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Early Middle Ages

The Early Middle Ages (or early medieval period), sometimes controversially referred to as the Dark Ages, is typically regarded by historians as lasting from the late 5th to the 10th century. They marked the start of the Middle Ages of European history, following the decline of the Western Roman Empire, and preceding the High Middle Ages (c. 11th to 14th centuries). The alternative term late antiquity, for the early part of the period, emphasizes elements of continuity with the Roman Empire, while early Middle Ages is used to emphasize developments characteristic of the earlier medieval period.

The period saw a continuation of trends evident since late classical antiquity, including population decline, especially in urban centres, a decline of trade, a small rise in average temperatures in the North Atlantic region and increased migration. In the 19th century the early Middle Ages were often labelled the Dark Ages, a characterization based on the relative scarcity of literary and cultural output from this time. The term is rarely used by academics today. The Eastern Roman Empire, or Byzantine Empire, survived, though in the 7th century the Rashidun Caliphate and the Umayyad Caliphate conquered the southern part of the Roman territory.

Many of the listed trends reversed later in the period. In 800, the title of Emperor was revived in Western Europe with Charlemagne, whose Carolingian Empire greatly affected later European social structure and history. Europe experienced a return to systematic agriculture in the form of the feudal system, which adopted such innovations as three-field planting and the heavy plough. Barbarian migration stabilized in much of Europe, although the Viking expansion greatly affected Northern Europe.

Collapse of Rome
Starting in the 2nd century, various indicators of Roman civilization began to decline, including urbanization, seaborne commerce, and population. Archaeologists have identified only 40 percent as many Mediterranean shipwrecks from the 3rd century as from the first. Estimates of the population of the Roman Empire during the period from 150 to 400 suggest a fall from 65 million to 50 million, a decline of more than 20 percent. Some scholars have connected this de-population to the Dark Ages Cold Period (300–700), when a decrease in global temperatures impaired agricultural yields.

Early in the 3rd century Germanic peoples migrated south from Scandinavia and reached the Black Sea, creating formidable confederations which opposed the local Sarmatians. In Dacia (present-day Romania) and on the steppes north of the Black Sea the Goths, a Germanic people, established at least two kingdoms: Therving and Greuthung.

The arrival of the Huns in 372–375 ended the history of these kingdoms. The Huns, a confederation of central Asian tribes, founded an empire. They had mastered the difficult art of shooting composite recurve bows from horseback. The Goths sought refuge in Roman territory (376), agreeing to enter the Empire as unarmed settlers. However many bribed the Danube border-guards into allowing them to bring their weapons.

The discipline and organization of a Roman legion made it a superb fighting unit. The Romans preferred infantry to cavalry because infantry could be trained to retain the formation in combat, while cavalry tended to scatter when faced with opposition. While a barbarian army could be raised and inspired by the promise of plunder, the legions required a central government and taxation to pay for salaries, constant training, equipment, and food. The decline in agricultural and economic activity reduced the empire's taxable income and thus its ability to maintain a professional army to defend itself from external threats.

In the Gothic War (376–382), the Goths revolted and confronted the main Roman army in the Battle of Adrianople (378). By this time, the distinction in the Roman army between Roman regulars and barbarian auxiliaries had broken down, and the Roman army was composed mainly of barbarians and soldiers recruited for a single campaign. The general decline in discipline also led to the use of smaller shields and lighter weaponry. Not wanting to share the glory, Eastern Emperor Valens ordered an attack on the Therving infantry under Fritigern without waiting for Western Emperor Gratian, who was on the way with reinforcements. While the Romans were fully engaged, the Greuthung cavalry arrived. Only one-third of the Roman army managed to escape. This represented the most shattering defeat that the Romans had suffered since the Battle of Cannae (216 BC), according to the Roman military writer Ammianus Marcellinus. The core army of the Eastern Roman Empire was destroyed, Valens was killed, and the Goths were freed to lay waste to the Balkans, including the armories along the Danube. As Edward Gibbon comments, "The Romans, who so coolly and so concisely mention the acts of justice which were exercised by the legions, reserve their compassion and their eloquence for their own sufferings, when the provinces were invaded and desolated by the arms of the successful Barbarians."

The empire lacked the resources, and perhaps the will, to reconstruct the professional mobile army destroyed at Adrianople, so it had to rely on barbarian armies to fight for it. The Eastern Roman Empire succeeded in buying off the Goths with tribute. The Western Roman Empire proved less fortunate. Stilicho, the western empire's half-Vandal military commander, stripped the Rhine frontier of troops to fend off invasions of Italy by the Visigoths in 402–03 and by other Goths in 406–07.

Fleeing before the advance of the Huns, the Vandals, Suebi, and Alans launched an attack across the frozen Rhine near Mainz; on 31 December 406, the frontier gave way and these tribes surged into Roman Gaul. There soon followed the Burgundians and bands of the Alamanni. In the fit of anti-barbarian hysteria which followed, the Western Roman Emperor Honorius had Stilicho summarily beheaded (408). Stilicho submitted his neck, "with a firmness not unworthy of the last of the Roman generals", wrote Gibbon. Honorius was left with only worthless courtiers to advise him. In 410, the Visigoths led by Alaric I captured the city of Rome and for three days fire and slaughter ensued as bodies filled the streets, palaces were stripped of their valuables, and the invaders interrogated and tortured those citizens thought to have hidden wealth. As newly converted Christians, the Goths respected church property, but those who found sanctuary in the Vatican and in other churches were the fortunate few.

Migration Period
The Goths and Vandals were only the first of many bands of peoples that flooded Western Europe in the absence of administrative governance. Some lived only for war and pillage and disdained Roman ways. Other peoples had been in prolonged contact with the Roman civilization, and were, to a certain degree, romanized. "A poor Roman plays the Goth, a rich Goth the Roman," said King Theoderic of the Ostrogoths. The subjects of the Roman empire were a mixture of Roman Christian, Arian Christian, Nestorian Christian, and pagan. The Germanic peoples knew little of cities, money, or writing, and were mostly pagan, though they were increasingly converting to Arianism, a non-trinitarian form of Christianity that considers God the Son to have been created by, and thus inferior to, God the Father, rather than the two being co-eternal, which is the position of Chalcedonian Christianity. Arianism found some favour in the Roman Empire before being eclipsed by the Chalcedonian position and then suppressed as heretical.

During the migrations, or Völkerwanderung (wandering of the peoples), the earlier settled populations were sometimes left intact though usually partially or entirely displaced. Roman culture north of the Po River was almost entirely displaced by the migrations. Whereas the peoples of France, Italy, Spain and Portugal continued to speak the dialects of Vulgar Latin that today constitute the Romance languages, the language of the smaller Roman-era population of what is now England disappeared with barely a trace in the territories settled by the Anglo-Saxons, although the Brittanic kingdoms of the west remained Brythonic speakers. The new peoples greatly altered established society, including law, culture, religion, and patterns of property ownership.

The pax Romana had provided safe conditions for trade and manufacture, and a unified cultural and educational milieu of far-ranging connections. As this was lost, it was replaced by the rule of local potentates, sometimes members of the established Romanized ruling elite, sometimes new lords of alien culture. In Aquitania, Gallia Narbonensis, southern Italy and Sicily, Baetica or southern Spain, and the Iberian Mediterranean coast, Roman culture lasted until the 6th or 7th centuries.

The gradual breakdown and transformation of economic and social linkages and infrastructure resulted in increasingly localized outlooks. This breakdown was often fast and dramatic as it became unsafe to travel or carry goods over any distance; there was a consequent collapse in trade and manufacture for export. Major industries that depended on trade, such as large-scale pottery manufacture, vanished almost overnight in places like Britain. Tintagel in Cornwall, as well as several other centres, managed to obtain supplies of Mediterranean luxury goods well into the 6th century, but then lost their trading links. Administrative, educational and military infrastructure quickly vanished, and the loss of the established cursus honorum led to the collapse of the schools and to a rise of illiteracy even among the leadership. The careers of Cassiodorus (died c. 585) at the beginning of this period and of Alcuin of York (died 804) at its close were founded alike on their valued literacy. For the formerly Roman area, there was another 20 per cent decline in population between 400 and 600, or a one-third decline for 150–600. In the 8th century, the volume of trade reached its lowest level. The very small number of shipwrecks found that dated from the 8th century supports this (which represents less than 2 per cent of the number of shipwrecks dated from the 1st century). There was also reforestation and a retreat of agriculture centred around 500.

The Romans had practiced two-field agriculture, with a crop grown in one field and the other left fallow and ploughed under to eliminate weeds. Systematic agriculture largely disappeared and yields declined. It is estimated that the Plague of Justinian which began in 541 and recurred periodically for 150 years thereafter killed as many as 100 million people across the world. Some historians such as Josiah C. Russell (1958) have suggested a total European population loss of 50 to 60 per cent between 541 and 700. After the year 750, major epidemic diseases did not appear again in Europe until the Black Death of the 14th century. The disease smallpox, which was eradicated in the late 20th century, did not definitively enter Western Europe until about 581 when Bishop Gregory of Tours provided an eyewitness account that describes the characteristic findings of smallpox. Waves of epidemics wiped out large rural populations. Most of the details about the epidemics are lost, probably due to the scarcity of surviving written records.

