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John Herschel

Sir John Frederick William Herschel, 1st Baronet (7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor and experimental photographer who invented the blueprint   and did botanical work.

Herschel originated the use of the Julian day system in astronomy. He named seven moons of Saturn and four moons of Uranus – the seventh planet, discovered by his father Sir William Herschel. He made many contributions to the science of photography, and investigated colour blindness and the chemical power of ultraviolet rays. His Preliminary Discourse (1831), which advocated an inductive approach to scientific experiment and theory-building, was an important contribution to the philosophy of science.

Early life and work on astronomy

Herschel was born in Slough, Buckinghamshire, the son of Mary Baldwin and astronomer Sir William Herschel. He was the nephew of astronomer Caroline Herschel. He studied shortly at Eton College and St John's College, Cambridge, graduating as Senior Wrangler in 1813. It was during his time as an undergraduate that he became friends with the mathematicians Charles Babbage and George Peacock. He left Cambridge in 1816 and started working with his father. He took up astronomy in 1816, building a reflecting telescope with a mirror 18 in in diameter, and with a 20 ft focal length. Between 1821 and 1823 he re-examined, with James South, the double stars catalogued by his father. He was one of the founders of the Royal Astronomical Society in 1820. For his work with his father, he was presented with the Gold Medal of the Royal Astronomical Society in 1826 (which he won again in 1836), and with the Lalande Medal of the French Academy of Sciences in 1825, while in 1821 the Royal Society bestowed upon him the Copley Medal for his mathematical contributions to their Transactions. Herschel was made a Knight of the Royal Guelphic Order in 1831. He also seemed to be aware of Indian thought and mathematics introduced to him by George Everest as claimed by Mary Boole:

"Some time about 1825, he came to England for two or three years, and made a fast and lifelong friendship with Herschel and with Babbage, who was then quite young.(.) My uncle returned from India. He never interfered with anyone's religious beliefs or customs. But no one under his influence could continue to believe in anything in the Bible being specially sacred, except the two elements which it has in common with other sacred books: the knowledge of our relation to others and of man's power to hold direct converse with the unseen truth."

He stated in his historical article Mathematics in Brewster's Cyclopedia:

"The Brahma Sidd'hanta, the work of Brahmagupta, an Indian astronomer at the beginning of the seventh century, contains a general method for the resolution of indeterminate problems of the second degree; an investigation which actually baffled the skill of every modern analyst till the time of Lagrange's solution, not excepting the all inventive Euler himself."

Herschel served as president of the Royal Astronomical Society three times: 1827–1829, 1839–1841 and 1847–1849.

Herschel's A preliminary discourse on the study of natural philosophy, published early in 1831 as part of Dionysius Lardner's Cabinet cyclopædia, set out methods of scientific investigation with an orderly relationship between observation and theorising. He described nature as being governed by laws which were difficult to discern or to state mathematically, and the highest aim of natural philosophy was understanding these laws through inductive reasoning, finding a single unifying explanation for a phenomenon. This became an authoritative statement with wide influence on science, particularly at the University of Cambridge where it inspired the student Charles Darwin with "a burning zeal" to contribute to this work.

He was elected as a member to the American Philosophical Society in 1854.

Herschel published a catalogue of his astronomical observations in 1864, as the General Catalogue of Nebulae and Clusters, a compilation of his own work and that of his father's, expanding on the senior Herschel's Catalogue of Nebulae. A further complementary volume was published posthumously, as the General Catalogue of 10,300 Multiple and Double Stars.

Herschel correctly considered astigmatism to be due to irregularity of the cornea and theorised that vision could be improved by the application of some animal jelly contained in a capsule of glass against the cornea. His views were published in an article entitled Light in 1828 and the Encyclopædia Metropolitana in 1845.

Discoveries of Herschel include the galaxies NGC 7, NGC 10, NGC 25, and NGC 28.

Visit to South Africa
He declined an offer from the Duke of Sussex that they travel to South Africa on a Navy ship. Herschel had his own inherited money and he paid £500 for passage on the S.S. Mountstuart Elphinstone. He, his wife, their three children and his 20 inch telescope departed from Portsmouth on 13 November 1833.

The voyage to South Africa was made to catalogue the stars, nebulae, and other objects of the southern skies. This was to be a completion as well as extension of the survey of the northern heavens undertaken initially by his father William Herschel. He arrived in Cape Town on 15 January 1834 and set up a private 21 ft telescope at Feldhausen at Claremont, a suburb of Cape Town. Amongst his other observations during this time was that of the return of Comet Halley. Herschel collaborated with Thomas Maclear, the Astronomer Royal at the Cape of Good Hope and the members of the two families became close friends. During this time, he also witnessed the Great Eruption of Eta Carinae (December 1837).

In addition to his astronomical work, however, this voyage to a far corner of the British empire also gave Herschel an escape from the pressures under which he found himself in London, where he was one of the most sought-after of all British men of science. While in southern Africa, he engaged in a broad variety of scientific pursuits free from a sense of strong obligations to a larger scientific community. It was, he later recalled, probably the happiest time in his life. A village in the contemporary province of Eastern Cape is named after him.

Herschel combined his talents with those of his wife, Margaret, and between 1834 and 1838 they produced 131 botanical illustrations of fine quality, showing the Cape flora. Herschel used a camera lucida to obtain accurate outlines of the specimens and left the details to his wife. Even though their portfolio had been intended as a personal record, and despite the lack of floral dissections in the paintings, their accurate rendition makes them more valuable than many contemporary collections. Some 112 of the 132 known flower studies were collected and published as Flora Herscheliana in 1996. The book also included work by Charles Davidson Bell and Thomas Bowler.

As their home during their stay in the Cape, the Herschels had selected 'Feldhausen' ("Field Houses"), an old estate on the south-eastern side of Table Mountain. Here John set up his reflector to begin his survey of the southern skies.

Herschel, at the same time, read widely. Intrigued by the ideas of gradual formation of landscapes set out in Charles Lyell's Principles of Geology, he wrote to Lyell on 20 February 1836 praising the book as a work that would bring "a complete revolution in [its] subject, by altering entirely the point of view in which it must thenceforward be contemplated" and opening a way for bold speculation on "that mystery of mysteries, the replacement of extinct species by others." Herschel himself thought catastrophic extinction and renewal "an inadequate conception of the Creator" and by analogy with other intermediate causes, "the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process". He prefaced his words with the couplet:

Taking a gradualist view of development and referring to evolutionary descent from a proto-language, Herschel commented:

"Words are to the Anthropologist what rolled pebbles are to the Geologist – battered relics of past ages often containing within them indelible records capable of intelligent interpretation – and when we see what amount of change 2000 years has been able to produce in the languages of Greece & Italy or 1000 in those of Germany France & Spain we naturally begin to ask how long a period must have lapsed since the Chinese, the Hebrew, the Delaware & the Malesass [Malagasy] had a point in common with the German & Italian & each other – Time! Time! Time! – we must not impugn the Scripture Chronology, but we must interpret it in accordance with whatever shall appear on fair enquiry to be the truth for there cannot be two truths. And really there is scope enough: for the lives of the Patriarchs may as reasonably be extended to 5000 or 50000 years apiece as the days of Creation to as many thousand millions of years."

The document was circulated, and Charles Babbage incorporated extracts in his ninth and unofficial Bridgewater Treatise, which postulated laws set up by a divine programmer. When HMS Beagle called at Cape Town, Captain Robert FitzRoy and the young naturalist Charles Darwin visited Herschel on 3 June 1836. Later on, Darwin would be influenced by Herschel's writings in developing his theory advanced in The Origin of Species. In the opening lines of that work, Darwin writes that his intent is "to throw some light on the origin of species – that mystery of mysteries, as it has been called by one of our greatest philosophers," referring to Herschel. However, Herschel ultimately rejected the theory of natural selection.

Herschel returned to England in 1838, was created a baronet, of Slough in the County of Buckingham, and published Results of Astronomical Observations made at the Cape of Good Hope in 1847. In this publication he proposed the names still used today for the seven then-known satellites of Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus. In the same year, Herschel received his second Copley Medal from the Royal Society for this work. A few years later, in 1852, he proposed the names still used today for the four then-known satellites of Uranus: Ariel, Umbriel, Titania, and Oberon. A stone obelisk, erected in 1842 and now in the grounds of The Grove Primary School, marks the site where his 20-ft reflector once stood.

Photography

Herschel made numerous important contributions to photography. He made improvements in photographic processes, particularly in inventing the cyanotype process, which became known as blueprints,  and variations, such as the chrysotype. In 1839, he made a photograph on glass, which still exists, and experimented with some colour reproduction, noting that rays of different parts of the spectrum tended to impart their own colour to a photographic paper. Herschel made experiments using photosensitive emulsions of vegetable juices, called phytotypes, also known as anthotypes, and published his discoveries in the Philosophical Transactions of the Royal Society of London in 1842. He collaborated in the early 1840s with Henry Collen, portrait painter to Queen Victoria. Herschel originally discovered the platinum process on the basis of the light sensitivity of platinum salts, later developed by William Willis.

Herschel coined the term photography in 1839. Herschel was also the first to apply the terms negative and positive to photography.

Herschel discovered sodium thiosulfate to be a solvent of silver halides in 1819, and informed Talbot and Daguerre of his discovery that this "hyposulphite of soda" ("hypo") could be used as a photographic fixer, to "fix" pictures and make them permanent, after experimentally applying it thus in early 1839.

Herschel's ground-breaking research on the subject was read at the Royal Society in London in March 1839 and January 1840.

