes historically
List of telescope types
Mirror support cell
PLate OPtimizer
Refracting telescope

References

External links

 Who was James Gregory? Reflecting Telescopes, Explore, National Museums Scotland

Telescope types
1663 introductions
1663 in science
17th-century inventions

Reflecting telescope


A body is the physical material of a person or organism. It is only used for organisms which are in one part or whole. There are organisms which change from single cells to whole organisms: for example, slime moulds. For them the term 'body' would mean the multicellular stage. Other uses:
Plant body
Cell body: here it may be used for cells like neurons which have long axons (nerve fibres). The cell body is the part with the nucleus in it.

The body of a dead person is also called a corpse or cadaver. The dead bodies of vertebrate animals and insects are sometimes called carcasses.

The human body has a head, neck, torso, two arms, two legs and the genitals of the groin, which differ between males and females.

The study of the body is anatomy.

The study of the workings of the body is physiology.

Here are the names of the body parts of a woman and a man.

The word body is sometimes used in other ways.  For example, a lake is a body of water. The laws on a topic are a "body of law".

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 Human body

Body parts
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Body


Elihu Vedder (26 February 183629 January 1923) was an American symbolist painter, book illustrator and poet from New York City. He is best known for his fifty-five illustrations for Edward FitzGerald's translation of The Rubaiyat of Omar Khayyam (deluxe edition, published by Houghton Mifflin).

Biography

Elihu Vedder was born on 26 February 1836 in New York City, the son of Dr. Elihu Vedder Sr. and Elizabeth Vedder.
His parents were cousins. His father, a dentist, decided to try his luck in Cuba, and this had a profound impact on Elihu Jr.'s childhood.
The remainder of his childhood was spent between his maternal grandfather Alexander Vedder's house in Schenectady and a boarding school. His mother supported his goals to be an artist while his father reluctantly assented, convinced that his son should try a different occupation. His brother, Dr. Alexander Madison Vedder, was a Navy surgeon who witnessed the transformation of Japan into a modern culture while he was stationed there.

Vedder trained in New York City with Tompkins H. Matteson, then in Paris with François-Édouard Picot.
Finally, he completed his studies in Italy - where he was strongly influenced not only by Italian Renaissance work but also by the modern Macchiaioli painters and the living Italian landscape.
He first visited Italy from 1858 until 1860, becoming deeply emotionally attached to fellow painter Giovanni Costa.
Their idyllic trips through the Italian countryside were cut short because Vedder's father cut off his financial allowance.

Penniless, Vedder returned to the United States during the American Civil War and made a small living undertaking commercial illustrations. He was involved in the bohemian 'Pfaff's' coffee house group and painted some of his most memorable paintings notable for their visionary nature, romantic imagery and often Oriental influences. Paintings of this time include The Roc's Egg, The Fisherman and the Genii, and one of his most famous works, Lair of the Sea Serpent. In the United States, Vedder sought out and befriended Walt Whitman, Herman Melville and William Morris Hunt. Vedder became a member of the American Academy of Arts and Letters in 1865. At the end of the Civil War, Vedder left America to live in Italy.

He married Caroline Rosekrans on 13 July 1869 in Glen Falls, New York.
Elihu Vedder and his wife had four children, only two of whom survived. His daughter Anita Herriman Vedder played a vital role in handling the business of her father, who was notorious for his general aloofness towards details. Elihu's son Enoch Rosekrans Vedder was a promising architect who married jewelry designer Angela Reston. Enoch died while visiting his parents in Italy on 2 April 1916. Elihu had a home in Rome and - after the financial success of his 1884 Rubaiyat work - on the Isle of Capri, then a haven for male aesthetes.

Vedder visited England many times, and was influenced by the Pre-Raphaelites, and was a friend of Simeon Solomon. He was also influenced by the work of English and Irish mystics such as William Blake and William Butler Yeats. In 1890 Vedder helped establish the In Arte Libertas group in Italy.