For almost a thousand years, Rome was the most politically important, richest and largest city in Europe. Around 100 AD, it had a population of about 450,000, and declined to a mere 20,000 during the early Middle Ages, reducing the sprawling city to groups of inhabited buildings interspersed among large areas of ruins and vegetation.

Eastern Roman Empire
The death of Theodosius I in 395 was followed by the division of the empire between his two sons. The Western Roman Empire disintegrated into a mosaic of warring Germanic kingdoms in the 5th century, effectively making the Eastern Roman Empire in Constantinople the Greek-speaking successor to the classical Roman Empire. The inhabitants continued to regard themselves as Romans, or Romaioi, until the fall of Constantinople to the Ottoman Empire in 1453. Despite this, to distinguish it from its predominantly Latin-speaking predecessor, historians began referring to the empire as "Byzantine", after the original name of Constantinople, Byzantium

The Eastern Roman or "Byzantine" Empire aimed to retain control of the trade routes between Europe and the Orient, which made the Empire the richest polity in Medieval Europe. Making use of their sophisticated warfare and superior diplomacy, the Byzantines managed to fend off assaults by the migrating barbarians. Their dreams of subduing the Western potentates briefly materialized during the reign of Justinian I in 527–565. Not only did Justinian restore some western territories to the Roman Empire, including Rome and the Italian peninsula itself, but he also codified Roman law (with his codification remaining in force in many areas of Europe until the 19th century) and commissioned the building of the largest and most architecturally advanced edifice of the early Middle Ages, the Hagia Sophia. However, his reign also saw the outbreak of a bubonic plague pandemic, now known retroactively as the Plague of Justinian. The Emperor himself was afflicted, and within the span of less than a year, an estimated 200,000 Constantinopolites—two out of every five city residents—had died of the disease.

Justinian's successors Maurice and Heraclius confronted invasions by the Avar and Slavic tribes. After the devastations by the Slavs and the Avars, large areas of the Balkans became depopulated. In 626 Constantinople, by far the largest city of early medieval Europe, withstood a combined siege by Avars and Persians. Within several decades, Heraclius completed a holy war against the Persians, taking their capital and having a Sassanid monarch assassinated. Yet Heraclius lived to see his spectacular success undone by the Muslim conquests of Syria, three Palaestina provinces, Egypt, and North Africa which was considerably facilitated by religious disunity and the proliferation of heretical movements (notably Monophysitism and Nestorianism) in the areas converted to Islam.

Although Heraclius's successors managed to salvage Constantinople from two Arab sieges (in 674–77 and 717), the empire of the 8th and early 9th century was rocked by the great Iconoclastic Controversy, punctuated by dynastic struggles between various factions at court. The Bulgar and Slavic tribes profited from these disorders and invaded Illyria, Thrace and even Greece. After the decisive victory at Ongala in 680 the armies of the Bulgars and Slavs advanced to the south of the Balkan mountains, defeating again the Byzantines who were then forced to sign a humiliating peace treaty which acknowledged the establishment of the First Bulgarian Empire on the borders of the Empire.

To counter these threats a new system of administration was introduced. The regional civil and military administration were combined in the hands of a general, or strategos. A theme, which formerly denoted a subdivision of the Byzantine army, came to refer to a region governed by a strategos. The reform led to the emergence of great landed families which controlled the regional military and often pressed their claims to the throne (see Bardas Phocas and Bardas Sklerus for characteristic examples).

By the early 8th century, notwithstanding the shrinking territory of the empire, Constantinople remained the largest and the wealthiest city west of China, comparable only to Sassanid Ctesiphon, and later Abbasid Baghdad. The population of the imperial capital fluctuated between 300,000 and 400,000 as the emperors undertook measures to restrain its growth. The only other large Christian cities were Rome (50,000) and Thessalonica (30,000). Even before the 8th century was out, the Farmer's Law signalled the resurrection of agricultural technologies in the Roman Empire. As the 2006 Encyclopædia Britannica noted, "the technological base of Byzantine society was more advanced than that of contemporary western Europe: iron tools could be found in the villages; water mills dotted the landscape; and field-sown beans provided a diet rich in protein".

The ascension of the Macedonian dynasty in 867 marked the end of the period of political and religious turmoil and introduced a new golden age of the empire. While the talented generals such as Nicephorus Phocas expanded the frontiers, the Macedonian emperors (such as Leo the Wise and Constantine VII) presided over the cultural flowering in Constantinople, known as the Macedonian Renaissance. The enlightened Macedonian rulers scorned the rulers of Western Europe as illiterate barbarians and maintained a nominal claim to rule over the West. Although this fiction had been exploded with the coronation of Charlemagne in Rome (800), the Byzantine rulers did not treat their Western counterparts as equals. Generally, they had little interest in political and economic developments in the barbarian (from their point of view) West.

Against this economic background the culture and the imperial traditions of the Eastern Roman Empire attracted its northern neighbours—Slavs, Bulgars, and Khazars—to Constantinople, in search of either pillage or enlightenment. The movement of the Germanic tribes to the south triggered the great migration of the Slavs, who occupied the vacated territories. In the 7th century, they moved westward to the Elbe, southward to the Danube and eastward to the Dnieper. By the 9th century, the Slavs had expanded into sparsely inhabited territories to the south and east from these natural frontiers, peacefully assimilating the indigenous Illyrian and Finnic populations.

Rise of Islam

 * 632–750

From the 7th century, Byzantine history was greatly affected by the rise of Islam and the Caliphates. Muslim Arabs first invaded historically Roman territory under Abū Bakr, first Caliph of the Rashidun Caliphate, who entered Roman Syria and Roman Mesopotamia. The Byzantines and neighbouring Persian Sasanids had been severely weakened by a long succession of Byzantine–Sasanian wars, especially the climactic Byzantine–Sasanian War of 602–628. Under Umar, the second Caliph, the Muslims decisively conquered Syria and Mesopotamia, as well as Roman Palestine, Roman Egypt, parts of Asia Minor and Roman North Africa, while they entirely toppled the Sasanids. In the mid 7th century, following the Muslim conquest of Persia, Islam penetrated into the Caucasus region, of which parts would later permanently become part of Russia. This expansion of Islam continued under Umar's successors and then the Umayyad Caliphate, which conquered the rest of Mediterranean North Africa and most of the Iberian Peninsula. Over the next centuries Muslim forces were able to take further European territory, including Cyprus, Malta, Septimania, Crete, and Sicily and parts of southern Italy.

The Muslim conquest of Hispania began when the Moors (mostly Berbers and some Arabs) invaded the Christian Visigothic Kingdom in the year 711, under their Berber leader Tariq ibn Ziyad. They landed at Gibraltar on 30 April and worked their way northward. Tariq's forces were joined the next year by those of his superior, Musa ibn Nusair. During the eight-year campaign most of the Iberian Peninsula was brought under Muslim rule—except for small areas in the north-northwest (Asturias) and largely Basque regions in the Pyrenees. This territory, under the Arab name Al-Andalus, became part of the expanding Umayyad empire.

The unsuccessful second siege of Constantinople (717) weakened the Umayyad dynasty and reduced their prestige. After their success in overrunning Iberia, the conquerors moved northeast across the Pyrenees. They were defeated by the Frankish leader Charles Martel at the Battle of Poitiers in 732. The Umayyads were overthrown in 750 by the Abbāsids and most of the Umayyad clan were massacred.

A surviving Umayyad prince, Abd-ar-rahman I, escaped to Spain and founded a new Umayyad dynasty in the Emirate of Cordoba in 756. Charles Martel's son Pippin the Short retook Narbonne, and his grandson Charlemagne established the Marca Hispanica across the Pyrenees in part of what today is Catalonia, reconquering Girona in 785 and Barcelona in 801. The Umayyads in Hispania proclaimed themselves caliphs in 929.

700–850

Climatic conditions in Western Europe began to improve after 700. In that year, the two major powers in western Europe were the Franks in Gaul and the Lombards in Italy. The Lombards had been thoroughly Romanized, and their kingdom was stable and well developed. The Franks, in contrast, were barely any different from their barbarian Germanic ancestors. The Kingdom of the Franks was weak and divided. Impossible to guess at the time, but by the end of the century, the Lombardic kingdom would be extinct, while the Frankish kingdom would have nearly reassembled the Western Roman Empire.

Though much of Roman civilization north of the Po River had been wiped out in the years after the end of the Western Roman Empire, between the 5th and 8th centuries, new political and social infrastructure began to develop. Much of this was initially Germanic and pagan. Arian Christian missionaries had been spreading Arian Christianity throughout northern Europe, though by 700 the religion of northern Europeans was largely a mix of Germanic paganism, Christianized paganism, and Arian Christianity. Chalcedonian Christianity had barely started to spread in northern Europe by this time. Through the practice of simony, local princes typically auctioned off ecclesiastical offices, causing priests and bishops to function as though they were yet another noble under the patronage of the prince. In contrast, a network of monasteries had sprung up as monks sought separation from the world. These monasteries remained independent from local princes, and as such constituted the "church" for most northern Europeans during this time. Being independent from local princes, they increasingly stood out as centres of learning, of scholarship, and as religious centres where individuals could receive spiritual or monetary assistance.

The interaction between the culture of the newcomers, their warband loyalties, the remnants of classical culture, and Christian influences, produced a new model for society, based in part on feudal obligations. The centralized administrative systems of the Romans did not withstand the changes, and the institutional support for chattel slavery largely disappeared. The Anglo-Saxons in England had also started to convert from Anglo-Saxon polytheism after the arrival of Christian missionaries in 597.

Italy

The Lombards, who first entered Italy in 568 under Alboin, carved out a state in the north, with its capital at Pavia. At first, they were unable to conquer the Exarchate of Ravenna, the Ducatus Romanus, and Calabria and Apulia. The next two hundred years were occupied in trying to conquer these territories from the Byzantine Empire.

The Lombard state was relatively Romanized, at least when compared to the Germanic kingdoms in northern Europe. It was highly decentralized at first, with the territorial dukes having practical sovereignty in their duchies, especially in the southern duchies of Spoleto and Benevento. For a decade following the death of Cleph in 575, the Lombards did not even elect a king; this period is called the Rule of the Dukes. The first written legal code was composed in poor Latin in 643: the Edictum Rothari. It was primarily the codification of the oral legal tradition of the people.

The Lombard state was well-organized and stabilized by the end of the long reign of Liutprand (717–744), but its collapse was sudden. Unsupported by the dukes, King Desiderius was defeated and forced to surrender his kingdom to Charlemagne in 774. The Lombard kingdom ended and a period of Frankish rule was initiated. The Frankish king Pepin the Short had, by the Donation of Pepin, given the pope the "Papal States" and the territory north of that swath of papally-governed land was ruled primarily by Lombard and Frankish vassals of the Holy Roman Emperor until the rise of the city-states in the 11th and 12th centuries.

In the south, a period of chaos began. The Duchy of Benevento maintained its sovereignty in the face of the pretensions of both the Western and Eastern Empires. In the 9th century, the Muslims conquered Sicily. The cities on the Tyrrhenian Sea departed from Byzantine allegiance. Various states owing various nominal allegiances fought constantly over territory until events came to a head in the early 11th century with the coming of the Normans, who conquered the whole of the south by the end of the century.

Britain
Roman Britain was in a state of political and economic collapse at the time of the Roman departure c. 400. A series of settlements (traditionally referred to as an invasion) by Germanic peoples began in the early fifth century, and by the sixth century the island would consist of many small kingdoms engaged in ongoing warfare with each other. The Germanic kingdoms are now collectively referred to as Anglo-Saxons. Christianity began to take hold among the Anglo-Saxons in the sixth century, with 597 given as the traditional date for its large-scale adoption.

Western Britain (Wales), eastern and northern Scotland (Pictland) and the Scottish highlands and isles continued their separate evolution. The Irish descended and Irish-influenced people of western Scotland were Christian from the fifth century onward, the Picts adopted Christianity in the sixth century under the influence of Columba, and the Welsh had been Christian since the Roman era.

The Kingdom of Northumbria was the pre-eminent power c. 600–700, absorbing several weaker Anglo-Saxon and Brythonic kingdoms, while Mercia held a similar status c. 700–800. Wessex would absorb all of the kingdoms in the south, both Anglo-Saxon and Briton. In Wales consolidation of power would not begin until the ninth century under the descendants of Merfyn Frych of Gwynedd, establishing a hierarchy that would last until the Norman invasion of Wales in 1081.

The first Viking raids on Britain began before 800, increasing in scope and destructiveness over time. In 865 a large, well-organized Danish Viking army (called the Great Heathen Army) attempted a conquest, breaking or diminishing Anglo-Saxon power everywhere but in Wessex. Under the leadership of Alfred the Great and his descendants, Wessex would at first survive, then coexist with, and eventually conquer the Danes. It would then establish the Kingdom of England and rule until the establishment of an Anglo-Danish kingdom under Cnut, and then again until the Norman Invasion of 1066.

Viking raids and invasion were no less dramatic for the north. Their defeat of the Picts in 839 led to a lasting Norse heritage in northernmost Scotland, and it led to the combination of the Picts and Gaels under the House of Alpin, which became the Kingdom of Alba, the predecessor of the Kingdom of Scotland. The Vikings combined with the Gaels of the Hebrides to become the Gall-Gaidel and establish the Kingdom of the Isles.

Frankish Empire

The Merovingians established themselves in the power vacuum of the former Roman provinces in Gaul, and Clovis I converted to Christianity following his victory over the Alemanni at the Battle of Tolbiac (496), laying the foundation of the Frankish Empire, the dominant state of early medieval Western Christendom. The Frankish kingdom grew through a complex development of conquest, patronage, and alliance building. Due to salic custom, inheritance rights were absolute, and all land was divided equally among the sons of a dead land holder. This meant that, when the king granted a prince land in reward for service, that prince and all of his descendants had an irrevocable right to that land that no future king could undo. Likewise, those princes (and their sons) could sublet their land to their own vassals, who could in turn sublet the land to lower sub-vassals. This all had the effect of weakening the power of the king as his kingdom grew, since the result was that the land became controlled not just by more princes and vassals, but by multiple layers of vassals. This also allowed his nobles to attempt to build their own power base, though given the strict salic tradition of hereditary kingship, few would ever consider overthrowing the king.

This increasingly fragmented arrangement was highlighted by Charles Martel, who as Mayor of the Palace was effectively the strongest prince in the kingdom. His accomplishments were highlighted, not just by his famous defeat of invading Muslims at the Battle of Tours, which is typically considered the battle that saved Europe from Muslim conquest, but by the fact that he greatly expanded Frankish influence. It was under his patronage that Boniface expanded Frankish influence into Germany by rebuilding the German church, with the result that, within a century, the German church was the strongest church in western Europe. Yet despite this, Charles Martel refused to overthrow the Frankish king. His son, Pepin the Short, inherited his power, and used it to further expand Frankish influence. Unlike his father, however, Pepin decided to seize the Frankish kingship. Given how strongly Frankish culture held to its principle of inheritance, few would support him if he attempted to overthrow the king. Instead, he sought the assistance of Pope Zachary, who was himself newly vulnerable due to fallout with the Byzantine Emperor over the Iconoclastic Controversy. Pepin agreed to support the pope and to give him land (the Donation of Pepin, which created the Papal States) in exchange for being consecrated as the new Frankish king. Given that Pepin's claim to the kingship was now based on an authority higher than Frankish custom, no resistance was offered to Pepin. With this, the Merovingian line of kings ended, and the Carolingian line began.

Pepin's son Charlemagne continued in the footsteps of his father and grandfather. He further expanded and consolidated the Frankish kingdom (now commonly called the Carolingian Empire). His reign also saw a cultural rebirth, commonly called the Carolingian Renaissance. Though the exact reasons are unclear, Charlemagne was crowned "Roman Emperor" by Pope Leo III on Christmas Day, 800. Upon Charlemagne's death, his empire had united much of modern-day France, western Germany and northern Italy. The years after his death illustrated how Germanic his empire remained. Rather than an orderly succession, his empire was divided in accordance with Frankish inheritance custom, which resulted in instability that plagued his empire until the last king of a united empire, Charles the Fat, died in 887, which resulted in a permanent split of the empire into West Francia and East Francia. West Francia would be ruled by Carolingians until 987 and East Francia until 911, after which time the partition of the empire into France and Germany was complete.

Feudalism
Around 800 there was a return to systematic agriculture in the form of the open field, or strip, system. A manor would have several fields, each subdivided into 1 acre strips of land. An acre measured one "furlong" of 220 yards by one "chain" of 22 yards (that is, about 200 m by 20 m). A furlong (from "furrow long") was considered to be the distance an ox could plough before taking a rest; the strip shape of the acre field also reflected the difficulty in turning early heavy ploughs. In the idealized form of the system, each family got thirty such strips of land. The three-field system of crop rotation was first developed in the 9th century: wheat or rye was planted in one field, the second field had a nitrogen-fixing crop, and the third was fallow.

Compared to the earlier two-field system, a three-field system allowed for significantly more land to be put under cultivation. Even more important, the system allowed for two harvests a year, reducing the risk that a single crop failure will lead to famine. Three-field agriculture created a surplus of oats that could be used to feed horses. This surplus allowed for the replacement of the ox by the horse after the introduction of the padded horse collar in the 12th century. Because the system required a major rearrangement of real estate and of the social order, it took until the 11th century before it came into general use. The heavy wheeled plough was introduced in the late 10th century. It required greater animal power and promoted the use of teams of oxen. Illuminated manuscripts depict two-wheeled ploughs with both a mouldboard, or curved metal ploughshare, and a coulter, a vertical blade in front of the ploughshare. The Romans had used light, wheel-less ploughs with flat iron shares that often proved unequal to the heavy soils of northern Europe.

The return to systemic agriculture coincided with the introduction of a new social system called feudalism. This system featured a hierarchy of reciprocal obligations. Each man was bound to serve his superior in return for the latter's protection. This made for confusion of territorial sovereignty since allegiances were subject to change over time and were sometimes mutually contradictory. Feudalism allowed the state to provide a degree of public safety despite the continued absence of bureaucracy and written records.

Manors became largely self-sufficient, and the volume of trade along long-distance routes and in market towns declined during this period, though never ceased entirely. Roman roads decayed and long-distance trade depended more heavily on water transport.

Viking Age
Viking Expansion.svg, called by the name of Norman

8th century homeland

9th century expansion

10th century expansion

Viking raiding regions]]

The Viking Age spans the period roughly between the late 8th and mid-11th centuries in Scandinavia and Britain, following the Germanic Iron Age (and the Vendel Age in Sweden). During this period, the Vikings, Scandinavian warriors and traders raided and explored most parts of Europe, south-western Asia, northern Africa, and north-eastern North America.

With the means to travel (longships and open water), desire for goods led Scandinavian traders to explore and develop extensive trading partnerships in new territories. Some of the most important trading ports during the period include both existing and ancient cities such as Aarhus, Ribe, Hedeby, Vineta, Truso, Kaupang, Birka, Bordeaux, York, Dublin, and Aldeigjuborg.

Viking raiding expeditions were separate from, though coexisted with, regular trading expeditions. Apart from exploring Europe via its oceans and rivers, with the aid of their advanced navigational skills, they extended their trading routes across vast parts of the continent. They also engaged in warfare, looting and enslaving numerous Christian communities of Medieval Europe for centuries, contributing to the development of feudal systems in Europe.

Eastern Europe

 * 600–1000

The Early Middle Ages marked the beginning of the cultural distinctions between Western and Eastern Europe north of the Mediterranean. Influence from the Byzantine Empire impacted the Christianization and hence almost every aspect of the cultural and political development of the East from the preeminence of Caesaropapism and Eastern Christianity to the spread of the Cyrillic alphabet. The turmoil of the so-called Barbarian invasions in the beginning of the period gradually gave way to more stabilized societies and states as the origins of contemporary Eastern Europe began to take shape during the High Middle Ages.

Turkic and Iranian invaders from Central Asia pressured the agricultural populations both in the Byzantine Balkans and in Central Europe creating a number of successor states in the Pontic steppes. After the dissolution of the Hunnic Empire, the Western Turkic and Avar Khaganates dominated territories from Pannonia to the Caspian Sea before being replaced by the short lived Old Great Bulgaria and the more successful Khazar Khaganate north of the Black Sea and the Magyars in Central Europe.

The Khazars were a nomadic Turkic people who managed to develop a multiethnic commercial state which owed its success to the control of much of the waterway trade between Europe and Central Asia. The Khazars also exacted tribute from the Alani, Magyars, various Slavic tribes, the Crimean Goths, and the Greeks of Crimea. Through a network of Jewish itinerant merchants, or Radhanites, they were in contact with the trade emporia of India and Spain.

Once they found themselves confronted by Arab expansionism, the Khazars pragmatically allied themselves with Constantinople and clashed with the Caliphate. Despite initial setbacks, they managed to recover Derbent and eventually penetrated as far south as Caucasian Iberia, Caucasian Albania and Armenia. In doing so, they effectively blocked the northward expansion of Islam into Eastern Europe even before khan Tervel achieved the same at the Second Arab Siege of Constantinople and several decades before the Battle of Tours in Western Europe. Islam eventually penetrated into Eastern Europe in the 920s when Volga Bulgaria exploited the decline of Khazar power in the region to adopt Islam from the Baghdad missionaries. The state religion of Khazaria, Judaism, disappeared as a political force with the fall of Khazaria, while Islam of Volga Bulgaria has survived in the region up to the present.

In the beginning of the period, the Slavic tribes started to expand aggressively into Byzantine possessions on the Balkans. The first attested Slavic polities were Serbia and Great Moravia, the latter of which emerged under the aegis of the Frankish Empire in the early 9th century. Great Moravia was ultimately overrun by the Magyars, who invaded the Pannonian Basin around 896. The Slavic state became a stage for confrontation between the Christian missionaries from Constantinople and Rome. Although West Slavs, Croats and Slovenes eventually acknowledged Roman ecclesiastical authority, the clergy of Constantinople succeeded in converting to Eastern Christianity two of the largest states of early medieval Europe, Bulgaria around 864, and Kievan Rus' c. 990.

Bulgaria
In 632 the Bulgars established the khanate of Old Great Bulgaria under the leadership of Kubrat. The Khazars managed to oust the Bulgars from Southern Ukraine into lands along middle Volga (Volga Bulgaria) and along lower Danube (Danube Bulgaria).

In 681 the Bulgars founded a powerful and ethnically diverse state that played a defining role in the history of early medieval Southeastern Europe. Bulgaria withstood the pressure from Pontic steppe tribes like the Pechenegs, Khazars, and Cumans, and in 806 destroyed the Avar Khanate. The Danube Bulgars were quickly slavicized and, despite constant campaigning against Constantinople, accepted Christianity from the Byzantine Empire. Through the efforts of missionaries Cyril and Methodius, mainly their disciples like Clement of Ohrid and Naum, the spread, initially of the Glagolitic, and later of the Cyrillic alphabet, developed in the capital Preslav. The local vernacular dialect, now known as Old Bulgarian or Old Church Slavonic, was established as the language of books and liturgy among Orthodox Christian Slavs.

After the adoption of Christianity in 864, Bulgaria became a cultural and spiritual hub of the Eastern Orthodox Slavic world. The Cyrillic script was developed around 885–886, and was afterwards also introduced with books to Serbia and Kievan Rus'. Literature, art, and architecture were thriving with the establishment of the Preslav and Ohrid Literary Schools along with the distinct Preslav Ceramics School. In 927 the Bulgarian Orthodox Church was the first European national Church to gain independence with its own Patriarch while conducting services in the vernacular Old Church Slavonic.

Under Simeon I (893–927), the state was the largest and one of the most powerful political entities of Europe, and it consistently threatened the existence of the Byzantine empire. From the middle of the 10th century Bulgaria was in decline as it entered a social and spiritual turmoil. It was in part due to Simeon's devastating wars, but was also exacerbated by a series of successful Byzantine military campaigns. Bulgaria was conquered after a long resistance in 1018.

Kievan Rus'
Led by a Varangian dynasty, the Kievan Rus' controlled the routes connecting Northern Europe to Byzantium and to the Orient (for example: the Volga trade route). The Kievan state began with the rule (882–912) of Prince Oleg, who extended his control from Novgorod southwards along the Dnieper river valley in order to protect trade from Khazar incursions from the east and moved his capital to the more strategic Kiev. Sviatoslav I (died 972) achieved the first major expansion of Kievan Rus' territorial control, fighting a war of conquest against the Khazar Empire and inflicting a serious blow on Bulgaria. A Rus' attack (967 or 968), instigated by the Byzantines, led to the collapse of the Bulgarian state and the occupation of the east of the country by the Rus'. An ensuing direct military confrontation between the Rus' and Byzantium (970–971) ended with a Byzantine victory (971). The Rus' withdrew and the Byzantine Empire incorporated eastern Bulgaria. Both before and after their conversion to Christianity (conventionally dated 988 under Vladimir I of Kiev—known as Vladimir the Great), the Rus' also embarked on predatory military campaigns against the Byzantine Empire, some of which resulted in trade treaties. The importance of Russo-Byzantine relations to Constantinople was highlighted by the fact that Vladimir I of Kiev, son of Svyatoslav I, became the only foreigner to marry (989) a Byzantine princess of the Macedonian dynasty (which ruled the Eastern Roman Empire from 867 to 1056), a singular honour sought in vain by many other rulers.

Transmission of learning

With the Fall of the Western Roman Empire and with urban centres in decline, literacy and learning decreased in the West. De-urbanization reduced the scope of education, and by the 6th century teaching and learning moved to monastic and cathedral schools, with the study of biblical texts at the centre of education. The education of the laity continued with little interruption in Italy, Spain, and the southern part of Gaul, where Roman influences lasted longer. In the 7th century, however, learning expanded in Ireland and the Celtic lands, where Latin was a foreign language and Latin texts were eagerly studied and taught. The Carolingian Renaissance of classical education appeared in the Carolingian Empire in the 8th century.

In the Eastern Roman Empire (Byzantium), learning (in the sense of formal education involving literature) was maintained at a higher level than in the West. The classical education system, which would persist for hundreds of years, emphasized grammar, Latin, Greek, and rhetoric. Pupils read and reread classic works and wrote essays imitating their style. By the 4th century, this education system was Christianized. In De Doctrina Christiana (started 396, completed 426), Augustine explained how classical education fits into the Christian worldview: Christianity is a religion of the book, so Christians must be literate. Tertullian was more skeptical of the value of classical learning, asking "What indeed has Athens to do with Jerusalem?"

Science
In the ancient world, Greek was the primary language of science. Advanced scientific research and teaching was mainly carried on in the Hellenistic side of the Roman empire, and in Greek. Late Roman attempts to translate Greek writings into Latin had limited success. As the knowledge of Greek declined, the Latin West found itself cut off from some of its Greek philosophical and scientific roots. For a time, Latin-speakers who wanted to learn about science had access to only a couple of books by Boethius (c. 470–524) that summarized Greek handbooks by Nicomachus of Gerasa. Isidore of Seville produced a Latin encyclopedia in 630. Private libraries would have existed, and monasteries would also keep various kinds of texts.

The study of nature was pursued more for practical reasons than as an abstract inquiry: the need to care for the sick led to the study of medicine and of ancient texts on drugs; the need for monks to determine the proper time to pray led them to study the motion of the stars; and the need to compute the date of Easter led them to study and teach mathematics and the motions of the Sun and Moon.

Carolingian Renaissance
In the late 8th century, there was renewed interest in Classical Antiquity as part of the Carolingian Renaissance. Charlemagne carried out a reform in education. The English monk Alcuin of York elaborated a project of scholarly development aimed at resuscitating classical knowledge by establishing programs of study based upon the seven liberal arts: the trivium, or literary education (grammar, rhetoric, and dialectic), and the quadrivium, or scientific education (arithmetic, geometry, astronomy, and music). From 787 on, decrees began to circulate recommending the restoration of old schools and the founding of new ones across the empire.

Institutionally, these new schools were either under the responsibility of a monastery (monastic schools), a cathedral, or a noble court. The teaching of dialectic (a discipline that corresponds to today's logic) was responsible for the increase in the interest in speculative inquiry; from this interest would follow the rise of the Scholastic tradition of Christian philosophy. In the 12th and 13th centuries, many of those schools founded under the auspices of Charlemagne, especially cathedral schools, would become universities.

Byzantium's golden age
Byzantium's great intellectual achievement was the Corpus Juris Civilis ("Body of Civil Law"), a massive compilation of Roman law made under Justinian (r. 528–65). The work includes a section called the Digesta which abstracts the principles of Roman law in such a way that they can be applied to any situation. The level of literacy was considerably higher in the Byzantine Empire than in the Latin West. Elementary education was much more widely available, sometimes even in the countryside. Secondary schools still taught the Iliad and other classics.

As for higher education, the Neoplatonic Academy in Athens was closed in 526. There was also a school in Alexandria which remained open until the Arab conquest (640). The University of Constantinople, founded by Emperor Theodosius II (425), seems to have dissolved around this time. It was refounded by Emperor Michael III in 849. Higher education in this period focused on rhetoric, although Aristotle's logic was covered in simple outline. Under the Macedonian dynasty (867–1056), Byzantium enjoyed a golden age and a revival of classical learning. There was little original research, but many lexicons, anthologies, encyclopedias, and commentaries.

Islamic learning
In the course of the 11th century, Islam's scientific knowledge began to reach Western Europe, via Islamic Spain. The works of Euclid and Archimedes, lost in the West, were translated from Arabic to Latin in Spain. The modern Hindu–Arabic numeral system, including a notation for zero, were developed by Hindu mathematicians in the 5th and 6th centuries. Muslim mathematicians learned of it in the 7th century and added a notation for decimal fractions in the 9th and 10th centuries. Around 1000, Gerbert of Aurillac (later Pope Sylvester II) made an abacus with counters engraved with Arabic numerals. A treatise by Al-Khwārizmī on how to perform calculations with these numerals was translated into Latin in Spain in the 12th century.

Monasteries
Monasteries were targeted in the eighth and ninth centuries by Vikings who invaded the coasts of northern Europe. They were targeted not only because they stored books but also precious objects that were looted by invaders. In the earliest monasteries, there were no special rooms set aside as a library, but from the sixth century onwards libraries became an essential aspect of monastic life in Western Europe. The Benedictines placed books in the care of a librarian who supervised their use. In some monastic reading rooms, valuable books would be chained to shelves, but there were also lending sections as well. Copying was also another important aspect of monastic libraries, this was undertaken by resident or visiting monks and took place in the scriptorium. In the Byzantine world, religious houses rarely maintained their own copying centres. Instead they acquired donations from wealthy donors. In the tenth century, the largest collection in the Byzantine world was found in the monasteries of Mount Athos (modern-day Greece), which accumulated over 10,000 books. Scholars travelled from one monastery to another in search of the texts they wished to study. Travelling monks were often given funds to buy books, and certain monasteries which held a reputation for intellectual activities welcomed travelling monks who came to copy manuscripts for their own libraries. One of these was the monastery of Bobbio in Italy, which was founded by the Irish abbot Columbanus in 614, and by the ninth century boasted a catalogue of 666 manuscripts, including religious works, classical texts, histories and mathematical treatises.

Christianity West and East
From the early Christians, early medieval Christians inherited a church united by major creeds, a stable Biblical canon, and a well-developed philosophical tradition. The history of medieval Christianity traces Christianity during the Middle Ages—the period after the fall of the Western Roman Empire until the Protestant Reformation. The institutional structure of Christianity in the west during this period is different from what it would become later in the Middle Ages. As opposed to the later church, the church of the early Middle Ages consisted primarily of the monasteries. The practice of simony has caused the ecclesiastical offices to become the property of local princes, and as such the monasteries constituted the only church institution independent of the local princes. In addition, the papacy was relatively weak, and its power was mostly confined to central Italy. Individualized religious practice was uncommon, as it typically required membership in a religious order, such as the Order of Saint Benedict. Religious orders would not proliferate until the high Middle Ages. For the typical Christian at this time, religious participation was largely confined to occasionally receiving mass from wandering monks. Few would receive this as often as once a month. By the end of this period, individual practice of religion was becoming more common, as monasteries started to transform into something approximating modern churches, where some monks might even give occasional sermons.

During the early Middle Ages, the divide between Eastern and Western Christianity widened, paving the way for the East-West Schism in the 11th century. In the West, the power of the Bishop of Rome expanded. In 607, Boniface III became the first Bishop of Rome to use the title Pope. Pope Gregory I used his office as a temporal power, expanded Rome's missionary efforts to the British Isles, and laid the foundations for the expansion of monastic orders. Roman church traditions and practices gradually replaced local variants, including Celtic Christianity in the British Isles. Various barbarian tribes went from raiding and pillaging the island to invading and settling. They were entirely pagan, having never been part of the Empire, though they experienced Christian influence from the surrounding peoples, such as those who were converted by the mission of Augustine of Canterbury, sent by Pope Gregory I. In the East, the conquests of Islam reduced the power of the Greek-speaking patriarchates.

Christianization of the West
The Roman Church, the only centralized institution to survive the fall of the Western Roman Empire intact, was the sole unifying cultural influence in the West, preserving Latin learning, maintaining the art of writing, and preserving a centralized administration through its network of bishops ordained in succession. The early Middle Ages are characterized by the urban control of bishops and the territorial control exercised by dukes and counts. The rise of urban communes marked the beginning of the High Middle Ages.

The Christianization of Germanic tribes began in the 4th century with the Goths and continued throughout the early Middle Ages, led in the 6th to 7th centuries by the Hiberno-Scottish mission and replaced in the 8th to 9th centuries by the Anglo-Saxon mission, with Anglo-Saxons like Alcuin playing an important role in the Carolingian Renaissance. Boniface, the Apostle of the Germans, propagated Christianity in the Frankish Empire during the 8th century. He helped shape Western Christianity, and many of the dioceses he proposed remain until today. After his martyrdom, he was quickly hailed as a saint. By 1000, even Iceland had become Christian, leaving only more remote parts of Europe (Scandinavia, the Baltic, and Finnic lands) to be Christianized during the High Middle Ages.

Europe in 1000
Speculation that the world would end in the year 1000 was confined to a few uneasy French monks. Ordinary clerks used regnal years, e.g. the 4th year of the reign of Robert II (the Pious) of France. The use of the modern "anno domini" system of dating was largely confined to chroniclers of universal history, such as the Venerable Bede.

Western Europe remained less developed compared to the Islamic world, with its vast network of caravan trade, or China, at this time the world's most populous empire under the Song Dynasty. Constantinople had a population of about 300,000, but Rome had a mere 35,000 and Paris 20,000. By contrast, Córdoba, in Islamic Spain, at this time the world's largest city contained 450,000 inhabitants. The Vikings had a trade network in northern Europe, including a route connecting the Baltic to Constantinople through Russia, as did the Radhanites.

With nearly the entire nation freshly ravaged by the Vikings, England was in a desperate state. The long-suffering English later responded with a massacre of Danish settlers in 1002, leading to a round of reprisals and finally to Danish rule (1013), though England regained independence shortly after. But Christianization made rapid progress and proved itself the long-term solution to the problem of barbarian raiding. The territories of Scandinavia were soon to be fully Christianized Kingdoms: Denmark in the 10th century, Norway in the 11th, and Sweden, the country with the least raiding activity, in the 12th. Kievan Rus, recently converted to Orthodox Christianity, flourished as the largest state in Europe. Iceland, Greenland, and Hungary were all declared Christian about 1000.

In Europe, a formalized institution of marriage was established. The proscribed degree of consanguinity varied, but the custom made marriages annullable by application to the Pope. North of Italy, where masonry construction was never extinguished, stone construction was replacing timber in important structures. Deforestation of the densely wooded continent was under way. The 10th century marked a return of urban life, with the Italian cities doubling in population. London, abandoned for many centuries, was again England's main economic centre by 1000. By 1000, Bruges and Ghent held regular trade fairs behind castle walls, a tentative return of economic life to western Europe.

In the culture of Europe, several features surfaced soon after 1000 that mark the end of the early Middle Ages: the rise of the medieval communes, the reawakening of city life, and the appearance of the burgher class, the founding of the first universities, the rediscovery of Roman law, and the beginnings of vernacular literature.

In 1000, the papacy was firmly under the control of German Emperor Otto III, or "emperor of the world" as he styled himself. But later church reforms enhanced its independence and prestige: the Cluniac movement, the building of the first great Transalpine stone cathedrals and the collation of the mass of accumulated decretals into a formulated canon law.

Rise of Islam
Consult particular article for details

The rise of Islam begins around the time Muhammad and his followers took flight, the Hijra, from Mecca to the city of Medina. Muhammad spent his last ten years in a series of battles to conquer the Arabian region. From 622 to 632, Muhammad as the leader of a Muslim community in Medina was engaged in a state of war with the Meccans. In the proceeding decades, the area of Basra was conquered by the Muslims. During the reign of Umar, the Muslim army found it a suitable place to construct a base. Later the area was settled and a mosque was erected. Madyan was conquered and settled by Muslims, but the environment was considered harsh and the settlers moved to Kufa. Umar defeated the rebellion of several Arab tribes in a successful campaign, unifying the entire Arabian peninsula and giving it stability. Under Uthman's leadership, the empire, through the Muslim conquest of Persia, expanded into Fars in 650, some areas of Khorasan in 651, and the conquest of Armenia was begun in the 640s. In this time, the Rashidun Caliphate extended over the whole Sassanid Persian Empire and to more than two-thirds of the Eastern Roman Empire. The First Fitna, or the First Islamic Civil War, lasted for the entirety of Ali ibn Abi Talib's reign. After the recorded peace treaty with Hassan ibn Ali and the suppression of early Kharijites' disturbances, Muawiyah I acceded to the position of Caliph.

Islamic expansion
The Muslim conquests of the Eastern Roman Empire and Arab wars occurred between 634 and 750. Starting in 633, Muslims conquered Iraq. The Muslim conquest of Syria would begin in 634 and would be complete by 638. The Muslim conquest of Egypt started in 639. Before the Muslim invasion of Egypt began, the Eastern Roman Empire had already lost the Levant and its Arab ally, the Ghassanid Kingdom, to the Muslims. The Muslims would bring Alexandria under control and the fall of Egypt would be complete by 642. Between 647 and 709, Muslims swept across North Africa and established their authority over that region. The site of the Grand Mosque was originally a pagan temple, then a Visigothic Christian church, before the Umayyad Moors at first converted the building into a mosque and then built a new mosque on the site.

The Transoxiana region was conquered by Qutayba ibn Muslim between 706 and 715 and loosely held by the Umayyads from 715 to 738. This conquest was consolidated by Nasr ibn Sayyar between 738 and 740. It was under the Umayyads from 740 to 748 and under the Abbasids after 748. Sindh, attacked in 664, would be subjugated by 712. Sindh became the easternmost province of the Umayyad. The Umayyad conquest of Hispania (Visigothic Spain) would begin in 711 and end by 718. The Moors, under Al-Samh ibn Malik, swept up the Iberian peninsula and by 719 overran Septimania; the area would fall under their full control in 720. With the Islamic conquest of Persia, the Muslim subjugation of the Caucasus would take place between 711 and 750. The end of the sudden Islamic Caliphate expansion ended around this time. The final Islamic dominion eroded the areas of the Iron Age Roman Empire in the Middle East and controlled strategic areas of the Mediterranean.

At the end of the 8th century, the former Western Roman Empire was decentralized and overwhelmingly rural. The Islamic conquest and rule of Sicily and Malta was a process which started in the 9th century. Islamic rule over Sicily was effective from 902, and the complete rule of the island lasted from 965 until 1061. The Islamic presence on the Italian Peninsula was ephemeral and limited mostly to semi-permanent soldier camps.

Caliphs and empire
The Abbasid Caliphate, ruled by the Abbasid dynasty of caliphs, was the third of the Islamic caliphates. Under the Abbasids, the Islamic Golden Age philosophers, scientists, and engineers of the Islamic world contributed enormously to technology, both by preserving earlier traditions and by adding their own inventions and innovations. Scientific and intellectual achievements blossomed in the period.

The Abbasids built their capital in Baghdad after replacing the Umayyad caliphs from all but the Iberian peninsula. The influence held by Muslim merchants over African-Arabian and Arabian-Asian trade routes was tremendous. As a result, Islamic civilization grew and expanded on the basis of its merchant economy, in contrast to their Christian, Indian, and Chinese peers who built societies from an agricultural landholding nobility.

The Abbasids flourished for two centuries but slowly went into decline with the rise to power of the Turkish army they had created, the Mamluks. Within 150 years of gaining control of Persia, the caliphs were forced to cede power to local dynastic emirs who only nominally acknowledged their authority. After the Abbasids lost their military dominance, the Samanids (or Samanid Empire) rose up in Central Asia. The Sunni Islam empire was a Tajik state and had a Zoroastrian theocratic nobility. It was the next native Persian dynasty after the collapse of the Sassanid Persian empire, caused by the Arab conquest.

Beginning years

 * Dates

 * 410: Visigoths under Alaric I sack Rome
 * 430: Death of Saint Augustine
 * 476: Odoacer deposes Romulus Augustus
 * 496: Battle of Tolbiac, Clovis I converts to Chalcedonianism
 * 507: Battle of Vouillé
 * 527–565: Justinian I
 * 535–552: Gothic Wars
 * 541–542: Plague of Justinian in Constantinople
 * 547: death of Benedict of Nursia
 * c. 570: birth of Muhammad
 * 590–604 Pope Gregory I
 * 597: death of Columba
 * 602–629: Last great Roman–Persian War
 * 615: death of Columbanus
 * 626: Joint Persian-Avar-Slav Siege of Constantinople
 * 632: death of Muhammad
 * 636: death of Isidore of Seville
 * 674–678: First Arab siege of Constantinople
 * 681: First Bulgarian Empire established

Ending years

 * Dates

 * 7th century: Khazar empire established
 * 711–718: Umayyad conquest of Hispania
 * 717: Second Arab siege of Constantinople
 * 721: death of Ardo, last king of the Visigoths
 * 718-722: Battle of Covadonga, establishment of the Kingdom of Asturias
 * 730: First Iconoclastic Controversy
 * 732: Battle of Tours/Poitiers
 * 735: death of Bede, British historian
 * 746: Blood court at Cannstatt
 * 751: Pepin the Short founds the Carolingian dynasty
 * 754: death of Saint Boniface
 * 768–814: Charlemagne
 * 778: Battle of Roncevaux Pass
 * 782: Bloody Verdict of Verden
 * 793: Viking raid on Lindisfarne; Viking Age begins
 * 796–804: Alcuin initiates the Carolingian Renaissance
 * 815: Byzantine Iconoclasm
 * 843: Treaty of Verdun
 * 862: Rurikid Dynasty established
 * 871–899: Alfred the Great
 * 872–930: Harald I of Norway
 * 874-930: Settlement of Iceland
 * 882: Kievan Rus' established
 * 911: Treaty of Saint-Clair-sur-Epte (Normandy)
 * 955: Battle of Lechfeld
 * 962: Otto I crowned Holy Roman Emperor
 * 969: Kievan Rus' subjugates Khazars
 * 987–996: Hugh Capet
 * 988: Christianization of Kievan Rus'
 * 991: Battle of Maldon

title: Early Middle Ages


text:
Edward R. Dewey

Edward Russel Dewey (1895–1978) was an economist who studied cycles in economics and other fields.

Dewey's cycles work
Dewey first became interested in cycles while Chief Economic Analyst of the Department of Commerce in 1930 or 1931 because President Hoover wanted to know the cause of the Great Depression. Dewey reported that each economist to whom he spoke gave him a different answer and he lost faith in the current economic methods. He received and took advice to study how business behaviour occurred rather than why. Therefore, his views are generally regarded as inconsistent with mainstream economics.

Dewey devoted his life to the study of cycles, claiming that "everything that has been studied has been found to have cycles present." He carried out extensive studies of cyclicity in economics, geology, biology, sociology, and other disciplines. In 1940, Edward R. Dewey learned of a 1931 Canadian conference on biological cycles held at Matamek. Under the guidance of Dewey and the conference leader, Copley Amory, the conference's Permanent Committee was reorganized into the Foundation for the Study of Cycles in 1941, and its scope was enlarged to encompass all disciplines. The foundation was set up with a board that included distinguished scientists and industrialists to act as a central clearing house of cycles studies from diverse areas.

The Foundation made studies of natural and social sciences as well as business and economics, and new methods were devised for isolating significant cycles present in time series. A magazine called Cycles was published from June 1950, and the foundation also published a four-volume collection of reports on cycles including some of Dewey's selected writings on cycles named "Cycles Classic Library Collection". Together with author Og Mandino, Dewey published a book entitled Cycles: The Mysterious Forces That Trigger Events.

As a result of his research, Dewey asserted that seemingly unrelated time series often had similar cycles periods present and that when they did the phase of these cycles was mostly very similar (cycle synchrony). He also said that there were many cycles with periods that were related by powers or products of 2 and 3. This is illustrated in the table below. To construct this table starting from the period 17.75 years, multiply by three as you proceed along diagonals from lower left to upper right, and multiply by two as you proceed along diagonals from lower right to upper left. Dewey reported that the underlined numbers are commonly occurring periods (in years):

142.0 213.9  319.5  479.3 -     71.0  106.5  159.8           -         35.5   53.3            17.75            -          5.92   8.88     1.97    2.96   4.44   0.66   0.99   1.48   2.22 0.22  0.33  0.49   0.74  1.11

Volume IV of the Cycles Classic Library Collection contains 1380 reports of cycles period determinations by scientists, doctors, economists and cycles researchers. In these reports there is a tendency for certain periods of cycles to be reported more commonly. These common periods include the underlined periods above and some other periods such as 9.6 years, found in the 9.6 year cycle of lynx abundance and 3.39 years (40.68 months), found in the US stock market, which Dewey says is the most commonly found period. Dewey further said that:
 * More than 500 different phenomena in 36 different areas of knowledge have been found to fluctuate in rhythmic cycles.

Cycles: The Science of Prediction
In 1947, Dewey and Edwin Franden Dakin published their book Cycles: The Science of Prediction which argued the United States economy was driven by four cycles of different length. Robert Gale Woolbert wrote that they "adduce interesting second-hand statistics to the effect that cyclical tendencies have been observed in industrial, biological and solar phenomena." Milton Friedman dismissed their theory as pseudoscience:

"[Cycles: The Science of Prediction] is not a scientific book: the evidence underlying the stated conclusions is not presented in full; data graphed are not identified so that someone else could reproduce them; the techniques employed are nowhere described in detail. [...] Its closest analogue is the modern high-power advertisement—here of book length and designed to sell an esoteric and supposedly scientific product. Like most modern advertising, the book seeks to sell its product by making exaggerated claims for it [...], showing it in association with other valued objects which really don't have anything to do with it [...], keeping discreetly silent about its defeats or mentioning them in only the vaguest form [...], and citing authorities who think highly of the product."

Joseph Schumpeter also put forward the idea that there were four economic cycles: Kitchin, Juglar, Kuznets and Kondratieff.

Business cycles
John Brookes describes Dewey's attitude to the business cycle:
 * I asked Dewey whether the cycles he was interested in had anything to do with the business cycle that economists were always talking about. Nothing at all, he replied. Although many of the cycles that the Foundation studies occur in economics and business affairs, what is usually called the business cycle is recurrent but not rhythmic – that is, its crests and bottoms do not necessarily occur at regular intervals and so, in the view of the Foundation, it isn't a true cycle.

Murray Rothbard critiques Schumpeter's and Dewey's approach in Man, Economy, and State:
 * Any such "multicyclic" approach must be set down as a mystical adoption of the fallacy of conceptual realism. There is no reality or meaning to the allegedly independent sets of "cycles." The market is one interdependent unit, and the more developed it is, the greater the interrelations among market elements. It is therefore impossible for several or numerous independent cycles to coexist as self-contained units. It is precisely the characteristic of a business cycle that it permeates all market activities.

and in America's Great Depression:
 * Some economists—such as Warren and Pearson or Dewey and Dakin —have believed that there are no such things as general business fluctuations—that general movements are but the results of different cycles that take place, at different specific time-lengths, in the various economic activities. To the extent that such varying cycles (such as the 20-year "building cycle" or the seventeen-year cicada cycle) may exist, however, they are irrelevant to a study of business cycles in general or to business depressions in particular. What we are trying to explain are general booms and busts in business.

The Foundation for the Study of Cycles
The Foundation for the Study of Cycles (FSC) is an international non-profit research organization for the study of cycles of events. It was incorporated in the State of Connecticut by Edward R. Dewey in 1941 and is currently under the directorship of Dr. Richard Smith.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Edward R. Dewey


text:
Principal quantum number

In quantum mechanics, the principal quantum number (symbolized n) is one of four quantum numbers assigned to each electron in an atom to describe that electron's state. Its values are natural numbers (from one) making it a discrete variable.

Apart from the principal quantum number, the other quantum numbers for bound electrons are the azimuthal quantum number ℓ, the magnetic quantum number ml, and the spin quantum number s.

Overview and history
As n increases, the electron is also at a higher energy and is, therefore, less tightly bound to the nucleus. For higher n the electron is farther from the nucleus, on average. For each value of n there are n accepted ℓ (azimuthal) values ranging from 0 to n − 1 inclusively, hence higher-n electron states are more numerous. Accounting for two states of spin, each n-shell can accommodate up to 2n2 electrons.

In a simplistic one-electron model described below, the total energy of an electron is a negative inverse quadratic function of the principal quantum number n, leading to degenerate energy levels for each n > 1. In more complex systems—those having forces other than the nucleus–electron Coulomb force—these levels split. For multielectron atoms this splitting results in "subshells" parametrized by ℓ. Description of energy levels based on n alone gradually becomes inadequate for atomic numbers starting from 5 (boron) and fails completely on potassium (Z = 19) and afterwards.

The principal quantum number was first created for use in the semiclassical Bohr model of the atom, distinguishing between different energy levels. With the development of modern quantum mechanics, the simple Bohr model was replaced with a more complex theory of atomic orbitals. However, the modern theory still requires the principal quantum number.

Derivation
There is a set of quantum numbers associated with the energy states of the atom. The four quantum numbers n, ℓ, m, and s specify the complete and unique quantum state of a single electron in an atom, called its wave function or orbital. Two electrons belonging to the same atom cannot have the same values for all four quantum numbers, due to the Pauli exclusion principle. The Schrödinger wave equation reduces to the three equations that when solved lead to the first three quantum numbers. Therefore, the equations for the first three quantum numbers are all interrelated. The principal quantum number arose in the solution of the radial part of the wave equation as shown below.

The Schrödinger wave equation describes energy eigenstates with corresponding real numbers En and a definite total energy, the value of En. The bound state energies of the electron in the hydrogen atom are given by: $$ E_n = \frac {E_1}{n^2} = \frac {-13.6\text{ eV}}{n^2}, \quad n=1,2,3,\ldots $$

The parameter n can take only positive integer values. The concept of energy levels and notation were taken from the earlier Bohr model of the atom. Schrödinger's equation developed the idea from a flat two-dimensional Bohr atom to the three-dimensional wavefunction model.

In the Bohr model, the allowed orbits were derived from quantized (discrete) values of orbital angular momentum, L according to the equation $$ L = n \cdot \hbar = n \cdot {h \over 2\pi} $$

where n = 1, 2, 3, … and is called the principal quantum number, and h is Planck's constant. This formula is not correct in quantum mechanics as the angular momentum magnitude is described by the azimuthal quantum number, but the energy levels are accurate and classically they correspond to the sum of potential and kinetic energy of the electron.

The principal quantum number n represents the relative overall energy of each orbital. The energy level of each orbital increases as its distance from the nucleus increases. The sets of orbitals with the same n value are often referred to as an electron shell.

The minimum energy exchanged during any wave–matter interaction is the product of the wave frequency multiplied by Planck's constant. This causes the wave to display particle-like packets of energy called quanta. The difference between energy levels that have different n determine the emission spectrum of the element.

In the notation of the periodic table, the main shells of electrons are labeled:

based on the principal quantum number.

The principal quantum number is related to the radial quantum number, nr, by:

$$ n = n_r + \ell + 1 $$

where ℓ is the azimuthal quantum number and nr is equal to the number of nodes in the radial wavefunction.

The definite total energy for a particle motion in a common Coulomb field and with a discrete spectrum, is given by: $$E_n = - \frac{Z^2 \hbar^2}{2 m_0 a_B^2 n^2} = -\frac {Z^2 e^4 m_0}{2 \hbar^2 n^2} ,$$ where $$a_B$$ is the Bohr radius.

This discrete energy spectrum resulted from the solution of the quantum mechanical problem on the electron motion in the Coulomb field, coincides with the spectrum that was obtained with the help application of the Bohr–Sommerfeld quantization rules to the classical equations. The radial quantum number determines the number of nodes of the radial wave function $$R(r)$$.

Values
In chemistry, values n = 1, 2, 3, 4, 5, 6, 7 are used in relation to the electron shell theory, with expected inclusion of n = 8 (and possibly 9) for yet-undiscovered period 8 elements. In atomic physics, higher n sometimes occur for description of excited states. Observations of the interstellar medium reveal atomic hydrogen spectral lines involving n on order of hundreds; values up to 766 were detected.

title: Principal quantum number


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Arachnoid granulation

Arachnoid granulations (also arachnoid villi, and pacchionian granulations or bodies) are small outpouchings of the arachnoid mater and subarachnoid space into the dural venous sinuses of the brain. The granulations are thought to mediate the draining of cerebrospinal fluid (CSF) from the subarachnoid space into the venous system.

The largest and most numerous granulations lie along the superior sagittal sinus; they are however present along other dural sinuses as well.

Anatomy
The granulations are often situated near where cerebral veins drain into the dural sinuses. They are most prominent along the superior sagittal sinus, particularly those lodged withinin the lateral lacunae. In order of decreasing frequency, the granulations occur within the: superior sagittal sinus, transverse sinuses, superior petrosal sinuses, and straight sinus.

The arachnoid granulations may be lodged within granular foveae - small pits upon the inner surface of the cranial bones.

Structure
The arachnoid granulations are local outpouchings of the arachnoid mater as well as the subarachnoid space enclosed within it into the dural venous sinuses. The granulations exhibit a thinner stalk that penetrates through the wall of a venous sinus, and a distended head formed within the lumen of the sinus. The head consists of a trabecular collagenous core that is largely covered by a dural cupula, except for an apical cap - some 0.3 mm in diameter - of arachnoid cells attached directly to the dural venous endothelium.

Development
The granulations develop during childhood as separate arachnoid villi gradually aggregate into macroscopic clumps.

Function
The arachnoid granulations are notably thought to be involved in resorption of cerebrospinal fluid, however, their function is not entirely understood.

Cerebrospinal fluid resorption
The arachnoid granulations act as one-way valves. Normally the pressure of the CSF is higher than that of the venous system, so CSF flows through the villi and granulations into the blood. If the pressure is reversed for some reason, fluid will not pass back into the subarachnoid space. The reason for this is not known. It has been suggested that the endothelial cells of the venous sinus create vacuoles of CSF, which move through the cell and out into the blood.

The importance of arachnoid granulations for the drainage of CSF is controversial. The granulations are sparse during early life, possibly underscoring the importance of alternate mechanisms of drainage. A large portion (perhaps the majority) of CSF may in fact drain through lymphatics associated with extracranial segments of cranial nerves - especially through axons of CN I (olfactory nerve) through their extension through the cribriform plate.

Subarachnoid systolic overpressure dampening
A suggested alternative or additional function of the granulation may be the dispersal of the overpressure wave formed within the subarachnoid space by the pulsation of arteries during systole. As the venous sinuses are enclosed in rigid dural structures, they represent a non-distensible compartment into which subarachnoid pressure increases may be dispersed.

Clinical significance
Age-related degenerative changes of the granulations and consequent decreased CSF resorption may underlie normal pressure hydrocephalus (which may in turn be pathogenetically implicated in additional age-related neurodegenerative disorders.

Eponym
Occasionally, they are referred to by their old name: Pacchioni's granulations or pacchionian bodies, named after Italian anatomist Antonio Pacchioni.

title: Arachnoid granulation


text:
Choroid plexus

The choroid plexus, or plica choroidea, is a plexus of cells that arises from the tela choroidea in each of the ventricles of the brain. Regions of the choroid plexus produce and secrete most of the cerebrospinal fluid (CSF) of the central nervous system. The choroid plexus consists of modified ependymal cells surrounding a core of capillaries and loose connective tissue. Multiple cilia on the ependymal cells move to circulate the cerebrospinal fluid.

Location
There is a choroid plexus in each of the four ventricles. In the lateral ventricles, it is found in the body, and continued in an enlarged amount in the atrium. There is no choroid plexus in the anterior horn. In the third ventricle, there is a small amount in the roof that is continuous with that in the body, via the interventricular foramina, the channels that connect the lateral ventricles with the third ventricle. A choroid plexus is in part of the roof of the fourth ventricle.

Microanatomy
The choroid plexus consists of a layer of cuboidal epithelial cells surrounding a core of capillaries and loose connective tissue. The epithelium of the choroid plexus is continuous with the ependymal cell layer (ventricular layer) that lines the ventricular system. Progenitor ependymal cells are monociliated but they differentiate into multiciliated ependymal cells. Unlike the ependyma, the choroid plexus epithelial layer has tight junctions between the cells on the side facing the ventricle (apical surface). These tight junctions prevent the majority of substances from crossing the cell layer into the cerebrospinal fluid (CSF); thus the choroid plexus acts as a blood–CSF barrier. The choroid plexus folds into many villi around each capillary, creating frond-like processes that project into the ventricles. The villi, along with a brush border of microvilli, greatly increase the surface area of the choroid plexus. CSF is formed as plasma is filtered from the blood through the epithelial cells. Choroid plexus epithelial cells actively transport sodium ions into the ventricles and water follows the resulting osmotic gradient.

The choroid plexus consists of many capillaries, separated from the ventricles by choroid epithelial cells. Fluid filters through these cells from blood to become cerebrospinal fluid. There is also much active transport of substances into, and out of, the CSF as it is made.

Function
The choroid plexus regulates the production and composition of cerebrospinal fluid (CSF), that provides the protective buoyancy for the brain. CSF acts as a medium for the glymphatic filtration system that facilitates the removal of metabolic waste from the brain, and the exchange of biomolecules and xenobiotics into and out of the brain. In this way the choroid plexus has a very important role in helping to maintain the delicate extracellular environment required by the brain to function optimally.

The choroid plexus is also a major source of transferrin secretion that plays a part in iron homeostasis in the brain.

Blood–cerebrospinal fluid barrier
The blood–cerebrospinal fluid barrier (BCSFB) is a fluid–brain barrier that is composed of a pair of membranes that separate blood from CSF at the capillary level and CSF from brain tissue. The blood–CSF boundary at the choroid plexus is a membrane composed of epithelial cells and tight junctions that link them. There is a CSF-brain barrier at the level of the pia mater, but only in the embryo.

Similar to the blood–brain barrier, the blood–CSF barrier functions to prevent the passage of most blood-borne substances into the brain, while selectively permitting the passage of specific substances (such as nutrients) into the brain and facilitating the removal of brain metabolites and metabolic products into the blood. Despite the similar function between the BBB and BCSFB, each facilitates the transport of different substances into the brain due to the distinctive structural characteristics of each of the two barrier systems. For a number of substances, the BCSFB is the primary site of entry into brain tissue.

The blood–cerebrospinal fluid barrier has also been shown to modulate the entry of leukocytes from the blood to the central nervous system. The choroid plexus cells secrete cytokines that recruit monocyte-derived macrophages, among other cells, to the brain. This cellular trafficking has implications both in normal brain homeostasis and in neuroinflammatory processes.

Choroid plexus cysts
During fetal development, some choroid plexus cysts may form. These fluid-filled cysts can be detected by a detailed second trimester ultrasound. The finding is relatively common, with a prevalence of ~1%. Choroid plexus cysts are usually an isolated finding. The cysts typically disappear later during pregnancy, and are usually harmless. They have no effect on infant and early childhood development.

Choroid plexus cysts are associated with a 1% risk of fetal aneuploidy. The risk of aneuploidy increases to 10.5-12% if other risk factors or ultrasound findings are noted. Size, location, disappearance or progression, and whether the cysts are found on both sides or not do not affect the risk of aneuploidy. 44-50% of Edwards syndrome (trisomy 18) cases will present with choroid plexus cysts, as well 1.4% of Down syndrome (trisomy 21) cases. ~75% of abnormal karyotypes associated with choroid plexus cysts are trisomy 18, while the remainder are trisomy 21.

Other
There are three graded types of choroid plexus tumor that mainly affect young children. These types of cancer are rare.

Etymology
Choroid plexus translates from the Latin plexus chorioides, which mirrors Ancient Greek χοριοειδές πλέγμα. The word chorion was used by Galen to refer to the outer membrane enclosing the fetus. Both meanings of the word plexus are given as pleating, or braiding. As often happens language changes and the use of both choroid or chorioid is both accepted. Nomina Anatomica (now Terminologia Anatomica) reflected this dual usage.

title: Choroid plexus