Polar bear
The polar bear is a hypercarnivorous bear whose native range lies within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is a large bear the same size as the omnivorous Kodiak bear. A boar weighs around 350 -- 700 kg. Although it is the sister species of the brown bear, it has evolved to occupy a narrower ecological niche, with many body characteristics adapted for cold temperatures, for moving across snow and open water, for hunting seals, which make up most of its diet. Although most polar bears are born on land, they spend most of their time on the sea ice, their scientific name derives from this fact. Polar bears hunt their preferred food of seals from the edge of sea ice living off fat reserves when no sea ice is present; because of their dependence on the sea ice, polar bears are classified as marine mammals. Because of expected habitat loss caused by climate change, the polar bear is classified as a vulnerable species. For decades, large-scale hunting raised international concern for the future of the species, but populations rebounded after controls and quotas began to take effect.
For thousands of years, the polar bear has been a key figure in the material and cultural life of circumpolar peoples, polar bears remain important in their cultures. The polar bear has been known as the white bear. Constantine John Phipps was the first to describe the polar bear as a distinct species in 1774, he chose the scientific name Ursus maritimus, the Latin for'maritime bear', due to the animal's native habitat. The Inuit refer to the animal as nanook; the Yupik refer to the bear as nanuuk in Siberian Yupik. The bear is umka in the Chukchi language. In Russian, it is called бе́лый медве́дь, though an older word still in use is ошку́й. In Quebec, the polar bear is referred to as ours polaire. In the Norwegian-administered Svalbard archipelago, the polar bear is referred to as Isbjørn; the polar bear was considered to be in its own genus, Thalarctos. However, evidence of hybrids between polar bears and brown bears, of the recent evolutionary divergence of the two species, does not support the establishment of this separate genus, the accepted scientific name is now therefore Ursus maritimus, as Phipps proposed.
The bear family, Ursidae, is thought to have split from other carnivorans about 38 million years ago. The subfamily Ursinae originated 4.2 million years ago. The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004. Fossils show that between 10,000 and 20,000 years ago, the polar bear's molar teeth changed from those of the brown bear. Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in the Pleistocene from the eastern part of Siberia; the evidence from DNA analysis is more complex. The mitochondrial DNA of the polar bear diverged from the brown bear, Ursus arctos 150,000 years ago. Further, some clades of brown bear, as assessed by their mtDNA, are more related to polar bears than to other brown bears, meaning that the polar bear might not be considered a species under some species concepts; the mtDNA of extinct Irish brown bears is close to polar bears. A comparison of the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged 603,000 years ago, although the latest research is based on analysis of the complete genomes of polar and brown bears, establishes the divergence of polar and brown bears at 400,000 years ago.
However, the two species have mated intermittently for all that time, most coming into contact with each other during warming periods, when polar bears were driven onto land and brown bears migrated northward. Most brown bears have about 2 percent genetic material from polar bears, but one population, the ABC Islands bears has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating. Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids. However, because neither species can survive long in the other's ecological niche, because they have different morphology, metabolism and feeding behaviours, other phenotypic characteristics, the two bears are classified as separate species; when the polar bear was documented, two subspecies were identified: the American polar bear by Constantine J. Phipps in 1774, the Siberian polar bear by Peter Simon Pallas in 1776; this distinction has since been invalidated. One alleged fossil subspecies has been identified: Ursus maritimus tyrannus, which became extinct during the Pleistocene.