Tiffany commissioned him to design glassware, mosaics and statuettes for the company. He decorated the hallway of the Reading Room of the Washington Library of Congress, and his mural paintings can still be seen there.

Vedder occasionally returned to the United States, but lived only in Italy from 1906 until his death on 29 January 1923. He is buried in the Protestant Cemetery, Rome. There are no known living descendants of Elihu Vedder as both surviving children died without issue.

Exhibitions
In 2008, the Smithsonian American Art Museum organized an exhibition of Vedder's Rubaiyat illustrations that toured several museums, including the Phoenix Art Museum.

Gallery

References

Further reading

External links

 www.ElihuVedder.org 80 works online at Elihu Vedder virtual Gallery
 Artcyclopedia: Elihu Vedder Online
 Smithsonian Archives of American Art: The Elihu Vedder Papers

1836 births
1923 deaths
19th-century American painters
20th-century American painters
20th-century American male artists
American muralists
American male painters
Burials in the Protestant Cemetery, Rome
Orientalist painters
Painters from New York City
Symbolist painters
19th-century American male artists
Pre-Raphaelite painters
Pre-Raphaelite illustrators

Elihu Vedder


A forward (or striker) is a football player who is the closest to the other team's goal. The forwards are the players that are known to score the most goals.

Coaches usually send a forward that scores all the goals (the centre forward) and sends another forward that makes the assists (second striker).

The attackers are normally the last part of the formation numbers, for example:

 4-3-3 is the formation. This formation shows that there are four defenders, three midfielders and three attackers.
 4-4-2 is the formation. This formation shows that there are four defenders, four midfielders and two attackers.
 3-4-3 is the formation. This formation shows that there are three defenders, four midfielders and three attackers.

Famous strikers
German Bundesliga:
Gerd Müller: 365 goals
Klaus Fischer: 265 goals
Jupp Heynckes: 220 goals
Premier League:
 Alan Shearer: 260 goals
 Andrew Cole: 187 goals
 Thierry Henry: 174 goals
La Liga (Spain):
Hugo Sánchez: 234 goals
 Raúl: 228 goals
 Ligue 1 (France):
 Carlos Bianchi: 179 goals
FIFA World Cup
Miroslav Klose (Germany): 16 goals
Ronaldo (Brazil): 15 goals
Gerd Müller (Germany): 14 goals
UEFA Champions League
Raúl: 71 goals
Ruud van Nistelrooy: 60 goals
Andriy Shevchenko: 59 goals
Thierry Henry: 51 goals
Alfredo Di Stéfano: 49 goals

Football (soccer) terminology

Forward


A religion is a set of beliefs about the origin, nature, and purpose of existence, usually including a belief in supernatural entities, such as deities or spirits that have power in the natural world. Religious practices include the rituals and devotions directed at the supernatural. Often religions believe in the spiritual nature of humans. There are many different religions, denominations or sects, each with a different set of beliefs. Some beliefs are also concerned with the moral behavior of humans.

Religious beliefs
Each religion has different ideas about these things. Each religion also has a "moral code" which is a set of beliefs about how humans should act. Each religion usually has their own type of "devotions" when people worship or pray. They often have rituals (special things that are always done in the same way) for certain times of the year or certain times of a person's life. Other words that are used for religion are "faith" and "belief system". Altogether, followers of religion can be known as 'believers', or 'the faithful'. Few people follow more than one religion at a time.

The largest religions are Christianity, Islam, Hinduism, Buddhism, Taoism, Sikhism, Judaism and Jainism. There are many other religions. People who do not believe in any gods are called atheists. People who say that there is no evidence are called agnostics.

God

In many religions, one of the main beliefs is that there is a "deity" (or god) who is a great creator spirit. In many religions, there is just one deity that the people believe in. In other religions, there are many deities who each have different roles in the universe. In many religions, there are other types of spirits. These may include angels, devils and other such things which can be both good and bad.

