where does the pacific crest trail start in california

The Pacific Crest Trail (commonly abbreviated as the PCT, and officially designated as the Pacific Crest National Scenic Trail) is a long-distance hiking and equestrian trail closely aligned with the highest portion of the Sierra Nevada and Cascade mountain ranges, which lie 100 to 150 miles (160 to 240 km) east of the U.S. Pacific coast. The trail's southern terminus is on the U.S. border with Mexico, just south of Campo, California, and its northern terminus on the Canada–US border on the edge of Manning Park in British Columbia; its corridor through the U.S. is in the states of California, Oregon, and Washington.

Pacific Crest Trail

The Pacific Crest Trail is 2,659 mi (4,279 km) long[1] and ranges in elevation from just above sea level at the Oregon–Washington border to 13,153 feet (4,009 m)[3] at Forester Pass in the Sierra Nevada. The route passes through 25 national forests and 7 national parks.[7] Its midpoint is near Chester, California (near Mt. Lassen), where the Sierra and Cascade mountain ranges meet.[8]

It was designated a National Scenic Trail in 1968, although it was not officially completed until 1993.[9] The PCT was conceived by Clinton Churchill Clarke in 1932.[10] It received official status under the National Trails System Act of 1968.

It is the westernmost and second longest component of the Triple Crown of Hiking and is part of the 6,875-mile Great Western Loop.

The route is mostly through National Forest and protected wilderness. The trail avoids civilization and covers scenic and pristine mountainous terrain with few roads. It passes through the Laguna, Santa Rosa, San Jacinto, San Bernardino, San Gabriel, Liebre, Tehachapi, Sierra Nevada, and Klamath ranges in California, and the Cascade Range in California, Oregon, and Washington.

A parallel route for bicycles, the Pacific Crest Bicycle Trail (PCBT) is a 2,500-mile (4,000Â km) route designed closely parallel to the PCT on roads. The PCT and PCBT cross in about 27 places along their routes.

The Pacific Crest Trail was first proposed by Clinton C. Clarke, as a trail running from Mexico to Canada along the crest of the mountains in California, Oregon, and Washington. The original proposal was to link the John Muir Trail, the Tahoe-Yosemite Trail (both in California), the Skyline Trail (in Oregon) and the Cascade Crest Trail (in Washington).[9]

The Pacific Crest Trail System Conference was formed by Clarke to both plan the trail and to lobby the federal government to protect the trail. The conference was founded by Clarke, the Boy Scouts, the YMCA, and Ansel Adams (amongst others). From 1935 through 1938, YMCA groups explored the 2000 miles of potential trail and planned a route, which has been closely followed by the modern PCT route.[9]

In 1968, President Lyndon B. Johnson defined the PCT and the Appalachian Trail with the National Trails System Act. The PCT was then constructed through cooperation between the federal government and volunteers organized by the Pacific Crest Trail Association. In 1993, the PCT was officially declared finished.[9]

The Trust for Public Land has purchased and conserved more than 3,000 acres along the Pacific Crest Trail in Washington.[11] Consolidation of this land has allowed for better recreational access as well as greater ease to manage conservation lands.[11]

Thru hiking is a term used in referring to hikers who complete long-distance trails from end to end in a single trip. The Pacific Crest Trail, Appalachian Trail, and Continental Divide Trail were the first three long-distance trails in the U.S.. Successfully thru-hiking all of these three trails is known as the Triple Crown of Hiking.[12]  Thru-hiking is a long commitment, usually taking between four and six months, that requires thorough preparation and dedication.
The Pacific Crest Trail Association estimates that it takes most hikers between six and eight months to plan their trip.[13]

While most hikers travel from the Southern Terminus at the Mexico–US border northward to Manning Park, British Columbia, some hikers prefer a southbound route. In a normal weather year, northbound hikes are most practical due to snow and temperature considerations.  Additionally, some hiker services are seasonal and may be better timed for northbound hikers.[14] If snowpack in the Sierra Nevada is high in early June and low in the Northern Cascades, some hikers may choose to 'flip-flop.' Flip-flopping can take many forms but often describes a process whereby a hiker begins at one end (on the PCT, usually the southern end) of the trail and then, at some point, like reaching the Sierra, 'flips' to the end of the trail at the Canada–US border and hikes southbound to complete the trail. However, it is not currently possible to legally enter the United States from Canada by using the Pacific Crest Trail.[15]

Hikers also have to determine their resupply points. Resupply points are towns or post offices where hikers replenish food and other supplies such as cooking fuel. Hikers can ship packages to themselves at the U.S. Post Offices along the trail, resupply at general and grocery stores along the trail, or any combination of the two.[16] The final major logistical step is to create an approximate schedule for completion. Thru hikers have to make sure they complete enough miles every day to reach the opposite end of the trail before weather conditions make sections impassable. For northbound thru-hikers, deep snow pack in the Sierra Nevada can prevent an early start. The timing is a balance between not getting to the Sierra too soon nor the Northern Cascades too late. Most hikers cover about 20 miles (32Â km) per day.[13]

In order to reduce their hiking time and thereby increase their chances of completing the trail, many hikers try to substantially reduce their pack weight. Since the creation of the Pacific Crest Trail there has been a large movement by hikers to get away from large heavy packs with a lot of gear. There are three general classifications for hikers: Traditional, Lightweight, and Ultralight.[17][18] Over the past few years[when?] the number of traditional hikers has dropped considerably.

Before the PCT was planned, Martin Papendick was the first known person to hike across three states of the PCT in 1952.[19] After being one of the first to finish the Appalachian trail in 1951, Papendick hiked between July 4 and December 1, 1952, from British Columbia to the Mexicoâ€“US border over the crests of the mountains along the Pacific Coast, a feat he reported in a periodical under the title "Pacific Crest Trails".[20]

On October 16, 1970, Eric Ryback, an 18-year-old student, completed the first PCT thru-hike. His personal congratulations came by telegram from Edward P. Cliff, Chief of the U.S. Forest Service.[21] Ryback is credited, recognized, and has been honored by the Pacific Crest Trail Association as the official first thru-hiker of the entire trail.[22] Ryback completed the Appalachian Trail in 1969 (as a 16-year-old); the Pacific Crest Trail in 1970; and a route approximating today's Continental Divide Trail in 1972.[23] Ryback's 1971 book The High Adventure of Eric Ryback: Canada to Mexico on Foot focused public attention on the PCT. Ryback carried an 80-pound pack on his 1970 thru-hike. He had only five resupply packages on the entire trip and was loaded with 40 pounds of food at the start of each leg. He often ran out of food and foraged or went hungry.[22] Ryback also helped the Forest Service lay out future plans for the PCT.[24]

However, Ryback's claim is disputed. When the guidebook publisher, Wilderness Press, stated that Ryback had used motor transport in places along the PCT, Ryback sued for $3 million but withdrew the suit after Wilderness Press revealed statements from the people who claim to have picked up the young hiker along highways parallel to the 2,600-mile trail. Ryback is in Smithsonian's top 9 list of people Cheating Their Way to Fame though it notes that The claims that Ryback "cheated" are still doubted by some.[25]

The first person to hike the PCT from south to north was Richard Watson, who completed the trail on September 1, 1972.[19] Watson was often credited as the first PCT thru-hiker, because Papendick was generally unknown and Ryback may have accepted rides.[19] The first woman to complete the PCT was Mary Carstens, who finished the journey later in 1972, accompanied by Jeff Smukler.[19]

The first person to thru-hike the entire PCT both ways in a single continuous round-trip was Scott Williamson, who completed the "yo-yo" circuit on his fourth attempt in November 2004. Williamson traveled a total of 5,300 miles (8,530 km) in 197 days, covering an average of 35 to 40 miles (56 to 64 km) per day when not in snow – an overall average of 27 miles (43 km) per day – wearing an extremely ultra-lightweight pack, which "without food, weighed about 8.5 pounds (3.9 kg)".[26] Williamson then went on to complete a second round trip on November 28, 2006, cutting two weeks off his 2004 time.[27]

The youngest person to hike the trail is Christian Thomas Geiger, who at the age of 6 completed the 2,686 mile trail with his parents Andrea Rego and Dion Pagonis.[28] Christian, also known by his trail name Buddy Backpacker, is also the youngest person to hike the Appalachian Trail [29][30]

Other notable young hikers include Sierra Burror and Reed Gjonnes.  Sierra Burror, who completed a continuous thru-hike of the trail in 2012 at the age of 9, is the youngest girl to thru-hike the trail. She completed her hike with her mother, Heather Burror.[31][32][33][34] Reed Gjonnes, who thru-hiked the trail in 2011 at age 11, went on to complete the Triple Crown of Hiking, becoming the youngest person ever to do so.[35]

Teddi Boston hiked from Canada to Mexico on the PCT in 1976 at age of 49. She was one of the first women to hike the trail alone.[36]

An autobiographical account of a woman hiking a portion of the PCT alone in 1995 at age 26 was written by Cheryl Strayed.  Her memoir Wild: From Lost to Found on the Pacific Crest Trail was published in 2012 and reached #1 on the New York Times Best Sellers list.[37] Her hike is the subject of the 2014 film Wild, starring Reese Witherspoon.

The first two reported deaths on the Pacific Crest Trail were in November 1995, when thru-hikers Jane and Flicka Rodman were killed during a detour down California State Route 138 in Southern California, when they were struck by a motorist who lost control of his vehicle. They were less than 400 miles from their goal of reaching the Mexicoâ€“US border.[38][39]

The oldest person to thru-hike the trail is not known.[40]

On August 7, 2013, Heather "Anish" Anderson of Bellingham, Washington, set the current unsupported speed record. She completed the entire PCT in 60 days, 17 hours, 12 minutes,[41] beating the previous record by almost 4 days.[42]

In 2014, Olive McGloin (from Ireland) became the first woman to thru-hike the PCT both ways in a single continuous round-trip.[43]

On August 10, 2014, Joseph McConaughy of Shoreline, Washington, and a former Boston College middle distance runner, set a new supported speed record and the overall fastest time for the PCT. The distance was covered in 53 days, 6 hours, and 37 minutes.[44][45][46] This overcame the previous record of 59 days, 8 hours, 14 minutes,[42][47][48][49] set by Josh Garret on August 8, 2013, by more than 6 days. Joe was supported by a team of three hikers, Jordan Hamm (a former Boston College distance runner), Michael Dillon, and Jack Murphy.

McConaughy's record was broken on August 14, 2016 by Karel Sabbe, a 27-year-old dentist from Ghent, Belgium. He covered the distance in a total time of 52 days, 8 hours, and 25 minutes, averaging over 50 miles a day and shaving almost a day (22 hours) off the previous record set by McConaughy.[50][51] Sabbe was supported by his friend Joren Biebuyck. With his run Sabbe was also awarded the official Guinness World Record for fastest completion of the Pacific Crest Trail.[52] For their record runs, McConaughy as well as Sabbe had to take some official detours because of wildfires.[53]

Don and June Mulford made the first verifiable equestrian Thru-Ride of the PCT in 1959.[54][55][56] In that year the Pacific Crest Trail stretched a poorly marked 2,400 miles from Mexico to Canada. More concept than footpath, the trail was an oft-broken, high-ridge track disappearing regularly from map and terrain. On April 19, 1959, on an empty scrub sage plain seven miles east of Tijuana, with four horses, Don and June Mulford began their journey north to the Washingtonâ€“Canada border. The Mulfords went to Hollywood for three months immediately after the ride and were featured on network television. June's old press book yields a half-dozen TV-Guide pages, and she recalls, "Art Linkletter was such a nice man. We appeared on his 'House Party' show and he had coffee with us afterward." "High Road to Danger," a syndicated TV show, made an episode on their ride. Even after they had returned home to the Northwest, there was continued TV coverage. A January 1961 TV Guide records their appearance on Portland's KOIN Red Dunning Show. The Mulfords even made a 90-minute movie and showed it around 12 western states for 10 years.

The Murray family – Barry, Bernice, Barry Jr. and Bennette – completed the trek on horseback on October 7, 1970.[57]

• In 2008, an agreement for realignment through Tejon Ranch in Southern California was reached.[58] This realignment would relocate 37 miles of the PCT from the Mojave Desert floor to the more scenic Tehachapi Mountains. While an agreement was reached, the realignment is a long-term project; many details remain to be determined, as well as an Optimal Location Review—a lengthy process through which the ideal path for the new section of trail is specified. Actual relocation of the trail is unlikely to happen before 2021.

â€¢ Portland, Oregon's 40-Mile Loop proposes to extend the Springwater Corridor hiking and bicycling spur trail to connect the Pacific Crest Trail[59] with the proposed Cazadero Trail.[60]

• Plans are currently in progress to add a dedicated pedestrian/equestrian lane to the Bridge of the Gods (modern structure) across the Columbia River. Currently, PCT hikers and equestrians must cross the bridge walking in vehicle traffic lanes—a potential danger which the new lane will eliminate. A completion date for this project is unknown.

The following notable locations are found along or adjacent to the route of the Pacific Crest Trail. They are listed from south to north to correspond with the itinerary typically followed by thru-hikers to take advantage of the best seasonal weather conditions.  The numbers in parentheses correspond to the numbers on the PCT overview map above.

Coordinates: 40°12′48″N 121°21′17″W / 40.2132°N 121.3546°W / 40.2132; -121.3546


you can tell a lot about a person by how they treat their waiter

The Waiter Rule refers to a common belief that one's true character can be gleaned from how one treats staff or service workers, such as a "waiter".[1] The rule was one of William H Swanson's 33 Unwritten Rules of Management, copied from Dave Barry's version "If someone is nice to you but rude to the waiter, they are not a nice person."[2]

Waiter Rule


who sings the country song merry go round

"Merry Go 'Round" is the debut single from American country music artist Kacey Musgraves. It was released in September 2012 as the lead single from Musgraves' debut album Same Trailer Different Park. Musgraves co-wrote and co-produced the song with Shane McAnally, with additional writing from Josh Osborne and production assistance from Luke Laird.

Merry Go 'Round (Kacey Musgraves song)

"Merry Go 'Round" won the Grammy Award for Best Country Song at the 56th Grammy Awards.[1]

The song is a cynical observation on life in the American heartland.[2] It also uses samples of the nursery rhyme Mary, Mary, Quite Contrary as a continued play-on-words. The latter part of the song retells another traditional nursery rhyme, Jack and Jill, in a modernized fashion. It is in the key of F-sharp major, in a 2/2 time signature with an approximate tempo of 88 beats per minute.[3] The song's main accompaniment is guitar and banjo.

Giving it 4.5 stars out of 5, Billy Dukes of Taste of Country compared the song's style and Musgraves' voice favorably to Miranda Lambert.[2] It received a "thumbs up" from Juli Thanki of Engine 145, who said that she "writes with a maturity beyond her 24 years, delivering an unflinching look into small-town life thatâ€™s more nuanced than the highly romanticized rural anthems offered up by many of her male counterparts."[4]

Rolling Stone named it the 49th best single of 2012.[5] It also made "NPR Music's 100 Favorite Songs of 2012" list.[6] It was number 1 on Billboard's "10 Best Country Singles of 2013" list.[7]

The music video was directed by Perry Bean and premiered in September 2012.[8]

"Merry Go 'Round" peaked at No. 14 on the Hot Country Songs chart.[1] The song reached the top 10 of the Billboard Country Airplay chart.  The song has sold 825,000 copies in the US as of February 2014.[9]

*sales figures based on certification alone^shipments figures based on certification alone


the pompidou center shown here is an early example of what 20th century style

Centre Georges Pompidou (French pronunciation: [sɑ̃tʁ ʒɔʁʒ pɔ̃pidu]), commonly shortened to Centre Pompidou and also known as the Pompidou Centre in English, is a complex building in the Beaubourg area of the 4th arrondissement of Paris, near Les Halles, rue Montorgueil, and the Marais. It was designed in the style of high-tech architecture by the architectural team of Richard Rogers and Renzo Piano, along with Gianfranco Franchini.

Centre Georges Pompidou

It houses the Bibliothèque publique d'information (Public Information Library), a vast public library; the Musée National d'Art Moderne, which is the largest museum for modern art in Europe; and IRCAM, a centre for music and acoustic research. Because of its location, the Centre is known locally as Beaubourg (IPA: [bobuʁ]).[1][2][3] It is named after Georges Pompidou, the President of France from 1969 to 1974 who commissioned the building, and was officially opened on 31 January 1977 by President Valéry Giscard d'Estaing. As of 2006[update], the Centre Pompidou has had over 180 million visitors since 1977[4] and more than 5,209,678 visitors in 2013,[5] including 3,746,899 for the museum.[6]

The sculpture Horizontal by Alexander Calder, a free-standing mobile that is 7.6 m (25 ft) tall, was placed in front of the Centre Pompidou in 2012.

The idea for a multicultural complex, bringing together in one place different forms of art and literature, developed, in part, from the ideas of France's first Minister of Cultural Affairs, André Malraux, a western proponent of the decentralisation of art and culture by impulse of the political power.[7] In the 1960s, city planners decided to move the foodmarkets of Les Halles, historically significant structures long prized by Parisians, with the idea that some of the cultural institutes be built in the former market area. Hoping to renew the idea of Paris as a leading city of culture and art, it was proposed to move the Musée d'Art Moderne to this new location. Paris also needed a large, free public library, as one did not exist at this time. At first the debate concerned Les Halles, but as the controversy settled, in 1968, President Charles de Gaulle announced the Plateau Beaubourg as the new site for the library. A year later in 1969, the new president adopted the Beaubourg project and decided it to be the location of both the new library and a centre for the contemporary arts. In the process of developing the project, the IRCAM (Institut de Recherche et Coordination Acoustique/Musique) was also housed in the complex.

By the mid-1980s, the Centre Pompidou was becoming the victim of its huge and unexpected popularity, its many activities, and a complex administrative structure. When Dominique Bozo returned to the Centre in 1981 as Director of the Musée National d'Art Moderne, he re-installed the museum, bringing out the full range of its collections and displayed the many major acquisitions that had been made.[8] By 1992, the Centre de Création Industrielle was incorporated into the Centre Pompidou.

Since re-opening in 2000 after a three-year renovation, the Centre Pompidou has improved accessibility for visitors. Now they can only access the escalators if they pay to enter the museum.[9]

The Centre was designed by Italian architect Renzo Piano; British architect Richard Rogers; and Italian architect Gianfranco Franchini, assisted by Ove Arup & Partners.[10] The project was awarded to this team in an architectural design competition, the results of which were announced in 1971. It was the first time in France that international architects were allowed to participate. World-renowned architects Oscar Niemeyer, Jean Prouvé and Philip Johnson made up the jury which would select one design out of the 681 entries.[11]

National Geographic described the reaction to the design as "love at second sight."[12] An article in Le Figaro declared "Paris has its own monster, just like the one in Loch Ness." But two decades later, while reporting on Rogers' winning the Pritzker Prize in 2007, The New York Times noted that the design of the Centre "turned the architecture world upside down" and that "Mr. Rogers earned a reputation as a high-tech iconoclast with the completion of the 1977 Pompidou Centre, with its exposed skeleton of brightly coloured tubes for mechanical systems. The Pritzker jury said the Pompidou "revolutionised museums, transforming what had once been elite monuments into popular places of social and cultural exchange, woven into the heart of the city."[13]

Initially, all of the functional structural elements of the building were colour-coded: green pipes are plumbing, blue ducts are for climate control, electrical wires are encased in yellow, and circulation elements and devices for safety (e.g., fire extinguishers) are red.[14]

The Centre was built by GTM and completed in 1977.[15] The building cost 993 million 1972 French francs. Renovation work conducted from October 1996 to January 2000 was completed on a budget of 576 million 1999 francs.[14]

The nearby Stravinsky Fountain (also called the Fontaine des automates), on Place Stravinsky, features 16 whimsical moving and water-spraying sculptures by Jean Tinguely and Niki de Saint-Phalle, which represent themes and works by composer Igor Stravinsky. The black-painted mechanical sculptures are by Tinguely, the coloured works by de Saint-Phalle. The fountain opened in 1983.[16]

Video footage of the fountain appeared frequently throughout the French language telecourse, French in Action.

The Place Georges Pompidou in front of the museum is noted for the presence of street performers, such as mimes and jugglers. In the spring, miniature carnivals are installed temporarily into the place in front with a wide variety of attractions: bands, caricature and sketch artists, tables set up for evening dining, and even skateboarding competitions.

The Centre Pompidou was intended to handle 8,000 visitors a day.[17] In its first two decades it attracted more than 145 million visitors, more than five times the number first predicted.[18] As of 2006[update], more than 180 million people have visited the centre since its opening in 1977.[4] However, until the 1997-2000 renovation, 20 percent of the centre's eight million annual visitors—in the main foreign tourists—rode the escalators up the outside of the building to the platform for the sights.[19]

Since 2006, the global attendance of the centre is no longer calculated at the main entrance, but only the one of the MusÃ©e National d'Art Moderne and of the public library (5,209,678 visitors for both in 2013),[6] but without the other visitors of the building (929,431 in 2004 or 928,380 in 2006, for only the panorama tickets or cinemas, festivals, lectures, bookshops, workshops, restaurants, etc.).[4]

The Musée National d'Art Moderne itself saw an increase in attendance from 3.1 million (2010) to 3.6 million visitors in 2011[20] and 3.75 million in 2013.[6]

The 2013 retrospective "Dalí" broke the museum’s daily attendance record; 7,364 people a day went to see the artist’s work (790,000 in total).[21]

Several major exhibitions are organised each year on either the first or sixth floors. Among them, many monographs:[22]

In 2010, the Centre Georges Pompidou opened a provincial branch, the Centre Pompidou-Metz, in Metz a city 170 miles east of Paris. The new museum is part of an effort to expand the display of contemporary arts beyond Paris's large museums. The new museum's building was designed by the architect Shigeru Ban with a curving and asymmetrical pagoda-like roof topped by a spire and punctured by upper galleries. The 77 metre central spire is a nod to the year the Centre Georges Pompidou of Paris was built – 1977. The Centre Pompidou-Metz displays unique, temporary exhibitions from the collection of the Musée National d'Art Moderne, which is not on display at the main Parisian museum. Since its inauguration, the institution has become the most visited cultural venue in France outside Paris, accommodating 550,000 visitors/year.[23][24]

Launched in 2011 in Chaumont, the museum for the first time went on the road to the French provinces with a selection of works from the permanent collection. To do this, it designed and constructed a mobile gallery, which, in the spirit of a circus, will make camp for a few months at a time in towns throughout the country.[25] However, in 2013, the Centre Pompidou halted its mobile-museum project because of the cost.[26]

In 2014, plans were released for a temporary satellite of the Centre Pompidou in the northern French town of Maubeuge close to the Belgian border. The 3,000 square metre outpost, to be designed by the architects Pierre Hebbelinck and Pierre de Wit, is said to be located at the 17th-century Maubeuge Arsenal for four years. The cost of the project is â‚¬5.8 million.[27]

In 2015, the city authorities in Libourne, a town in south-western France, proposed a Pompidou branch housed in a former military base called Esog.[28]

In 2015, around 70 works from the Centre Pompidou's collection are planned to go on show in a temporary glass-and-steel structure called The Cube (El Cubo) in Málaga. According to the Spanish newspaper El País, the annual €1 million cost of the five-year project will be funded by the city council.[29] The partnership with Málaga was announced by the city's mayor but was not confirmed by Pompidou Centre president Alain Seban until 24 April 2014.[30] Around 100 works from the Pompidou’s 20th and 21st century collection will be installed in the 2,000 square metres (22,000 square feet) space for two years, while a smaller area will be used for temporary exhibitions. Portraiture and the influence of Picasso will be among the subjects explored in the permanent display, organised by the Pompidou’s deputy director Brigitte Leal. Highlights will include works by Alberto Giacometti, René Magritte, Alexander Calder and Constantin Brancusi, and contemporary works by Sophie Calle, Bruce Nauman and Orlan. The city of Málaga also commissioned Daniel Buren to create a large-scale installation within El Cubo.[31]

The city of Málaga will pay the Centre Pompidou €1 million a year for the brand and the use of the collection.[31]

In a joint proposal with the Solomon R. Guggenheim Museum presented in 2005, the Centre Pompidou planned to build a museum of modern and contemporary art, design and the media arts in Hong Kong's West Kowloon Cultural District.[32] In 2007, the then president Bruno Racine announced plans to open a museum carrying the Pompidou's name in Shanghai, with its programming to be determined by the Pompidou. The location chosen for the new museum was a former fire station in the Luwan district's Huaihai Park. However, the scheme never materialised, reportedly due to the lack of a legal framework for a non-profit foreign institution to operate in China.[33] Other projects include the Pompidouâ€™s joint venture with the King Abdulaziz Centre for World Culture, an arts complex incorporating a museum in Dhahran, the building of which has stalled.[26]

In April 2014, Pompidou president Alain Seban confirmed that after Malaga (Spain), Mexico will be the next site for a pop-up Pompidou Centre.[30]

There have been rumours of a pop-up Pompidou satellite museum in Brazil since Alain Seban announced the plan for these temporary locations back in 2012.[33] At a talk on satellite museums at the Guggenheim on 24 April 2014, Alain Seban suggested that Brazil may be the third country to a host a temporary satellite museum, after Spain and Mexico.[30]

As a national museum, the Centre Pompidou is government-owned and subsidised by the Ministry of Culture (64.2% of its budget in 2012 : 82.8 on 129 million €), essentially for its staff. The Culture Ministry appoints its directors and controls its gestion, which is nevertheless independent, as Etablissement public à caractère administratif since its creation. In 2011, the museum earned $1.9 million from travelling exhibitions.[34]

Established in 1977 as the institution's US philanthropic arm, the Georges Pompidou Art and Culture Foundation acquires and encourages major gifts of art and design for exhibition at the museum.[35][36] Since 2006, the non-profit support group has brought in donations of 28 works, collectively valued at more than $14 million, and purchased many others.[37] In 2013, New York-based art collectors Thea Westreich Wagner and Ethan Wagner announced their intention to donate about 300 works by 27 European and international artists to the Centre Pompidou, thereby making one of the largest gifts in the institutionâ€™s history.[38]


refraction defect due to irregularities of the surface of the cornea or lens

Keratoconus (KC) is a disorder of the eye which results in progressive thinning of the cornea.[3] This may result in blurry vision, double vision, nearsightedness, astigmatism, and light sensitivity.[3] Usually both eyes are affected.[3] In more severe cases a scarring or a circle may be seen within the cornea.[4]

Keratoconus

While the cause is unknown, it is believed to occur due to a combination of genetic, environmental, and hormonal factors.[3] About seven percent of those affected have a family history of the condition.[3] Proposed environmental factors include rubbing the eyes and allergies.[4] The underlying mechanism involves changes of the cornea to a cone shape.[3] Diagnosis is by examination with a slit lamp.[3]

Initially the condition can typically be corrected with glasses or soft contact lenses.[3] As the disease worsens special contact lenses may be required.[3] In most people the disease stabilizes after a few years without severe vision problems.[3] In a small number of people scarring of the cornea occurs and a corneal transplantation is required.[3]

Keratoconus affects about 1 in 2000 people.[3][4] It occurs most commonly in late childhood to early adulthood.[3] While it occurs in all populations it may be more frequent in certain ethnic groups such as those of Asian descent.[4] The word is from the Greek kéras meaning cornea and the Latin cōnus meaning cone.[5]

People with early keratoconus typically notice a minor blurring of their vision and come to their clinician seeking corrective lenses for reading or driving.[7][8] At early stages, the symptoms of keratoconus may be no different from those of any other refractive defect of the eye. As the disease progresses, vision deteriorates, sometimes rapidly. Visual acuity becomes impaired at all distances, and night vision is often poor. Some individuals have vision in one eye that is markedly worse than that in the other. The disease is often bilateral, though asymmetrical. Some develop photophobia (sensitivity to bright light), eye strain from squinting in order to read, or itching in the eye,[7] but there is normally little or no sensation of pain. It may cause luminous objects to appear as cylindrical pipes with the same intensity at all points.

The classic symptom of keratoconus is the perception of multiple "ghost" images, known as monocular polyopia. This effect is most clearly seen with a high contrast field, such as a point of light on a dark background. Instead of seeing just one point, a person with keratoconus sees many images of the point, spread out in a chaotic pattern. This pattern does not typically change from day to day, but over time, it often takes on new forms. People also commonly notice streaking and flaring distortion around light sources. Some even notice the images moving relative to one another in time with their heart beat. The predominant optical aberration of the eye in keratoconus is coma.[9][10] The visual distortion experienced by the person comes from two sources, one being the irregular deformation of the surface of the cornea, and the other being scarring that occurs on its exposed highpoints. These factors act to form regions on the cornea that map an image to different locations on the retina. The effect can worsen in low light conditions, as the dark-adapted pupil dilates to expose more of the irregular surface of the cornea.

Six genes have been found to be associated with the condition. These genes include BANP-ZNF469, COL4A4, FOXO1, FNDC3B, IMMP2L and RXRA-COL5A1.[11] Others likely also exist.[11]

Despite considerable research, the cause of keratoconus remains unclear.[12] Several sources suggest that keratoconus likely arises from a number of different factors: genetic, environmental or cellular, any of which may form the trigger for the onset of the disease.[13][14][15] Once initiated, the disease normally develops by progressive dissolution of Bowman's layer,[8] which lies between the corneal epithelium and stroma. As the two come into contact, cellular and structural changes in the cornea adversely affect its integrity and lead to the bulging and scarring characteristic of the disorder. Within any individual keratoconic cornea, regions of degenerative thinning coexisting with regions undergoing wound healing may be found. Scarring appears to be an aspect of the corneal degradation; however, a recent, large, multicenter study suggests abrasion by contact lenses may increase the likelihood of this finding by a factor over two.[16][17]

A number of studies have indicated keratoconic corneas show signs of increased activity by proteases,[13] a class of enzymes that break some of the collagen cross-linkages in the stroma, with a simultaneous reduced expression of protease inhibitors.[18] Other studies have suggested that reduced activity by the enzyme aldehyde dehydrogenase may be responsible for a build-up of free radicals and oxidising species in the cornea.[19] Whatever the pathogenetical process, the damage caused by activity within the cornea likely results in a reduction in its thickness and biomechanical strength. At an ultrastructural level the weakening of the corneal tissue is associated with a disruption of the regular arrangement of the collagen layers and collagen fibril orientation.[20] While keratoconus is considered a noninflammatory disorder, one study shows wearing rigid contact lenses by people leads to overexpression of proinflammatory cytokines, such as IL-6, TNF-alpha, ICAM-1, and VCAM-1 in the tear fluid.[21]

A genetic predisposition to keratoconus has been observed,[22] with the disease running in certain families,[23] and incidences reported of concordance in identical twins.[24] The frequency of occurrence in close family members is not clearly defined, though it is known to be considerably higher than that in the general population,[12] and studies have obtained estimates ranging between 6% and 19%.[25] Two studies involving isolated, largely homogenetic communities have contrarily mapped putative gene locations to chromosomes 16q and 20q.[25] Most genetic studies agree on an autosomal dominant model of inheritance.[7] A rare, autosomal dominant form of severe keratoconus with anterior polar cataract is caused by a mutation in the seed region of mir-184, a microRNA that is highly expressed in the cornea and anterior lens.[26] Keratoconus is diagnosed more often in people with Down's syndrome, though the reasons for this link have not yet been determined.[27]

Keratoconus has been associated with atopic diseases,[28] which include asthma, allergies, and eczema, and it is not uncommon for several or all of these diseases to affect one person. Keratoconus is also associated with Alport syndrome, Down syndrome and Marfan syndrome.[28] A number of studies suggest vigorous eye rubbing contributes to the progression of keratoconus, and people should be discouraged from the practice.[29][30][31][32][33][34] Keratoconus differs from ectasia which is caused by LASIK eye surgery. Post-LASIK Ectasia has been associated with the excessive removal of the eye's stromal bed tissue during surgery.

Prior to any physical examination, the diagnosis of keratoconus frequently begins with an ophthalmologist's or optometrist's assessment of the person's medical history, particularly the chief complaint and other visual symptoms, the presence of any history of ocular disease or injury which might affect vision, and the presence of any family history of ocular disease. An eye chart, such as a standard Snellen chart of progressively smaller letters, is then used to determine the person's visual acuity. The eye examination may proceed to measurement of the localized curvature of the cornea with a manual keratometer,[35] with detection of irregular astigmatism suggesting a possibility of keratoconus. Severe cases can exceed the instrument's measuring ability.[8] A further indication can be provided by retinoscopy, in which a light beam is focused on the person's retina and the reflection, or reflex, observed as the examiner tilts the light source back and forth. Keratoconus is amongst the ophthalmic conditions that exhibit a scissor reflex action of two bands moving toward and away from each other like the blades of a pair of scissors.[8][36]

If keratoconus is suspected, the ophthalmologist or optometrist will search for other characteristic findings of the disease by means of slit lamp examination of the cornea.[24] An advanced case is usually readily apparent to the examiner, and can provide for an unambiguous diagnosis prior to more specialized testing. Under close examination, a ring of yellow-brown to olive-green pigmentation known as a Fleischer ring can be observed in around half of keratoconic eyes.[37] The Fleischer ring, caused by deposition of the iron oxide hemosiderin within the corneal epithelium, is subtle and may not be readily detectable in all cases, but becomes more evident when viewed under a cobalt blue filter.[8] Similarly, around 50% of subjects exhibit Vogt's striae, fine stress lines within the cornea caused by stretching and thinning.[37] The striae temporarily disappear while slight pressure is applied to the eyeball.[8] A highly pronounced cone can create a V-shaped indentation in the lower eyelid when the person's gaze is directed downwards, known as Munson's sign.[7] Other clinical signs of keratoconus will normally have presented themselves long before Munson's sign becomes apparent,[38] and so this finding, though a classic sign of the disease, tends not to be of primary diagnostic importance.

A handheld keratoscope, sometimes known as "Placido's disk", can provide a simple noninvasive visualization of the surface of the cornea by projecting a series of concentric rings of light onto the cornea. A more definitive diagnosis can be obtained using corneal topography, in which an automated instrument projects the illuminated pattern onto the cornea and determines its topography from analysis of the digital image. The topographical map indicates any distortions or scarring in the cornea, with keratoconus revealed by a characteristic steepening of curvature which is usually below the centreline of the eye.[12] The technique can record a snapshot of the degree and extent of the deformation as a benchmark for assessing its rate of progression. It is of particular value in detecting the disorder in its early stages when other signs have not yet presented.[39]

Once keratoconus has been diagnosed, its degree may be classified by several metrics:[40]

Increasing use of corneal topography has led to a decline in use of these terms.[40]

Corneal thickness is the thinnest measured spot of the cornea.

In early stages of keratoconus, glasses or soft contact lenses can suffice to correct for the mild astigmatism. As the condition progresses, these may no longer provide the person with a satisfactory degree of visual acuity, and most practitioners will move to manage the condition with rigid contact lenses, known as rigid, gas-permeable, (RGP) lenses. RGP lenses provide a good level of visual correction, but do not arrest progression of the condition.[44]

In people with keratoconus, rigid contact lenses improve vision by means of tear fluid filling the gap between the irregular corneal surface and the smooth regular inner surface of the lens, thereby creating the effect of a smoother cornea. Many specialized types of contact lenses have been developed for keratoconus, and affected people may seek out both doctors specialized in conditions of the cornea, and contact lens fitters who have experience managing people with keratoconus. The irregular cone presents a challenge[13] and the fitter will endeavor to produce a lens with the optimal contact, stability and steepness. Some trial-and-error fitting may prove necessary.[38]

Traditionally, contact lenses for keratoconus have been the 'hard' or RGP variety, although manufacturers have also produced specialized'soft' or hydrophilic lenses and, most recently, silicone hydrogel lenses. A soft lens has a tendency to conform to the conical shape of the cornea, thus diminishing its effect. To counter this, hybrid lenses have been developed which are hard in the centre and encompassed by a soft skirt. However, soft or earlier generation hybrid lenses did not prove effective for every person.[45] Early generation lenses have been discontinued.[46] The fourth generation of hybrid lens technology has improved, giving more people an option that combines the comfort of a soft lens with the visual acuity of an RGP lens.[47]

Scleral lenses are sometimes prescribed for cases of advanced or very irregular keratoconus; these lenses cover a greater proportion of the surface of the eye and hence can offer improved stability.[48] Easier handling can find favor with people with reduced dexterity, such as the elderly.

Some people find good vision correction and comfort with a "piggyback" lens combination, in which RGP lenses are worn over soft lenses, both providing a degree of vision correction.[49] One form of piggyback lens makes use of a soft lens with a countersunk central area to accept the rigid lens. Fitting a piggyback lens combination requires experience on the part of the lens fitter, and tolerance on the part of the person with keratoconus.

Between 11% and 27% of cases of keratoconus[14][50][51] will progress to a point where vision correction is no longer possible, thinning of the cornea becomes excessive, or scarring as a result of contact lens wear causes problems of its own, and a corneal transplantation or penetrating keratoplasty becomes required. Keratoconus is the most common grounds for conducting a penetrating keratoplasty, generally accounting for around a quarter of such procedures.[52] The corneal transplant surgeon trephines a lenticule of corneal tissue and then grafts the donor cornea to the existing eye tissue, usually using a combination of running and individual sutures. The cornea does not have a direct blood supply, so the donor tissue is not required to be blood type matched. Eye banks check the donor corneas for any disease or cellular irregularities.

The acute recovery period can take four to six weeks, and full postoperative vision stabilization often takes a year or more, but most transplants are very stable in the long term.[51] The National Keratoconus Foundation reports that penetrating keratoplasty has the most successful outcome of all transplant procedures, and when performed for keratoconus in an otherwise healthy eye, its success rate can be 95% or greater.[14] The sutures used usually dissolve over a period of three to five years, but individual sutures can be removed during the healing process if they are causing irritation to the person.

In the USA, corneal transplants (also known as corneal grafts) for keratoconus are usually performed under sedation as outpatient surgery. In other countries, such as Australia and the UK, the operation is commonly performed with the person undergoing a general anaesthetic. All cases require a careful follow-up with an eye doctor (ophthalmologist or optometrist) for a number of years. Frequently, vision is greatly improved after the surgery, but even if the actual visual acuity does not improve, because the cornea is a more normal shape after the healing is completed, people can more easily be fitted with corrective lenses. Complications of corneal transplants are mostly related to vascularization of the corneal tissue and rejection of the donor cornea. Vision loss is very rare, though difficult-to-correct vision is possible. When rejection is severe, repeat transplants are often attempted, and are frequently successful.[53] Keratoconus will not normally reoccur in the transplanted cornea; incidences of this have been observed, but are usually attributed to incomplete excision of the original cornea or inadequate screening of the donor tissue.[54] The long-term outlook for corneal transplants performed for keratoconus is usually favorable once the initial healing period is completed and a few years have elapsed without problems.

One way of reducing the risk of rejection is to use a technique called deep anterior lamellar keratoplasty (DALK). In a DALK graft, only the outermost epithelium and the main bulk of the cornea, the stroma, are replaced; the person's rearmost endothelium layer and the Descemet's membrane are left, giving some additional structural integrity to the postgraft cornea.[55] Furthermore, it is possible to transplant freeze-dried donor tissue. The freeze-drying process ensures this tissue is dead, so there is no chance of rejection.[55] Research from two trials in Iran provide low to moderate evidence that graft rejection is more likely to occur in penetrating keratoplasty than in DALK, though the likelihood for graft failure were similar with both procedures.[56]

Rarely, a nonpenetrating keratoplasty known as an epikeratophakia (or epikeratoplasty) may be performed in cases of keratoconus. The corneal epithelium is removed and a lenticule of donor cornea is grafted on top of it.[13] The procedure requires a greater level of skill on the part of the surgeon, and is less frequently performed than a penetrating keratoplasty, as the outcome is generally less favorable. However, it may be seen as an option in a number of cases, particularly for young people.[57]

A possible surgical alternative to corneal transplant is the insertion of intrastromal corneal ring segments. A small incision is made in the periphery of the cornea and two thin arcs of polymethyl methacrylate are slid between the layers of the stroma on either side of the pupil before the incision is closed by a suture.[58] The segments push out against the curvature of the cornea, flattening the peak of the cone and returning it to a more natural shape. The procedure offers the benefit of being reversible and even potentially exchangeable as it involves no removal of eye tissue.[58][59]

Corneal collagen cross-linking is a developing treatment which aims to strengthen the cornea, however, according to a 2015 Cochrane review, there is insufficient evidence to determine if it is useful in keratoconus.[60]

In 2016, the US Food and Drug Administration approved riboflavin ophthalmic solution and KXL system for crosslinking based on three 12-month clinical trials.[61]

Radial keratotomy is a refractive surgery procedure where the surgeon makes a spoke-like pattern of incisions into the cornea to modify its shape. This early surgical option for myopia has been largely superseded by LASIK and other similar procedures. LASIK is absolutely contraindicated in keratoconus and other corneal thinning conditions as removal of corneal stromal tissue will further damage an already thin and weak cornea.[62] For similar reasons, radial keratotomy has also generally not been used for people with keratoconus.[63][64]

Patients with keratoconus typically present initially with mild astigmatism and myopia, commonly at the onset of puberty, and are diagnosed by the late teenage years or early 20s. The disease can, however, present or progress at any age; in rare cases, keratoconus can present in children or not until later adulthood.[8] A diagnosis of the disease at an early age may indicate a greater risk of severity in later life.[12][65] Patients' vision will seem to fluctuate over a period of months, driving them to change lens prescriptions frequently, but as the condition worsens, contact lenses are required in the majority of cases. The course of the disorder can be quite variable, with some patients remaining stable for years or indefinitely, while others progress rapidly or experience occasional exacerbations over a long and otherwise steady course. Most commonly, keratoconus progresses for a period of 10 to 20 years[38] before the course of the disease generally ceases in the third and fourth decades of life.

In advanced cases, bulging of the cornea can result in a localized rupture of Descemet's membrane, an inner layer of the cornea. Aqueous humor from the eye's anterior chamber seeps into the cornea before Descemet's membrane reseals. The patient experiences pain and a sudden severe clouding of vision, with the cornea taking on a translucent milky-white appearance known as a corneal hydrops.[66]

Although disconcerting to the patient, the effect is normally temporary and after a period of six to eight weeks, the cornea usually returns to its former transparency. The recovery can be aided nonsurgically by bandaging with an osmotic saline solution. Although a hydrops usually causes increased scarring of the cornea, occasionally it will benefit a patient by creating a flatter cone, aiding the fitting of contact lenses.[66] Corneal transplantation is not usually indicated during corneal hydrops.

The National Eye Institute reports keratoconus is the most common corneal dystrophy in the United States, affecting about one in 2,000 Americans,[67][68] but some reports place the figure as high as one in 500.[69] The inconsistency may be due to variations in diagnostic criteria,[8] with some cases of severe astigmatism interpreted as those of keratoconus, and vice versa.[38] A long-term study found a mean incidence rate of 2.0 new cases per 100,000 population per year.[68] Some studies have suggested a higher prevalence amongst females,[70] or that people of South Asian ethnicity are 4.4 times as likely to suffer from keratoconus as Caucasians, and are also more likely to be affected with the condition earlier.[71]

Keratoconus is normally bilateral[68] (affecting both eyes) although the distortion is usually asymmetric and is rarely completely identical in both corneas.[8] Unilateral cases tend to be uncommon, and may in fact be very rare if a very mild condition in the better eye is simply below the limit of clinical detection.[38] It is common for keratoconus to be diagnosed first in one eye and not until later in the other. As the condition then progresses in both eyes, the vision in the earlier-diagnosed eye will often remain poorer than that in its fellow.

The German oculist Burchard Mauchart provided an early description in a 1748 doctoral dissertation of a case of keratoconus,[12] which he called staphyloma diaphanum. However, it was not until 1854 that British physician John Nottingham (1801–1856) clearly described keratoconus and distinguished it from other ectasias of the cornea.[12] Nottingham reported the cases of "conical cornea" that had come to his attention, and described several classic features of the disease, including polyopia, weakness of the cornea, and difficulty matching corrective lenses to the patient's vision.[6] In 1859, British surgeon William Bowman used an ophthalmoscope (recently invented by Hermann von Helmholtz) to diagnose keratoconus, and described how to angle the instrument's mirror so as to best see the conical shape of the cornea.[72] Bowman also attempted to restore vision by pulling on the iris with a fine hook inserted through the cornea and stretching the pupil into a vertical slit, like that of a cat. He reported that he had had a measure of success with the technique, restoring vision to an 18-year-old woman who had previously been unable to count fingers at a distance of 8 inches (20 cm).

By 1869, when the pioneering Swiss ophthalmologist Johann Horner wrote a thesis entitled On the treatment of keratoconus,[73] the disorder had acquired its current name. The treatment at that time, endorsed by the leading German ophthalmologist Albrecht von Graefe, was an attempt to physically reshape the cornea by chemical cauterization with a silver nitrate solution and application of a miosis-causing agent with a pressure dressing.[12] In 1888, the treatment of keratoconus became one of the first practical applications of the then newly invented contact lens, when the French physician Eugène Kalt manufactured a glass scleral shell that improved vision by compressing the cornea into a more regular shape.[74] Since the start of the 20th century, research on keratoconus has both improved understanding of the disease and greatly expanded the range of treatment options. The first successful corneal transplantation to treat keratoconus was done in 1936 by Ramon Castroviejo.[75][76]

According to the findings of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK), people who have keratoconus could be expected to pay more than $25,000 over their lifetime post-diagnosis, with a standard deviation of $19,396.[77] There is limited evidence on the costs of corneal cross-linking,[78] a cost-effectiveness study estimated the costs of the total treatment for one person as Â£928 ($1,392 U.S.) in the UK National Health Service,[79] but this may be as high as $2,500 per eye in other countries.[80] A 2013 cost-benefit analysis by the Lewin Group for Eye Bank Association of America, estimated an average cost of $16,500 for each corneal transplant.[81]

Several other corneal ectatic disorders also cause thinning of the cornea:[38]


which astronomical bodies emit light in the visible spectrum

There are three main types of nebula: absorption, reflection, and emission nebulae. Absorption (or dark) nebulae are made of dust and gas in such quantities that they obscure the starlight behind them, making photometry difficult. Reflection nebulae, as their name suggest, reflect the light of nearby stars. Their spectra are the same as the stars surrounding them, though the light is bluer; shorter wavelengths scatter better than longer wavelengths. Emission nebulae emit light at specific wavelengths depending on their chemical composition.[34]

Astronomical spectroscopy

Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light and radio, which radiates from stars and other hot celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition, temperature, density, mass, distance, luminosity, and relative motion using Doppler shift measurements.

Astronomical spectroscopy is used to measure three major bands of radiation: visible spectrum, radio, and X-ray. While all spectroscopy looks at specific areas of the spectrum, different methods are required to acquire the signal depending on the frequency. Ozone (O3) and molecular oxygen (O2) absorb light with wavelengths under 300Â nm, meaning that X-ray and ultraviolet spectroscopy require the use of a satellite telescope or rocket mounted detectors.[1]:27 Radio signals have much longer wavelengths than optical signals, and require the use of antennas or radio dishes. Infrared light is absorbed by atmospheric water and carbon dioxide, so while the equipment is similar to that used in optical spectroscopy, satellites are required to record much of the infrared spectrum.[2]

Physicists have been looking at the solar spectrum since Isaac Newton first used a simple prism to observe the refractive properties of light.[3] In the early 1800s Joseph von Fraunhofer used his skills as a glass maker to create very pure prisms, which allowed him to observe 574 dark lines in a seemingly continuous spectrum.[4] Soon after he combined telescope and prism to observe the spectrum of Venus, the Moon, Mars, and various stars such as Betelgeuse; his company continued to manufacture and sell high-quality refracting telescopes based on his original designs until its closure in 1884.[5]:28â€“29

The resolution of a prism is limited by its size; a larger prism will provide a more detailed spectrum, but the increase in mass makes it unsuitable for highly detailed work.[6] This issue was resolved in the early 1900s with the development of high-quality reflection gratings by J.S. Plaskett at the Dominion Observatory in Ottawa, Canada.[5]:11 Light striking a mirror will reflect at the same angle, however a small portion of the light will be refracted at a different angle; this is dependent upon the indices of refraction of the materials and the wavelength of the light.[7] By creating a "blazed" grating which utilizes a large number of parallel mirrors, the small portion of light can be focused and visualized. These new spectroscopes were more detailed than a prism, required less light, and could be focused on a specific region of the spectrum by tilting the grating.[6]

The limitation to a blazed grating is the width of the mirrors, which can only be ground a finite amount before focus is lost; the maximum is around 1000 lines/mm. In order to overcome this limitation holographic gratings were developed. Volume phase holographic gratings use a thin film of dichromated gelatin on a glass surface, which is subsequently exposed to a wave pattern created by an interferometer. This wave pattern sets up a reflection pattern similar to the blazed gratings but utilizing Bragg diffraction, a process where the angle of reflection is dependent on the arrangement of the atoms in the gelatin. The holographic gratings can have up to 6000 lines/mm and can be up to twice as efficient in collecting light as blazed gratings. Because they are sealed between two sheets of glass, the holographic gratings are very versatile, potentially lasting decades before needing replacement.[8]

Light dispersed by the grating or prism in a spectrograph can be recorded by a detector. Historically, photographic plates were widely used to record spectra until electronic detectors were developed, and today optical spectrographs most often employ charge-coupled devices (CCDs). The wavelength scale of a spectrum can be calibrated by observing the spectrum of emission lines of known wavelength from a gas-discharge lamp. The flux scale of a spectrum can be calibrated as a function of wavelength by comparison with an observation of a standard star with corrections for atmospheric absorption of light; this is known as spectrophotometry.[9]

Radio astronomy was founded with the work of Karl Jansky in the early 1930s, while working for Bell Labs. He built a radio antenna to look at potential sources of interference for transatlantic radio transmissions. One of the sources of noise discovered came not from Earth, but from the center of the Milky Way, in the constellation Sagittarius.[10] In 1942, JS Hey captured the sun's radio frequency using military radar receivers.[1]:26 Radio spectroscopy started with the discovery of the 21-centimeter HÂ I line in 1951.

Radio interferometry was pioneered in 1946, when Joseph Lade Pawsey, Ruby Payne-Scott and Lindsay McCready used a single antenna atop a sea cliff to observe 200Â MHz solar radiation. Two incident beams, one directly from the sun and the other reflected from the sea surface, generated the necessary interference.[11] The first multi-receiver interferometer was built in the same year by Martin Ryle and Vonberg.[12][13] In 1960, Ryle and Antony Hewish published the technique of aperture synthesis to analyze interferometer data.[14] The aperture synthesis process, which involves autocorrelating and discrete Fourier transforming the incoming signal, recovers both the spatial and frequency variation in flux.[15] The result is a 3D image whose third axis is frequency. For this work, Ryle and Hewish were jointly awarded the 1974 Nobel Prize in Physics.[16]

Newton used a prism to split white light into a spectrum of color, and Fraunhofer's high-quality prisms allowed scientists to see dark lines of an unknown origin. In the 1850s, Gustav Kirchhoff and Robert Bunsen described the phenomena behind these dark lines. Hot solid objects produce light with a continuous spectrum, hot gasses emit light at specific wavelengths, and hot solid objects surrounded by cooler gasses show a near-continuous spectrum with dark lines corresponding to the emission lines of the gasses.[5]:42â€“44[17] By comparing the absorption lines of the sun with emission spectra of known gasses, the chemical composition of stars can be determined.

The major Fraunhofer lines, and the elements with which they are associated, appear in the following table. Designations from the early Balmer Series are shown in parentheses.

Not all of the elements in the sun were immediately identified. Two examples are listed below.

To date more than 20 000 absorption lines have been listed for the Sun between 293.5 and 877.0Â nm, yet only approximately 75% of these lines have been linked to elemental absorption.[1]:69

By analyzing the width of each spectral line in an emission spectrum, both the elements present in a star and their relative abundances can be determined.[7] Using this information stars can be categorized into stellar populations; Population I stars are the youngest stars and have the highest metal content (our Sun is a Pop I star), while Population III stars are the oldest stars with a very low metal content.[19][20]

In 1860 Gustav Kirchhoff proposed the idea of a black body, a material that emits electromagnetic radiation at all wavelengths.[21][22] In 1894 Wilhelm Wien derived an expression relating the temperature (T) of a black body to its peak emission wavelength (Î»max).[23]

b is a constant of proportionality called Wien's displacement constant, equal to 6997289777290000000♠2.8977729(17)×10−3 m⋅K.[24] This equation is called Wien's Law. By measuring the peak wavelength of a star, the surface temperature can be determined.[17] For example, if the peak wavelength of a star is 502 nm the corresponding temperature will be 5778 kelvins.

The luminosity of a star is a measure of the electromagnetic energy output in a given amount of time.[25] Luminosity (L) can be related to the temperature (T) of a star by

where R is the radius of the star and σ is the Stefan–Boltzmann constant, with a value of 6992567036700000000♠5.670367(13)×10−8 W⋅m−2⋅K−4.[26] Thus, when both luminosity and temperature are known (via direct measurement and calculation) the radius of a star can be determined.

The spectra of galaxies look similar to stellar spectra, as they consist of the combined light of millions of stars.

Doppler shift studies of galaxy clusters by Fritz Zwicky in 1937 found that most galaxies were moving much faster than seemed to be possible from what was known about the mass of the cluster. Zwicky hypothesized that there must be a great deal of non-luminous matter in the galaxy clusters, which became known as dark matter.[27] Since his discovery, astronomers have determined that a large portion of galaxies (and most of the universe) is made up of dark matter. In 2003, however, four galaxies (NGC 821, NGC 3379, NGC 4494, and NGC 4697) were found to have little to no dark matter influencing the motion of the stars contained within them; the reason behind the lack of dark matter is unknown.[28]

In the 1950s, strong radio sources were found to be associated with very dim, very red objects. When the first spectrum of one of these objects was taken there were absorption lines at wavelengths where none were expected. It was soon realised that what was observed was a normal galactic spectrum, but highly red shifted.[29][30] These were named quasi-stellar radio sources, or quasars, by Hong-Yee Chiu in 1964.[31] Quasars are now thought to be galaxies formed in the early years of our universe, with their extreme energy output powered by super-massive black holes.[30]

The properties of a galaxy can also be determined by analyzing the stars found within them. NGC 4550, a galaxy in the Virgo Cluster, has a large portion of its stars rotating in the opposite direction as the other portion. It is believed that the galaxy is the combination of two smaller galaxies that were rotating in opposite directions to each other.[32] Bright stars in galaxies can also help determine the distance to a galaxy, which may be a more accurate method than parallax or standard candles.[33]

The interstellar medium is matter that occupies the space between star systems in a galaxy. 99% of this matter is gaseous - hydrogen, helium, and smaller quantities of other ionized elements such as oxygen. The other 1% is dust particles, thought to be mainly graphite, silicates, and ices.[34] Clouds of the dust and gas are referred to as nebulae.

In the early years of astronomical spectroscopy, scientists were puzzled by the spectrum of gaseous nebulae. In 1864 William Huggins noticed that many nebulae showed only emission lines rather than a full spectrum like stars. From the work of Kirchhoff, he concluded that nebulae must contain "enormous masses of luminous gas or vapour."[35] However, there were several emission lines that could not be linked to any terrestrial element, brightest among them lines at 495.9 nm and 500.7 nm.[36] These lines were attributed to a new element, nebulium, until Ira Bowen determined in 1927 that the emission lines were from highly ionised oxygen (O+2).[37][38] These emission lines could not be replicated in a laboratory because they are forbidden lines; the low density of a nebula (one atom per cubic centimetre)[34] allows for metastable ions to decay via forbidden line emission rather than collisions with other atoms.[36]

Not all emission nebulae are found around or near stars where solar heating causes ionisation. The majority of gaseous emission nebulae are formed of neutral hydrogen. In the ground state neutral hydrogen has two possible spin states: the electron has either the same spin or the opposite spin of the proton. When the atom transitions between these two states, it releases an emission or absorption line of 21Â cm.[34] This line is within the radio range and allows for very precise measurements:[36]

Using this information the shape of the Milky Way has been determined to be a spiral galaxy, though the exact number and position of the spiral arms is the subject of ongoing research.[39]

Dust and molecules in the interstellar medium not only obscures photometry, but also causes absorption lines in spectroscopy. Their spectral features are generated by transitions of component electrons between different energy levels, or by rotational or vibrational spectra. Detection usually occurs in radio, microwave, or infrared portions of the spectrum.[40] The chemical reactions that form these molecules can happen in cold, diffuse clouds[41] or in the hot ejecta around a white dwarf star from a nova or supernova.[42] Polycyclic aromatic hydrocarbons such as acetylene (C2H2) generally group together to form graphites or other sooty material,[43] but other organic molecules such as acetone ((CH3)2CO)[44] and buckminsterfullerenes (C60 and C70) have been discovered.[42]

Stars and interstellar gas are bound by gravity to form galaxies, and groups of galaxies can be bound by gravity in galaxy clusters.[45] With the exception of stars in the Milky Way and the galaxies in the Local Group, almost all galaxies are moving away from us due to the expansion of the universe.[18]

The motion of stellar objects can be determined by looking at their spectrum. Because of the Doppler effect, objects moving towards us are blueshifted, and objects moving away are redshifted. The wavelength of redshifted light is longer, appearing redder than the source. Conversely, the wavelength of blueshifted light is shorter, appearing bluer than the source light:

where

λ

0

{\displaystyle \lambda _{0}}

 is the emitted wavelength,

v

0

{\displaystyle v_{0}}

 is the velocity of the object, and

λ

{\displaystyle \lambda }

 is the observed wavelength. Note that v<0 corresponds to λ<λ0, a blueshifted wavelength. A redshifted absorption or emission line will appear more towards the red end of the spectrum than a stationary line. In 1913 Vesto Slipher determined the Andromeda Galaxy was blueshifted, meaning it was moving towards the Milky Way. He recorded the spectra of 20 other galaxies — all but 4 of which were redshifted — and was able to calculate their velocities relative to the Earth. Edwin Hubble would later use this information, as well as his own observations, to define Hubble's law: The further a galaxy is from the Earth, the faster it is moving away from us.[18][46] Hubble's law can be generalised to

where

v

{\displaystyle v}

 is the velocity (or Hubble Flow),

H

0

{\displaystyle H_{0}}

 is the Hubble Constant, and

d

{\displaystyle d}

 is the distance from Earth.

Redshift (z) can be expressed by the following equations:[47]

In these equations, frequency is denoted by

f

{\displaystyle f}

 and wavelength by

λ

{\displaystyle \lambda }

. The larger the value of z, the more redshifted the light and the farther away the object is from the Earth. As of January 2013, the largest galaxy redshift of z~12 was found using the Hubble Ultra-Deep Field, corresponding to an age of over 13 billion years (the universe is approximately 13.82 billion years old).[48][49][50]

The Doppler effect and Hubble's law can be combined to form the equation

z
=

v

H
u
b
b
l
e

c

{\displaystyle z={\frac {v_{Hubble}}{c}}}

, where c is the speed of light.

Objects that are gravitationally bound will rotate around a common center of mass. For stellar bodies, this motion is known as peculiar velocity, and can alter the Hubble Flow. Thus, an extra term for the peculiar motion needs to be added to Hubble's law:[51]

This motion can cause confusion when looking at a solar or galactic spectrum, because the expected redshift based on the simple Hubble law will be obscured by the peculiar motion. For example, the shape and size of the Virgo Cluster has been a matter of great scientific scrutiny due to the very large peculiar velocities of the galaxies in the cluster.[52]

Just as planets can be gravitationally bound to stars, pairs of stars can orbit each other. Some binary stars are visual binaries, meaning they can be observed orbiting each other through a telescope. Some binary stars, however, are too close together to be resolved.[53] These two stars, when viewed through a spectrometer, will show a composite spectrum: the spectrum of each star will be added together. This composite spectrum becomes easier to detect when the stars are of similar luminosity and of different spectral class.[54]

Spectroscopic binaries can be also detected due to their radial velocity; as they orbit around each other one star may be moving towards the Earth whilst the other moves away, causing a Doppler shift in the composite spectrum. The orbital plane of the system determines the magnitude of the observed shift: if the observer is looking perpendicular to the orbital plane there will be no observed radial velocity.[53][54] For example, if you look at a carousel from the side, you will see the animals moving toward and away from you, whereas if you look from directly above they will only be moving in the horizontal plane.

Planets, asteroids, and comets all reflect light from their parent stars and emit their own light. For cooler objects, including solar-system planets and asteroids, most of the emission is at infrared wavelengths we cannot see, but that are routinely measured with spectrometers. For objects surrounded by gas, such as comets and planets with atmospheres, further emission and absorption happens at specific wavelengths in the gas, imprinting the spectrum of the gas on that of the solid object. In the case of worlds with thick atmospheres or complete cloud cover (such as the gas giants, Venus, and Saturn's satellite Titan (moon)), the spectrum is mostly or completely due to the atmosphere alone.[55]

The reflected light of a planet contains absorption bands due to minerals in the rocks present for rocky bodies, or due to the elements and molecules present in the atmosphere. To date over 3,500 exoplanets have been discovered. These include so-called Hot Jupiters, as well as Earth-like planets. Using spectroscopy, compounds such as alkali metals, water vapor, carbon monoxide, carbon dioxide, and methane have all been discovered.[56]

Asteroids can be classified into three major types according to their spectra. The original categories were created by Clark R. Chapman, David Morrison, and Ben Zellner in 1975, and further expanded by David J. Tholen in 1984. In what is now known as the Tholen classification, the C-types are made of carbonaceous material, S-types consist mainly of silicates, and X-types are'metallic'. There are other classifications for unusual asteroids. C- and S-type asteroids are the most common asteroids. In 2002 the Tholen classification was further "evolved" into the SMASS classification, expanding the number of categories from 14 to 26 to account for more precise spectroscopic analysis of the asteroids.[57][58]

The spectra of comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet, as well as emission lines from gaseous atoms and molecules excited to fluorescence by sunlight and/or chemical reactions. For example, the chemical composition of Comet ISON[59] was determined by spectroscopy due to the prominent emission lines of cyanogen (CN), as well as two- and three-carbon atoms (C2 and C3).[60] Nearby comets can even be seen in X-ray as solar wind ions flying to the coma are neutralized. The cometary X-ray spectra therefore reflect the state of the solar wind rather than that of the comet.[61]


who is the mother of harry bosch's daughter

Bosch has a daughter named Maddie. Maddie spent most of her time with her mother, Bosch's ex-wife Eleanor, in Hong Kong, seeing him only twice a year. Eleanor, who was a star attraction at a Macau casino, had a personal security guard, Sun Yee, who was also her boyfriend. She was killed during an attempt to rescue Maddie from Chinese thugs. Maddie then started to live with her father in Los Angeles.

Harry Bosch

Detective Hieronymus 'Harry' Bosch (pronounced "He-ron-ee-mus" "Bosh") is a Fictional character created by American author Michael Connelly. Bosch debuted as the lead character in the 1992 novel The Black Echo, the first in a best-selling police procedural series now numbering twenty-one novels.

The novels are more or less coincident in time frame with the year in which they were published. Harry, as he is commonly known by his associates, is a veteran police homicide detective with the Los Angeles Police Department. Bosch was named after the 15th century Dutch artist, Hieronymus Bosch.

Since 2015, Titus Welliver has portrayed Bosch in a series adapted from the novels.

Bosch's mother was a prostitute in Hollywood and was murdered in 1961 (10/28/61) when Bosch was 11 years old. His father, whom he met later in life, was a powerful defense attorney. Bosch spent his youth in various orphanages, youth halls, and the occasional foster family. When he learned of his mother's murder, Bosch, then living at a youth hall, dived to the bottom of the pool, screamed until he ran out of air and then swam back to the surface. This event is referenced in several Bosch novels. He joined the United States Army when he was seventeen years old, after getting his foster father to sign the enlistment papers that gave his consent for his minor foster son.

In Vietnam, Bosch was a "tunnel rat" (nicknamed Hari Kari Bosch), with the 1st Infantry Divisionâ€”a specialized soldier whose job it was to go into the maze of tunnels used as barracks, hospitals, and on some occasions, morgues, by the Vietcong and North Vietnamese Army.[1] While in the enemy tunnels, the Tunnel Rats would kill enemy soldiers they encountered, gather documents for analysis by military intelligence, and then plant C-4 high explosive charges that they would set to detonate after they exited the tunnels. Once, while on R&R leave in Hawaii, Bosch went AWOL but returned to his unit and served two tours of duty in Vietnam.

After his return from Vietnam and an honorable discharge from the Army, Bosch joined the LAPD and rose to the rank of Detective III, a position which entails both investigative and supervisory duties and is the LAPD equivalent of Detective Sergeant.

While in the LAPD, Bosch worked in the prestigious Robbery Homicide Division (RHD) for five years, but was drummed out by an Internal Affairs Division (IAD) investigation involving Bosch's shooting of a suspect (The Dollmaker) who was later linked to nine murders. Following the IAD investigation, which was conducted by Detectives Pierce Lewis and Don Clarke, Bosch was sent to Hollywood Division and assigned to the homicide desk. Lewis and Clarke also investigated Bosch in The Black Echo. At one point, Bosch left the LAPD and worked as a private investigator for three years. He left retirement and returned to the LAPD at the conclusion of The Narrows. After his return, Bosch was assigned to RHD's Open-Unsolved Unit, a cold case squad. Bosch transferred out of Open-Unsolved and into Homicide Special during the time between Echo Park and The Overlook.

During his time in the LAPD as covered in the novels, Bosch was in Hollywood homicide and worked with Frankie Sheehan (Bosch's partner in RHD, who was later murdered in Angels Flight), Jerry Edgar (his longest serving partner) and Kizmin 'Kiz' Rider, the other members of Bosch's team in Hollywood homicide. The 2004 Limited Edition DVD that was available with The Narrows included an excerpt of a speech real-life LAPD Chief William Bratton made at the Police Academy, in which he publicly asked Harry Bosch to return from retirement (Connelly was on the stand behind listening to the speech; see Blue Neon Night: Michael Connelly's Los Angeles). In The Overlook, Connelly gave Bratton an approving endorsement (albeit without using his name) for "raising the morale of the rank and file" and for giving "Bosch his job back" (pp. 145–146 of the Vision paperback edition).

In The Closers, Bosch was once again partnered with Rider, while Edgar remained in Hollywood homicide. Bosch remained partnered with Rider until her transfer to the Chief's office after being shot during Echo Park. During The Overlook, Bosch partnered with Detective Ignacio 'Iggy' Ferras, a younger detective with whom Bosch had not yet developed a solid rapport. Not a stranger to being second-guessed, Bosch was investigated by the LAPD's IAD multiple times, and was always cleared.

Bosch lived in a house on stilts at 7203 Woodrow Wilson Drive[2] in the Hollywood Hills. The money that financed Bosch's home came from a production studio for a TV mini-series in which actor Dan Lacey portrays Bosch in a case the detective had worked that involved a serial killer; Bosch worked as a technical advisor on the mini-series. His house was later damaged during the Northridge earthquake shortly before the book The Last Coyote. After his house was condemned and demolished, he had a new one built on the same road, still facing out over the valley.

Bosch has an active love life, with usually one love interest per book. He has a daughter, who, as of 9 Dragons, is living with him. She had lived with her mother, Harry's ex-wife Eleanor Wish (a former FBI agent, ex-con and professional poker player, whom Bosch met in The Black Echo and married while on a case in Las Vegas, and who left Bosch in Angels Flight), who was killed in Hong Kong. Recent stories find Bosch linked in a close relationship with FBI agent Rachel Walling. The liaison formulated in The Narrows and heightened romantically during Echo Park, but Walling broke off the relationship at its conclusion. Walling returned in The Overlook on a strictly professional basis, and she has since resumed a relationship with reporter Jack McEvoy. Walling notes in The Scarecrow that her relationship with Bosch broke up in part because Bosch was still in love with Eleanor Wish.

Bosch is left-handed. He stands a few inches short of six feet and is described as wiry. His muscles are like nylon cords, strength concealed by economy of scale. He has a moustache and has brown hair that is by now graying (The Black Echo). Bosch's eyes are the other key aspect of his appearance; they are brown and nearly black, and were mentioned often for this reason in A Darkness More Than Night. Connelly gives a good clue as to how he visualizes Bosch when, in The Overlook, Rachel Walling tells Bosch that "You look like House" (actor Hugh Laurie).

Bosch is always finding himself in conflict with authority, whether with his lieutenant, or a deputy chief of police (specifically Irvin Irving, Bosch's recurring nemesis until forced to retire at the end of The Closers, and now a city councilman), or the FBI. His confrontational side is usually attributed to his strong sense of right and wrong, coupled with little regard for his career. At the end of The Overlook, Connelly states this trait can be described in a single word: "relentless". He also uses this word in Lost Light, describing jazz, and implying reference to his own work and personality.

Bosch has a half-brother, Mickey Haller, a Los Angeles attorney who makes his first appearance in the novel The Lincoln Lawyer, although he briefly appears in a flashback in The Black Ice as a boy. Haller is the legitimate son of the attorney who fathered Bosch. In the second Mickey Haller novel, The Brass Verdict, it is revealed that Harry Bosch has known for years of the relationship, but Haller was unaware of it until the end of the book.[3]

Bosch's namesake, the Dutch painter, was famous for his religious portrayal of earthly sins (mostly debauchery) and their violent consequences. In several of the books there are parallels suggested between the hell in the paintings and the events of the fictional Bosch's life.

Besides the Connelly series, Harry Bosch has made cameos in several other authors' books. Bosch has appeared in books by Paula Woods, Joe Gores, and Robert Crais.[4] Likewise, during an October 16, 2008 book-signing in San Mateo, California, to promote The Brass Verdict, Michael Connelly informed the audience that Bosch also appeared in a cameo without identification in Connelly's novel Chasing the Dime.

Bosch carried a Smith & Wesson Model 5906 9mm semiautomatic pistol as his duty weapon for many years. The S&W Model 5906 is authorized by the LAPD for duty carry by its officers. It was a popular weapon in the LAPD prior to the North Hollywood shootout. He kept a Heckler & Koch P7 9mm semiautomatic pistol at his house, in the front closet wrapped in an oil wrap, as a back-up weapon. When he retired from the police department, he carried a Glock 27.40 Caliber semiautomatic subcompact pistol. When he returned to active duty with the LAPD, he started to carry a Kimber Ultra Carry II.45 ACP Caliber semiautomatic pistol. Many officers in the LAPD started carrying.45 caliber pistols after the North Hollywood Shootout bank robbery incident. The LAPD authorized the heavier caliber pistol ammunition for its officers in response to officers facing heavily armed bandits in the North Hollywood shootout. In "The Burning Room", Bosch starts carrying a Glock 30,.45 ACP caliber, semiautomatic pistol. He still kept the Kimber.45 as his second pistol but uses the Glock as his duty weapon. He had an additional handgun, a Smith & Wesson.44-caliber revolver, in The Black Ice. It was given to him by the father of a murdered girl. It was kept at home, but he took it with him on his trip to Mexico to have a gun which he could check at the border crossing so as to show he had not carried it into Mexico. That done, he hid his carry gun, the 9mm S&W pistol, under the spare tire of his Chevy Caprice. The cop at the border station observed that the left-handed Harry was checking in a right-handed revolver.

In February 2015, Amazon Prime showed the series Bosch, based on the novels. The ten-episode series starred Titus Welliver as Harry Bosch, with Annie Wersching and Jamie Hector co-starring as Officer Julia Brasher and Det. Jerry Edgar respectively. Henrik Bastin of Fabrik Entertainment produced and Jim McKay directed. The series followed Bosch "as he pursues the killer of a 12-year-old boy while standing trial in federal court on accusations that he murdered a suspected serial killer in cold blood."[8]

According to Connelly, a number of changes were made "to the world of Harry Bosch... in making the shift from page to screen". In the series, Harry "is 47 years old and a veteran of the first Gulf War in 1991, where he was part of a Special Forces team that cleared tunnels. He has now been a police officer for twenty years with a one-year exception when he re-upped with the Army after 9/11, as many LAPD officers did. He came back to the force after serving in Afghanistan and again encountering tunnel warfare."[9]

A "13 day shoot" on the pilot episode of the series began in Los Angeles on 4 November 2013, while Connelly kept a daily "set journal".[10] The pilot was released on Amazon Prime in February 2014, and the full series was shown in February 2015.[11][12] On March 18, 2015, Bosch was renewed for a second season,[13] which takes inspiration from Connelly's novels Trunk Music, The Drop, and The Last Coyote.[14] The second season premiered on March 11, 2016. Amazon renewed Bosch for a third season soon after the second season was released, which will adapt novels The Black Echo and elements of A Darkness More Than Night. The third season premiered on April 21, 2017.[15] On October 16, 2016, Bosch was renewed for a fourth season.[16]


the bottom environments of the oceans are referred to as the

The pelagic zone can be contrasted with the benthic and demersal zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom of the sea. It includes the sediment surface and some subsurface layers. Marine organisms living in this zone, such as clams and crabs, are called benthos. The demersal zone is just above the benthic zone. It can be significantly affected by the seabed and the life that lives there. Fish that live in the demersal zone are called demersal fish, and can be divided into benthic fish, which are denser than water so they can rest on the bottom, and benthopelagic fish, which swim in the water column just above the bottom. Demersal fish are also known as bottom feeders and groundfish.

Pelagic zone

The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth. The word "pelagic" is derived from Greek πέλαγος (pélagos), meaning 'open sea'. The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes from the surface of the sea almost to the bottom. Conditions differ deeper in the water column such that as pressure increases with depth, the temperature drops and less light penetrates. Depending on the depth, the water column, rather like the Earth's atmosphere, may be divided into different layers.

The pelagic zone occupies 1,330 million km3 (320 million mi3) with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi).[1][2][3] Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. It is affected by light intensity, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography, which is called bathymetry. In deep water, the pelagic zone is sometimes called the open-ocean zone and can be contrasted with water that is near the coast or on the continental shelf. In other contexts, coastal water not near the bottom is still said to be in the pelagic zone.

Depending on how deep the sea is, the pelagic zone can extend over up to five horizontal regions in the ocean. From the top down, these are:

From the surface (MSL) down to around 200 m (660 ft)

This is the illuminated zone at the surface of the sea where enough light is available for photosynthesis. Nearly all primary production in the ocean occurs here. Consequently, plants and animals are largely concentrated in this zone. Examples of organisms living in this zone are plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins.

From 200 m (660 ft) down to around 1,000 m (3,300 ft)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria.[4] Examples of animals that live here are swordfish, squid, Anarhichadidae or "wolffish" and some species of cuttlefish. Many organisms that live in this zone are bioluminescent.[5] Some creatures living in the mesopelagic zone rise to the epipelagic zone at night to feed.[5]

From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)

The name stems from Greek βαθύς, meaning 'deep'. At this depth, the ocean is pitch black, apart from occasional bioluminescent organisms, such as anglerfish. No living plant exists here. Most animals living here survive by consuming the detritus falling from the zones above, which is known as "marine snow", or, like the marine hatchetfish, by preying on other inhabitants of this zone. Other examples of this zone's inhabitants are giant squid, smaller squids and the grimpoteuthis or "dumbo octopus". The giant squid is hunted here by deep-diving sperm whales.

From around 4,000 m (13,000 ft) down to above the ocean floor

The name is derived from Greek ἄβυσσος, meaning 'bottomless' (a holdover from the times when the deep ocean, or abyss, was believed to be bottomless). Very few creatures live in the cold temperatures, high pressures and complete darkness of this depth.[5] Among the species found in this zone are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider.[5] Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone.[5]

The deep water in ocean trenches

The name is derived from the realm of Hades, the Greek underworld. However, many organisms live in hydrothermal vents in this and other zones. Some define the hadopelagic as waters below 6,000 m (20,000 ft), whether in a trench or not.

The pelagic ecosystem is based on phytoplankton. Phytoplankton manufacture their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or neritic zone. Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears.[7]

Pelagic birds, also called oceanic birds, live on the open sea, rather than around waters adjacent to land or around inland waters. Pelagic birds feed on planktonic crustaceans, squid and forage fish. Examples are the Atlantic puffin, macaroni penguins, sooty terns, shearwaters, and Procellariiformes such as the albatross, Procellariidae and petrels.

The term seabird includes birds which live around the sea adjacent to land, as well as pelagic birds.

Pelagic fish live in the water column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake. They can be contrasted with demersal fish, which live on or near the bottom, and coral reef fish.[8]

These fish are often migratory forage fish, which feed on plankton, and the larger fish that follow and feed on the forage fish. Examples of migratory forage fish are herring, anchovies, capelin, and menhaden. Examples of larger pelagic fish which prey on the forage fish are billfish, tuna, and oceanic sharks.

Some examples of pelagic invertebrates include krill, copepods, jellyfish, decapod larvae, hyperiid amphipods, rotifers and cladocerans.

Thorson's rule states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to widely dispersing pelagic larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic (yolk-feeding) eggs and larger offspring.[9][10]

Pelamis platura, the pelagic sea snake, is the only one of the 65 species of marine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The pelagic sea snake is the worldâ€™s most widely distributed snake species.

Many species of sea turtles spend the first years of their lives in the pelagic zone, moving closer to shore as they reach maturity.