Simon Newcomb was a Canadian–American astronomer, applied mathematician and autodidactic polymath, Professor of Mathematics in the U. S. Navy and at Johns Hopkins. Though he had little conventional schooling, he made important contributions to timekeeping as well as other fields in applied mathematics such as economics and statistics in addition to writing a science fiction novel. Simon Newcomb was born in the town of Nova Scotia, his parents were Emily Prince, the daughter of a New Brunswick magistrate, itinerant school teacher John Burton Newcomb. John moved around teaching in different parts of Canada in different villages in Nova Scotia and Prince Edward Island. Emily was a daughter of Thomas Prince and Miriam Steeves, making Simon a great-great-grandson of Heinrich Stief, a not-too-distant cousin of William Henry Steeves, a Canadian Father of Confederation. Newcomb seems to have had little conventional schooling other than from his father and from a short apprenticeship to Dr. Foshay, a charlatan herbalist, in New Brunswick in 1851.
His father provided him with an excellent foundation for his future studies. Newcomb's apprenticeship with Dr. Foshay occurred, they entered an agreement that Newcomb would serve a five-year apprenticeship during which time Foshay would train him in using herbs to treat illnesses. For two years he was an apprentice but became unhappy and disillusioned with his apprenticeship and about Foshay's unscientific approach, realizing that the man was a charlatan, he made the decision to break their agreement. He walked the 120 miles to the port of Calais in Maine where he met the captain of a ship who agreed to take him to Salem, Massachusetts so that he could join his father. In about 1854, he joined his father in Salem, the two journeyed together to Maryland. After arriving in Maryland, Newcomb taught for two years from 1854 to 1856. In his spare time he studied a variety of subjects such as political economy and religion, but his deepest studies were made in mathematics and astronomy. In particular he read Newton's Principia at this time.
In 1856 he took up a position as a private tutor close to Washington and he travelled to that city to study mathematics in the libraries there. He was able to borrow a copy of Bowditch's translation of Laplace's Traité de mécanique céleste from the library of the Smithsonian Institution but found the mathematics beyond him. Newcomb studied mathematics and physics and supported himself by teaching before becoming a human computer at the Nautical Almanac Office in Cambridge, Massachusetts in 1857. At around the same time, he enrolled at the Lawrence Scientific School of Harvard University, graduating BSc in 1858. Newcomb studied mathematics under Benjamin Peirce and the impecunious Newcomb was a welcome guest at the Peirce home. However, he was said to develop a dislike of Peirce's son, Charles Sanders Peirce and has been accused of a "successful destruction" of C. S. Peirce's career. In particular, Daniel Coit Gilman, president of Johns Hopkins University, is alleged to have been on the point of awarding tenure to C. S. Peirce, before Newcomb intervened behind the scenes to dissuade him.
About 20 years Newcomb influenced the Carnegie Institution Trustees, to prevent C. S. Peirce's last chance to publish his life's work, through a denial of a Carnegie grant to Peirce though Andrew Carnegie himself, Theodore Roosevelt, William James and others, wrote to support it. In the prelude to the American Civil War, many US Navy staff of Confederate sympathies left the service and, in 1861, Newcomb took advantage of one of the ensuing vacancies to become professor of mathematics and astronomer at the United States Naval Observatory, Washington D. C.. Newcomb set to work on the measurement of the position of the planets as an aid to navigation, becoming interested in theories of planetary motion. By the time Newcomb visited Paris, France in 1870, he was aware that the table of lunar positions calculated by Peter Andreas Hansen was in error. While in Paris, he realised that, in addition to the data from 1750 to 1838 that Hansen had used, there was further data stretching as far back as 1672.
His visit allowed little serenity for analysis as he witnessed the defeat of French emperor Napoleon III in the Franco-Prussian War and the coup that ended the Second French Empire. Newcomb managed to escape from the city during the ensuing rioting that led up to the formation of the Paris Commune and which engulfed the Paris Observatory. Newcomb was able to use the "new" data to revise Hansen's tables, he was offered the post of director of the Harvard College Observatory in 1875 but declined, having by now settled that his interests lay in mathematics rather than observation. In 1877 he became director of the Nautical Almanac Office where, ably assisted by George William Hill, he embarked on a program of recalculation of all the major astronomical constants. Despite fulfilling a further demanding role as professor of mathematics and astronomy at Johns Hopkins University from 1884, he conceived with A. M. W. Downing a plan to resolve much international confusion on the subject. By the time he attended a standardisation conference in Paris, France, in May 1896, the international consensus was that all ephemerides should be based on Newcomb's calculations—Newcomb's Tables of the Sun.