U.m. tyrannus was larger than the living subspecies. However, recent reanalysis of the fossil suggests that it was a brown bear; the polar bear is found in the Arctic Circle and adjacent land masses as far south as Newfoundland. Due to the absence of human development in i
Big Dipper
The Big Dipper or the Plough is a large asterism consisting of seven bright stars of the constellation Ursa Major. Four define a "bowl" or "body" and three define a "handle" or "head", it is recognized as a distinct grouping in many cultures. The North Star, the current northern pole star and the tip of the handle of the Little Dipper, can be located by extending an imaginary line through the front two stars of the asterism and Dubhe; this makes it useful in celestial navigation. The constellation of Ursa Major has been seen as a wagon, or a ladle; the "bear" tradition is Greek, but the name "bear" has parallels in Siberian or North American traditions. The name "Bear" is Homeric, native to Greece, while the "Wain" tradition is Mesopotamian. Book XVIII of Homer's Iliad mentions it as "the Bear, which men call the Wain". In Latin, these seven stars were known as the "Seven Oxen"; the classical mythographer identified the "Bear" as the nymph Callisto, changed into a she-bear by Hera, the jealous wife of Zeus.
In Ireland and the United Kingdom, this pattern is known as the Plough. The symbol of the Starry Plough has been used as a political symbol by Irish Republican and left wing movements. Former names include Butcher's Cleaver; the terms Charles's Wain and Charles his Wain are derived from the still older Carlswæn. A folk etymology holds that this derived from Charlemagne, but the name is common to all the Germanic languages and intended the churls' wagon, in contrast with the women's wagon. An older "Odin's Wain" may have preceded these Nordic designations. In German, it is known as the "Great Wagon" and, less the "Great Bear". In Scandinavia, it is known by variations of "Charles's Wagon", but the "Great Bear". In Dutch, its official name is the "Great Bear", but it is popularly known as the "Saucepan". In Italian, too, it is called the "Great Wagon". In Romanian and most Slavic languages, it is known as the "Great Wagon" as well. In Hungarian, it is called "Göncöl's Wagon" or, less "Big Göncöl" after a táltos in Hungarian mythology who carried medicine that could cure any disease.
In Finnish, the figure is known as Otava with established etymology in the archaic meaning'salmon net', although other uses of the word refer to'bear' and'wheel'. The bear relation is claimed to stem from the animal's resemblance to—and mythical origin from—the asterism rather than vice versa. In the Lithuanian language, the stars of Ursa Major are known as Didieji Grįžulo Ratai. Other names for the constellation include Perkūno Ratai, Kaušas, Vežimas, Samtis. In traditional Chinese astronomy, which continues to be used throughout East Asia, these stars are considered to compose the Right Wall of the Purple Forbidden Enclosure which surrounds the Northern Celestial Pole, although numerous other groupings and names have been made over the centuries; each star has a distinct name, which has varied over time and depending upon the asterism being constructed. The Western asterism is now known as the "Northern Dipper" or the "Seven Stars of the Northern Dipper"; the personification of the Big Dipper itself is known as "Doumu" in Chinese folk religion and Taoism, Marici in Buddhism.
In Shinto, the seven largest stars of Ursa Major belong to Amenominakanushi, the oldest and most powerful of all kami. In North Korea, the constellation is featured on the flag of the country's special forces. In South Korea, the constellation is referred to as "the seven stars of the north". In the related myth, a widow with seven sons found comfort with a widower, but to get to his house required crossing a stream; the seven sons, sympathetic to their mother, placed stepping stones in the river. Their mother, not knowing who put the stones in place, blessed them and, when they died, they became the constellation. In Malay, it is known as the "Boat Constellation". In Burmese, these stars are known as Pucwan Tārā. Pucwan is a general term for a crustacean, such as prawn, crab, etc. In Javanese, as known as "Bintang Kartika"; this name comes from Sanskrit. In ancient Javanese this brightest seven stars are known as Lintang Wuluh means "seven stars"; this star cluster is so popular because its emergence into the sky signals the time marker for planting.
In Hindu astronomy, it is referred to as the "Collection of Seven Great Sages", as each star is named after a mythical Hindu sage. An Arabian story has the four stars of the Plough's bowl as a coffin, with the three stars in the handle as mourners, following it. In Mongolian, it is known as the "Seven Gods". In Kazakh, they are known as the Jetiqaraqshi and, in Kyrgyz, as the Jetigen. While its Western origins come from its resemblance to the kitchen utensil, In Filipino, the Big Dipper and its sister constellation Little Dipper are more associated with the tabo, a hygiene tool akin to a bucket with a handl
American Legion
The American Legion is a U. S. war veterans' organization headquartered in Indiana. It is made up of state, U. S. territory, overseas departments, these are in turn made up of local posts. The legislative body of The American Legion is a national convention, held annually; the organization was founded on March 15, 1919, at the American Club near Place de la Concorde in Paris, France, by members of the American Expeditionary Forces, it was chartered on September 16, 1919, by the U. S. Congress; the organization played the leading role in drafting and passing of the Servicemen's Readjustment Act of 1944, otherwise known as the "GI Bill." In addition to organizing commemorative events, members provide assistance at VA hospitals and clinics. It is active in issue-oriented U. S. politics. Its primary political activity is lobbying on behalf of interests of veterans and service members, including support for benefits such as pensions and the Veterans Health Administration; the organization has historically promoted "Americanism."
Veterans who served at least one day of active duty during wartime, or are serving now, are eligible for membership in The American Legion. Members must have been honorably discharged/discharged under honorable conditions or are still serving honorably. Merchant Marines who served from December 7, 1941, to December 31, 1946, are eligible. World War I veterans were eligible during their lifetimes. Membership peaked for The American Legion right after World War II, when enrollments doubled from 1.7 million to 3.3 million. After the Korean War, there were 2.5 million Legionnaires. As the baby boomers joined, its membership increased to 3.1 million in 1992. However, membership has been decreasing since then. In 2013, National Headquarters of The American Legion reported 2.3 million members. The aftermath of two American wars in the second half of the 19th century had seen the formation of several ex-soldiers' organizations. Former Union Army soldiers of the American Civil War of 1861–65 established a fraternal organization called the Grand Army of the Republic, while their Southern brethren would join together in the United Confederate Veterans.
Both organizations emerged as powerful political entities, with the GAR serving as a mainstay of the Republican Party, which controlled the Presidency from the Civil War through William Howard Taft's administration except for the two terms of office of Grover Cleveland. In Southern politics the UCV maintained an more dominant position as a bulwark of the Democratic Party which dominated there; the conclusion of the brief Spanish–American conflict of 1898 ushered in another soldiers' organization, the American Veterans of Foreign Service, today known as the Veterans of Foreign Wars of the United States. Concerned about the United States' absence from the world war and the preparedness of its army and navy, magazine editor Arthur Sullivant Hoffman and writer Stephen Allan Reynolds founded The American Legion in February 1915, inspired by a letter from reader E. D. Cook, they lobbied government to strengthen the military. They held a preparedness parade in New York City and made a film America Prepare Officers included Theodore Roosevelt, Arthur S. Hoffman, William Howard Taft, Elihu Root, Jacob M. Dickinson, Henry L. Stimson and Luke E. Wright, George von L. Meyer, Truman H. Newberry and Charles J. Bonaparte.
Its officers were at New York City. In 1917, when war was declared the Legion had 23,000 members skilled in 77 professions pledged to fight, their pledge cards were shared with the government and used to raise two regiments of air mechanics. The Legion was discorporated in 1917. With the termination of hostilities in World War I in November 1918, some American officers, participants in the conflict began to think about creating a similar organization for the two million men, on European duty; the need for an organization for former members of the AEF was immediate. With the war at an end, hundreds of thousands of impatient draftees found themselves trapped in France and pining for home, certain only that untold weeks or months lay ahead of them before their return would be logistically possible. Morale plummeted. Cautionary voices were raised about an apparent correlation between disaffected and discharged troops and the Bolshevik uprisings taking place in Russia, Finland and Hungary; this situation was a particular matter of concern to Lt. Col. Theodore Roosevelt, Jr. eldest son of the 26th President.
One day in January 1919, he had a discussion at General Headquarters with a mobilized National Guard officer named George A. White, a former newspaper editor with the Portland Oregonian. After long discussion, he suggested the establishment at once of a new servicemen's organization including all members of the AEF, as well as those soldiers who remained stateside as members of the Army and Marine Corps during the war without having been shipped abroad, he and White advocated ceaselessly for this proposal until they found sufficient support at headquarters to move forward with the plan. General John J. Pershing issued orders to a group of 20 non-career officers to report to the YMCA in Paris on February 15, 1919; the selection of these individuals had been made by Roosevelt. They were joined with a number of regular Army officers Pershing selected himself; the session of reserve and regular officers was instructed to provide a set of laws to curb the problem of declining morale. After three days, the officers presented a series of proposals, including eliminating restrictive regulations, organizing additional athletic and recreational events, expanding leave time and entertainment programs
Gold panning
Gold panning, or panning, is a form of placer mining and traditional mining that extracts gold from a placer deposit using a pan. The process is one of the simplest ways to extract gold, is popular with geology enthusiasts because of its low cost and relative simplicity; the first recorded instances of placer mining are from ancient Rome, where gold and other precious metals were extracted from streams and mountainsides using sluices and panning. However, the productivity rate is comparatively smaller compared to other methods such as the rocker box or large extractors, such as those used at the Super Pit gold mine, in Kalgoorlie, Western Australia, which has led to panning being replaced in the commercial market. Gold panning is a simple process. Once a suitable placer deposit is located, some alluvial deposits are scooped into a pan, where they are gently agitated in water and the gold sinks to the bottom of the pan. Materials with a low specific gravity are allowed to spill out of the pan, whereas materials with a higher specific gravity sink to the bottom of the sediment during agitation and remain within the pan for examination and collection by the prospector.
These dense materials consist of a black, magnetite sand with whatever stones or metal dust that may be found in the deposit, used for source material. While an effective method with certain kinds of deposits, essential for prospecting skilled panners can work but a limited amount of material less than the other methods which have replaced it in larger operation. Pans remain in use in places where there is limited capital or infrastructure, as well as in recreational gold mining. In many situations, gold panning turns up only minor gold dust, collected as a souvenir in small clear tubes by hobbyists. Nuggets and considerable amounts of dust are found, but panning mining is not lucrative. Panning for gold can be used to locate the parent gold veins which are the source of most placer deposits. Gold pans of various designs have been developed over the years, the common features being a means for trapping the heavy materials during agitation, or for removing them at the end of the process; some are intended for use with mercury, include screens, sharp corners for breaking ice, are non-round, or are designed for use "with or without water".
Edward Otho Cresap Ord, II, a former Army officer and co-owner of several mines, patented several pan designs including designs for use with mercury or dry. Pans are measured by their diameter in centimeters. Common sizes of gold pans today range between 10–17 inches, with 14 inches being the most used size; the sides are angled between 30° to 45°. Pans are manufactured in high-impact plastic. Russia iron or heavy gauge steel pans are traditional. Steel pans are stronger than plastic pans; some are made of lightweight alloys for structural stability. Plastic gold pans resist rust and corrosion, most are designed with moulded riffles along one side of the pan. Of the plastic gold pans and red ones are preferred among prospectors, as both the gold and the black sand stands out in the bottom of the pan, although many opt for black pans instead to identify gold deposits; the batea, Spanish for "gold pan", is a particular variant of gold pan. Traditionally made of a solid piece of wood, it may be made of metal.
Bateas are used in areas where there is less water available for use than with traditional gold pans, such as Mexico and South America, where it was introduced by the Spanish. Bateas are larger than other gold pans; the yuri-ita, Japanese for "rocking plate" is a traditional wooden gold pan used in Japan. Unlike other gold pans, it is rectangular in shape with a concave cross section and is sealed off at one end with the other end open; as the Japanese name implies, the gold is panned with a rocking motion
Midnight sun
The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. Around the summer solstice, the Sun is visible for given fair weather; the number of days per year with potential midnight sun increases the closer towards either pole one goes. Although defined by the polar circles, in practice the midnight sun can be seen as much as 55 miles outside the polar circle, as described below, the exact latitudes of the farthest reaches of midnight sun depend on topography and vary year-to-year; because there are no permanent human settlements south of the Antarctic Circle, apart from research stations, the countries and territories whose populations experience the midnight sun are limited to those crossed by the Arctic Circle: the Canadian Yukon and Northwest Territories. A quarter of Finland's territory lies north of the Arctic Circle, at the country's northernmost point the sun does not set at all for 60 days during summer.
In Svalbard, the northernmost inhabited region of Europe, there is no sunset from 19 April to 23 August. The extreme sites are the poles; the North Pole has midnight sun for 6 months from late March to late September. The South Pole experiences this from 23 September to 20 March; the opposite phenomenon, polar night, occurs in winter, when the Sun stays below the horizon throughout the day. Since the axial tilt of the Earth is considerable, the Sun does not set at high latitudes in local summer; the Sun remains continuously visible for one day during the summer solstice at the polar circle, for several weeks only 100 km closer to the pole, for six months at the pole. At extreme latitudes, the midnight sun is referred to as polar day. At the poles themselves, the Sun sets only once each year on the equinox. During the six months that the Sun is above the horizon, it spends the days continuously moving in circles around the observer spiralling higher and reaching its highest circuit of the sky at the summer solstice.
Because of atmospheric refraction, because the Sun is a disc rather than a point, the midnight sun may be experienced at latitudes south of the Arctic Circle or north of the Antarctic Circle, though not exceeding one degree. For example, Iceland is known for its midnight sun though most of it is south of the Arctic Circle. For the same reasons, the period of sunlight at the poles is longer than six months; the northern extremities of Scotland experience twilight throughout the night in the northern sky at around the summer solstice. Observers at heights appreciably above sea level can experience extended periods of midnight sun as a result of the "dip" of the horizon viewed from altitude; the term "midnight sun" refers to the consecutive 24-hour periods of sunlight experienced north of the Arctic Circle and south of the Antarctic Circle. Other phenomena are sometimes referred to as "midnight sun", but they are caused by time zones and the observance of daylight saving time. For instance, in Fairbanks, south of the Arctic Circle, the Sun sets at 12:47 am at the summer solstice.
This is because Fairbanks is 51 minutes ahead of its idealized time zone and Alaska observes daylight saving time. This means. If a precise moment for the genuine "midnight sun" is required, the observer's longitude, the local civil time and the equation of time must be taken into account; the moment of the Sun's closest approach to the horizon coincides with its passing due north at the observer's position, which occurs only at midnight in general. Each degree of longitude east of the Greenwich meridian makes the vital moment 4 minutes earlier than midnight as shown on the clock, while each hour that the local civil time is ahead of coordinated universal time makes the moment an hour later; these two effects must be added. Furthermore, the equation of time must be added: a positive value on a given date means that the Sun is running ahead of its average position, so the value must be subtracted; as an example, at the North Cape of Norway at midnight on June 21/22, the longitude of 25.9 degrees east makes the moment 103.2 minutes earlier by clock time.
The equation of time at that date is -2.0 minutes. Therefore, the sun's lowest elevation occurs 120 - 103.2 + 2.0 minutes after midnight: at 00.19 Central European Summer time. On other nearby dates the only thing different is the equation of time, so this remains a reasonable estimate for a considerable period; the Sun's altitude remains within half a degree of the minimum of about 5 degrees for about 45 minutes either side of this time. Locations where the Sun remains less than 6 degrees below the horizon—above 60° 34’ latitude south of the Arctic Circle or north of the Antarctic Circle—experience midnight twilight instead of midnight
Ursa Major
Ursa Major is a constellation in the northern sky, whose associated mythology dates back into prehistory. Its Latin name means "greater she-bear", standing as a reference to and in direct contrast with nearby Ursa Minor, the lesser bear. In antiquity, it was one of the original 48 constellations listed by Ptolemy, is now the third largest constellation of the 88 modern constellations. Ursa Major is known from the asterism of its main seven bright stars comprising the "Big Dipper", "the Wagon", "Charles's Wain" or "the Plough", with its stellar configuration mimicking the shape of the "Little Dipper"; the general constellation outline significantly features in numerous world cultures, is used as a symbol of the north. E.g. as the flag of Alaska. The asterism's two brightest stars, named Dubhe and Merak, can be used as the navigational pointer towards the place of the current northern pole star, Polaris in Ursa Minor. Ursa Major is visible throughout the year from most of the northern hemisphere, appears circumpolar above the mid-northern latitudes.
From southern temperate latitudes, the main asterism is invisible, but the southern parts of the constellation can still be viewed. Appearing in the northern sky, Ursa Major occupies a large area covering 1279.66 square degrees or 3.10% of the total sky, making it the third largest constellations in the night sky. Eugène Delporte in 1930, who set the official International Astronomical Union constellation boundaries, formed a 28-sided irregular polygon, which according to the equatorial coordinate system, stretches between the right ascension coordinates of 08h 08.3m and 14h 29.0m and the declination coordinates of +28.30° and +73.14°. Ursa Major borders eight other constellations: Draco to the north and northeast, Boötes to the east, Canes Venatici to the east and southeast, Coma Berenices to the southeast and Leo Minor to the south, Lynx to the southwest and Camelopardalis to the northwest; the three-letter constellation abbreviation'UMa' was adopted by the IAU in 1922. The "Big Dipper" is an asterism within Ursa Major composed of seven bright stars that together comprise one of the best-known patterns in the sky.
Like many of its common names allude to, its shape is said to resemble either a ladle, an agricultural plough or wagon. Starting with the "ladle" portion of the dipper and extending clockwise through the handle, these stars are the following: α Ursae Majoris, known by the Arabic name Dubhe, which at a magnitude of 1.79 is the 35th-brightest star in the sky and the second-brightest of Ursa Major. Β Ursae Majoris, called Merak, with a magnitude of 2.37. Γ Ursae Majoris, known as either Phecda or Phad, with a magnitude of 2.44. Δ Ursae Majoris, or Megrez, meaning "root of the tail," referring to its location as the intersection of the body and tail of the bear. Ε Ursae Majoris, known as Alioth, a name which refers not to a bear but to a "black horse," the name corrupted from the original and mis-assigned to the named Alcor, the naked-eye binary companion of Mizar. Alioth is the brightest star of Ursa Major and the 33rd-brightest in the sky, with a magnitude of 1.76. It is the brightest of the "peculiar A stars," magnetic stars whose chemical elements are either depleted or enhanced, appear to change as the star rotates.
Ζ Ursae Majoris, the second star in from the end of the handle of the Big Dipper, the constellation's fourth-brightest star. Mizar, which means "girdle," forms a famous double star, with its optical companion Alcor, the two of which were termed the "horse and rider" by the Arabs; the ability to resolve the two stars with the naked eye is quoted as a test of eyesight, although people with quite poor eyesight can see the two stars. Η Ursae Majoris, known as either Alkaid or Benetnash, both meaning the "end of the tail." With a magnitude of 1.85, Alkaid is the third-brightest star of Ursa Major. Except for Dubhe and Alkaid, the stars of the Big Dipper all have proper motions heading toward a common point in Sagittarius. A few other such stars have been identified, together they are called the Ursa Major Moving Group; the stars Merak and Dubhe are known as the "pointer stars" because they are helpful for finding Polaris known as the North Star or Pole Star. By visually tracing a line from Merak through Dubhe and continuing for 5 units, one's eye will land on Polaris indicating true north.
