International Astronomical Union
The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies and any surface features on them; the IAU is a member of the International Council for Science. Its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation; the IAU maintains friendly relations with organizations that include amateur astronomers in their membership. The IAU has its head office on the second floor of the Institut d'Astrophysique de Paris in the 14th arrondissement of Paris. Working groups include the Working Group for Planetary System Nomenclature, which maintains the astronomical naming conventions and planetary nomenclature for planetary bodies, the Working Group on Star Names, which catalogs and standardizes proper names for stars.
The IAU is responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center operates under the IAU, is a "clearinghouse" for all non-planetary or non-moon bodies in the Solar System; the Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers. The IAU was founded on 28 July 1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. Two subsidiaries of the IAU were created at this assembly: the International Time Commission seated at the International Time Bureau in Paris and the International Central Bureau of Astronomical Telegrams seated in Copenhagen, Denmark; the 7 initial member states were Belgium, France, Great Britain, Greece and the United States, soon to be followed by Italy and Mexico. The first executive committee consisted of Benjamin Baillaud, Alfred Fowler, four vice presidents: William Campbell, Frank Dyson, Georges Lecointe, Annibale Riccò.
Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets. The reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, 2–10 May 1922. By the end of the first General Assembly, ten additional nations had joined the Union, bringing the total membership to 19 countries. Although the Union was formed eight months after the end of World War I, international collaboration in astronomy had been strong in the pre-war era; the first 50 years of the Union's history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents and General Secretaries. Twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Union's history in IAU Information Bulletin No. 100. Six past IAU Presidents in the period 1976–2003 contributed their recollections in IAU Information Bulletin No. 104. The IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide.
83% of all individual members are male, while 17% are female, among them the union's former president, Mexican astronomer Silvia Torres-Peimbert. Membership includes 79 national members, professional astronomical communities representing their country's affiliation with the IAU. National members include the Australian Academy of Science, the Chinese Astronomical Society, the French Academy of Sciences, the Indian National Science Academy, the National Academies, the National Research Foundation of South Africa, the National Scientific and Technical Research Council, KACST, the Council of German Observatories, the Royal Astronomical Society, the Royal Astronomical Society of New Zealand, the Royal Swedish Academy of Sciences, the Russian Academy of Sciences, the Science Council of Japan, among many others; the sovereign body of the IAU is its General Assembly. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union and elects various committees; the right to vote on matters brought before the Assembly varies according to the type of business under discussion.
The Statutes consider such business to be divided into two categories: issues of a "primarily scientific nature", upon which voting is restricted to individual members, all other matters, upon which voting is restricted to the representatives of national members. On budget matters, votes are weighted according to the relative subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid. An absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union. Since 1922, the IAU General Assembly meets every three years, with the ex
Royal Air Force
The Royal Air Force is the United Kingdom's aerial warfare force. Formed towards the end of the First World War on 1 April 1918, it is the oldest independent air force in the world. Following victory over the Central Powers in 1918 the RAF emerged as, at the time, the largest air force in the world. Since its formation, the RAF has taken a significant role in British military history. In particular, it played a large part in the Second World War where it fought its most famous campaign, the Battle of Britain; the RAF's mission is to support the objectives of the British Ministry of Defence, which are to "provide the capabilities needed to ensure the security and defence of the United Kingdom and overseas territories, including against terrorism. The RAF describes its mission statement as "... an agile and capable Air Force that, person for person, is second to none, that makes a decisive air power contribution in support of the UK Defence Mission". The mission statement is supported by the RAF's definition of air power.
Air power is defined as "the ability to project power from the air and space to influence the behaviour of people or the course of events". Today the Royal Air Force maintains an operational fleet of various types of aircraft, described by the RAF as being "leading-edge" in terms of technology; this consists of fixed-wing aircraft, including: fighter and strike aircraft, airborne early warning and control aircraft, ISTAR and SIGINT aircraft, aerial refueling aircraft and strategic and tactical transport aircraft. The majority of the RAF's rotary-wing aircraft form part of the tri-service Joint Helicopter Command in support of ground forces. Most of the RAF's aircraft and personnel are based in the UK, with many others serving on operations or at long-established overseas bases. Although the RAF is the principal British air power arm, the Royal Navy's Fleet Air Arm and the British Army's Army Air Corps deliver air power, integrated into the maritime and land environments. While the British were not the first to make use of heavier-than-air military aircraft, the RAF is the world's oldest independent air force: that is, the first air force to become independent of army or navy control.
