World War II
World War II known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries; the major participants threw their entire economic and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China, it included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, the only use of nuclear weapons in war. Japan, which aimed to dominate Asia and the Pacific, was at war with China by 1937, though neither side had declared war on the other. World War II is said to have begun on 1 September 1939, with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Finland and the Baltic states. Following the onset of campaigns in North Africa and East Africa, the fall of France in mid 1940, the war continued between the European Axis powers and the British Empire. War in the Balkans, the aerial Battle of Britain, the Blitz, the long Battle of the Atlantic followed. On 22 June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history; this Eastern Front trapped most crucially the German Wehrmacht, into a war of attrition. In December 1941, Japan launched a surprise attack on the United States as well as European colonies in the Pacific. Following an immediate U. S. declaration of war against Japan, supported by one from Great Britain, the European Axis powers declared war on the U.
S. in solidarity with their Japanese ally. Rapid Japanese conquests over much of the Western Pacific ensued, perceived by many in Asia as liberation from Western dominance and resulting in the support of several armies from defeated territories; the Axis advance in the Pacific halted in 1942. Key setbacks in 1943, which included a series of German defeats on the Eastern Front, the Allied invasions of Sicily and Italy, Allied victories in the Pacific, cost the Axis its initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned toward Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in Central China, South China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands; the war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet troops, the suicide of Adolf Hitler and the German unconditional surrender on 8 May 1945.
Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings, the Soviet entry into the war against Japan and its invasion of Manchuria, Japan announced its intention to surrender on 15 August 1945, cementing total victory in Asia for the Allies. Tribunals were set up by fiat by the Allies and war crimes trials were conducted in the wake of the war both against the Germans and the Japanese. World War II changed the political social structure of the globe; the United Nations was established to foster international co-operation and prevent future conflicts. The Soviet Union and United States emerged as rival superpowers, setting the stage for the nearly half-century long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia.
Most countries whose industries had been damaged moved towards economic expansion. Political integration in Europe, emerged as an effort to end pre-war enmities and create a common identity; the start of the war in Europe is held to be 1 September 1939, beginning with the German invasion of Poland. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred and the two wars merged in 1941; this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935; the British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the fo
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system; the force applied on a surface in a direction perpendicular or normal to the surface is called thrust. Force, thus thrust, is measured using the International System of Units in newtons, represents the amount needed to accelerate 1 kilogram of mass at the rate of 1 meter per second per second. In mechanical engineering, force orthogonal to the main load is referred to as thrust. A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight; this can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream.
Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, vector sum of this thrust fore and aft to control forward speed. A motorboat generates thrust; the resulting thrust pushes the boat in the opposite direction to the sum of the momentum change in the water flowing through the propeller. A rocket is propelled forward by a thrust force equal in magnitude, but opposite in direction, to the time-rate of momentum change of the exhaust gas accelerated from the combustion chamber through the rocket engine nozzle; this is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled, or in mathematical terms: T = v d m d t Where T is the thrust generated, d m d t is the rate of change of mass with respect to time, v is the speed of the exhaust gases measured relative to the rocket.
For vertical launch of a rocket the initial thrust at liftoff must be more than the weight. Each of the three Space Shuttle Main Engines could produce a thrust of 1.8 MN, each of the Space Shuttle's two Solid Rocket Boosters 14.7 MN, together 29.4 MN. By contrast, the simplified Aid For EVA Rescue has 24 thrusters of 3.56 N each. In the air-breathing category, the AMT-USA AT-180 jet engine developed for radio-controlled aircraft produce 90 N of thrust; the GE90-115B engine fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the "World's Most Powerful Commercial Jet Engine," has a thrust of 569 kN. The power needed to generate thrust and the force of the thrust can be related in a non-linear way. In general, P 2 ∝ T 3; the proportionality constant varies, can be solved for a uniform flow: d m d t = ρ A v T = d m d t v, P = 1 2 d m d t v 2 T = ρ A v 2, P = 1 2 ρ A v 3 P 2 = T 3 4 ρ A Note that these calculations are only valid for when the incoming air is accelerated from a standstill – for example when hovering.
The inverse of the proportionality constant, the "efficiency" of an otherwise-perfect thruster, is proportional to the area of the cross section of the propelled volume of fluid and the density of the fluid. This helps to explain why moving through water is easier and why aircraft have much larger propellers than watercraft. A common question is how to contrast the thrust rating of a jet engine with the power rating of a piston engine; such comparison is difficult. A piston engine does not move the aircraft by itself, so piston engines are rated by how much power they deliver to the propeller. Except for changes in temperature and air pressure, this quantity depends on the throttle setting. A jet engine has no propeller, so the propulsive power of a jet engine is determined from its thrust as follows. Power is the force it takes to move something over some distance divided by the time it takes to move that distance: P = F d t In case of
A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is known as a burner, combustion chamber or flame holder. In a gas turbine engine, the combustor or combustion chamber is fed high pressure air by the compression system; the combustor heats this air at constant pressure. After heating, air passes from the combustor through the nozzle guide vanes to the turbine. In the case of a ramjet or scramjet engines, the air is directly fed to the nozzle. A combustor must contain and maintain stable combustion despite high air flow rates. To do so combustors are designed to first mix and ignite the air and fuel, mix in more air to complete the combustion process. Early gas turbine engines used a single chamber known as a can type combustor. Today three main configurations exist: can and cannular. Afterburners are considered another type of combustor. Combustors play a crucial role in determining many of an engine's operating characteristics, such as fuel efficiency, levels of emissions and transient response.
The objective of the combustor in a gas turbine is to add energy to the system to power the turbines, produce a high velocity gas to exhaust through the nozzle in aircraft applications. As with any engineering challenge, accomplishing this requires balancing many design considerations, such as the following: Completely combust the fuel. Otherwise, the engine wastes the unburnt fuel and creates unwanted emissions of unburnt hydrocarbons, carbon monoxide and soot. Low pressure loss across the combustor; the turbine which the combustor feeds needs high pressure flow to operate efficiently. The flame must be held inside of the combustor. If combustion happens further back in the engine, the turbine stages can be overheated and damaged. Additionally, as turbine blades continue to grow more advanced and are able to withstand higher temperatures, the combustors are being designed to burn at higher temperatures and the parts of the combustor need to be designed to withstand those higher temperatures.
It should be capable of relighting at high altitude in an event of engine flame-out. Uniform exit temperature profile. If there are hot spots in the exit flow, the turbine may be subjected to thermal stress or other types of damage; the temperature profile within the combustor should avoid hot spots, as those can damage or destroy a combustor from the inside. Small physical size and weight. Space and weight is at a premium in aircraft applications, so a well designed combustor strives to be compact. Non-aircraft applications, like power generating gas turbines, are not as constrained by this factor. Wide range of operation. Most combustors must be able to operate with a variety of inlet pressures and mass flows; these factors change with environmental conditions. Environmental emissions. There are strict regulations on aircraft emissions of pollutants like carbon dioxide and nitrogen oxides, so combustors need to be designed to minimize those emissions. Sources: Advancements in combustor technology focused on several distinct areas.
Early jet engines produced large amounts of smoke, so early combustor advances, in the 1950s, were aimed at reducing the smoke produced by the engine. Once smoke was eliminated, efforts turned in the 1970s to reducing other emissions, like unburned hydrocarbons and carbon monoxide; the 1970s saw improvement in combustor durability, as new manufacturing methods improved liner lifetime by nearly 100 times that of early liners. In the 1980s combustors began to improve their efficiency across the whole operating range. Development over that decade improved efficiencies at lower levels; the 1990s and 2000s saw a renewed focus on reducing emissions nitrogen oxides. Combustor technology is still being researched and advanced, much modern research focuses on improving the same aspects. CaseThe case is the outer shell of the combustor, is a simple structure; the casing requires little maintenance. The case is protected from thermal loads by the air flowing in it, so thermal performance is of limited concern.
However, the casing serves as a pressure vessel that must withstand the difference between the high pressures inside the combustor and the lower pressure outside. That mechanical load is a driving design factor in the case. DiffuserThe purpose of the diffuser is to slow the high speed compressed, air from the compressor to a velocity optimal for the combustor. Reducing the velocity results in an unavoidable loss in total pressure, so one of the design challenges is to limit the loss of pressure as much as possible. Furthermore, the diffuser must be designed to limit the flow distortion as much as possible by avoiding flow effects like boundary layer separation. Like most other gas turbine engine components, the diffuser is designed to be as short and light as possible. LinerThe liner contains the combustion process and introduces the various airflows into the combustion zone; the liner must be built to withstand extended high temperature cycles. For that reason liners tend to be made from superalloys like Hastelloy X. Furthermore though high
The Soviet Union the Union of Soviet Socialist Republics, was a socialist state in Eurasia that existed from 1922 to 1991. Nominally a union of multiple national Soviet republics, its government and economy were centralized; the country was a one-party state, governed by the Communist Party with Moscow as its capital in its largest republic, the Russian Soviet Federative Socialist Republic. Other major urban centres were Leningrad, Minsk, Alma-Ata, Novosibirsk, it spanned over 10,000 kilometres east to west across 11 time zones, over 7,200 kilometres north to south. It had five climate zones: tundra, steppes and mountains; the Soviet Union had its roots in the 1917 October Revolution, when the Bolsheviks, led by Vladimir Lenin, overthrew the Russian Provisional Government which had replaced Tsar Nicholas II during World War I. In 1922, the Soviet Union was formed by a treaty which legalized the unification of the Russian, Transcaucasian and Byelorussian republics that had occurred from 1918. Following Lenin's death in 1924 and a brief power struggle, Joseph Stalin came to power in the mid-1920s.
Stalin committed the state's ideology to Marxism–Leninism and constructed a command economy which led to a period of rapid industrialization and collectivization. During his rule, political paranoia fermented and the Great Purge removed Stalin's opponents within and outside of the party via arbitrary arrests and persecutions of many people, resulting in at least 600,000 deaths. In 1933, a major famine struck the country. Before the start of World War II in 1939, the Soviets signed the Molotov–Ribbentrop Pact, agreeing to non-aggression with Nazi Germany, after which the USSR invaded Poland on 17 September 1939. In June 1941, Germany broke the pact and invaded the Soviet Union, opening the largest and bloodiest theatre of war in history. Soviet war casualties accounted for the highest proportion of the conflict in the effort of acquiring the upper hand over Axis forces at intense battles such as Stalingrad and Kursk; the territories overtaken by the Red Army became satellite states of the Soviet Union.
The post-war division of Europe into capitalist and communist halves would lead to increased tensions with the United States-led Western Bloc, known as the Cold War. Stalin died in 1953 and was succeeded by Nikita Khrushchev, who in 1956 denounced Stalin and began the de-Stalinization; the Cuban Missile Crisis occurred during Khrushchev's rule, among the many factors that led to his downfall in 1964. In the early 1970s, there was a brief détente of relations with the United States, but tensions resumed with the Soviet–Afghan War in 1979. In 1985, the last Soviet premier, Mikhail Gorbachev, sought to reform and liberalize the economy through his policies of glasnost and perestroika, which caused political instability. In 1989, Soviet satellite states in Eastern Europe overthrew their respective communist governments; as part of an attempt to prevent the country's dissolution due to rising nationalist and separatist movements, a referendum was held in March 1991, boycotted by some republics, that resulted in a majority of participating citizens voting in favor of preserving the union as a renewed federation.
Gorbachev's power was diminished after Russian President Boris Yeltsin's high-profile role in facing down a coup d'état attempted by Communist Party hardliners. In late 1991, Gorbachev resigned and the Supreme Soviet of the Soviet Union met and formally dissolved the Soviet Union; the remaining 12 constituent republics emerged as independent post-Soviet states, with the Russian Federation—formerly the Russian SFSR—assuming the Soviet Union's rights and obligations and being recognized as the successor state. The Soviet Union was a powerhouse of many significant technological achievements and innovations of the 20th century, including the world's first human-made satellite, the first humans in space and the first probe to land on another planet, Venus; the country had the largest standing military in the world. The Soviet Union was recognized as one of the five nuclear weapons states and possessed the largest stockpile of weapons of mass destruction, it was a founding permanent member of the United Nations Security Council as well as a member of the Organization for Security and Co-operation in Europe, the World Federation of Trade Unions and the leading member of the Council for Mutual Economic Assistance and the Warsaw Pact.
The word "Soviet" is derived from a Russian word сове́т meaning council, advice, harmony and all deriving from the proto-Slavic verbal stem of vět-iti, related to Slavic věst, English "wise", the root in "ad-vis-or", or the Dutch weten. The word sovietnik means "councillor". A number of organizations in Russian history were called "council". For example, in the Russian Empire the State Council, which functioned from 1810 to 1917, was referred to as a Council of Ministers after the revolt of 1905. During the Georgian Affair, Vladimir Lenin envisioned an expression of Great Russian ethnic chauvinism by Joseph Stalin and his supporters, calling for these nation-states to join Russia as semi-independent parts of a greater union, which he named as the Union of Soviet Republics of Europe and Asia. Stalin resisted the proposal, but accepted it, although with Lenin's agreement changed the name of the newly proposed sta
A railroad car or railcar, railway wagon or railway carriage called a train car or train wagon, is a vehicle used for the carrying of cargo or passengers on a rail transport system. Such cars, when coupled together and hauled by one or more locomotives, form a train. Alternatively, some passenger cars are self-propelled in which case they may be either single railcars or make up multiple units; the term "car" is used by itself in American English when a rail context is implicit. Indian English sometimes uses "bogie" in the same manner, though the term has other meanings in other variants of English. In American English, "railcar" is a generic term for a railway vehicle. Although some cars exist for the railroad's own use – for track maintenance purposes, for example – most carry a revenue-earning load of passengers or freight, may be classified accordingly as passenger cars or coaches on the one hand or freight cars on the other. Passenger cars, or coaches, vary in their internal fittings: In standard-gauge cars, seating is configured into ranges of between three and five seats across the width of the car, with an aisle in between or at the side.
Tables may be provided between seats facing one another. Alternatively, seats facing in the same direction may have access to a fold-down ledge on the back of the seat in front. If the aisle is located between seats, seat rows may face the same direction, or be grouped, with twin rows facing each other. In some vehicles intended for commuter services, seats are positioned with their backs to the side walls, either on one side or more on both, facing each other across the aisle; this gives a wide accessway and allows room for standing passengers at peak times, as well as improving loading and unloading speeds. If the aisle is at the side, the car is divided into small compartments; these contain six seats, although sometimes in second class they contain eight, sometimes in first class they contain four. Passenger cars can take the electricity supply for heating and lighting equipment from either of two main sources: directly from a head end power generator on the locomotive via bus cables, or by an axle-powered generator which continuously charges batteries whenever the train is in motion.
Modern cars have either air-conditioning or windows that can be opened, or sometimes both. Various types of onboard train toilet facilities may be provided. Other types of passenger car exist for long journeys, such as the dining car, parlor car, disco car, in rare cases theater and movie theater car. In some cases another type of car is temporarily converted to one of these for an event. Observation cars were built for the rear of many famous trains to allow the passengers to view the scenery; these proved popular, leading to the development of dome cars multiple units of which could be placed mid-train, featured a glass-enclosed upper level extending above the normal roof to provide passengers with a better view. Sleeping cars outfitted with small bedrooms allow passengers to sleep through their night-time trips, while couchette cars provide more basic sleeping accommodation. Long-distance trains require baggage cars for the passengers' luggage. In European practice it used to be common for day coaches to be formed of compartments seating 6 or 8 passengers, with access from a side corridor.
In the UK, Corridor coaches fell into disfavor in the 1960s and 1970s because open coaches are considered more secure by women traveling alone. Another distinction is between single- and double deck train cars. An example of a double decker is the Amtrak superliner. A "trainset" is a semi-permanently arranged formation of cars, rather than one created "ad hoc" out of whatever cars are available; these are only broken up and reshuffled'on shed'. Trains are built of one or more of these'sets' coupled together as needed for the capacity of that train, but not always, passenger cars in a train are linked together with enclosed, flexible gangway connections through which passengers and crewmen can walk. Some designs incorporate semi-permanent connections between cars and may have a full-width connection making them one long, articulated'car'. In North America, passenger cars employ tightlock couplings to keep a train together in the event of a derailment or other accident. Many multiple unit trains consist of cars which are semi-permanently coupled into sets: these sets may be joined together to form larger trains, but passengers can only move around between cars within a set.
This "closed" arrangement keeps parties of travellers and their luggage together, hence allows the separate sets to be split to go separate ways. Some multiple-unit trainsets are designed so that corridor connections can be opened between coupled sets; these cabs or driving trailers are useful for reversing the train. Freight cars, goods wagons, or trucks exist in a wide variety of types, adapted to carry a host of goods. There were few types of cars. Freight cars or goods wagons are categorized as follows: Boxcar, covered wagon or van: enclosed car with side
The Mikoyan-Gurevich MiG-17 is a high-subsonic fighter aircraft produced in the USSR from 1952 and operated by numerous air forces in many variants. It is an advanced development of the similar looking MiG-15 of the Korean War; the MiG-17 was license-built in China as the Shenyang J-5 and Poland as the PZL-Mielec Lim-6. MiG-17s first saw combat in 1958 in the Second Taiwan Strait Crisis and proved to be an effective threat against more modern supersonic fighters of the United States in the Vietnam War, it was briefly known as the Type 38 by U. S. Air Force designation prior to the development of NATO codes. While the MiG-15bis introduced swept wings to air combat over Korea, the Mikoyan-Gurevich design bureau had begun work on its replacement in 1949 in order to fix any problems found with the MiG-15 in combat; the result was one of the most successful transonic fighters introduced before the advent of true supersonic types such as the Mikoyan-Gurevich MiG-19 and North American F-100 Super Sabre.
The design would still prove effective into the 1960s when pressed into subsonic dogfights over Vietnam against much faster planes which were not optimized for maneuvering in such slower speed, short-range engagements. While the MiG-15 used a Mach sensor to deploy airbrakes because it could not safely exceed Mach 0.92, the MiG-17 was designed to be controllable at higher Mach numbers. Early versions which retained the original Soviet copy of the Rolls-Royce Nene VK-1 engine were heavier with equal thrust. MiG-17s would be the first Soviet fighter application of an afterburner which offered increased thrust on demand by dumping fuel in the exhaust of the basic engine. Though the MiG-17 still resembles its forebear, it had an new thinner and more swept wing and tailplane for speeds approaching Mach 1. While the F-86 introduced the "all-flying" tailplane which helped controllability near the speed of sound, this would not be adopted on MiGs until the supersonic MiG-19; the wing had a "sickle sweep" compound shape with a 45° angle like the U.
S. F-100 Super Sabre near the fuselage, a 42° angle for the outboard part of the wings; the stiffer wing resisted the tendency to bend its wingtips and lose aerodynamic symmetry unexpectedly at high speeds and wing loads. Like its forebearer, the MiG-17 inherited a major design deficit which caused its fuel tanks to develop an under-pressure condition if more than half the fuel had been used which could lead to tank implosions, crushing the main fuselage of the aircraft in mid-flight with always fatal results. 30% of the fatal accidents of Soviet MiG-17 were attributed to this problem. Other visible differences to its predecessor were the addition of a third wing fence on each wing, the addition of a ventral fin and a longer and less tapered rear fuselage that added about one meter in length; the MiG-17 shared the same Klimov VK-1 engine, much of the rest of its construction such as the forward fuselage, landing gear and gun installation was carried over. The first prototype, designated I-330 "SI" by the construction bureau, was flown on the 14 January 1950, piloted by Ivan Ivashchenko.
In the midst of testing, pilot Ivan Ivashchenko was killed when his aircraft developed flutter which tore off his horizontal tail, causing a spin and crash on 17 March 1950. Lack of wing stiffness resulted in aileron reversal, discovered and fixed. Construction and tests of additional prototypes "SI-2" and experimental series aircraft "SI-02" and "SI-01" in 1951, were successful. On 1 September 1951, the aircraft was accepted for production, formally given its own MiG-17 designation after so many changes from the original MiG-15, it was estimated that with the same engine as the MiG-15's, the MiG-17's maximum speed is higher by 40–50 km/h, the fighter has greater manoeuvrability at high altitude. Serial production started in August 1951, but large quantity production was delayed in favor of producing more MiG-15s so it was never introduced in the Korean War, it did not enter service until October 1952, when the MiG-19 was ready to be flight tested. During production, the aircraft was modified several times.
The basic MiG-17 was a general-purpose day fighter, armed with three cannons, one Nudelman N-37 37mm cannon and two 23mm with 80 rounds per gun, 160 rounds total. It could act as a fighter-bomber, but its bombload was considered light relative to other aircraft of the time, it carried additional fuel tanks instead of bombs. Although a canopy which provided clear vision to the rear necessary for dogfighting like the F-86 was designed, production MiG-17Fs got a cheaper rear-view periscope which would still appear on Soviet fighters as late as the MiG-23. By 1953, pilots got safer ejection seats with protective face curtain and leg restraints like the Martin-Baker seats in the west; the MiG-15 had suffered for its lack of a radar gunsight, but in 1951, Soviet engineers obtained a captured F-86 Sabre from Korea and they copied the optical gunsight and SRD-3 gun ranging radar to produce the ASP-4N gunsight and SRC-3 radar. The combination would prove deadly over the skies of Vietnam against aircraft such as the F-4 Phantom whose pilots lamented that guns and radar gunsights had been omitted as obsolescent.
The second prototype variant, "SP-2", was an interceptor equipped with a radar. Soon a number of MiG-17P all-weather fighters were produced with the Izumrud radar and front air intake modifications. In early 1953 the MiG-17F day fighter entered production; the "F" indicated it was fitted with the VK-1F engine with an afterburner by modifying the rear fuselage with a n
Beijing romanized as Peking, is the capital of the People's Republic of China, the world's third most populous city proper, most populous capital city. The city, located in northern China, is governed as a municipality under the direct administration of central government with 16 urban and rural districts. Beijing Municipality is surrounded by Hebei Province with the exception of neighboring Tianjin Municipality to the southeast. Beijing is an important world capital and global power city, one of the world's leading centers for politics and business, education, culture and technology, architecture and diplomacy. A megacity, Beijing is the second largest Chinese city by urban population after Shanghai and is the nation's political and educational center, it is home to the headquarters of most of China's largest state-owned companies and houses the largest number of Fortune Global 500 companies in the world, as well as the world's four biggest financial institutions. It is a major hub for the national highway, expressway and high-speed rail networks.
The Beijing Capital International Airport has been the second busiest in the world by passenger traffic since 2010, and, as of 2016, the city's subway network is the busiest and second longest in the world. Combining both modern and traditional architecture, Beijing is one of the oldest cities in the world, with a rich history dating back three millennia; as the last of the Four Great Ancient Capitals of China, Beijing has been the political center of the country for most of the past eight centuries, was the largest city in the world by population for much of the second millennium A. D. Encyclopædia Britannica notes that "few cities in the world have served for so long as the political headquarters and cultural center of an area as immense as China." With mountains surrounding the inland city on three sides, in addition to the old inner and outer city walls, Beijing was strategically poised and developed to be the residence of the emperor and thus was the perfect location for the imperial capital.
The city is renowned for its opulent palaces, parks, tombs and gates. It has seven UNESCO World Heritage Sites—the Forbidden City, Temple of Heaven, Summer Palace, Ming Tombs and parts of the Great Wall and the Grand Canal— all tourist locations. Siheyuans, the city's traditional housing style, hutongs, the narrow alleys between siheyuans, are major tourist attractions and are common in urban Beijing. Many of Beijing's 91 universities rank among the best in China, such as the Peking University and Tsinghua University. Beijing CBD is a center for Beijing's economic expansion, with the ongoing or completed construction of multiple skyscrapers. Beijing's Zhongguancun area is known as China's Silicon Valley and a center of innovation and technology entrepreneurship. Over the past 3,000 years, the city of Beijing has had numerous other names; the name Beijing, which means "Northern Capital", was applied to the city in 1403 during the Ming dynasty to distinguish the city from Nanjing. The English spelling is based on the pinyin romanization of the two characters as they are pronounced in Standard Mandarin.
An older English spelling, Peking, is the postal romanization of the same two characters as they are pronounced in Chinese dialects spoken in the southern port towns first visited by European traders and missionaries. Those dialects preserve the Middle Chinese pronunciation of 京 as kjaeng, prior to a phonetic shift in the northern dialects to the modern pronunciation. Although Peking is no longer the common name for the city, some of the city's older locations and facilities, such as Beijing Capital International Airport, with IATA Code PEK, Peking University, still use the former romanization; the single Chinese character abbreviation for Beijing is 京, which appears on automobile license plates in the city. The official Latin alphabet abbreviation for Beijing is "BJ"; the earliest traces of human habitation in the Beijing municipality were found in the caves of Dragon Bone Hill near the village of Zhoukoudian in Fangshan District, where Peking Man lived. Homo erectus fossils from the caves date to 230,000 to 250,000 years ago.
Paleolithic Homo sapiens lived there more about 27,000 years ago. Archaeologists have found neolithic settlements throughout the municipality, including in Wangfujing, located in downtown Beijing; the first walled city in Beijing was Jicheng, the capital city of the state of Ji and was built in 1045 BC. Within modern Beijing, Jicheng was located around the present Guang'anmen area in the south of Xicheng District; this settlement was conquered by the state of Yan and made its capital. After the First Emperor unified China, Jicheng became a prefectural capital for the region. During the Three Kingdoms period, it was held by Gongsun Zan and Yuan Shao before falling to the Wei Kingdom of Cao Cao; the AD 3rd-century Western Jin demoted the town, placing the prefectural seat in neighboring Zhuozhou. During the Sixteen Kingdoms period when northern China was conquered and divided by the Wu Hu, Jicheng was the capital of the Xianbei Former Yan Kingdom. After China was reunified during the Sui dynasty, Jicheng known as Zhuojun, became the northern terminus of the Grand Canal.
Under the Tang dynasty, Jicheng as Youzhou, served as a military frontier command center. During the An-Shi Rebellion and again amidst the turmoil of the late Tang, local military commanders founded their own shor