A further conference as late as 1950 confirmed Newcomb's constants as the int
The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, refined and reduced. The Imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825; the system came into official use across the British Empire. By the late 20th century, most nations of the former empire had adopted the metric system as their main system of measurement, although some imperial units are still used in the United Kingdom and other countries part of the British Empire; the imperial system developed from what were first known as English units, as did the related system of United States customary units. The Weights and Measures Act of 1824 was scheduled to go into effect on 1 May 1825. However, the Weights and Measures Act of 1825 pushed back the date to 1 January 1826; the 1824 Act allowed the continued use of pre-imperial units provided that they were customary known, marked with imperial equivalents. Apothecaries' units are mentioned neither in the act of 1824 nor 1825.
At the time, apothecaries' weights and measures were regulated "in England and Berwick-upon-Tweed" by the London College of Physicians, in Ireland by the Dublin College of Physicians. In Scotland, apothecaries' units were unofficially regulated by the Edinburgh College of Physicians; the three colleges published, at infrequent intervals, the London and Dublin editions having the force of law. Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836, the Edinburgh Pharmacopoeia of 1839, the Dublin Pharmacopoeia of 1850; the Medical Act of 1858 transferred to The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures. Metric equivalents in this article assume the latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was 0.914398415 metres. In 1824, the various different gallons in use in the British Empire were replaced by the imperial gallon, a unit close in volume to the ale gallon.
It was defined as the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury at a temperature of 62 °F. In 1963, the gallon was redefined as the volume of 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL, which works out to 4.546096 l or 277.4198 cu in. The Weights and Measures Act of 1985 switched to a gallon of 4.54609 L. These measurements were in use from 1826, when the new imperial gallon was defined, but were abolished in the United Kingdom on 1 January 1971. In the US, though no longer recommended, the apothecaries' system is still used in medicine in prescriptions for older medications. In the 19th and 20th centuries, the UK used three different systems for weight. Troy weight, used for precious metals; the distinction between mass and weight is not always drawn. A pound is a unit of mass, although it is referred to as a weight; when a distinction is necessary, the term pound-force may be used to refer to a unit of force rather than mass.
The troy pound was made the primary unit of mass by the 1824 Act. The Weights and Measures Act 1855 made the avoirdupois pound the primary unit of mass. In all the systems, the fundamental unit is the pound, all other units are defined as fractions or multiples of it. Although the 1824 act defined the yard and pound by reference to the prototype standards, it defined the values of certain physical constants, to make provision for re-creation of the standards if they were to be damaged. For the yard, the length of a pendulum beating seconds at the latitude of Greenwich at Mean Sea Level in vacuo was defined as 39.01393 inches. For the pound, the mass of a cubic inch of distilled water at an atmospheric pressure of 30 inches of mercury and a temperature of 62° Fahrenheit was defined as 252.458 grains, with there being 7,000 grains per pound. However, following the destruction of the original prototypes in the 1834 Houses of Parliament fire, it proved impossible to recreate the standards from these definitions, a new Weights and Measures Act was passed in 1855 which permitted the recreation of the prototypes from recognized secondary standards.
The imperial system is one of many systems of English units. Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other; the distinctions between these systems are not drawn precisely. One such distinction is that between these systems and older British/English units/systems or newer additions; the term imperial should not be applied to English units that were outlawed in the Weights and Measures Act 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions, such as the slug or poundal. The US customary system is derived from the English units that were in use at the time of settlement; because the United States was independent at the time, these units were unaffected b
The metre or meter is the base unit of length in the International System of Units. The SI unit symbol is m; the metre is defined as the length of the path travelled by light in vacuum in 1/299 792 458 of a second. The metre was defined in 1793 as one ten-millionth of the distance from the equator to the North Pole – as a result the Earth's circumference is 40,000 km today. In 1799, it was redefined in terms of a prototype metre bar. In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted; the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, i.e. about 39 3⁄8 inches. Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Other Germanic languages, such as German and the Scandinavian languages spell the word meter. Measuring devices are spelled "-meter" in all variants of English.