Following publication of the "Smuts report" prepared by Jan Smuts the RAF was founded on 1 April 1918, with headquarters located in the former Hotel Cecil, during the First World War, by the amalgamation of the Royal Flying Corps and the Royal Naval Air Service. At that time it was the largest air force in the world. After the war, the service was drastically cut and its inter-war years were quiet, with the RAF taking responsibility for the control of Iraq and executing a number of minor actions in other parts of the British Empire; the RAF's naval aviation branch, the Fleet Air Arm, was founded in 1924 but handed over to Admiralty control on 24 May 1939. The RAF developed the doctrine of strategic bombing which led to the construction of long-range bombers and became its main bombing strategy in the Second World War; the RAF underwent rapid expansion prior to and during the Second World War. Under the British Commonwealth Air Training Plan of December 1939, the air forces of British Commonwealth countries trained and formed "Article XV squadrons" for service with RAF formations.
Many individual personnel from these countries, exiles from occupied Europe served with RAF squadrons. By the end of the war the Royal Canadian Air Force had contributed more than 30 squadrons to serve in RAF formations approximately a quarter of Bomber Command's personnel were Canadian. Additionally, the Royal Australian Air Force represented around nine percent of all RAF personnel who served in the European and Mediterranean theatres. In the Battle of Britain in 1940, the RAF defended the skies over Britain against the numerically superior German Luftwaffe. In what is the most prolonged and complicated air campaign in history, the Battle of Britain contributed to the delay and subsequent indefinite postponement of Hitler's plans for an invasion of the United Kingdom. In the House of Commons on 20 August, prompted by the ongoing efforts of the RAF, Prime Minister Winston Churchill eloquently made a speech to the nation, where he said "Never in the field of human conflict was so much owed by so many to so few".
The largest RAF effort during the war was the strategic bombing campaign against Germany by Bomber Command. While RAF bombing of Germany began immediately upon the outbreak of war, under the leadership of Air Chief Marshal Harris, these attacks became devastating from 1942 onward as new technology and greater numbers of superior aircraft became available; the RAF adopted night-time area bombing on German cities such as Hamburg and Dresden, developed precision bombing techniques for specific operations, such as the "Dambusters" raid by No. 617 Squadron, or the Amiens prison raid known as Operation Jericho. Following victory in the Second World War, the RAF underwent significant re-organisation, as technological advances in air warfare saw the arrival of jet fighters and bombers. During the early stages of the Cold War, one of the first major operations undertaken by the Royal Air Force was in 1948 and the Berlin Airlift, codenamed Operation Plainfire. Between 26 June and the lifting of the Russian blockade of the city on 2 May, the RAF provided 17% of the total supplies delivered du
Sumer is the earliest known civilization in the historical region of southern Mesopotamia, modern-day southern Iraq, during the Chalcolithic and Early Bronze ages, one of the first civilizations in the world along with Ancient Egypt and the Indus Valley. Living along the valleys of the Tigris and Euphrates, Sumerian farmers were able to grow an abundance of grain and other crops, the surplus of which enabled them to settle in one place. Prehistoric proto-writing dates back before 3000 BC; the earliest texts, from c. 3300 BC, come from the cities of Jemdet Nasr. Modern historians have suggested that Sumer was first permanently settled between c. 5500 and 4000 BC by a West Asian people who spoke the Sumerian language, an agglutinative language isolate. These prehistoric people are now called "proto-Euphrateans" or "Ubaidians", are theorized to have evolved from the Samarra culture of northern Mesopotamia; the Ubaidians, though never mentioned by the Sumerians themselves, are assumed by modern-day scholars to have been the first civilizing force in Sumer.
They drained the marshes for agriculture, developed trade, established industries, including weaving, metalwork and pottery. Some scholars contest the idea of a Proto-Euphratean language or one substrate language. Reliable historical records begin much later. Juris Zarins believes the Sumerians lived along the coast of Eastern Arabia, today's Persian Gulf region, before it was flooded at the end of the Ice Age. Sumerian civilization took form in the Uruk period, continuing into the Jemdet Nasr and Early Dynastic periods. During the 3rd millennium BC, a close cultural symbiosis developed between the Sumerians, who spoke a language isolate, Akkadians, which gave rise to widespread bilingualism; the influence of Sumerian on Akkadian is evident in all areas, from lexical borrowing on a massive scale, to syntactic and phonological convergence. This has prompted scholars to refer to Sumerian and Akkadian in the 3rd millennium BC as a Sprachbund. Sumer was conquered by the Semitic-speaking kings of the Akkadian Empire around 2270 BC, but Sumerian continued as a sacred language.
Native Sumerian rule re-emerged for about a century in the Third Dynasty of Ur at 2100–2000 BC, but the Akkadian language remained in use. The Sumerian city of Eridu, on the coast of the Persian Gulf, is considered to have been one of the oldest cities, where three separate cultures may have fused: that of peasant Ubaidian farmers, living in mud-brick huts and practicing irrigation; the term Sumerian is the common name given to the ancient non-Semitic-speaking inhabitants of Mesopotamia by the East Semitic-speaking Akkadians. The Sumerians referred to themselves as ùĝ saĝ gíg ga, phonetically /uŋ saŋ ɡi ɡa/ meaning "the black-headed people", to their land as ki-en-gi, meaning "place of the noble lords"; the Akkadian word Shumer may represent the geographical name in dialect, but the phonological development leading to the Akkadian term šumerû is uncertain. Hebrew Shinar, Egyptian Sngr, Hittite Šanhar, all referring to southern Mesopotamia, could be western variants of Shumer. In the late 4th millennium BC, Sumer was divided into many independent city-states, which were divided by canals and boundary stones.
Each was centered on a temple dedicated to the particular patron god or goddess of the city and ruled over by a priestly governor or by a king, intimately tied to the city's religious rites. The five "first" cities, said to have exercised pre-dynastic kingship "before the flood": Eridu Bad-tibira Larsa Sippar Shuruppak Other principal cities: Minor cities: Kuara Zabala Kisurra Marad Dilbat Borsippa Kutha Der Eshnunna Nagar 2 Apart from Mari, which lies full 330 kilometres north-west of Agade, but, credited in the king list as having "exercised kingship" in the Early Dynastic II period, Nagar, an outpost, these cities are all in the Euphrates-Tigris alluvial plain, south of Baghdad in what are now the Bābil, Diyala, Wāsit, Dhi Qar, Basra, Al-Muthannā and Al-Qādisiyyah governorates of Iraq; the Sumerian city-states rose to power during the prehistoric Uruk periods. Sumerian written history reaches back to the 27th century BC and before, but the historical record remains obscure until the Early Dynastic III period, c. the 23rd century BC, when a now deciphered syllabary writing system was developed, which has allowed archaeologists to read contemporary records and inscriptions.
Classical Sumer ends with the rise of the Akkadian Empire in the 23rd century BC. Following the Gutian period, there was a brief Sumerian Renaissance in the 21st century BC, cut short in the 20th century BC by invasions by the Amorites; the Amorite "dynasty of Isin"
A star catalogue or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, this article covers only some of the more quoted ones. Star catalogues were compiled by many different ancient people, including the Babylonians, Chinese and Arabs, they were sometimes accompanied by a star chart for illustration. Most modern catalogues are available in electronic format and can be downloaded from space agencies data centres. Completeness and accuracy is described by the weakest apparent magnitude V and the accuracy of the positions. From their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock; the Egyptians called the circumpolar star "the star that cannot perish" and, although they made no known formal star catalogues, they nonetheless created extensive star charts of the night sky which adorn the coffins and ceilings of tomb chambers.
Although the ancient Sumerians were the first to record the names of constellations on clay tablets, the earliest known star catalogues were compiled by the ancient Babylonians of Mesopotamia in the late 2nd millennium BC, during the Kassite Period. They are better known by their Assyrian-era name'Three Stars Each'; these star catalogues, written on clay tablets, listed thirty-six stars: twelve for "Anu" along the celestial equator, twelve for "Ea" south of that, twelve for "Enlil" to the north. The Mul. Apin lists, dated to sometime before the Neo-Babylonian Empire, are direct textual descendants of the "Three Stars Each" lists and their constellation patterns show similarities to those of Greek civilization. In Ancient Greece, the astronomer and mathematician Eudoxus laid down a full set of the classical constellations around 370 BC, his catalogue Phaenomena, rewritten by Aratus of Soli between 275 and 250 BC as a didactic poem, became one of the most consulted astronomical texts in antiquity and beyond.
It contains descriptions of the positions of the stars, the shapes of the constellations and provided information on their relative times of rising and setting. In the 3rd century BC, the Greek astronomers Timocharis of Alexandria and Aristillus created another star catalogue. Hipparchus completed his star catalogue in 129 BC, which he compared to Timocharis' and discovered that the longitude of the stars had changed over time; this led him to determine the first value of the precession of the equinoxes. In the 2nd century, Ptolemy of Roman Egypt published a star catalogue as part of his Almagest, which listed 1,022 stars visible from Alexandria. Ptolemy's catalogue was based entirely on an earlier one by Hipparchus, it remained the standard star catalogue in the Arab worlds for over eight centuries. The Islamic astronomer al-Sufi updated it in 964, the star positions were redetermined by Ulugh Beg in 1437, but it was not superseded until the appearance of the thousand-star catalogue of Tycho Brahe in 1598.
Although the ancient Vedas of India specified how the ecliptic was to be divided into twenty-eight nakshatra, Indian constellation patterns were borrowed from Greek ones sometime after Alexander's conquests in Asia in the 4th century BC. The earliest known inscriptions for Chinese star names were written on oracle bones and date to the Shang Dynasty. Sources dating from the Zhou Dynasty which provide star names include the Zuo Zhuan, the Shi Jing, the "Canon of Yao" in the Book of Documents; the Lüshi Chunqiu written by the Qin statesman Lü Buwei provides most of the names for the twenty-eight mansions. An earlier lacquerware chest found in the Tomb of Marquis Yi of Zeng contains a complete list of the names of the twenty-eight mansions. Star catalogues are traditionally attributed to Shi Shen and Gan De, two rather obscure Chinese astronomers who may have been active in the 4th century BC of the Warring States period; the Shi Shen astronomy is attributed to Shi Shen, the Astronomic star observation to Gan De.
It was not until the Han Dynasty that astronomers started to observe and record names for all the stars that were apparent in the night sky, not just those around the ecliptic. A star catalogue is featured in one of the chapters of the late 2nd-century-BC history work Records of the Grand Historian by Sima Qian and contains the "schools" of Shi Shen and Gan De's work. Sima's catalogue—the Book of Celestial Offices —includes some 90 constellations, the stars therein named after temples, ideas in philosophy, locations such as markets and shops, different people such as farmers and soldiers. For his Spiritual Constitution of the Universe of 120 AD, the astronomer Zhang Heng compiled a star catalogue comprising 124 constellations. Chinese constellation names were adopted by the Koreans and Japanese. A large number of star catalogues were published by Muslim astronomers in the medieval Islamic world; these were Zij treatises, including Arzachel's Tables of Toledo, the Maragheh observatory's Zij-i Ilkhani and Ulugh Beg's Zij-i-Sultani.
Fomalhaut designated Alpha Piscis Austrini is the brightest star in the constellation of Piscis Austrinus and one of the brightest stars in the sky. It is a class A star on the main sequence 25 light-years from the Sun as measured by the Hipparcos astrometry satellite. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified, it is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk. Fomalhaut, K-type main-sequence star TW Piscis Austrini, M-type, red dwarf star LP 876-10 constitute a triple system though the companions are separated by several degrees. Fomalhaut holds a special significance in extrasolar planet research, as it is the center of the first stellar system with an extrasolar planet candidate imaged at visible wavelengths; the image was published in Science in November 2008. Fomalhaut is the third-brightest star known to have a planetary system, after the Pollux.
Α Piscis Austrini is the system's Bayer designation. It bears the Flamsteed designation of 24 Piscis Austrini; the classical astronomer Ptolemy put it in Aquarius, as well as Piscis Austrinus. In the 1600s Johann Bayer planted it in the primary position of Piscis Austrinus. Following Ptolemy, John Flamsteed in 1725 additionally denoted it 79 Aquarii; the current designation reflects modern consensus on Bayer's decision, that the star belongs in Piscis Austrinus. Under the rules for naming objects in multiple star systems, the three components – Fomalhaut, TW Piscis Austrini and LP 876-10 – are designated A, B and C, respectively. On its discovery, the planet was designated Fomalhaut b; the star's traditional name derives from Fom al-Haut from scientific Arabic فم الحوت fam al-ḥūt "the mouth of the Fish", a translation of how Ptolemy labeled it. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars; the WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included the name Fomalhaut for this star.
In July 2014, the International Astronomical Union launched a process for giving proper names to certain exoplanets. The process involved public voting for the new names. In December 2015, the IAU announced; the winning name was proposed by Todd Vaccaro and forwarded by the St. Cloud State University Planetarium of St. Cloud, United States of America, to the IAU for consideration. Dagon was a Semitic deity represented as half-man, half-fish. At a declination of −29.6°, Fomalhaut is located south of the celestial equator, hence is best viewed from the Southern Hemisphere. However, its southerly declination is not as great as that of stars such as Acrux, Alpha Centauri and Canopus, meaning that, unlike them, Fomalhaut is visible from a large part of the Northern Hemisphere as well, its declination is similar to that of Antares. At 40°N, Fomalhaut rises above the horizon for eight hours and reaches only 20° above the horizon, while Capella, which rises at the same time, will stay above the horizon for twenty hours.
Fomalhaut can be located in northern latitudes by the fact that the western side of the Square of Pegasus points to it. Continuing the line from Beta to Alpha Pegasi towards the southern horizon, Fomalhaut is about 45˚ south of Alpha Pegasi, with no bright stars in between. Fomalhaut is a young star, for many years thought to be only 100 to 300 million years old, with a potential lifespan of a billion years. A 2012 study gave a higher age of 440±40 million years; the surface temperature of the star is around 8,590 K. Fomalhaut's mass is about 1.92 times that of the Sun, its luminosity is about 16.6 times greater, its diameter is 1.84 times as large. Fomalhaut is metal-deficient compared to the Sun, which means it is composed of a smaller percentage of elements other than hydrogen and helium; the metallicity is determined by measuring the abundance of iron in the photosphere relative to the abundance of hydrogen. A 1997 spectroscopic study measured a value equal to 93% of the Sun's abundance of iron.
A second 1997 study deduced a value of 78%, by assuming Fomalhaut has the same metallicity as the neighboring star TW Piscis Austrini, which has since been argued to be a physical companion. In 2004, a stellar evolutionary model of Fomalhaut yielded a metallicity of 79%. In 2008, a spectroscopic measurement gave a lower value of 46%. Fomalhaut has been claimed to be one of 16 stars belonging to the Castor Moving Group; this is an association of stars which share a common motion through space, have been claimed to be physically associated. Other members of this group include Vega; the moving group has an estimated age of 200±100 million years and originated from the same location. More recent work has found that purported members of the Castor Moving Group appear to not only have a wide range of ages, but their velocities are too different to have been associated with one another in the distant past. Hence, "membership" to this dynamical group has no bearing on the age of the Fomalhaut system. Fomalhaut is surrounded by several debris disks.
The inner disk is a high-carbon small-grain ash disk, clustering at 0.1 AU from the star. Next is a disk of larger particles, with inner edge 0.4-1 AU of the star. The innermost dis
Naked eye called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical instrument, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is still considered "naked". In astronomy, the naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions, passing comets, meteor showers, the brightest asteroids, including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye; the basic accuracies of the human eye are: Quick autofocus from distances of 25 cm to 50 cm to infinity. Angular resolution: about 1 arcminute 0.02° or 0.0003 radians, which corresponds to 0.3 m at a 1 km distance. Field of view: simultaneous visual perception in an area of about 160° × 175°. Faint stars up to +8 magnitude under a dark sky. Photometry to ±10% or 1% of intensity – in a range between night and day of 1:10,000,000,000.
Symmetries of 10–20', see the measurements of Tycho Brahe. Interval estimations to 3–5%. Unconscious recognizing of movement. Visual perception allows a person to gain much information about their surroundings: the distances and 3-dimensional position of things and persons the vertical and the slope of plain objects luminosities and colors and their changes by time and direction The visibility of astronomical objects is affected by light pollution. A few hundred kilometers away from a metropolitan area where the sky can appear to be dark, it is still the residual light pollution that sets the limit on the visibility of faint objects. For most people, these are to be the best observing conditions within their reach. Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8m might be visible; the angular resolution of the naked eye is about 1′. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented.
Uranus and Vesta had most been seen but could not be recognized as planets because they appear so faint at maximum brightness. Uranus, when discovered in 1781, was the first planet discovered using technology rather than being spotted by the naked eye. Theoretically, in a typical dark sky, the dark adapted human eye would see the about 5,600 stars brighter than +6m while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible. In practice, the atmospheric extinction and dust reduces this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution can be as low as 2m, with as few as 50 stars visible. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars; the visibility of diffuse objects such as star clusters and galaxies is much more affected by light pollution than is that of planets and stars. Under typical dark conditions only a few such objects are visible.
These include the Pleiades, h/χ Persei, the Andromeda galaxy, the Carina Nebula, the Orion Nebula, Omega Centauri, 47 Tucanae, the Ptolemy Cluster Messier 7 near the tail of Scorpius and the globular cluster M13 in Hercules. The Triangulum Galaxy is a difficult averted vision object and only visible at all if it is higher than 50° in the sky; the globular clusters M 3 in Canes Venatici and M 92 in Hercules are visible with the naked eye under such conditions. Under dark sky conditions, however, M33 is easy to see in direct vision. Many other Messier objects are visible under such conditions; the most distant objects that have been seen by the naked eye are nearby bright galaxies such as Centaurus A, Bode's Galaxy, Sculptor Galaxy, Messier 83. Five planets can be recognized as planets from Earth with the naked eye: Mercury, Mars and Saturn. Under typical dark sky conditions Uranus can be seen as well with averted vision, as can the asteroid Vesta at its brighter oppositions; the Sun and the Moon—the remaining noticeable naked-eye objects of the solar system—are sometimes added to make seven "planets."
During daylight only the Moon and Sun are obvious naked eye objects, but in many cases Venus can be spotted in daylight and in rarer cases Jupiter. Close to sunset and sunrise bright stars like Sirius or Canopus can be spotted with the naked eye as long as one knows the exact position in which to look; the zenith of naked-eye astronomy was the work of Tycho Brahe. He built an extensive observatory to make precise measurements of the heavens without any instruments for magnification. In 1610, Galileo Galilei pointed a telescope towards the sky, he discovered the moons of Jupiter and the phases of Venus, among other things. Meteor showers are better observed by naked eye than with binoculars; such showers include the December Geminids. Some 100 satellites per night, the International Space Station and the Milky Way are other popular objects visible to the naked eye. Many other things can be estimated without an instrument. If an arm is stretched the span of the hand corresponds to an angle of 18 to 20°.
The distance of a person, just covered up by the outstretched thumbnail, is about 100 meters. The vertical can be esti
Arabic is a Central Semitic language that first emerged in Iron Age northwestern Arabia and is now the lingua franca of the Arab world. It is named after the Arabs, a term used to describe peoples living in the area bounded by Mesopotamia in the east and the Anti-Lebanon mountains in the west, in northwestern Arabia, in the Sinai Peninsula. Arabic is classified as a macrolanguage comprising 30 modern varieties, including its standard form, Modern Standard Arabic, derived from Classical Arabic; as the modern written language, Modern Standard Arabic is taught in schools and universities, is used to varying degrees in workplaces and the media. The two formal varieties are grouped together as Literary Arabic, the official language of 26 states, the liturgical language of the religion of Islam, since the Quran and Hadith were written in Arabic. Modern Standard Arabic follows the grammatical standards of Classical Arabic, uses much of the same vocabulary. However, it has discarded some grammatical constructions and vocabulary that no longer have any counterpart in the spoken varieties, has adopted certain new constructions and vocabulary from the spoken varieties.
Much of the new vocabulary is used to denote concepts that have arisen in the post-classical era in modern times. Due to its grounding in Classical Arabic, Modern Standard Arabic is removed over a millennium from everyday speech, construed as a multitude of dialects of this language; these dialects and Modern Standard Arabic are described by some scholars as not mutually comprehensible. The former are acquired in families, while the latter is taught in formal education settings. However, there have been studies reporting some degree of comprehension of stories told in the standard variety among preschool-aged children; the relation between Modern Standard Arabic and these dialects is sometimes compared to that of Latin and vernaculars in medieval and early modern Europe. This view though does not take into account the widespread use of Modern Standard Arabic as a medium of audiovisual communication in today's mass media—a function Latin has never performed. During the Middle Ages, Literary Arabic was a major vehicle of culture in Europe in science and philosophy.
As a result, many European languages have borrowed many words from it. Arabic influence in vocabulary, is seen in European languages Spanish and to a lesser extent Portuguese, Catalan, owing to both the proximity of Christian European and Muslim Arab civilizations and 800 years of Arabic culture and language in the Iberian Peninsula, referred to in Arabic as al-Andalus. Sicilian has about 500 Arabic words as result of Sicily being progressively conquered by Arabs from North Africa, from the mid-9th to mid-10th centuries. Many of these words relate to related activities; the Balkan languages, including Greek and Bulgarian, have acquired a significant number of Arabic words through contact with Ottoman Turkish. Arabic has influenced many languages around the globe throughout its history; some of the most influenced languages are Persian, Spanish, Kashmiri, Bosnian, Bengali, Malay, Indonesian, Punjabi, Assamese, Sindhi and Hausa, some languages in parts of Africa. Conversely, Arabic has borrowed words from other languages, including Greek and Persian in medieval times, contemporary European languages such as English and French in modern times.
Classical Arabic is the liturgical language of 1.8 billion Muslims, Modern Standard Arabic is one of six official languages of the United Nations. All varieties of Arabic combined are spoken by as many as 422 million speakers in the Arab world, making it the fifth most spoken language in the world. Arabic is written with the Arabic alphabet, an abjad script and is written from right to left, although the spoken varieties are sometimes written in ASCII Latin from left to right with no standardized orthography. Arabic is a Central Semitic language related to the Northwest Semitic languages, the Ancient South Arabian languages, various other Semitic languages of Arabia such as Dadanitic; the Semitic languages changed a great deal between Proto-Semitic and the establishment of the Central Semitic languages in grammar. Innovations of the Central Semitic languages—all maintained in Arabic—include: The conversion of the suffix-conjugated stative formation into a past tense; the conversion of the prefix-conjugated preterite-tense formation into a present tense.
The elimination of other prefix-conjugated mood/aspect forms in favor of new moods formed by endings attached to the prefix-conjugation forms. The development of an internal passive. There are several features which Classical Arabic, the modern Arabic varieties, as well as the Safaitic and Hismaic inscriptions share which are unattested in any other Central Semitic language variety, including the Dadanitic and Taymanitic languages of the northern Hejaz; these features are evidence of common descent from Proto-Arabic. The following features can be reconstructed with confidence for Proto-Arabic: negative particles m *mā.