Other aspects of Herschel's career
Herschel wrote many papers and articles, including entries on meteorology, physical geography and the telescope for the eighth edition of the Encyclopædia Britannica. He also translated the Iliad of Homer.

In 1823, Herschel published his findings on the optical spectra of metal salts.

Herschel invented the actinometer in 1825 to measure the direct heating power of the Sun's rays, and his work with the instrument is of great importance in the early history of photochemistry.

Herschel proposed a correction to the Gregorian calendar, making years that are multiples of 4000 common years rather than leap years, thus reducing the average length of the calendar year from 365.2425 days to 365.24225. Although this is closer to the mean tropical year of 365.24219 days, his proposal has never been adopted because the Gregorian calendar is based on the mean time between vernal equinoxes (currently 365.242 374 days).

Herschel was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1832, and in 1836, a foreign member of the Royal Swedish Academy of Sciences.

In 1835, the New York Sun newspaper wrote a series of satiric articles that came to be known as the Great Moon Hoax, with statements falsely attributed to Herschel about his supposed discoveries of animals living on the Moon, including batlike winged humanoids.

Several locations are named for him: the village of Herschel in western Saskatchewan, Canada, site of the discovery of Dolichorhynchops herschelensis, a type of plesiosaur; Mount Herschel in Antarctica; the crater J. Herschel on the Moon; and the settlement of Herschel, Eastern Cape and the Herschel Girls' School in Cape Town, South Africa.

While it is commonly accepted that Herschel Island, in the Arctic Ocean, part of the Yukon Territory, was named after him, the entries in the expedition journal of Sir John Franklin state that the latter wished to honour the Herschel family, of which John Herschel's father, Sir William Herschel, and his aunt, Caroline Herschel, are as notable as John.

Family
Herschel married Margaret Brodie Stewart (1810–1884) on 3 March 1829 at St. Marlyebone Church in London, and was father of the following children:

 * 1) Caroline Emilia Mary Herschel (31 March 1830 – 29 January 1909), who married the soldier and politician Alexander Hamilton-Gordon
 * 2) Isabella Herschel (5 June 1831 – 1893)
 * 3) Sir William James Herschel, 2nd Bt. (9 January 1833 – 1917),
 * 4) Margaret Louisa Herschel (1834–1861), an accomplished artist
 * 5) Alexander Stewart Herschel (1836–1907), FRS, FRAS
 * 6) Col. John Herschel FRS, FRAS, (1837–1921) surveyor
 * 7) Maria Sophia Herschel (1839–1929)
 * 8) Amelia Herschel (1841–1926) married Sir Thomas Francis Wade, diplomat and sinologist
 * 9) Julia Herschel (1842–1933) married on 4 June 1878 to Captain (later Admiral) John Fiot Lee Pearse Maclear
 * 10) Matilda Rose Herschel (1844–1914), a gifted artist, married William Waterfield (Indian Civil Service)
 * 11) Francisca Herschel (1846–1932)
 * 12) Constance Anne Herschel (1855–20 June 1939), mathematician and scientist who became lecturer in natural sciences at Girton College, Cambridge

Death

Herschel died on 11 May 1871 at age 79 at Collingwood, his home near Hawkhurst in Kent. On his death, he was given a national funeral and buried in Westminster Abbey.

His obituary by Henry W Field of London was read to the American Philosophical Society on 1 December 1871.

title: John Herschel


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De facto

De facto (, ; lit. 'in fact') describes practices that exist in reality, regardless of whether they are officially recognized by laws or other formal norms. It is commonly used to refer to what happens in practice, in contrast with de jure ("by law").

Jurisprudence and de facto law
In jurisprudence, a de facto law (also known as a de facto regulation) is a law or regulation that is followed but "is not specifically enumerated by a law." By definition, de facto 'contrasts' de jure which means "as defined by law" or "as a matter of law." For example, if a particular law exists in one jurisdiction, but is followed in another where it has no legal effect (such as in another country), then the law could be considered a de facto regulation (a "de facto regulation" is not an officially prescribed legal classification for a type of law in a particular jurisdiction, rather, it is a concept about law(s)).

A de facto regulation may be followed by an organization as a result of the market size of the jurisdiction imposing the regulation as a proportion of the overall market; wherein the market share is so large that it results in the organization choosing to comply by implementing one standard of business with respect to the given de facto law instead of altering standards between different jurisdictions and markets (e.g. data protection, manufacturing, etc.). The decision to voluntarily comply may be the result of: a desire to simplify manufacturing processes & cost-effectiveness (such as adopting a one size fits all approach), consumer demand & expectation, or other factors known only to the complier.

In prison sentences, the term de facto life sentence (also known as a "virtual" life sentence) is used to describe a "non-life sentence" that is long enough to end after the convicted person would have likely died due to old age, or one long enough to cause the convicted person to "live out the vast majority of their life in jail prior to their release."

Technical standards
A de facto standard is a standard (formal or informal) that has achieved a dominant position by tradition, enforcement, or market dominance. It has not necessarily received formal approval by way of a standardization process, and may not have an official standards document.

Technical standards are usually voluntary, such as ISO 9000 requirements, but may be obligatory, enforced by government norms, such as drinking water quality requirements. The term "de facto standard" is used for both: to contrast obligatory standards (also known as "de jure standards"); or to express a dominant standard, when there is more than one proposed standard.

In social sciences, a voluntary standard that is also a de facto standard, is a typical solution to a coordination problem.

National languages
Several countries, including Australia, Japan, Mexico, the United Kingdom and the United States, have a de facto national language but no official, de jure national language.

Some countries have a de facto national language in addition to an official language. In Lebanon and Morocco, Arabic is an official language (in addition to Tamazight in the case of Morocco), but an additional de facto language is also French. In New Zealand, the official languages are Māori and New Zealand Sign Language; however, English is a third de facto language.

Russian was the de facto official language of the central government and, to a large extent, republican governments of the former Soviet Union, but was not declared de jure state language until 1990. A short-lived law, effected April 24, 1990, installed Russian as the sole de jure official language of the Union prior to its dissolution in 1991.

In Hong Kong and Macau, the special administrative regions of China, the official languages are English and Portuguese respectively, together with Chinese. However, no particular variety of "Chinese" referred to in law is specified. Cantonese (Hong Kong Cantonese) in traditional Chinese characters is the de facto standard in both territories.

Governance and sovereignty

A de facto government is a government wherein all the attributes of sovereignty have, by usurpation, been transferred from those who had been legally invested with them to others, who, sustained by a power above the forms of law, claim to act and do really act in their stead.

In politics, a de facto leader of a country or region is one who has assumed authority, regardless of whether by lawful, constitutional, or legitimate means; very frequently, the term is reserved for those whose power is thought by some faction to be held by unlawful, unconstitutional, or otherwise illegitimate means, often because it had deposed a previous leader or undermined the rule of a current one. De facto leaders sometimes do not hold a constitutional office and may exercise power informally.

Not all dictators are de facto rulers. For example, Augusto Pinochet of Chile initially came to power as the chairperson of a military junta, which briefly made him de facto leader of Chile, but he later amended the nation's constitution and made himself president until new elections were called, making him the formal and legal ruler of Chile. Similarly, Saddam Hussein's formal rule of Iraq is often recorded as beginning in 1979, the year he assumed the Presidency of Iraq. However, his de facto rule of the nation began earlier: during his time as vice president; he exercised a great deal of power at the expense of the elderly Ahmed Hassan al-Bakr, the de jure president.

In Argentina, the successive military coups that overthrew constitutional governments installed de facto governments in 1930–1932, 1943–1946, 1955–1958, 1966–1973 and 1976–1983, the last of which combined the powers of the presidential office with those of the National Congress. The subsequent legal analysis of the validity of such actions led to the formulation of a doctrine of the de facto governments, a case law (precedential) formulation which essentially said that the actions and decrees of past de facto governments, although not rooted in legal legitimacy when taken, remained binding until and unless such time as they were revoked or repealed de jure by a subsequent legitimate government.

That doctrine was nullified by the constitutional reform of 1994. Article 36 states:

"*(1) This Constitution shall rule even when its observance is interrupted by acts of force against the institutional order and the democratic system. These acts shall be irreparably null.
 * (2) Their authors shall be punished with the penalty foreseen in Section 29, disqualified in perpetuity from holding public offices and excluded from the benefits of pardon and commutation of sentences.
 * (3) Those who, as a consequence of these acts, were to assume the powers foreseen for the authorities of this Constitution or for those of the provinces, shall be punished with the same penalties and shall be civil and criminally liable for their acts. The respective actions shall not be subject to prescription.
 * (4) All citizens shall have the right to oppose resistance to those committing the acts of force stated in this section.
 * (5) He who, procuring personal enrichment, incurs in serious fraudulent offense against the Nation shall also attempt subversion against the democratic system, and shall be disqualified to hold public office for the term specified by law.
 * (6) Congress shall enact a law on public ethics which shall rule the exercise of public office."

Two examples of de facto leaders are Deng Xiaoping of the People's Republic of China and general Manuel Noriega of Panama. Both of these men exercised nearly all control over their respective nations for many years despite not having either legal constitutional office or the legal authority to exercise power. These individuals are today commonly recorded as the "leaders" of their respective nations; recording their legal, correct title would not give an accurate assessment of their power.

Another example of a de facto ruler is someone who is not the actual ruler but exerts great or total influence over the true ruler, which is quite common in monarchies. Some examples of these de facto rulers are Empress Dowager Cixi of China (for son Tongzhi Emperor and nephew Guangxu Emperor), Prince Alexander Menshikov (for his former lover Empress Catherine I of Russia), Cardinal Richelieu of France (for Louis XIII), Queen Elisabeth of Parma (for her husband, King Philip V) and Queen Maria Carolina of Naples and Sicily (for her husband King Ferdinand I of the Two Sicilies).

Borders
The de facto boundaries of a country are defined by the area that its government is actually able to enforce its laws in, and to defend against encroachments by other countries that may also claim the same territory de jure. The Durand Line is an example of a de facto boundary. As well as cases of border disputes, de facto boundaries may also arise in relatively unpopulated areas in which the border was never formally established or in which the agreed border was never surveyed and its exact position is unclear. The same concepts may also apply to a boundary between provinces or other subdivisions of a federal state.

Segregation
In South Africa, although de jure apartheid formally began in 1948, de facto racist policies and practices discriminating against black South Africans, People of Colour, and Indians dated back decades before.

De facto racial discrimination and segregation in the United States (outside of the South) until the 1950s and 1960s was simply discrimination that was segregation by law (de jure). "Jim Crow laws", which were enacted in the 1870s, brought legal racial segregation against black Americans residing in the American South. These laws were legally ended in 1964 by the Civil Rights Act of 1964.

De facto state of war
Most commonly used to describe large scale conflicts of the 20th century, the phrase de facto state of war refers to a situation where two nations are actively engaging, or are engaged, in aggressive military actions against the other without a formal declaration of war.

Relationships
A domestic partner outside marriage is referred to as a de facto husband or wife by some authorities.

In Australia and New Zealand
In Australian law, a de facto relationship is a legally recognized, committed relationship of a couple living together (opposite-sex or same-sex). De facto unions are defined in the federal Family Law Act 1975. De facto relationships provide couples who are living together on a genuine domestic basis with many of the same rights and benefits as married couples. Two people can become a de facto couple by entering into a registered relationship (i.e.: civil union or domestic partnership) or by being assessed as such by the Family Court or Federal Circuit Court. Couples who are living together are generally recognised as a de facto union and thus able to claim many of the rights and benefits of a married couple, even if they have not registered or officially documented their relationship, although this may vary by state. It has been noted that it is harder to prove de facto relationship status, particularly in the case of the death of one of the partners.

In April 2014, an Australian federal court judge ruled that a heterosexual couple who had a child and lived together for 13 years were not in a de facto relationship and thus the court had no jurisdiction to divide up their property under family law following a request for separation. In his ruling, the judge stated "de facto relationship(s) may be described as 'marriage like' but it is not a marriage and has significant differences socially, financially and emotionally."

The above sense of de facto is related to the relationship between common law traditions and formal (statutory, regulatory, civil) law, and common-law marriages. Common law norms for settling disputes in practical situations, often worked out over many generations to establishing precedent, are a core element informing decision making in legal systems around the world. Because its early forms originated in England in the Middle Ages, this is particularly true in Anglo-American legal traditions and in former colonies of the British Empire, while also playing a role in some countries that have mixed systems with significant admixtures of civil law.

Relationships not recognised outside Australia
Due to Australian federalism, de facto partnerships can only be legally recognised whilst the couple lives within a state in Australia. This is because the power to legislate on de facto matters relies on referrals by States to the Commonwealth in accordance with Section 51(xxxvii) of the Australian Constitution, where it states the new federal law can only be applied back within a state. There must be a nexus between the de facto relationship itself and the Australian state.

If an Australian de facto couple moves out of a state, they do not take the state with them and the new federal law is tied to the territorial limits of a state. The legal status and rights and obligations of the de facto or unmarried couple would then be recognised by the laws of the country where they are ordinarily resident.

This is unlike marriage and "matrimonial causes" which are recognised by sections 51(xxi) and (xxii) of the Constitution of Australia and internationally by marriage law and conventions, Hague Convention on Marriages (1978).

Non-marital relationship contract
A de facto relationship is comparable to non-marital relationship contracts (sometimes called "palimony agreements") and certain limited forms of domestic partnership, which are found in many jurisdictions throughout the world.

A de facto Relationship is not comparable to common-law marriage, which is a fully legal marriage that has merely been contracted in an irregular way (including by habit and repute). Only nine U.S. states and the District of Columbia still permit common-law marriage; but common law marriages are otherwise valid and recognised by and in all jurisdictions whose rules of comity mandate the recognition of any marriage that was legally formed in the jurisdiction where it was contracted.

Family law – custody
De facto joint custody is comparable to the joint legal decision-making authority a married couple has over their child(ren) in many jurisdictions (Canada as an example). Upon separation, each parent maintains de facto joint custody, until such time a court order awards custody, either sole or joint.

Monopoly
A de facto monopoly is a system where many suppliers of a product are allowed but the market is so completely dominated by one that the other players are unable to compete or even survive. The related terms oligopoly and monopsony are similar in meaning and this is the type of situation that antitrust laws are intended to eliminate.

Finance
In finance, the World Bank has a pertinent definition:

"A 'de facto government' comes into, or remains in, power by means not provided for in the country's constitution, such as a coup d'état, revolution, usurpation, abrogation or suspension of the constitution."

Intellectual property
In engineering, is a system in which the intellectual property and know-how is privately held. Usually only the owner of the technology manufactures the related equipment. Meanwhile, a consists of systems that have been publicly released to a certain degree so that anybody can manufacture equipment supporting the technology. For instance, in cell phone communications, CDMA1X is a de facto technology, while GSM is a standard technology.

Sports
Examples of a de facto General Manager in sports include Syd Thrift who acted as the GM of the Baltimore Orioles between 1999 and 2002. Bill Belichick, the former head coach of the New England Patriots in the NFL did not hold the official title of GM, but served as de facto general manager as he had control over drafting and other personnel decisions.

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title: De facto


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United States Naval Observatory

The United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depot of Charts and Instruments, it is one of the oldest scientific agencies in the United States, and remains the country's leading facility for astronomical and timing data.

The observatory is located in Northwest Washington, D.C. at the northwestern end of Embassy Row. It is among the few pre-20th century astronomical observatories located in an urban area. In 1893, in an effort to escape light pollution, it was relocated from Foggy Bottom near the city's center, to its Northwest Washington, D.C. location.

The USNO has conducted significant scientific studies throughout its history, including measuring the speed of light, observing solar eclipses, and discovering the moons of Mars. Its achievements including providing data for the first radio time signals, constructing some of the earliest and most accurate telescopes of their kind, and helping develop universal time. The Naval Observatory performs radio VLBI-based positions of quasars for astrometry and geodesy with numerous global collaborators (IERS), in order to produce Earth orientation parameters and to realize the celestial reference system (ICRF).

Aside from its scientific mission, since the 1970s the Naval Observatory campus hosts the official residence of the vice president of the United States.

History
President John Quincy Adams, who in 1825 signed the bill for the creation of a national observatory just before leaving presidential office, had intended for it to be called the National Observatory.

The names "National Observatory" and "Naval Observatory" were both used for 10 years, until the Secretary of the Navy officially adopted the latter.

Adams had made protracted efforts to bring astronomy to a national level. He spent many nights at the observatory, watching and charting the stars, which had always been one of his interests.

Established by order of the United States Secretary of the Navy John Branch on 6 December 1830 as the Depot of Charts and Instruments, the Observatory rose from humble beginnings: Placed under the command of Lieutenant Louis M. Goldsborough, with an annual budget of $330; its primary function was the restoration, repair, and rating of navigational instruments.

Old Naval Observatory
It was established as a national observatory in 1842 by federal law and a Congressional appropriation of $25,000. Lt. J.M. Gilliss was put in charge of "obtaining the instruments needed and books." Lt. Gilliss visited the principal observatories of Europe with the mission to purchase telescopes and other scientific devices, and books.

The observatory's primary mission was to care for the United States Navy's marine chronometers, charts, and other navigational equipment. It calibrated ships' chronometers by timing the transit of stars across the meridian. It opened in 1844 in Foggy Bottom, north of the site of the Lincoln Memorial and west of the White House.

In 1893, the observatory moved to its current location in Northwest Washington, D.C. located on a 2000 foot circle of land atop "Observatory Hill", overlooking Massachusetts Avenue.

In 2017, the facilities were listed on the National Register of Historic Places.

The time ball
The first superintendent was Navy Commander M.F. Maury. Maury had the world's first vulcanized time ball, created to his specifications by Charles Goodyear for the U.S. Observatory. Placed into service in 1845, it was the first time ball in the United States and the 12th in the world. Maury kept accurate time by the stars and planets.

The time ball was dropped every day except Sunday, precisely at the astronomically defined moment of mean solar noon; this enabled all ships and civilians within sight to know the exact time. By the end of the American Civil War, the Observatory's clocks were linked via telegraph to ring the alarm bells in all of the Washington, D.C. firehouses three times a day.

The USNO held a one-off time-ball re-enactment for the year-2000 celebration.

Nautical Almanac Office
In 1849, the Nautical Almanac Office (NAO) was established in Cambridge, Massachusetts as a separate organization. In 1866, it was moved to Washington, D.C., operating near Fort Myer. It relocated to the U.S. Naval Observatory grounds in 1893.

On 20 September 1894, the NAO became a "branch" of USNO; however, it remained autonomous for several years.

The site houses the largest astronomy library in the United States (and the largest astrophysical periodicals collection in the world). The library includes a large collection of rare physics and astronomy books from the past millennium.

Measuring the astronomical unit
An early scientific duty assigned to the Observatory was the U.S. contribution to the definition of the Astronomical Unit, or the, which defines a standard mean distance between the Sun and the Earth. This was conducted under the auspices of the congressionally-funded U.S. Transit of Venus Commission. The astronomical measurements taken of the transit of Venus by a number of countries since 1639 resulted in a progressively more accurate definition of the.

Relying strongly on photographic methods, the naval observers returned 350 photographic plates in 1874, and 1,380 measurable plates in 1882. The results of the surveys conducted simultaneously from several locations around the world (for each of the two transits) produced a final value of the solar parallax, after adjustments, of 8.809″, with a probable error of 0.0059″, yielding a U.S.-determined Earth-Sun distance of 92,797,000 mi, with a probable error of 59,700 mi. The calculated distance was a significant improvement over several previous estimates.

The 26 inch and 40 inch refractors
The telescope used for the discovery of the Moons of Mars was the 26 inch (66 cm) refractor telescope, then located at Foggy Bottom, Washington, DC. In 1893 it was moved to its Northwest DC location.

In 1934, the largest optical telescope installed at USNO saw "first light". This 40 inch aperture instrument was also the second (and final) telescope made by famed optician, George Willis Ritchey. The Ritchey–Chrétien telescope design has since become the de facto optical design for nearly all major telescopes, including the famed Keck telescopes and the space-borne Hubble Space Telescope.

Because of light pollution in the Washington metropolitan area, USNO relocated the 40 inch telescope to Flagstaff, Arizona. A new Navy command, now called the USNO Flagstaff Station (NOFS), was established there. Those operations began in 1955. Within a decade, the Navy's largest telescope, the 61 inch "Kaj Strand Astrometric Reflector" was built; it saw light at Flagstaff in 1964.

USNO continues to maintain its dark-sky observatory, NOFS, near Flagstaff. This facility now oversees the Navy Precision Optical Interferometer.

History of the time service
By the early 1870s the USNO daily noon-time signal was distributed electrically, nationwide, via the Western Union Telegraph Company. Time was also "sold" to the railroads and was used in conjunction with railroad chronometers to schedule American rail transport. Early in the 20th century, the service was broadcast by radio, with Arlington time signal available to those with wireless receivers.

In November 1913 the Paris Observatory, using the Eiffel Tower as an antenna, exchanged sustained wireless (radio) signals with the U.S. Naval Observatory to determine the exact difference of longitude between the two institutions, via an antenna in Arlington, Virginia.

The U.S. Naval Observatory in Washington continues to be a major authority in the areas of Precise Time and Time Interval, Earth orientation, astrometry, and celestial observation. In collaboration with many national and international scientific establishments, it determines the timing and astronomical data required for accurate navigation, astrometry, and fundamental astronomy, and calculation methods — and distributes this information (such as star catalogs) on-line and in the annual publications The Astronomical Almanac and The Nautical Almanac.

Former USNO director Gernot M. R. Winkler initiated the "Master clock" service that the USNO still operates, and which provides precise time to the GPS satellite constellation run by the United States Space Force. The alternate Master Clock time service continues to operate at Schriever Space Force Base in Colorado.

Departments
In 1990 two departments were established: Orbital Mechanics and Astronomical Applications, with the Nautical Almanac Office a division in Astronomical Applications. The Orbital Mechanics Department operated under P. Kenneth Seidelmann until 1994, when the department was abolished and its functions transferred to a group within the Astronomical Applications Department.

In 2010, USNO's astronomical 'department' known as the Naval Observatory Flagstaff Station (NOFS) was officially made autonomous as an Echelon 5 command, separate from, but still reporting to the USNO in Washington. In the alpine woodlands above 7,000 feet altitude outside Flagstaff, Arizona, NOFS performs its national, Celestial Reference Frame (CRF) mission under dark skies in that region.

Official residence of the vice president of the United States

A house situated on the grounds of the observatory, at Number One Observatory Circle, has been the official residence of the vice president of the United States since 1974. It is protected by tight security control enforced by the Secret Service. The house is separated from the Naval Observatory.

It formerly served as the residence of the observatory's superintendent, and later was the residence of the chief of naval operations, and finally the vice president.

Time service
The U.S. Naval Observatory operates two “Master Clock” facilities, one in Washington, DC, and the other at Schriever SFB near Colorado Springs, CO.
 * The primary facility, in Washington, D.C. maintains 57 HP/Agilent/Symmetricom 5071A-001 high performance cesium atomic clocks and 24 hydrogen masers.
 * The alternate facility, at Schriever Space Force Base, maintains 12 cesium clocks and 3 masers.

The observatory also operates four rubidium atomic fountain clocks, which have a stability reaching 7. The observatory plans to build several more of this type for use at its two facilities.

The clocks used for the USNO timescale are kept in 19 environmental chambers, whose temperatures are kept constant to within 0.1°C. The relative humidities are kept constant in all maser, and most cesiums enclosures, to within 1%. Time-scale management only uses the clocks in Washington, DC, and of those, preferentially uses the clocks that currently conform reliably to the time reports of the majority. It is the combined ‘vote’ of the ensemble that constitutes the otherwise-fictitious “Master Clock”. The time-scale computations on 7 June 2007 weighted 70 of the clocks into the standard.

The U.S. Naval Observatory provides public time service via 26 NTP servers on the public Internet, and via telephone voice announcements: The voice of actor Fred Covington (1928–1993) has been announcing the USNO time since 1978.
 * +1 202 762-1401 (Washington, DC)
 * +1 202 762-1069 (Washington, DC)
 * +1 719 567-6742 (Colorado Springs, CO)

The voice announcements always begin with the local time (daylight or standard), and include a background of 1 second ticks. Local time announcements are made on the minute, and 15, 30, and 45 seconds after the minute. Coordinated Universal Time (UTC) is announced 5 seconds after the local time. Upon connecting, only the second-marking ticks are heard for the few seconds before the next scheduled local time announcement

The USNO also operates a modem time service, and provides time to the Global Positioning System.

Instrument shop
The United States Naval Observatory Instrument shop has been designing and manufacturing precise instrumentation since the early 1900s.

Publications

 * Astronomical Observations made at the U.S. Naval Observatory (USNOA) (v. 1–6: 1846–1867)
 * Astronomical and Meteorological Observations made at the U.S. Naval Observatory (USNOM) (v. 1–22: 1862–1880)
 * Observations made at the U.S. Naval Observatory (USNOO) (v. 1–7: 1887–1893)
 * Publications of the U.S. Naval Observatory, Second Series (PUSNO) (v. 1–16: 1900–1949)
 * U.S. Naval Observatory Circulars
 * The Astronomical Almanac
 * The Nautical Almanac
 * The Air Almanac
 * Astronomical Phenomena

title: United States Naval Observatory


text:
Exciton

An electron and an electron hole that are attracted to each other by the Coulomb force can form a bound state called an exciton. It is an electrically neutral quasiparticle that exists mainly in condensed matter, including insulators, semiconductors, some metals, but also in certain atoms, molecules and liquids. The exciton is regarded as an elementary excitation that can transport energy without transporting net electric charge.

An exciton can form when an electron from the valence band of a crystal is promoted in energy to the conduction band e.g., when a material absorbs a photon. Promoting the electron to the conduction band leaves a positively charged hole in the valence band. Here 'hole' represents the unoccupied quantum mechanical electron state with a positive charge, an analogue in crystal of a positron. Because of the attractive coulomb force between the electron and the hole, a bound state is formed, akin to that of the electron and proton in a hydrogen atom or the electron and positron in positronium. Excitons are composite bosons since they are formed from two fermions which are electron and hole.

The concept of excitons was first proposed by Yakov Frenkel in 1931, when he described the excitation of an atomic lattice considering what is now called the tight-binding description of the band structure. In his model the electron and the hole bound by the coulomb interaction are located either on the same or on the nearest neighbouring sites of the lattice, but the exciton as a composite quasi-particle is able to travel through the lattice without any net transfer of charge, which lead to many propositions for optoelectronic devices.

Excitons are often treated in the two limiting cases:

(i) The small radius excitons, or Frenkel excitons, where the electron-hole relative distance is restricted to one or only a few nearest neighbour unit cells. Frenkel excitons typically occur in insulators and organic semiconductors with relatively narrow allowed energy bands and accordingly, rather heavy Effective mass.

(ii) the large radius excitons are called Wannier-Mott excitons, for which the relative motion of electron and hole in the crystal covers many unit cells. Wannier-Mott excitons are considered as hydrogen-like quasiparticles. The wavefunction of the bound state then is said to be hydrogenic, resulting in a series of energy states in analogy to a hydrogen atom. Compared to a hydrogen atom, the exciton binding energy in a crystal is much smaller and the exciton's size (radius) is much larger. This is mainly because of two effects: (a) Coulomb forces are screened in a crystal, which is expressed as a relative permittivity εr significantly larger than 1 and (b) the Effective mass of the electron and hole in a crystal are typically smaller compared to that of free electrons. Wannier-Mott excitons with binding energies ranging from a few to hundreds of meV, depending on the crystal, occur in many semiconductors including Cu2 O, GaAs, other III-V and II-VI semiconductors, transition metal dichalcogenides such as MoS2.

Excitons give rise to spectrally narrow lines in optical absorption, reflection, transmission and luminescence spectra with the energies below the free-particle band gap of an insulator or a semiconductor. Exciton binding energy and radius can be extracted from optical absorption measurements in applied magnetic fields.

The exciton as a quasiparticle is characterized by the momentum (or wavevector K) describing free propagation of the electron-hole pair as a composite particle in the crystalline lattice in agreement with the Bloch theorem. The exciton energy depends on K and is typically parabolic for the wavevectors much smaller than the reciprocal lattice vector of the host lattice. The exciton energy also depends on the respective orientation of the electron and hole spins, whether they are parallel or anti-parallel. The spins are coupled by the exchange interaction, giving rise to exciton energy fine structure.

In metals and highly doped semiconductors a concept of the Gerald Mahan exciton is invoked where the hole in a valence band is correlated with the Fermi sea of conduction electrons. In that case no bound state in a strict sense is formed, but the Coulomb interaction leads to a significant enhancement of absorption in the vicinity of the fundamental absorption edge also known as the Mahan or Fermi-edge singularity.

Frenkel exciton
In materials with a relatively small dielectric constant, the Coulomb interaction between an electron and a hole may be strong and the excitons thus tend to be small, of the same order as the size of the unit cell. Molecular excitons may even be entirely located on the same molecule, as in fullerenes. This Frenkel exciton, named after Yakov Frenkel, has a typical binding energy on the order of 0.1 to 1 eV. Frenkel excitons are typically found in alkali halide crystals and in organic molecular crystals composed of aromatic molecules, such as anthracene and tetracene. Another example of Frenkel exciton includes on-site d-d excitations in transition metal compounds with partially filled d-shells. While d-d transitions are in principle forbidden by symmetry, they become weakly-allowed in a crystal when the symmetry is broken by structural relaxations or other effects. Absorption of a photon resonant with a d-d transition leads to the creation of an electron-hole pair on a single atomic site, which can be treated as a Frenkel exciton.

Wannier–Mott exciton
In semiconductors, the dielectric constant is generally large. Consequently, electric field screening tends to reduce the Coulomb interaction between electrons and holes. The result is a Wannier–Mott exciton, which has a radius larger than the lattice spacing. Small effective mass of electrons that is typical of semiconductors also favors large exciton radii. As a result, the effect of the lattice potential can be incorporated into the effective masses of the electron and hole. Likewise, because of the lower masses and the screened Coulomb interaction, the binding energy is usually much less than that of a hydrogen atom, typically on the order of 0.01 eV. This type of exciton was named for Gregory Wannier and Nevill Francis Mott. Wannier–Mott excitons are typically found in semiconductor crystals with small energy gaps and high dielectric constants, but have also been identified in liquids, such as liquid xenon. They are also known as large excitons.

In single-wall carbon nanotubes, excitons have both Wannier–Mott and Frenkel character. This is due to the nature of the Coulomb interaction between electrons and holes in one-dimension. The dielectric function of the nanotube itself is large enough to allow for the spatial extent of the wave function to extend over a few to several nanometers along the tube axis, while poor screening in the vacuum or dielectric environment outside of the nanotube allows for large (0.4 to 1.0 eV) binding energies.

Often more than one band can be chosen as source for the electron and the hole, leading to different types of excitons in the same material. Even high-lying bands can be effective as femtosecond two-photon experiments have shown. At cryogenic temperatures, many higher excitonic levels can be observed approaching the edge of the band, forming a series of spectral absorption lines that are in principle similar to hydrogen spectral series.

3D semiconductors
In a bulk semiconductor, a Wannier exciton has an energy and radius associated with it, called exciton Rydberg energy and exciton Bohr radius respectively. For the energy, we have

 * $$E(n)=- \frac{ \left( \frac{\mu}{m_0 \varepsilon_r^2}\text{Ry} \right)}{n^2} \equiv -\frac{R_\text{X}}{n^2}$$

where $$\text{Ry}$$ is the Rydberg unit of energy (cf. Rydberg constant), $$\varepsilon_r$$ is the (static) relative permittivity, $$\mu= (m^*_e m^*_h)/(m^*_e+m^*_h)$$ is the reduced mass of the electron and hole, and $$m_0$$ is the electron mass. Concerning the radius, we have

 * $$r_n = \left(\frac{m_0 \varepsilon_r a_\text{H}}{\mu} \right)n^2 \equiv a_\text{X}n^2$$

where $$a_\text{H}$$ is the Bohr radius.

For example, in GaAs, we have relative permittivity of 12.8 and effective electron and hole masses as 0.067m0 and 0.2m0 respectively; and that gives us $$R_\text{X}=4.2$$ meV and $$a_\text{X}=13$$ nm.

2D semiconductors
In two-dimensional (2D) materials, the system is quantum confined in the direction perpendicular to the plane of the material. The reduced dimensionality of the system has an effect on the binding energies and radii of Wannier excitons. In fact, excitonic effects are enhanced in such systems.

For a simple screened Coulomb potential, the binding energies take the form of the 2D hydrogen atom

 * $$E(n)= -\frac{R_\text{X}}{\left(n-\tfrac{1}{2}\right)^2}$$.

In most 2D semiconductors, the Rytova–Keldysh form is a more accurate approximation to the exciton interaction

 * $$V(r)= -\frac{e^2}{8 \epsilon_0 r_0}\left[\text{H}_0\left(\frac{\kappa r}{r_0}\right)-Y_0\left(\frac{\kappa r}{r_0}\right)\right].$$

where $$r_0$$ is the so-called screening length, $$\epsilon_0$$ is the vacuum permittivity, $$e$$ is the elementary charge, $$\kappa$$ the average dielectric constant of the surrounding media, and $$r$$ the exciton radius. For this potential, no general expression for the exciton energies may be found. One must instead turn to numerical procedures, and it is precisely this potential that gives rise to the nonhydrogenic Rydberg series of the energies in 2D semiconductors.

Example: excitons in transition metal dichalcogenides (TMDs)
Monolayers of a transition metal dichalcogenide (TMD) are a good and cutting-edge example where excitons play a major role. In particular, in these systems, they exhibit a bounding energy of the order of 0.5 eV with a Coulomb attraction between the hole and the electrons stronger than in other traditional quantum wells. As a result, optical excitonic peaks are present in these materials even at room temperatures.

0D semiconductors
In nanoparticles which exhibit quantum confinement effects and hence behave as quantum dots (also called 0-dimensional semiconductors), excitonic radii are given by
 * $$a_\text{X} = \frac{\varepsilon_r}{\mu/m_0}a_0$$

where $$\varepsilon_r$$ is the relative permittivity, $$\mu \equiv (m_e^*m_h^*)/(m_e^*+m_h^*)$$ is the reduced mass of the electron-hole system, $$m_0$$ is the electron mass, and $$a_0$$ is the Bohr radius.

Hubbard exciton
Hubbard excitons are linked to electrons not by a Coulomb's interaction, but by a magnetic force. Their name derives by the English physicist John Hubbard.

Hubbard excitons were observed for the first time in 2023 through the Terahertz time-domain spectroscopy. Those particles have been obtained by applying a light to a Mott antiferromagnetic insulator.

Charge-transfer exciton
An intermediate case between Frenkel and Wannier excitons is the charge-transfer (CT) exciton. In molecular physics, CT excitons form when the electron and the hole occupy adjacent molecules. They occur primarily in organic and molecular crystals; in this case, unlike Frenkel and Wannier excitons, CT excitons display a static electric dipole moment. CT excitons can also occur in transition metal oxides, where they involve an electron in the transition metal 3d orbitals and a hole in the oxygen 2p orbitals. Notable examples include the lowest-energy excitons in correlated cuprates or the two-dimensional exciton of TiO2. Irrespective of the origin, the concept of CT exciton is always related to a transfer of charge from one atomic site to another, thus spreading the wave-function over a few lattice sites.

Surface exciton
At surfaces it is possible for so called image states to occur, where the hole is inside the solid and the electron is in the vacuum. These electron-hole pairs can only move along the surface.

Dark exciton
Dark excitons are those where the electrons have a different momentum from the holes to which they are bound that is they are in an optically forbidden transition which prevents them from photon absorption and therefore to reach their state they need phonon scattering. They can even outnumber normal bright excitons formed by absorption alone.

Atomic and molecular excitons
Alternatively, an exciton may be described as an excited state of an atom, ion, or molecule, if the excitation is wandering from one cell of the lattice to another.

When a molecule absorbs a quantum of energy that corresponds to a transition from one molecular orbital to another molecular orbital, the resulting electronic excited state is also properly described as an exciton. An electron is said to be found in the lowest unoccupied orbital and an electron hole in the highest occupied molecular orbital, and since they are found within the same molecular orbital manifold, the electron-hole state is said to be bound. Molecular excitons typically have characteristic lifetimes on the order of nanoseconds, after which the ground electronic state is restored and the molecule undergoes photon or phonon emission. Molecular excitons have several interesting properties, one of which is energy transfer (see Förster resonance energy transfer) whereby if a molecular exciton has proper energetic matching to a second molecule's spectral absorbance, then an exciton may transfer (hop) from one molecule to another. The process is strongly dependent on intermolecular distance between the species in solution, and so the process has found application in sensing and molecular rulers.

The hallmark of molecular excitons in organic molecular crystals are doublets and/or triplets of exciton absorption bands strongly polarized along crystallographic axes. In these crystals an elementary cell includes several molecules sitting in symmetrically identical positions, which results in the level degeneracy that is lifted by intermolecular interaction. As a result, absorption bands are polarized along the symmetry axes of the crystal. Such multiplets were discovered by Antonina Prikhot'ko and their genesis was proposed by Alexander Davydov. It is known as 'Davydov splitting'.

Giant oscillator strength of bound excitons
Excitons are lowest excited states of the electronic subsystem of pure crystals. Impurities can bind excitons, and when the bound state is shallow, the oscillator strength for producing bound excitons is so high that impurity absorption can compete with intrinsic exciton absorption even at rather low impurity concentrations. This phenomenon is generic and applicable both to the large radius (Wannier–Mott) excitons and molecular (Frenkel) excitons. Hence, excitons bound to impurities and defects possess giant oscillator strength.

Self-trapping of excitons
In crystals, excitons interact with phonons, the lattice vibrations. If this coupling is weak as in typical semiconductors such as GaAs or Si, excitons are scattered by phonons. However, when the coupling is strong, excitons can be self-trapped. Self-trapping results in dressing excitons with a dense cloud of virtual phonons which strongly suppresses the ability of excitons to move across the crystal. In simpler terms, this means a local deformation of the crystal lattice around the exciton. Self-trapping can be achieved only if the energy of this deformation can compete with the width of the exciton band. Hence, it should be of atomic scale, of about an electron volt.

Self-trapping of excitons is similar to forming strong-coupling polarons but with three essential differences. First, self-trapped exciton states are always of a small radius, of the order of lattice constant, due to their electric neutrality. Second, there exists a self-trapping barrier separating free and self-trapped states, hence, free excitons are metastable. Third, this barrier enables coexistence of free and self-trapped states of excitons. This means that spectral lines of free excitons and wide bands of self-trapped excitons can be seen simultaneously in absorption and luminescence spectra. While the self-trapped states are of lattice-spacing scale, the barrier has typically much larger scale. Indeed, its spatial scale is about $$r_b\sim m\gamma^2/\omega^2$$ where $$m$$ is effective mass of the exciton, $$\gamma$$ is the exciton-phonon coupling constant, and $$\omega$$ is the characteristic frequency of optical phonons. Excitons are self-trapped when $$m$$ and $$\gamma$$ are large, and then the spatial size of the barrier is large compared with the lattice spacing. Transforming a free exciton state into a self-trapped one proceeds as a collective tunneling of coupled exciton-lattice system (an instanton). Because $$r_b$$ is large, tunneling can be described by a continuum theory. The height of the barrier $$W\sim \omega^4/m^3\gamma^4$$. Because both $$m$$ and $$\gamma$$ appear in the denominator of $$W$$, the barriers are basically low. Therefore, free excitons can be seen in crystals with strong exciton-phonon coupling only in pure samples and at low temperatures. Coexistence of free and self-trapped excitons was observed in rare-gas solids, alkali-halides, and in molecular crystal of pyrene.

Interaction
Excitons are the main mechanism for light emission in semiconductors at low temperature (when the characteristic thermal energy kT is less than the exciton binding energy), replacing the free electron-hole recombination at higher temperatures.

The existence of exciton states may be inferred from the absorption of light associated with their excitation. Typically, excitons are observed just below the band gap.

When excitons interact with photons a so-called polariton (or more specifically exciton-polariton) is formed. These excitons are sometimes referred to as dressed excitons.

Provided the interaction is attractive, an exciton can bind with other excitons to form a biexciton, analogous to a dihydrogen molecule. If a large density of excitons is created in a material, they can interact with one another to form an electron-hole liquid, a state observed in k-space indirect semiconductors.

Additionally, excitons are integer-spin particles obeying Bose statistics in the low-density limit. In some systems, where the interactions are repulsive, a Bose–Einstein condensed state, called excitonium, is predicted to be the ground state. Some evidence of excitonium has existed since the 1970s but has often been difficult to discern from a Peierls phase. Exciton condensates have allegedly been seen in a double quantum well systems. In 2017 Kogar et al. found "compelling evidence" for observed excitons condensing in the three-dimensional semimetal 1T-TiSe2.

Spatially direct and indirect excitons
Normally, excitons in a semiconductor have a very short lifetime due to the close proximity of the electron and hole. However, by placing the electron and hole in spatially separated quantum wells with an insulating barrier layer in between so called 'spatially indirect' excitons can be created. In contrast to ordinary (spatially direct), these spatially indirect excitons can have large spatial separation between the electron and hole, and thus possess a much longer lifetime. This is often used to cool excitons to very low temperatures in order to study Bose–Einstein condensation (or rather its two-dimensional analog).

title: Exciton


text:
Fjord

In physical geography, a fjord or fiord is a long, narrow sea inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Antarctica, the Arctic, and surrounded landmasses of the northern and southern hemispheres. Norway's coastline is estimated to be 29000 km long with its nearly 1,200 fjords, but only 2500 km long excluding the fjords.

Formation
A true fjord is formed when a glacier cuts a U-shaped valley by ice segregation and abrasion of the surrounding bedrock. According to the standard model, glaciers formed in pre-glacial valleys with a gently sloping valley floor. The work of the glacier then left an overdeepened U-shaped valley that ends abruptly at a valley or trough end. Such valleys are fjords when flooded by the ocean. Thresholds above sea level create freshwater lakes. Glacial melting is accompanied by the rebounding of Earth's crust as the ice load and eroded sediment is removed (also called isostasy or glacial rebound). In some cases, this rebound is faster than sea level rise. Most fjords are deeper than the adjacent sea; Sognefjord, Norway, reaches as much as 1300 m below sea level. Fjords generally have a sill or shoal (bedrock) at their mouth caused by the previous glacier's reduced erosion rate and terminal moraine. In many cases this sill causes extreme currents and large saltwater rapids (see skookumchuck). Saltstraumen in Norway is often described as the world's strongest tidal current. These characteristics distinguish fjords from rias (e.g. the Bay of Kotor), which are drowned valleys flooded by the rising sea. Drammensfjorden is cut almost in two by the Svelvik "ridge", a sandy moraine that was below sea level when it was covered by ice, but after the post-glacial rebound reaches 60 m above the fjord.

Jens Esmark in the 19th century introduced the theory that fjords are or have been created by glaciers and that large parts of Northern Europe had been covered by thick ice in prehistory. Thresholds at the mouths and overdeepening of fjords compared to the ocean are the strongest evidence of glacial origin, and these thresholds are mostly rocky. Thresholds are related to sounds and low land where the ice could spread out and therefore have less erosive force. John Walter Gregory argued that fjords are of tectonic origin and that glaciers had a negligible role in their formation. Gregory's views were rejected by subsequent research and publications. In the case of Hardangerfjord the fractures of the Caledonian fold has guided the erosion by glaciers, while there is no clear relation between the direction of Sognefjord and the fold pattern. This relationship between fractures and direction of fjords is also observed in Lyngen. Preglacial, tertiary rivers presumably eroded the surface and created valleys that later guided the glacial flow and erosion of the bedrock. This may in particular have been the case in Western Norway where the tertiary uplift of the landmass amplified eroding forces of rivers.

Confluence of tributary fjords led to excavation of the deepest fjord basins. Near the very coast, the typical West Norwegian glacier spread out (presumably through sounds and low valleys) and lost their concentration and reduced the glaciers' power to erode leaving bedrock thresholds. Bolstadfjorden is 160 m deep with a threshold of only 1.5 m, while the 1300 m deep Sognefjorden has a threshold around 100 to 200 m deep. Hardangerfjord is made up of several basins separated by thresholds: The deepest basin Samlafjorden between Jonaneset (Jondal) and Ålvik with a distinct threshold at Vikingneset in Kvam.

Hanging valleys are common along glaciated fjords and U-shaped valleys. A hanging valley is a tributary valley that is higher than the main valley and was created by tributary glacier flows into a glacier of larger volume. The shallower valley appears to be 'hanging' above the main valley or a fjord. Often, waterfalls form at or near the outlet of the upper valley. Small waterfalls within these fjords are also used as freshwater resources. Hanging valleys also occur underwater in fjord systems. The branches of Sognefjord are for instance much shallower than the main fjord. The mouth of Fjærlandsfjord is about 400 m deep while the main fjord is 1200 m nearby. The mouth of Ikjefjord is only 50 meters deep while the main fjord is around 1300 m at the same point.

Hydrology
During the winter season, there is usually little inflow of freshwater. Surface water and deeper water (down to 100 m or more) are mixed during winter because of the steady cooling of the surface and wind. In the deep fjords, there is still fresh water from the summer with less density than the saltier water along the coast. Offshore wind, common in the fjord areas during winter, sets up a current on the surface from the inner to the outer parts. This current on the surface in turn pulls dense salt water from the coast across the fjord threshold and into the deepest parts of the fjord. Bolstadfjorden has a threshold of only 1.5 m and strong inflow of freshwater from Vosso river creates a brackish surface that blocks circulation of the deep fjord. The deeper, salt layers of Bolstadfjorden are deprived of oxygen and the seabed is covered with organic material. The shallow threshold also creates a strong tidal current.

During the summer season, there is usually a large inflow of river water in the inner areas. This freshwater gets mixed with saltwater creating a layer of brackish water with a slightly higher surface than the ocean which in turn sets up a current from the river mouths towards the ocean. This current is gradually more salty towards the coast and right under the surface current there is a reverse current of saltier water from the coast. In the deeper parts of the fjord the cold water remaining from winter is still and separated from the atmosphere by the brackish top layer. This deep water is ventilated by mixing with the upper layer causing it to warm and freshen over the summer. In fjords with a shallow threshold or low levels of mixing this deep water is not replaced every year and low oxygen concentration makes the deep water unsuitable for fish and animals. In the most extreme cases, there is a constant barrier of freshwater on the surface and the fjord freezes over such that there is no oxygen below the surface. Drammensfjorden is one example. The mixing in fjords predominantly results from the propagation of an internal tide from the entrance sill or internal seiching.

The Gaupnefjorden branch of Sognefjorden is strongly affected by freshwater as a glacial river flows in. Velfjorden has little inflow of freshwater.

Coral reefs
In 2000, some coral reefs were discovered along the bottoms of the Norwegian fjords. These reefs were found in fjords from the north of Norway to the south. The marine life on the reefs is believed to be one of the most important reasons why the Norwegian coastline is such a generous fishing ground. Since this discovery is fairly new, little research has been done. The reefs are host to thousands of lifeforms such as plankton, coral, anemones, fish, several species of shark, and many more. Most are specially adapted to life under the greater pressure of the water column above it, and the total darkness of the deep sea.

New Zealand's fjords are also host to deep-water corals, but a surface layer of dark fresh water allows these corals to grow in much shallower water than usual. An underwater observatory in Milford Sound allows tourists to view them without diving.

Skerries
In some places near the seaward margins of areas with fjords, the ice-scoured channels are so numerous and varied in direction that the rocky coast is divided into thousands of island blocks, some large and mountainous while others are merely rocky points or rock reefs, menacing navigation. These are called skerries. The term skerry is derived from the Old Norse sker, which means a rock in the sea.

Skerries most commonly formed at the outlet of fjords where submerged glacially formed valleys perpendicular to the coast join with other cross valleys in a complex array. The island fringe of Norway is such a group of skerries (called a skjærgård); many of the cross fjords are so arranged that they parallel the coast and provide a protected channel behind an almost unbroken succession of mountainous islands and skerries. By this channel, one can travel through a protected passage almost the entire 1601 km route from Stavanger to North Cape, Norway. The Blindleia is a skerry-protected waterway that starts near Kristiansand in southern Norway and continues past Lillesand. The Swedish coast along Bohuslän is likewise skerry guarded. The Inside Passage provides a similar route from Seattle, Washington, and Vancouver, British Columbia, to Skagway, Alaska. Yet another such skerry-protected passage extends from the Straits of Magellan north for 800 km.

Phytoplankton
Fjords provide unique environmental conditions for phytoplankton communities. In polar fjords, glacier and ice sheet outflow add cold, fresh meltwater along with transported sediment into the body of water. Nutrients provided by this outflow can significantly enhance phytoplankton growth. For example, in some fjords of the West Antarctic Peninsula (WAP), nutrient enrichment from meltwater drives diatom blooms, a highly productive group of phytoplankton that enable such fjords to be valuable feeding grounds for other species. It is possible that as climate change reduces long-term meltwater output, nutrient dynamics within such fjords will shift to favor less productive species, destabilizing the food web ecology of fjord systems.

In addition to nutrient flux, sediment carried by flowing glaciers can become suspended in the water column, increasing turbidity and reducing light penetration into greater depths of the fjord. This effect can limit the available light for photosynthesis in deeper areas of the water mass, reducing phytoplankton abundance beneath the surface.

Overall, phytoplankton abundance and species composition within fjords is highly seasonal, varying as a result of seasonal light availability and water properties that depend on glacial melt and the formation of sea ice. The study of phytoplankton communities within fjords is an active area of research, supported by groups such as FjordPhyto, a citizen science initiative to study phytoplankton samples collected by local residents, tourists, and boaters of all backgrounds.

Epishelf lakes
An epishelf lake forms when meltwater is trapped behind a floating ice shelf and the freshwater floats on the denser saltwater below. Its surface may freeze forming an isolated ecosystem.

Etymology

The word fjord is borrowed from Norwegian, where it is pronounced, , or  in various dialects and has a more general meaning, referring in many cases to any long, narrow body of water, inlet or channel (for example, see Oslofjord).

The Norwegian word is inherited from Old Norse fjǫrðr, a noun which refers to a 'lake-like' body of water used for passage and ferrying and is closely related to the noun ferð "travelling, ferrying, journey". Both words go back to Indo-European "crossing", from the root  "cross". The words fare and ferry are of the same origin.

The Scandinavian fjord, Proto-Scandinavian *ferþuz, is the origin for similar Germanic words: Icelandic fjörður, Faroese fjørður, Swedish fjärd (for Baltic waterbodies), Scots firth (for marine waterbodies, mainly in Scotland and northern England). The Norse noun fjǫrðr was adopted in German as Förde, used for the narrow long bays of Schleswig-Holstein, and in English as firth "fjord, river mouth". The English word ford (compare German Furt, Low German Ford or Vörde, in Dutch names voorde such as Vilvoorde, Ancient Greek πόρος, poros, and Latin portus) is assumed to originate from Germanic ferþu- and Indo-European root * meaning "crossing point". Fjord/firth/Förde as well as ford/Furt/Vörde/voorde refer to a Germanic noun for a travel: North Germanic ferd or färd and of the verb to travel, Dutch varen, German fahren; English to fare.

As a loanword from Norwegian, it is one of the few words in the English language to start with the sequence fj. The word was for a long time normally spelled fiord, a spelling preserved in place names such as Grise Fiord. The fiord spelling mostly remains only in New Zealand English, as in the place name Fiordland.

Scandinavian usage

The use of the word fjord in Norwegian, Danish and Swedish is more general than in English and in international scientific terminology. In Scandinavia, fjord is used for a narrow inlet of the sea in Norway, Denmark and western Sweden, but this is not its only application. In Norway and Iceland, the usage is closest to the Old Norse, with fjord used for both a firth and for a long, narrow inlet. In eastern Norway, the term is also applied to long narrow freshwater lakes (Randsfjorden and Tyrifjorden) and sometimes even to rivers (for instance in Flå in Hallingdal, the Hallingdal river is referred to as fjorden). In southeast Sweden, the name fjard fjärd is a subdivision of the term 'fjord' used for bays, bights and narrow inlets on the Swedish Baltic Sea coast, and in most Swedish lakes. This latter term is also used for bodies of water off the coast of Finland where Finland Swedish is spoken. In Danish, the word may even apply to shallow lagoons. In modern Icelandic, fjörður is still used with the broader meaning of firth or inlet. In Faroese fjørður is used both about inlets and about broader sounds, whereas a narrower sound is called sund. In the Finnish language, a word vuono is used although there is only one fjord in Finland.

In old Norse genitive was fjarðar whereas dative was firði. The dative form has become common place names like Førde (for instance Førde), Fyrde or Førre (for instance Førre).

The German use of the word Föhrde for long narrow bays on their Baltic Sea coastline, indicates a common Germanic origin of the word. The landscape consists mainly of moraine heaps. The Föhrden and some "fjords" on the east side of Jutland, Denmark are also of glacial origin. But while the glaciers digging "real" fjords moved from the mountains to the sea, in Denmark and Germany they were tongues of a huge glacier covering the basin of which is now the Baltic Sea. See Förden and East Jutland Fjorde.

Whereas fjord names mostly describe bays (though not always geological fjords), straits in the same regions typically are named Sund, in Scandinavian languages as well as in German. The word is related to "to sunder" in the meaning of "to separate". So the use of Sound to name fjords in North America and New Zealand differs from the European meaning of that word.

The name of Wexford in Ireland is originally derived from Veisafjǫrðr ("inlet of the mud flats") in Old Norse, as used by the Viking settlers—though the inlet at that place in modern terms is an estuary, not a fjord. Similarly the name of Milford (now Milford Haven) in Wales is derived from Melrfjǫrðr ("sandbank fjord/inlet"), though the inlet on which it is located is actually a ria.

Before or in the early phase of Old Norse angr was another common noun for fjords and other inlets of the ocean. This word has survived only as a suffix in names of some Scandinavian fjords and has in same cases also been transferred to adjacent settlements or surrounding areas for instance Hardanger, Stavanger and Geiranger.

Differences in definitions

The differences in usage between the English and the Scandinavian languages have contributed to confusion in the use of the term fjord. Bodies of water that are clearly fjords in Scandinavian languages are not considered fjords in English; similarly bodies of water that would clearly not be fjords in the Scandinavian sense have been named or suggested to be fjords. Examples of this confused usage follow.

In the Danish language some inlets are called a fjord, but are, according to the English language definition, technically not a fjord. For example, Roskilde Fjord. Limfjord in English terminology is a sound, since it separates the North Jutlandic Island (Vendsyssel-Thy) from the rest of Jutland. However, the Limfjord once was a fjord until the sea broke through from the west. Ringkøbing Fjord on the western coast of Jutland is a lagoon. The long narrow fjords of Denmark's Baltic Sea coast like the German Förden were dug by ice moving from the sea upon land, while fjords in the geological sense were dug by ice moving from the mountains down to the sea. However, some definitions of a fjord is: "A long narrow inlet consisting of only one inlet created by glacial activity". Examples of Danish fjords are: Kolding Fjord, Vejle Fjord and Mariager Fjord.

The fjords in Finnmark (Norway), which are fjords in the Scandinavian sense of the term, are not universally considered to be fjords by the scientific community. Although glacially formed, most Finnmark fjords lack the steep-sided valleys of the more southerly Norwegian fjords since the glacial pack was deep enough to cover even the high grounds when they were formed. The Oslofjord on the other hand is a rift valley, and not glacially formed.

The indigenous Māori people of New Zealand see a fjord as a kind of sea (tai) that runs by a bluff (matapari, altogether tai matapari "bluff sea").

"Fjords" not created by glaciers
The term "fjord" is sometimes applied to steep-sided inlets which were not created by glaciers. Most such inlets are drowned river canyons or rias. Examples include:
 * In Acapulco, Mexico, the calanques—narrow, rocky inlets—on the western side of the city, where the famous cliff-divers perform daily, are described in the city's tourist literature as being fjords.
 * The calanques of Parc national des Calanques, Provence, France, are also referred to as fjords.
 * Camel Estuary at Padstow, Cornwall, England, sometimes referred to as a "fjord" despite being classified as a ria.
 * The Fiordo di Furore in Italy is a ria
 * Golfo Dulce in Puntarenas, Costa Rica. Like the Saco de Mamangua below, it is sometimes described as a "tropical fjord".
 * The Khor ash Sham in the Musandam Peninsula in Oman, and other "khors" or inlets in the deeply indented coast of Musandam, are often described as "fjords". They were formed by the subduction of the Arabian tectonic plate beneath the Eurasian plate.
 * Bay of Kotor in Montenegro
 * the Lim bay in Istria, Croatia, is sometimes called "Lim fjord" although it is a ria dug by the river Pazinčica. The Croats call it Limski kanal, which does not translate precisely to the English equivalent either.
 * Milford Haven Waterway in Pembrokeshire, Wales. This inlet is a ria. The place-name is derived from Old Norse Melrfjordr meaning "sandbank fjord".
 * Port Davey in Tasmania, Australia is popularly believed to be a "fjord", but is now thought to be part of a drowned river valley system.
 * Saco De Mamangua in Rio de Janeiro, Brazil. Colloquially labeled the world's "only tropical fjord".

Freshwater fjords
Some Norwegian freshwater lakes that have formed in long glacially carved valleys with sill thresholds, ice front deltas or terminal moraines blocking the outlet follow the Norwegian naming convention; they are frequently named fjords. Ice front deltas developed when the ice front was relatively stable for long time during the melting of the ice shield. The resulting landform is an isthmus between the lake and the saltwater fjord, in Norwegian called "eid" as in placename Eidfjord or Nordfjordeid. The post-glacial rebound changed these deltas into terraces up to the level of the original sea level. In Eidfjord, Eio has dug through the original delta and left a 110 m terrace while lake is only 19 m above sea level. Such deposits are valuable sources of high-quality building materials (sand and gravel) for houses and infrastructure. Eidfjord village sits on the eid or isthmus between Eidfjordvatnet lake and Eidfjorden branch of Hardangerfjord. Nordfjordeid is the isthmus with a village between Hornindalsvatnet lake and Nordfjord. Such lakes are also denoted fjord valley lakes by geologists.

One of Norway's largest is Tyrifjorden at 63 m above sea level and an average depth at 97 m most of the lake is under sea level. Norway's largest lake, Mjøsa, is also referred to as "the fjord" by locals. Another example is the freshwater fjord Movatnet (Mo lake) that until 1743 was separated from Romarheimsfjorden by an isthmus and connected by a short river. During a flood in November 1743, the river bed eroded and sea water could flow into the lake at high tide. Eventually, Movatnet became a saltwater fjord and renamed Mofjorden (Mofjorden). Like fjords, freshwater lakes are often deep. For instance Hornindalsvatnet is at least 500 m deep and water takes an average of 16 years to flow through the lake. Such lakes created by glacial action are also called fjord lakes or moraine-dammed lakes.

Some of these lakes were salt after the ice age but later cut off from the ocean during the post-glacial rebound. At the end of the ice age Eastern Norway was about 200 m lower (the marine limit). When the ice cap receded and allowed the ocean to fill valleys and lowlands, and lakes like Mjøsa and Tyrifjorden were part of the ocean while Drammen valley was a narrow fjord. At the time of the Vikings Drammensfjord was still 4 or higher than today and reached the town of Hokksund, while parts of what is now the city of Drammen was under water. After the ice age the ocean was about 150 m at Notodden. The ocean stretched like a fjord through Heddalsvatnet all the way to Hjartdal. Post-glacial rebound eventually separated Heddalsvatnet from the ocean and turned it into a freshwater lake. In neolithic times Heddalsvatnet was still a saltwater fjord connected to the ocean, and was cut off from the ocean around 1500 BC.

Some freshwater fjords such as Slidrefjord are above the marine limit.

Like freshwater fjords, the continuation of fjords on land are in the same way denoted as fjord-valleys. For instance Flåmsdal (Flåm valley) and Måbødalen. Outside of Norway, the three western arms of New Zealand's Lake Te Anau are named North Fiord, Middle Fiord and South Fiord. Another freshwater "fjord" in a larger lake is Western Brook Pond, in Newfoundland's Gros Morne National Park; it is also often described as a fjord, but is actually a freshwater lake cut off from the sea, so is not a fjord in the English sense of the term. Locally they refer to it as a "landlocked fjord". Such lakes are sometimes called "fjord lakes". Okanagan Lake was the first North American lake to be so described, in 1962. The bedrock there has been eroded up to 650 m below sea level, which is 2000 m below the surrounding regional topography. Fjord lakes are common on the inland lea of the Coast Mountains and Cascade Range; notable ones include Lake Chelan, Seton Lake, Chilko Lake, and Atlin Lake. Kootenay Lake, Slocan Lake and others in the basin of the Columbia River are also fjord-like in nature, and created by glaciation in the same way. Along the British Columbia Coast, a notable fjord-lake is Owikeno Lake, which is a freshwater extension of Rivers Inlet. Quesnel Lake, located in central British Columbia, is claimed to be the deepest fjord formed lake on Earth.

Great Lakes
A family of freshwater fjords are the embayments of the North American Great Lakes. Baie Fine is located on the northwestern coast of Georgian Bay of Lake Huron in Ontario, and Huron Bay is located on the southern shore of Lake Superior in Michigan.

Locations
The principal mountainous regions where fjords have formed are in the higher middle latitudes and the high latitudes reaching to 80°N (Svalbard, Greenland), where, during the glacial period, many valley glaciers descended to the then-lower sea level. The fjords develop best in mountain ranges against which the prevailing westerly marine winds are orographically lifted over the mountainous regions, resulting in abundant snowfall to feed the glaciers. Hence coasts having the most pronounced fjords include the west coast of Norway, the west coast of North America from Puget Sound to Alaska, the southwest coast of New Zealand, and the west and to south-western coasts of South America, chiefly in Chile.

Principal fjord regions

 * West coast of Europe
 * Faroe Islands
 * Westfjords of Iceland
 * Eastern Region of Iceland
 * West Highlands of Scotland
 * Norway, the whole coast including Svalbard
 * Kola Peninsula in Russia
 * West coast of New Zealand
 * Fiordland, in the southwest of the South Island
 * Northwest coast of North America
 * Coast of Alaska, United States: Lynn Canal, Glacier Bay, etc.
 * British Columbia Coast, Canada: from the Alaskan Border along the Portland Canal to Indian Arm; Kingcome Inlet is a typical West Coast fjord.
 * Hood Canal in Washington, United States and various of the arms of Puget Sound
 * Northeast coast of North America
 * Labrador: Saglek Fjord, Nachvak Fjord, Hebron Fjord
 * The east coast of Ungava Bay.
 * Baffin Island
 * Ellesmere Island
 * Greenland: Kangerlussuaq, Ilulissat Icefjord, Scoresby Sund, Disko Island
 * Southwest coast of South America
 * Fjords and channels of Chile
 * Isla de los Estados, Argentina

Other glaciated or formerly glaciated regions
Other regions have fjords, but many of these are less pronounced due to more limited exposure to westerly winds and less pronounced relief. Areas include:
 * Europe
 * Ireland
 * Lough Swilly
 * Carlingford Lough
 * Killary Harbour
 * Russia (see also List of fjords of Russia)
 * Chukchi Peninsula
 * Kola Peninsula
 * Scotland (where they are called firths, the Scots language cognate of fjord; lochs or sea lochs). Notable examples are:
 * Loch Long
 * Loch Fyne, Scotland's longest fjord at 65 km
 * Loch Etive
 * Sweden
 * Gullmarsfjorden, in Bohuslän, Sweden
 * Wales
 * Mawddach Estuary, a fjord in-filled by glacial deposits.
 * North America
 * Canada:
 * the west and south coasts of Newfoundland, particularly:
 * Facheux Bay
 * Bonne Bay in Gros Morne National Park
 * Aviron Bay
 * La Hune Bay
 * Bay de Vieux
 * White Bear Bay
 * Baie d'Espoir
 * La Poile Bay
 * Bay Le Moine
 * the Canadian Arctic Archipelago
 * Quebec, Saguenay Fjord
 * United States:
 * Somes Sound, Acadia National Park, Maine
 * Hudson River
 * most clearly seen at The Palisades
 * Puget Sound
 * South America
 * Argentina:
 * Isla de los Estados
 * Arctic
 * Arctic islands
 * Novaya Zemlya
 * Severnaya Zemlya
 * Antarctica
 * South Georgia (UK)
 * Kerguelen Islands (France)
 * particularly the Antarctic Peninsula
 * Sub-Antarctic islands

Extreme fjords
The longest fjords in the world are: Deep fjords include:
 * 1) Nansen Sound/Greely Fiord/Tanquary Fiord in Canada—420 km
 * 2) Chatham Strait/Lynn Canal in United States—403 km
 * 3) Scoresby Sund in Greenland—382 km
 * 4) Concepción Channel-Puerto Simpson in Chile—245 km
 * 5) Sognefjord in Norway—204 km
 * 6) Independence Fjord in Greenland—200 km
 * 7) Matochkin Shar, Novaya Zemlya, Russia—125 km (a strait with a fjord structure)
 * 1) Skelton Inlet in Antarctica—1933 m
 * 2) Sognefjord in Norway—1308 m (the mountains then rise to up to 1500 m and more, Hurrungane reaches 2400 m)
 * 3) Messier Channel in Tortel, Chile—1358 m
 * 4) Baker Channel in Tortel, Chile—1251 m

title: Fjord