The suffix "-meter" has the same Greek origin as the unit of length. The etymological roots of metre can be traced to the Greek verb μετρέω and noun μέτρον, which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism; this range of uses is found in Latin, French and other languages. The motto ΜΕΤΡΩ ΧΡΩ in the seal of the International Bureau of Weights and Measures, a saying of the Greek statesman and philosopher Pittacus of Mytilene and may be translated as "Use measure!", thus calls for both measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, the universal measure or standard based on a pendulum with a two-second period; the use of the seconds pendulum to define length had been suggested to the Royal Society in 1660 by Christopher Wren. Christiaan Huygens had observed that length to be 39.26 English inches. No official action was taken regarding these suggestions.
In 1670 Gabriel Mouton, Bishop of Lyon suggested a universal length standard with decimal multiples and divisions, to be based on a one-minute angle of the Earth's meridian arc or on a pendulum with a two-second period. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the phrase metro cattolico, derived from the Greek μέτρον καθολικόν, to denote the standard unit of length derived from a pendulum; as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, on 19 March 1791 advised the adoption of the term mètre, a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator. In 1793, the French National Convention adopted the proposal. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies over the surface of the Earth, which affects the period of a pendulum.
To establish a universally accepted foundation for the definition of the metre, more accurate measurements of this meridian were needed. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona to estimate the length of the meridian arc through Dunkerque; this portion of the meridian, assumed to be the same length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. The problem with this approach is that the exact shape of the Earth is not a simple mathematical shape, such as a sphere or oblate spheroid, at the level of precision required for defining a standard of length; the irregular and particular shape of the Earth smoothed to sea level is represented by a mathematical model called a geoid, which means "Earth-shaped". Despite these issues, in 1793 France adopted this definition of the metre as its official unit of length based on provisional results from this expedition.
However, it was determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the French standard and was progressively adopted by other countries in Europe; the expedition was fictionalised in Le mètre du Monde. Ken Alder wrote factually about the expedition in The Measure of All Things: the seven year odyssey and hidden error that transformed the world. In 1867 at the second general conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth; the conference recommended the adoption of the metre and the creation of an internatio
A quasar is an luminous active galactic nucleus. It has been theorized that most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of times the mass of the Sun. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk; as gas falls toward the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous: the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way; the term "quasar" originated as a contraction of quasi-stellar radio source, because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin, when identified in photographic images at visible wavelengths they resembled faint star-like points of light. High-resolution images of quasars from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, that some host-galaxies are interacting or merging galaxies.
As with other categories of AGN, the observed properties of a quasar depend on many factors including the mass of the black hole, the rate of gas accretion, the orientation of the accretion disk relative to the observer, the presence or absence of a jet, the degree of obscuration by gas and dust within the host galaxy. Quasars are found over a broad range of distances, quasar discovery surveys have demonstrated that quasar activity was more common in the distant past; the peak epoch of quasar activity was 10 billion years ago. As of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z = 7.54. The supermassive black hole in this quasar, estimated at 800 million solar masses, is the most distant black hole identified to date; the term "quasar" was first used in a paper by Chinese-born U. S. astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, to describe certain astronomically-puzzling objects: So far, the clumsily long name'quasi-stellar radio sources' is used to describe these objects.
Because the nature of these objects is unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form'quasar' will be used throughout this paper. Between 1917 and 1922, it became clear from work by Heber Curtis, Ernst Öpik and others, that some objects seen by astronomers were in fact distant galaxies like our own, but when radio astronomy commenced in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation. The objects emitted large amounts of radiation of many frequencies, but no source could be located optically, or in some cases only a faint and point-like object somewhat like a distant star; the spectral lines of these objects, which identify the chemical elements of which the object is composed, were extremely strange and defied explanation. Some of them changed their luminosity rapidly in the optical range and more in the X-ray range, suggesting an upper limit on their size no larger than our own Solar System.
This implies an high power density. Considerable discussion took place over, they were described as "quasi-stellar radio sources", or "quasi-stellar objects", a name which reflected their unknown nature, this became shortened to "quasar". The first quasars were discovered as radio sources in all-sky radio surveys, they were first noted as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a small angular size. Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalogue as astronomers scanned the skies for their optical counterparts. In 1963, a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A. Matthews. Astronomers had detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum, which contained many unknown broad emission lines; the anomalous spectrum defied interpretation.