Giving honour to God, the gods or the spirits is an important part of most religions. While this may often be done privately, it is also often done with gatherings of people and rituals. These rituals are often based on old traditions, and may have been done in almost the same way for hundreds, or even thousands of years.

Human spirit
Another main belief is that humans have a "soul" or spirit which lives on after their body has died. The person's spirit is on a journey through life that continues after death. Most religions believe that what a person does during their lifetime will affect what happens to their spirit in the afterlife. Many religions teach that a good person's spirit can reach a special place of peace and happiness such as Heaven or Nirvana, and that a bad person's spirit can travel to a place of pain and suffering such as Hell. Still other religions believe in reincarnation - that instead of going either to Heaven or Hell, spirits of the dead return to earth in a new body.

Morality
"Morals" are the way a human behaves to other humans. Most religions make rules about human morals. The rules of how people should act to each other are different in different religions.

For some religions, following a "path" of goodness, truth and duty is very important. This is called Tao in China. In the teachings of Judaism, people were told to "love your neighbour as yourself". In the teachings of Jesus, people were told to think of every single person as their "neighbour" and treat them with love.

Not every religion teaches people to be kind to all other people. In many religions, it has been common for people to believe that they have to act kindly only to some people and not to others. In some religions, people believed that they could please a god by killing or sacrificing another person.

Traditions

Teaching
A religion is passed on from one person to another through teachings and stories (which are often called "myths") which may be written down like the Bible, or told from memory like the Dreamtime stories of Australian Aboriginal people. In many religions, there are people who take the role of "priest" and spend their lives teaching others about the religion. There are also people who take the role of "pastor" and spend their life caring for other people. A person may be both a priest and a pastor. They are called by different names in different religions.

Symbols
Symbols are used to remind people of their religious beliefs. They are also used or worn as a sign to other people that the person belongs to a particular religion. A symbol might be something that is drawn or written, it might be a piece of clothing or jewellery, it might be a sign that a person makes with their body, or it might be a building or monument or artwork. Picture symbols for different religions are shown in the box in the introduction to this article.

Witness and conversion
In many religions, it is thought important that people should show other people that they are following a particular religion. This might be done in a general way by wearing a symbol or a type of clothing. Many people believe that it is important to tell other people about their religion, so that they can believe as well. This is called "witnessing".

There are many ways to witness. A young person might simply say to their friends "I do not use drugs or get drunk because of my religion". This is a witness. A person may tell their classmates, workmates and friends about their beliefs. A person might go to other people's houses and talk about their beliefs, or invite the people to join in the rituals of the religion, such as going to church or to a religious festival. A person might have printed material such as books or leaflets that they give to other people to read. A person might travel to a different country to teach, to work in a health service or to help people in some other way. (People who do this are called "missionaries".) These are different ways that people witness to their religion.

When a person hears a witness and decides that they will join the religion, this is called a "conversion". Usually a person decides to join a religion because they like what they have read or been told, and they believe that they are hearing the truth. They join the religion because they choose. However, throughout history there have been many times when people have been forced to join a religion by violence and threats. This is still happening today.

In most countries of the world, people are free to belong to whatever religion they choose. This is generally thought of as a basic human right. However, there are parts of the world where it is illegal (against the law) to witness to any religion except the one accepted by the government of the country. People who belong to other religions may be threatened, put in jail or murdered.

Ritual
Rituals are an important part of the tradition of many religions. In many religions, it is the tradition for people to meet for a celebration on one day in every week. There are also major celebrations that may be held only at certain times of the year, for example, on the birthday of a person who is honoured in that religion. Some religions have celebrations for different seasons of the year, or when the sun or moon is in a certain part of the sky.

In nearly every religion, the important stages of a person's life have a religious celebration. Birth, naming, reaching an age to think for oneself, reaching adulthood, marriage, childbirth, sickness and death are all celebrated by some religions. Having a celebration or special traditions when a person dies is very common.

It is the traditions that are about death that give the earliest evidence of religious beliefs. Scientists have discovered that 120,000 years ago, Neanderthal people started burying their dead. Early Homo sapiens put tools and other things into graves with the bodies, as if they could use them in the afterlife. From 40,000 years ago, many of the objects in graves are small artworks. Scientists believe that these objects were put there for religious reasons.

Groups and institutions
An institution is one name for an organization. Many religions have organizations that manage the way that people who follow the religion are to act. The organization might employ religious leaders, educate people into the ideas of the religion, manage money, own buildings and make rules. Many religions have sub-groups which are called denominations. In Islam, for example, there is Ahmadiyya, Sunnism, Shi'ism and Sufism.

Buildings

Most religions have special buildings where people meet. They are often called temples. In Judaism, they are called synagogues. In Christianity, they are called churches. In Islam, they are called mosques. In Buddhism there are pagodas, temples and monasteries. In Hinduism they are called Mandirs. People often try to make their religious building as beautiful as possible. Some religious buildings are great works of architecture.

Art and music
People often make artworks that are about their religion, or that are used in religious celebration, or are put in a religious building. Religious art comes in all shapes and sizes, from tiny pieces of jewellery to huge statues and paintings. Artworks often give important clues to historians about different ancient religions that are not well understood.

Music is often important in religious celebrations. Singing, chanting and playing musical instruments are often part of regular religious gatherings of people. Special music is often used on special occasions. Many famous composers have written religious music. The words of songs that are 3,000 years old are used every day in Christian churches and Jewish synagogues.

Related pages
 Freedom of religion
 List of religions
 Philosophy of religion
 Religious denomination
 Secular religion

References

Further reading

 Chopra, R.M., "A Study of Religions", 2015, Anuradha Prakashan, New Delhi,

Other websites
 Religion Facts
 Religion on the BBC website

Basic English 850 words

Religion


A lake (from Latin: lacus) is a large body of water (larger and deeper than a pond) within a body of land. As a lake is separated from the ocean, it is not a sea. Some lakes are very big, and people in the past sometimes called them seas. Lakes do not flow like rivers, but many have rivers flowing into and out of them.

Most lakes on the surface of the Earth are fresh water and most are in the Northern Hemisphere. More than 60% of the lakes of the world are in Canada. Finland is known as The Land of the Thousand Lakes (there are 187,888 lakes in Finland, of which 60,000 are large).

Many lakes are man-made reservoirs built to produce electricity, for recreation, or to use the water for irrigation or industry, or in houses.

If there are not rivers flowing out of the lake (see Endorheic basin), or they are few and small, the lake loses water only by evaporation or because the water flows through the soil pores. Where the water evaporates rapidly and the soil around the lake has a high salt level, as in very dry places, the water of the lake has a high concentration of salt and the lake is called a salt lake. Examples of salt lakes are the Great Salt Lake, the Caspian Sea, the Aral Sea, and the Dead Sea.

Largest lakes by continent

 Africa - Lake Victoria, also the second largest freshwater lake on Earth. It is one of the Great Lakes of Africa.
 Antarctica - Lake Vostok.
 Asia - Lake Baikal is the largest lake that is completely in Asia. The Caspian Sea, the largest lake on Earth, is on the Europe-Asia border (an artificial border) and so both continents share this lake.
 Australia - Lake Eyre, that most of the time is without water; it takes water when it rains a lot.
 Europe - Lake Ladoga, followed by Lake Onega, both in northwestern Russia.
 North America - Lake Superior.
 South America - Lake Maracaibo but it is like a bay because it has a wide opening to sea. The largest freshwater lake of South America is Lake Titicaca, which is also the highest body of water on Earth at 3,821 m (12,507 ft) above sea level where boats can travel.