Another asterism known as the "Three Leaps of the Gazelle" is recognized in Arab culture, a series of three pairs of stars found along the southern border of the constellation. W Ursae Majoris is the prototype of a class of contact binary variable stars, ranges between 7.75m and 8.48m. 47 Ursae Majoris is a Sun-like star with a three-planet system. 47 Ursae Majoris b, discovered in 1996, orbits every 1078 days and is 2.53 times the mass of Jupiter. 47 Ursae Majoris c, discovered in 2001, orbits every 2391 days and is 0.54 times the
Aurora
An aurora, sometimes referred to as polar lights, northern lights, southern lights, is a natural light display in the Earth's sky, predominantly seen in the high-latitude regions. Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma in the form of electrons and protons, precipitate them into the upper atmosphere due to Earth's magnetic field, where their energy is lost; the resulting ionization and excitation of atmospheric constituents emits light of varying color and complexity. The form of the aurora, occurring within bands around both polar regions, is dependent on the amount of acceleration imparted to the precipitating particles. Precipitating protons produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton auroras are observed at lower latitudes; the word "aurora" is derived from the name of the Roman goddess of the dawn, who traveled from east to west announcing the coming of the sun.
Ancient Roman poets used the name metaphorically to refer to dawn mentioning its play of colours across the otherwise dark sky. Most auroras occur in a band known as the "auroral zone", 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles at all local times, most seen at night against a dark sky. A region that displays an aurora is called the "auroral oval", a band displaced towards the night side of the Earth. Early evidence for a geomagnetic connection comes from the statistics of auroral observations. Elias Loomis, Hermann Fritz and S. Tromholt in more detail, established that the aurora appeared in the auroral zone. Day-to-day positions of the auroral ovals are posted on the Internet. In northern latitudes, the effect is known as the northern lights; the former term was coined by Galileo in 1619, from the Roman goddess of the dawn and the Greek name for the north wind. The southern counterpart, the aurora australis or the southern lights, has features identical to the aurora borealis and changes with changes in the northern auroral zone.
The Aurora Australis is visible from high southern latitudes in Antarctica, Argentina, New Zealand, Australia. A geomagnetic storm causes the auroral ovals to expand, bring the aurora to lower latitudes; the instantaneous distribution of auroras is different, being centered about 3–5° nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun. The aurora can be seen best at this time, called magnetic midnight. Auroras seen within the auroral oval may be directly overhead, but from farther away, they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras occur poleward of the auroral zone as either diffuse patches or arcs, which can be subvisual. Auroras are seen in latitudes below the auroral zone, when a geomagnetic storm temporarily enlarges the auroral oval. Large geomagnetic storms are most common during the peak of the 11-year sunspot cycle or during the three years after the peak.
An aurora may appear overhead as a "corona" of rays, radiating from a distant and apparent central location, which results from perspective. An electron spirals about a field line at an angle, determined by its velocity vectors and perpendicular to the local geomagnetic field vector B; this angle is known as the "pitch angle" of the particle. The distance, or radius, of the electron from the field line at any time is known as its Larmor radius; the pitch angle increases as the electron travels to a region of greater field strength nearer to the atmosphere. Thus, it is possible for some particles to return, or mirror, if the angle becomes 90° before entering the atmosphere to collide with the denser molecules there. Other particles that do not mirror enter the atmosphere and contribute to the auroral display over a range of altitudes. Other types of auroras have been observed from space, e.g."poleward arcs" stretching sunward across the polar cap, the related "theta aurora", "dayside arcs" near noon.
These are infrequent and poorly understood. Other interesting effects occur such as flickering "black aurora" and subvisual red arcs. In addition to all these, a weak glow observed around the two polar cusps, the field lines separating the ones that close through the Earth from those that are swept into the tail and close remotely; the altitudes where auroral emissions occur were revealed by Carl Størmer and his colleagues, who used cameras to triangulate more than 12,000 auroras. They discovered that most of the light is produced between 90 and 150 km above the ground, while extending at times to more than 1000 km. Images of auroras are more common today than in the past due to the increase in the use of digital cameras that have high enough sensitivities. Film and digital exposure to auroral displays is fraught with difficulties. Due to the different color spectra present, the temporal changes occurring during the exposure, the results are somewhat unpredictable. Different layers of the film emulsion respond differently to lower light levels, choice of a film can be important.
Longer exposures superimpose changing features, blanket the dynamic attribute of a display. Hi
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