British-Australian astronomer John Bolton made many early observations of quasars, including a breakthrough in 1962. Another radio source, 3C 273, was predicted to undergo five occultations by the Moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to find a visible counterpart to the radio source and obtain an optical spectrum using the 200-inch Hale Telescope on Mount Palomar; this spectrum revealed the same strange emission lines. Schmidt was able to demonstrate that these were to be the ordinary spectral lines of hydrogen redshifted by 15.8 percent - an extreme redshift never seen in astronomy before. If this was due to the physical motion of the "star" 3C 273 was receding at an enormous velocity, around 47,000 km/s, far beyond the speed of any known star and defying any obvious explanation. Nor would an extreme velocity help to explain 3C 273's huge radio emissions. Although it raised many questions, Schmidt's discovery revolutionized quasar observation.
The strange spectrum of 3C 48 was identified by Schmidt and Oke as hydrogen and magnesium redshifted by 37%. Shortly afterwards, two more quasar spectra in 1964 and five more in 1965, were confirmed as ordinary
Sir Arthur Stanley Eddington was an English astronomer and mathematician of the early 20th century who did his greatest work in astrophysics. He was a philosopher of science and a populariser of science; the Eddington limit, the natural limit to the luminosity of stars, or the radiation generated by accretion onto a compact object, is named in his honour. Around 1920, he anticipated the discovery and mechanism of nuclear fusion processes in stars, in his paper "The Internal Constitution of the Stars". At that time, the source of stellar energy was a complete mystery, he is famous for his work concerning the theory of relativity. Eddington wrote a number of articles that announced and explained Einstein's theory of general relativity to the English-speaking world. World War I severed many lines of scientific communication and new developments in German science were not well known in England, he conducted an expedition to observe the solar eclipse of 29 May 1919 that provided one of the earliest confirmations of general relativity, he became known for his popular expositions and interpretations of the theory.
Eddington was born 28 December 1882 in Kendal, England, the son of Quaker parents, Arthur Henry Eddington, headmaster of the Quaker School, Sarah Ann Shout. His father taught at a Quaker training college in Lancashire before moving to Kendal to become headmaster of Stramongate School, he died in the typhoid epidemic which swept England in 1884. His mother was left to bring up her two children with little income; the family moved to Weston-super-Mare where at first Stanley was educated at home before spending three years at a preparatory school. The family lived at a house called 42 Walliscote Road, Weston-super-Mare. There is a commemorative plaque on the building explaining Sir Arthur's contribution to science. In 1893 Eddington entered Brynmelyn School, he proved to be a most capable scholar in mathematics and English literature. His performance earned him a scholarship to Owens College, Manchester in 1898, which he was able to attend, having turned 16 that year, he turned to physics for the next three years.
Eddington was influenced by his physics and mathematics teachers, Arthur Schuster and Horace Lamb. At Manchester, Eddington lived at Dalton Hall, where he came under the lasting influence of the Quaker mathematician J. W. Graham, his progress was rapid, winning him several scholarships and he graduated with a B. Sc. in physics with First Class Honours in 1902. Based on his performance at Owens College, he was awarded a scholarship to Trinity College, Cambridge in 1902, his tutor at Cambridge was Robert Alfred Herman and in 1904 Eddington became the first second-year student to be placed as Senior Wrangler. After receiving his M. A. in 1905, he began research on thermionic emission in the Cavendish Laboratory. This did not go well, meanwhile he spent time teaching mathematics to first year engineering students; this hiatus was brief. Through a recommendation by E. T. Whittaker, his senior colleague at Trinity College, he secured a position at the Royal Observatory in Greenwich where he was to embark on his career in astronomy, a career whose seeds had been sown as a young child when he would "try to count the stars".
In January 1906, Eddington was nominated to the post of chief assistant to the Astronomer Royal at the Royal Greenwich Observatory. He left Cambridge for Greenwich the following month, he was put to work on a detailed analysis of the parallax of 433 Eros on photographic plates that had started in 1900. He developed a new statistical method based on the apparent drift of two background stars, winning him the Smith's Prize in 1907; the prize won him a Fellowship of Cambridge. In December 1912 George Darwin, son of Charles Darwin and Eddington was promoted to his chair as the Plumian Professor of Astronomy and Experimental Philosophy in early 1913; that year, Robert Ball, holder of the theoretical Lowndean chair died, Eddington was named the director of the entire Cambridge Observatory the next year. In May 1914 he was elected a Fellow of the Royal Society: he was awarded the Royal Medal in 1928 and delivered the Bakerian Lecture in 1926. Eddington investigated the interior of stars through theory, developed the first true understanding of stellar processes.