Notable lakes
 The largest lake in the world by area is the Caspian Sea, with 394,299 km². The largest freshwater lake by area is Lake Superior (82,414 km²), part of the Great Lakes.
 The longest freshwater lake is Lake Tanganyika, with a length of about 660 km. Lake Baikal is the second longest (about 630 km from tip to tip).
 The deepest lake is Lake Baikal in Siberia, with a bottom at 1,637 m (5,371 ft). Lake Tanganyika (1,470 m) is the second deepest lake.
 The highest lake of the world is a small lake (pond) without a name on Ojos del Salado at 6,390 m (20,965 ft). But the highest navigable lake is Lake Titicaca in Peru and Bolivia at 3,812 m (12,507 ft).
 The lowest lake of the world is the Dead Sea, bordering Israel and Jordan at 418 m (1,371 ft) below sea level. It is also one of the lakes with highest salt concentration.
 Lake Enriquillo in Dominican Republic is the only saltwater lake in the world where crocodiles live.

Related pages
 List of lakes
 Salt lake

References

Biomes

Lake


Crystallography is the study of the arrangement of atoms in solids that are mostly in three dimensional pattern.

Before the development of X-ray crystallography, the study of crystals was based on their geometry. This involves measuring the angles of crystal faces, and establishing the symmetry of the crystal in question.

X-ray diffraction
Crystal structure is now found by analysis of the diffraction patterns of a sample targeted by a beam of some type.

The technique was jointly invented by Sir William Bragg (1862–1942) and his son Sir Lawrence Bragg (1890–1971), who jointly won the Nobel Prize in Physics for 1915. Lawrence Bragg was the youngest to become a Nobel Laureate. He was the Director of the Cavendish Laboratory, Cambridge University, when the discovery of the structure of DNA was made by James D. Watson and Francis Crick in February 1953.

X-rays are most commonly used, but for some purposes electrons or neutrons are used. Because of the different forms of interaction, the three types of radiation are suitable for different crystallographic studies.

Technique
Some materials studied using crystallography, proteins for example, do not always occur naturally as crystals. Normally such molecules are placed in solution and allowed to crystallize over days, weeks, or months.

Once a crystal is obtained, data can be collected using a beam of radiation. Although X-ray equipment is commonplace, crystallography often uses special synchrotron light sources to make X-rays. They produce purer and more complete patterns. Synchrotron sources also have a much higher intensity of X-ray beams, so data collection takes a fraction of the time normally necessary at weaker sources.

Producing an image from a diffraction pattern requires sophisticated mathematics.

The mathematical methods for the analysis of diffraction data only apply to patterns, which in turn result only when waves diffract from orderly arrays. Hence crystallography applies for the most part only to crystals, or to molecules which can be got to crystallize.

In spite of this, a certain amount of molecular information can be deduced from the patterns that are generated by fibres and powders. For example, the double-helical structure of DNA was deduced from an X-ray diffraction pattern that had been got from a fibrous sample.

Electron diffraction

Electron crystallography is a method to determine the arrangement of atoms in solids using a transmission electron microscope (TEM). The method was invented by Aaron Klug, who won the Nobel Prize in Chemistry for this, and his studies on virus structures and transfer RNA, in 1982.

The first electron crystallographic protein structure to achieve atomic resolution was bacteriorhodopsin in 1990.

Examples

Crystallography in materials engineering

Crystallography is a tool that is often employed in materials science. The understanding of crystal structures is needed to understand crystallographic defects.

A number of other physical properties are linked to crystallography. For example, the minerals in clay form small, flat, platelike structures. Clay can be easily deformed because the platelike particles can slip along each other in the plane of the plates, yet remain strongly connected in the direction perpendicular to the plates. Such mechanisms can be studied by crystallographic texture measurements.

Crystallography includes the symmetry patterns which can be formed by atoms in a crystal.

Biology
X-ray crystallography was the primary method for determining the 3-D molecular structure of biological macromolecules. The most important of these are enzymes, and nucleic acids such as DNA and RNA. In fact, the double helix structure of DNA was worked out from crystallographic data.

The first crystal structure of a macromolecule was solved in 1958  The Protein Data Bank (PDB) is a freely accessible repository for the structures of proteins and other biological macromolecules. Computer programs can be used to help visualize biological molecular structures.

X-ray crystallography has now given way to electron crystallography for macromolecules which do not form large 3-D crystals.

References

Technology
Branches of chemistry
Molecular biology
Subfields of physics

Crystallography


A zombie is a mythical dead person who has returned to life as a walking corpse. Mythical things that have been "re-animated" are called undead, or the living dead. The zombie myth came from the Caribbean.

Voodoo zombies
In Haiti people believe that Voodoo sorcerers or 'bokor' can bring dead people back to life, making a zombie. The zombie is under the control of the sorcerer because zombies have no free will. It's believed a zombie is a trapped human soul. If a sorcerer can catch the trapped soul, the sorcerer becomes more powerful.

Studies on Haitian zombies
Wade Davis wrote about zombies in two books, The Serpent and the Rainbow (1985) and Passage of Darkness: The Ethnobiology of the Haitian Zombie (1988). Davis went to Haiti in 1982 and wrote about how a living person can be turned into a zombie by two special powders being rubbed into a wound. The first powder brings a 'deathlike' state because of a certain poison in it, called tetrodotoxin (the same deadly poison found in the Japanese blowfish). At just the right amount, it can make a person almost die, but not quite. The second powder puts the person in a zombie-like state where they seem to have no free will of their own.

Many people outside Haiti do not believe what Davis wrote about, but in Haiti, lots of people believe in the "zombie drugs".

Zombies in history

Ancient civilizations
In Mesopotamian mythology, Ishtar, the goddess of love and fertility once said in anger:
"Father give me the Bull of Heaven,
So he can kill Gilgamesh in his delling.
If you do not give me the Bull of Heaven,
I will knock down the Gates of the Netherworld,
I will smash the doorposts, and leave the doors flat down,
and will let the dead go up to eat the living!
And the dead will outnumber the living!"

Middle Ages
In the Middle Ages, many people believed that the souls of the dead could come back as ghosts and haunt the living, often because of a crime that the living person had committed. Sometimes, the ghost could actually take on a physical shape and touch or attack things or people.

Zombies in fiction

Zombies can walk, think (in some cases), and attack living persons. Most zombies eat the brains of living humans. In zombies, the heart, lungs, and a small part of their brain still work. They may react to their environment, but they do not have consciousness. Zombies can use their skeleton and muscles to move. The skin is rotten. Its hair and nails are falling off. Zombies are usually covered in blood. They often have open wounds and are dressed in ripped clothes.

Zombies appear a lot in horror and fantasy films. Normally, the zombie is a mindless, clumsy corpse which eats human flesh. Zombies can not be called cannibals because they do not eat each other, only living humans. The first zombie stories appeared in the 1600s. Since that time, zombies have appeared in lots of books and films. They have become more popular in modern media, such as Capcom's Resident Evil series and AMC's The Walking Dead.

Characteristics
In zombie movies, zombies are almost always:
 Mobile (able to move) but technically dead, without a heartbeat or other vital signs
 In a decaying (rotting) state, with discolored skin and eyes
 Non-communicative (groaning and howling instead of speaking)
 Unemotional, with no mercy toward victims
 Hungry for human flesh (zombies usually ignore animals)
 Clumsy and violent
 Vulnerable to destruction of the brain (which kills them).
 Unaffected by injuries, even normally fatal ones, as long as the brain isn't damaged badly.
 Contagious: a person that is bitten by a zombie will become a zombie
 Extremely persistent

Related pages
 List of zombie movies
 Ghost
 Ghoul stories

Monsters
Superstitions

Zombie