The United Kingdom the United Kingdom of Great Britain and Northern Ireland, sometimes referred to as Britain, is a sovereign country located off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland, many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world; the Irish Sea lies between Great Ireland. With an area of 242,500 square kilometres, the United Kingdom is the 78th-largest sovereign state in the world, it is the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017. The UK is constitutional monarchy; the current monarch is Queen Elizabeth II, who has reigned since 1952, making her the longest-serving current head of state.
The United Kingdom's capital and largest city is London, a global city and financial centre with an urban area population of 10.3 million. Other major urban areas in the UK include Greater Manchester, the West Midlands and West Yorkshire conurbations, Greater Glasgow and the Liverpool Built-up Area; the United Kingdom consists of four constituent countries: England, Scotland and Northern Ireland. Their capitals are London, Edinburgh and Belfast, respectively. Apart from England, the countries have their own devolved governments, each with varying powers, but such power is delegated by the Parliament of the United Kingdom, which may enact laws unilaterally altering or abolishing devolution; the nearby Isle of Man, Bailiwick of Guernsey and Bailiwick of Jersey are not part of the UK, being Crown dependencies with the British Government responsible for defence and international representation. The medieval conquest and subsequent annexation of Wales by the Kingdom of England, followed by the union between England and Scotland in 1707 to form the Kingdom of Great Britain, the union in 1801 of Great Britain with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland.
Five-sixths of Ireland seceded from the UK in 1922, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. There are fourteen British Overseas Territories, the remnants of the British Empire which, at its height in the 1920s, encompassed a quarter of the world's land mass and was the largest empire in history. British influence can be observed in the language and political systems of many of its former colonies; the United Kingdom is a developed country and has the world's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity. It has a high-income economy and has a high Human Development Index rating, ranking 14th in the world, it was the world's first industrialised country and the world's foremost power during the 19th and early 20th centuries. The UK remains a great power, with considerable economic, military and political influence internationally, it is sixth in military expenditure in the world. It has been a permanent member of the United Nations Security Council since its first session in 1946.
It has been a leading member state of the European Union and its predecessor, the European Economic Community, since 1973. The United Kingdom is a member of the Commonwealth of Nations, the Council of Europe, the G7, the G20, NATO, the Organisation for Economic Co-operation and Development and the World Trade Organization; the 1707 Acts of Union declared that the kingdoms of England and Scotland were "United into One Kingdom by the Name of Great Britain". The term "United Kingdom" has been used as a description for the former kingdom of Great Britain, although its official name from 1707 to 1800 was "Great Britain"; the Acts of Union 1800 united the kingdom of Great Britain and the kingdom of Ireland in 1801, forming the United Kingdom of Great Britain and Ireland. Following the partition of Ireland and the independence of the Irish Free State in 1922, which left Northern Ireland as the only part of the island of Ireland within the United Kingdom, the name was changed to the "United Kingdom of Great Britain and Northern Ireland".
Although the United Kingdom is a sovereign country, Scotland and Northern Ireland are widely referred to as countries. The UK Prime Minister's website has used the phrase "countries within a country" to describe the United Kingdom; some statistical summaries, such as those for the twelve NUTS 1 regions of the United Kingdom refer to Scotland and Northern Ireland as "regions". Northern Ireland is referred to as a "province". With regard to Northern Ireland, the descriptive name used "can be controversial, with the choice revealing one's political preferences"; the term "Great Britain" conventionally refers to the island of Great Britain, or politically to England and Wales in combination. However, it is sometimes used as a loose synonym for the United Kingdom as a whole; the term "Britain" is used both as a synonym for Great Britain, as a synonym for the United Kingdom. Usage is mixed, with the BBC preferring to use Britain as shorthand only for Great Britain and the UK Government, while accepting that both terms refer to the United K
A navigational aid is any sort of marker which aids the traveler in navigation nautical or aviation travel. Common types of such aids include lighthouses, fog signals, day beacons. According to the glossary of terms in the United States Coast Guard Light list, an Aid to Navigation is any device external to a vessel or aircraft intended to assist navigators in determining their position or safe course, or to warn them of dangers or obstructions to navigation. Red ATONs always have numbers, green ATONs have odd numbers. Under the IALA B standard used in North and South America, when you are going to sea, the red ATON is on your left, the green on your right. Under the IALA A standard used in Europe and most of Asia, the colors are reversed. In the IALA B system, the red ATONs are on your right when you return from sea and the green on your left. Red daybeacons are triangles and green daybeacons are squares. All of these ATONs are Lateral Markers where it is safe to travel. For waters where the direction of the related fairway is not obvious, such as where there are fairways leading in several directions or where there is navigable water outside designated fairways, cardinal marks are used instead of lateral ones.
These give the direction of safe water as a cardinal direction relative to the mark. There are other markers that give information other than the edges of safe waters. Most are white with black lettering, they are used to give direction and information, warn of hazards and destructions, mark controlled areas, mark off-limits areas. These ATONs do not mark traffic channels. On non-lateral markers, there are some shapes that show certain things: Squares show information, including places to find food and repairs, they sometimes show directions. Diamonds warn about dangers like rocks, dams, or stumps. Circles mark a controlled area such as idles speed, speed limit, or ski zone. Crossed diamonds show areas like swimming areas and dams. There are some special colored ATONs; when there are red and green horizontal stripes, you are at the junction of two channels. The ATONs indicate the primary channel. If the green is on the top, the preferred channel is to the right. If the red is on top, the preferred channel is to the left.
The light matches the top stripe color. These ATONs are sometimes called "junction buoys". ATONs are integrated with Automatic Identification System, e.g. a lighthouse can be equipped with an AIS transmitter. Sometimes it is impractical to equip the ATON with an AIS transponder; this is known as a synthetic ATON. In other cases, such as marking a wreck until a physical buoy can be deployed, a so-called virtual ATON is created: A shore-based AIS system is configured to transmit AIS messages indicating the existence of an ATON at a specified location. Lead marks and lights are fixed markers that are laterally displaced to allow a mariner to navigate a fixed channel along the preferred route, they are known as "channel markers". They can be used coming into and out of the channel; when lit, they are usable at night. Customarily, the upper mark is up-hill from the lower mark; the mariner will know the geometry of the marks/lights from the navigational chart and can understand that when "open" the ship needs to be navigated to "close" the marks and be in the preferred line of the channel.
In some cases, the lead marks/lights are provided by lasers, as in the laser channel under the Tasman Bridge on the Derwent River at Hobart, Tasmania. USCG aids to navigation boat Buoy Daymark Distance Measuring Equipment Foghorn Global Positioning System Instrument Landing System landmark Lighthouse LORAN Non-Directional Beacon Racon Radio navigation Range light Sea mark Signal station Tactical Air Navigation VHF Omni-directional Range International Regulations for Preventing Collisions at Sea United States Coast Guard. Aids to Navigation. Scott T. Price. "U. S. Coast Guard Aids to Navigation: A Historical Bibliography". United States Coast Guard Historian's Office. UK Department for Transport. UK Government Strategy for AIS. IALA. IALA Standard A-126: On the Use of the Automatic Identification System in Marine Aids to Navigation Service. Trevor Diamond's Aviation Navaid Gallery. Terry Pepper. Aids to Navigation in the Gulf of Gdansk
Federal Aviation Administration
The Federal Aviation Administration is a governmental body of the United States with powers to regulate all aspects of civil aviation in that nation as well as over its surrounding international waters. Its powers include the construction and operation of airports, air traffic management, the certification of personnel and aircraft, the protection of U. S. assets during the launch or re-entry of commercial space vehicles. Powers over neighboring international waters were delegated to the FAA by authority of the International Civil Aviation Organization. Created in August 1958, the FAA replaced the former Civil Aeronautics Administration and became an agency within the US Department of Transportation; the FAA's roles include: Regulating U. S. commercial space transportation Regulating air navigation facilities' geometric and flight inspection standards Encouraging and developing civil aeronautics, including new aviation technology Issuing, suspending, or revoking pilot certificates Regulating civil aviation to promote transportation safety in the United States through local offices called Flight Standards District Offices Developing and operating a system of air traffic control and navigation for both civil and military aircraft Researching and developing the National Airspace System and civil aeronautics Developing and carrying out programs to control aircraft noise and other environmental effects of civil aviation The FAA is divided into four "lines of business".
Each LOB has a specific role within the FAA. Airports: plans and develops projects involving airports, overseeing their construction and operations. Ensures compliance with federal regulations. Air Traffic Organization: primary duty is to safely and efficiently move air traffic within the National Airspace System. ATO employees manage air traffic facilities including Airport Traffic Control Towers and Terminal Radar Approach Control Facilities. See Airway Operational Support. Aviation Safety: Responsible for aeronautical certification of personnel and aircraft, including pilots and mechanics. Commercial Space Transportation: ensures protection of U. S. assets during the launch or reentry of commercial space vehicles. The FAA is headquartered in Washington, D. C. as well as the William J. Hughes Technical Center in Atlantic City, New Jersey, the Mike Monroney Aeronautical Center in Oklahoma City and its nine regional offices: Alaskan Region – Anchorage, Alaska Northwest Mountain – Seattle, Washington Western Pacific – Los Angeles, California Southwest – Fort Worth, Texas Central – Kansas City, Missouri Great Lakes – Chicago, Illinois Southern – Atlanta, Georgia Eastern – New York, New York New England – Boston, Massachusetts The Air Commerce Act of May 20, 1926, is the cornerstone of the federal government's regulation of civil aviation.
This landmark legislation was passed at the urging of the aviation industry, whose leaders believed the airplane could not reach its full commercial potential without federal action to improve and maintain safety standards. The Act charged the Secretary of Commerce with fostering air commerce and enforcing air traffic rules, licensing pilots, certifying aircraft, establishing airways, operating and maintaining aids to air navigation; the newly created Aeronautics Branch, operating under the Department of Commerce assumed primary responsibility for aviation oversight. In fulfilling its civil aviation responsibilities, the Department of Commerce concentrated on such functions as safety regulations and the certification of pilots and aircraft, it took over the building and operation of the nation's system of lighted airways, a task initiated by the Post Office Department. The Department of Commerce improved aeronautical radio communications—before the founding of the Federal Communications Commission in 1934, which handles most such matters today—and introduced radio beacons as an effective aid to air navigation.
The Aeronautics Branch was renamed the Bureau of Air Commerce in 1934 to reflect its enhanced status within the Department. As commercial flying increased, the Bureau encouraged a group of airlines to establish the first three centers for providing air traffic control along the airways. In 1936, the Bureau itself began to expand the ATC system; the pioneer air traffic controllers used maps and mental calculations to ensure the safe separation of aircraft traveling along designated routes between cities. In 1938, the Civil Aeronautics Act transferred the federal civil aviation responsibilities from the Commerce Department to a new independent agency, the Civil Aeronautics Authority; the legislation expanded the government's role by giving the CAA the authority and the power to regulate airline fares and to determine the routes that air carriers would serve. President Franklin D. Roosevelt split the authority into two agencies in 1940: the Civil Aeronautics Administration and the Civil Aeronautics Board.
CAA was responsible for ATC, airman and aircraft certification, safety enforcement, airway development. CAB was entrusted with safety regulation, accident investigation, economic regulation of the airlines; the CAA was part of the Department of Commerce. The CAB was an independent federal agency. On the eve of America's entry into World War II, CAA began to extend its ATC responsibilities to takeoff and landing operations at airports; this expanded role became permanent after the war. The application of radar to ATC helped controllers in their drive to keep abreast of the postwar boom in commercial air transportation. In 1946, Congress gave CAA the added task of administering the federal-aid airport program, the first peacetime program of financial assistance aimed exclusivel
Volcanic ash consists of fragments of pulverized rock and volcanic glass, created during volcanic eruptions and measuring less than 2 mm in diameter. The term volcanic ash is often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere; the force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away. Due to its wide dispersal, ash can have a number of impacts on society, including human and animal health, disruption to aviation, disruption to critical infrastructure, primary industries and structures.
Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents. Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles to exsolve into gas bubbles; as more bubbles nucleate a foam is produced, which decreases the density of the magma, accelerating it up the conduit. Fragmentation occurs; when fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a efficient process of ash formation and is capable of generating fine ash without the addition of water. Volcanic ash is produced during phreatomagmatic eruptions. During these eruptions fragmentation occurs when magma comes into contact with bodies of water groundwater, snow or ice; as the magma, hotter than the boiling point of water, comes into contact with water an insulating vapor film forms. This vapor film will collapse leading to direct coupling of the cold water and hot magma.
This increases the heat transfer which leads to the rapid expansion of water and fragmentation of the magma into small particles which are subsequently ejected from the volcanic vent. Fragmentation causes an increase in contact area between magma and water creating a feedback mechanism, leading to further fragmentation and production of fine ash particles. Pyroclastic density currents can produce ash particles; these are produced by lava dome collapse or collapse of the eruption column. Within pyroclastic density currents particle abrasion occurs as particles interact with each other resulting in a reduction in grain size and production of fine grained ash particles. In addition, ash can be produced during secondary fragmentation of pumice fragments, due to the conservation of heat within the flow; these processes produce large quantities of fine grained ash, removed from pyroclastic density currents in co-ignimbrite ash plumes. Physical and chemical characteristics of volcanic ash are controlled by the style of volcanic eruption.
Volcanoes display a range of eruption styles which are controlled by magma chemistry, crystal content and dissolved gases of the erupting magma and can be classified using the volcanic explosivity index. Effusive eruptions of basaltic composition produce <105 m3 of ejecta, whereas explosive eruptions of rhyolitic and dacitic composition can inject large quantities of ejecta into the atmosphere. Another parameter controlling the amount of ash produced is the duration of the eruption: the longer the eruption is sustained, the more ash will be produced. For example, the second phase of the 2010 eruptions of Eyjafjallajökull was classified as VEI 4 despite a modest 8 km high eruption column, but the eruption continued for a month, which allowed a large volume of ash to be ejected into the atmosphere; the types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it erupted. Considering that the most abundant elements found in silicate magma are silicon and oxygen, the various types of magma produced during volcanic eruptions are most explained in terms of their silica content.
Low energy eruptions of basalt produce a characteristically dark coloured ash containing ~45 - 55% silica, rich in iron and magnesium. The most explosive rhyolite eruptions produce a felsic ash, high in silica while other types of ash with an intermediate composition have a silica content between 55-69%; the principal gases released during volcanic activity are water, carbon dioxide, sulfur dioxide, hydrogen sulfide, carbon monoxide and hydrogen chloride. These sulfur and halogen gases and metals are removed from the atmosphere by processes of chemical reaction and wet deposition, by adsorption onto the surface of volcanic ash, it has long been recognised that a range of sulfate and halide compounds are mobilised from fresh volcanic ash.. While some 55 ionic species have been reported in fresh ash leachates the most
Heathrow Airport known as London Heathrow, is a major international airport in London, United Kingdom. Heathrow is the second busiest airport in the world by international passenger traffic, as well as the busiest airport in Europe by passenger traffic, the seventh busiest airport in the world by total passenger traffic, it is one of six international airports serving Greater London. In 2018, it handled a record 80.1 million passengers, a 2.7% increase from 2017 as well as 480,339 aircraft movements, a 4,715 increase from 2017. Heathrow lies 14 miles west of Central London, has two parallel east–west runways along with four operational terminals on a site that covers 12.27 square kilometres. The airport is owned and operated by Heathrow Airport Holdings, which itself is owned by FGP TopCo Limited, an international consortium led by Ferrovial that includes Qatar Holding LLC, Caisse de dépôt et placement du Québec, GIC Private Limited, Alinda Capital Partners, China Investment Corporation and Universities Superannuation Scheme.
London Heathrow is the primary hub for British Airways and the primary operating base for Virgin Atlantic. In September 2012, the Government of the United Kingdom established the Airports Commission, an independent commission chaired by Sir Howard Davies to examine various options for increasing capacity at UK airports. In July 2015, the commission backed a third runway at Heathrow, which the government approved in October 2016. Heathrow is 14 mi west of central London, near the south end of the London Borough of Hillingdon on a parcel of land, designated part of the Metropolitan Green Belt; the airport is surrounded by the villages of Harlington, Harmondsworth and Cranford to the north and by Hounslow and Hatton to the east. To the south lie Bedfont and Stanwell while to the west Heathrow is separated from Slough in Berkshire by the M25 motorway. Heathrow falls under the Twickenham postcode area, with the postcode TW6; as the airport is located west of London and as its runways run east–west, an airliner's landing approach is directly over the conurbation of London when the wind is from the west, most of the time.
Along with Gatwick, Luton and London City, Heathrow is one of six airports with scheduled services serving the London area. Heathrow Airport originated in 1929 as a small airfield on land south-east of the hamlet of Heathrow from which the airport takes its name. At that time there were farms, market gardens and orchards there: there was a "Heathrow Farm" about where the old Terminal 1 was and where Terminal 2 is, a "Heathrow Hall" and a "Heathrow House"; this hamlet was along a country lane which ran along the east and south edges of the present central terminals area. Development of the whole Heathrow area as a much larger airport began in 1944: it was stated to be for long-distance military aircraft bound for the Far East, but by the time the airfield was nearing completion, World War II had ended. The government continued to develop the airport as a civil airport; the airport was opened on 25 March 1946 as London Airport and was renamed Heathrow Airport in 1966. The masterplan for the airport was designed by Sir Frederick Gibberd, who designed the original terminals and central area buildings, including the original control tower and the multi-faith chapel of St George's.
Heathrow Airport is used by over 80 airlines flying to 185 destinations in 84 countries. The airport is a base for Virgin Atlantic, it has a cargo terminal. Of Heathrow's 78 million passengers in 2017, 94% were international travellers; the busiest single destination in passenger numbers is New York, with over 3 million passengers flying between Heathrow and JFK Airport in 2013. In the 1950s, Heathrow had six runways, arranged in three pairs at different angles in the shape of a hexagram with the permanent passenger terminal in the middle and the older terminal along the north edge of the field; as the required length for runways has grown, Heathrow now has only two parallel runways running east–west. These are extended versions of the two east–west runways from the original hexagram. From the air all of the original runways can still be seen, incorporated into the present system of taxiways. North of the northern runway and the former taxiway and aprons, now the site of extensive car parks, is the entrance to the access tunnel and the site of Heathrow's unofficial "gate guardian".
For many years the home of a 40% scale model of a British Airways Concorde, G-CONC, the site has been occupied by a model of an Emirates Airbus A380 since 2008. Heathrow Airport has Anglican, Free Church, Jewish and Sikh chaplains. There is a multi-faith prayer room and counselling room in each terminal, in addition to St. George's Interdenominational Chapel in an underground vault adjacent to the old control tower, where Christian services take place; the chaplains lead prayers at certain times in the prayer room. The airport has its own resident press corps, consisting of six photographers and one TV crew, serving all the major newspapers and television stations around the world. Most of Heathrow's internal roads are initial letter coded by area: N in the north, E in the east, S in the south, W in the west, C in the centre. Aircraft destined for Heathrow are routed to one of four holding points. Air tra
International Civil Aviation Organization
The International Civil Aviation Organization is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth, its headquarters is located in the Quartier International of Montreal, Canada. The ICAO Council adopts standards and recommended practices concerning air navigation, its infrastructure, flight inspection, prevention of unlawful interference, facilitation of border-crossing procedures for international civil aviation. ICAO defines the protocols for air accident investigation followed by transport safety authorities in countries signatory to the Chicago Convention on International Civil Aviation; the Air Navigation Commission is the technical body within ICAO. The Commission is composed of 19 Commissioners, nominated by the ICAO's contracting states, appointed by the ICAO Council. Commissioners serve as independent experts, who although nominated by their states, do not serve as state or political representatives.
The development of international Standards And Recommended Practices is done under the direction of the ANC through the formal process of ICAO Panels. Once approved by the Commission, standards are sent to the Council, the political body of ICAO, for consultation and coordination with the Member States before final adoption. ICAO is distinct from other international air transport organizations, like the International Air Transport Association, a trade association representing airlines; the forerunner to ICAO was the International Commission for Air Navigation. It held its first convention in 1903 in Berlin, but no agreements were reached among the eight countries that attended. At the second convention in 1906 held in Berlin, 27 countries attended; the third convention, held in London in 1912 allocated the first radio callsigns for use by aircraft. ICAN continued to operate until 1945. Fifty-two countries signed the Chicago Convention on International Civil Aviation known as the Chicago Convention, in Chicago, Illinois, on 7 December 1944.
Under its terms, a Provisional International Civil Aviation Organization was to be established, to be replaced in turn by a permanent organization when 26 countries ratified the convention. Accordingly, PICAO began operating on 6 June 1945, replacing ICAN; the 26th country ratified the Convention on 5 March 1947 and PICAO was disestablished on 4 April 1947 and replaced by ICAO, which began operations the same day. In October 1947, ICAO became an agency of the United Nations linked to the United Nations Economic and Social Council. In April 2013 Qatar offered to serve as the new permanent seat of the Organization. Qatar promised to construct a massive new headquarters for ICAO and cover all moving expenses, stating that Montreal "was too far from Europe and Asia", "had cold winters," was hard to attend due to the refusal of the Canadian government to provide visas in a timely manner, that the taxes imposed on ICAO by Canada were too high. According to The Globe and Mail, Qatar's move was at least motivated by the pro-Israel foreign policy of Canadian Prime Minister Stephen Harper.
One month Qatar withdrew its bid after a separate proposal to the ICAO's governing council to move the ICAO triennial conference to Doha was defeated by a vote of 22–14. The 9th edition of the Convention on International Civil Aviation includes modifications from 1948 up to year 2006. ICAO refers to its current edition of the Convention as the Statute, designates it as ICAO Document 7300/9; the Convention has 19 Annexes that are listed by title in the article Convention on International Civil Aviation. As of January 2019, there are 192 ICAO members, consisting of 191 of the 193 UN members, plus the Cook Islands. Liechtenstein has delegated Switzerland to enter into the treaty on its behalf and the treaty applies in the territory of Liechtenstein; the Republic of China was a founding member of ICAO but was replaced by People's Republic of China as the legal representative of China in 1971 and as such, did not take part in the organization. In 2013, the Republic of China was for the first time invited to attend 38th session of ICAO Assembly as a guest under the name of Chinese Taipei.
The Council of ICAO is elected by the Assembly every 3 years and consists of 36 members elected in 3 groups. The present Council was elected on 4 October 2016 at the 39th Assembly of ICAO at Montreal; the structure of the present Council is as follows: ICAO standardizes certain functions for use in the airline industry, such as the Aeronautical Message Handling System. This makes it a standards organization; each country should have an accessible Aeronautical Information Publication, based on standards defined by ICAO, containing information essential to air navigation. Countries are required to update their AIP manuals every 28 days and so provide definitive regulations and information for each country about airspace and airports. ICAO's standards dictate that temporary hazards to aircraft are published using NOTAMs. ICAO defines an International Standard Atmosphere, a model of the standard variation of pressure, temperature and viscosity with altitude in the Earth's atmosphere; this is useful in designing aircraft.
A day is the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, was designated the SI base unit of time; the unit of measurement "day", was symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. A civil day is 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time, plus or minus an hour in those locations that change from or to daylight saving time. Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, corresponding to a rotation of the earth on its axis; however its use depends on its context, for example when people say'day and night','day' will have a different meaning. It will mean the interval of light between two successive nights.
However, in order to be clear when using'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period. The word day may refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle. Several definitions of this universal human concept are used according to context and convenience. Besides the day of 24 hours, the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the Sun to return to its culmination point; because celestial orbits are not circular, thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than 24 hours.
In recent decades, the average length of a solar day on Earth has been about 86 400.002 seconds and there are about 365.2422 solar days in one mean tropical year. Ancient custom has a new day start at either the setting of the Sun on the local horizon; the exact moment of, the interval between, two sunrises or sunsets depends on the geographical position, the time of year. A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon or midnight; the exact moment is dependent on the geographical longitude, to a lesser extent on the time of the year. The length of such a day is nearly constant; this is the time as indicated by modern sundials. A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator. A day, understood as the span of time it takes for the Earth to make one entire rotation with respect to the celestial background or a distant star, is called a stellar day; this period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 stellar days in one mean tropical year.
Other planets and moons have solar days of different lengths from Earth's. A day, in the sense of daytime, distinguished from night time, is defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles; the length of daytime averages more than half of the 24-hour day. Two effects make daytime on average longer than nights; the Sun has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. Thus, daytime is on average around 7 minutes longer than 12 hours; the term comes from the Old English dæg, with its cognates such as dagur in Icelandic, Tag in German, dag in Norwegian, Danish and Dutch. All of them from the Indo-European root dyau which explains the similarity with Latin dies though the word is known to come from the Germanic branch.
As of October 17, 2015, day is the 205th most common word in US English, the 210th most common in UK English. A day, symbol d, defined as 86 400 seconds, is not an SI unit, but is accepted for use with SI; the Second is the base unit of time in SI units. In 1967–68, during the 13th CGPM, the International Bureau of Weights and Measures redefined a second as … the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom; this makes the SI-based day last 794 243 384 928 000 of those periods. Due to tidal effects, the