He began this in 1916 with investigations of possible physical explanations for Cepheid variable stars. He began by extending Karl Schwarzschild's earlier work on radiation pressure in Emden polytropic models; these models treated a star as a sphere of gas held up against gravity by internal thermal pressure, one of Eddington's chief additions was to show that radiation pressure was necessary to prevent collapse of the sphere. He developed his model despite knowingly lacking firm foundations for understanding opacity and energy generation in the stellar interior. However, his results allowed for calculation of temperature and pressure at all points inside a star, Eddington argued that his theory was so useful for further astrophysical investigation that it should be retained despite not being based on accepted physics. James Jeans contributed the important suggestion that stellar matter would be ionized, but, the end of any collaboration between the pair, who became famous for their lively debates.
Eddington defended his method by pointing to the utility of his results his important mass-luminosity
Minute and second of arc
A minute of arc, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn – it is for this reason that the Earth's circumference is exactly 21,600 nautical miles. A minute of arc is π/10800 of a radian. A second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, π/648000 of a radian; these units originated in Babylonian astronomy as sexagesimal subdivisions of the degree. To express smaller angles, standard SI prefixes can be employed; the number of square arcminutes in a complete sphere is 4 π 2 = 466 560 000 π ≈ 148510660 square arcminutes. The names "minute" and "second" have nothing to do with the identically named units of time "minute" or "second"; the identical names reflect the ancient Babylonian number system, based on the number 60. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted.
One arcminute is thus written 1′. It is abbreviated as arcmin or amin or, less the prime with a circumflex over it; the standard symbol for the arcsecond is the double prime, though a double quote is used where only ASCII characters are permitted. One arcsecond is thus written 1″, it is abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations, the preference being for degrees and decimals of a minute, for example, written as 42° 25.32′ or 42° 25.322′. This notation has been carried over into marine GPS receivers, which display latitude and longitude in the latter format by default; the full moon's average apparent size is about 31 arcminutes. An arcminute is the resolution of the human eye. An arcsecond is the angle subtended by a U. S. dime coin at a distance of 4 kilometres. An arcsecond is the angle subtended by an object of diameter 725.27 km at a distance of one astronomical unit, an object of diameter 45866916 km at one light-year, an object of diameter one astronomical unit at a distance of one parsec, by definition.
A milliarcsecond is about the size of a dime atop the Eiffel Tower. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth. A nanoarcsecond is about the size of a penny on Neptune's moon Triton as observed from Earth. Notable examples of size in arcseconds are: Hubble Space Telescope has calculational resolution of 0.05 arcseconds and actual resolution of 0.1 arcseconds, close to the diffraction limit. Crescent Venus measures between 66 seconds of arc. Since antiquity the arcminute and arcsecond have been used in astronomy. In the ecliptic coordinate system and longitude; the principal exception is right ascension in equatorial coordinates, measured in time units of hours and seconds. The arcsecond is often used to describe small astronomical angles such as the angular diameters of planets, the proper motion of stars, the separation of components of binary star systems, parallax, the small change of position of a star in the course of a year or of a solar system body as the Earth rotates.
These small angles may be written in milliarcseconds, or thousandths of an arcsecond. The unit of distance, the parsec, named from the parallax of one arc second, was developed for such parallax measurements, it is the distance at which the mean radius of the Earth's orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia, launched in 2013, can approximate star positions to 7 microarcseconds. Apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red giant with a diameter of 0.05 arcsecond. Because of the effects of atmospheric seeing, ground-based telescopes will smear the image of a star to an angular diameter of about 0.5 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond. Space telescopes are diffraction limited. For example, the Hubble Space Telescope can reach an angular size of stars down to about 0.1″. Techniques exist for improving seeing on the ground. Adaptive optics, for example, can produce images around 0.05 arcsecond on a 10 m class telescope.
Minutes and seconds of arc are used in cartography and navigation. At sea level one minute of arc
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics