In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth with the aid of optical telescopes. This occurs because the pair either forms a binary star or is an optical double, a chance line-of-sight alignment of two stars at different distances from the observer. Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical. Multiple stars are studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars.
The following are three types of paired stars: Optical doubles are unrelated stars that appear close together through chance alignment with Earth. Visual binaries are gravitationally-bound stars. Non-visual binaries are stars whose binary status was deduced through more esoteric means, such as occultation, spectroscopy, or anomalies in proper motion. Improvements in telescopes can shift non-visual binaries into visual binaries, as happened with Polaris A in 2006, it is only the inability to telescopically observe two separate stars that distinguish non-visual and visual binaries. Mizar, in Ursa Major, was observed to be double by Benedetto Galileo; the identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis, in 1664, while the bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685. Since that time, the search has been carried out and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0.
At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36-inch telescope. The unrelated categories of optical doubles and true binaries are lumped together for historical and practical reasons; when Mizar was found to be a binary, it was quite difficult to determine whether a double star was a binary system or only an optical double. Improved telescopes and photography are the basic tools used to make the distinction. After it was determined to be a visual binary, Mizar's components were found to be spectroscopic binaries themselves. Observation of visual double stars by visual measurement will yield the separation, or angular distance, between the two component stars in the sky and the position angle; the position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°. These measurements are called measures. In the measures of a visual binary, the position angle will change progressively and the separation between the two stars will oscillate between maximum and minimum values.
Plotting the measures in the plane will produce an ellipse. This is the projection of the orbit of the two stars onto the celestial sphere. Although it is expected that the majority of catalogued visual doubles are visual binaries, orbits have been computed for only a few thousand of the over 100,000 known visual double stars. Confirmation of a visual double star as a binary star can be achieved by observing the relative motion of the components. If the motion is part of an orbit, or if the stars have similar radial velocities or the difference in their proper motions is small compared to their common proper motion, the pair is physical; when observed over a short period of time, the components of both optical doubles and long-period visual binaries will appear to be moving in straight lines. Some bright visual double stars have a Bayer designation. In this case, the components may be denoted by superscripts. An example of this is α Crucis, whose components are α2 Crucis. Since α1 Crucis is a spectroscopic binary, this is a multiple star.
Superscripts are used to distinguish more distant, physically unrelated, pairs of stars with the same Bayer designation, such as α1,2 Capricorni, ξ1,2 Centauri, ξ1,2 Sagittarii. These optical pairs are resolvable by the naked eye. Apart from these pairs, the components of a double star are denoted by the letters A and B appended to the designation, of whatever sort, of the double star. For example, the components of α Canis Majoris are α Canis Majoris A and α Canis Majoris B; the letters AB may be used together to designate the pair. In the case of multiple stars, the letters C, D, so on may be used to denote additional components in order of increasing separation from the brightest star, A. Visual doubles are designated by an abbreviation for the name of their discoverer followed by a catalogue number unique to that observer. For example, the pair α Centauri AB was discovered by Father Ri
The South Pole known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of Earth and lies on the opposite side of Earth from the North Pole. Situated on the continent of Antarctica, it is the site of the United States Amundsen–Scott South Pole Station, established in 1956 and has been permanently staffed since that year; the Geographic South Pole is distinct from the South Magnetic Pole, the position of, defined based on Earth's magnetic field. The South Pole is at the center of the Southern Hemisphere. For most purposes, the Geographic South Pole is defined as the southern point of the two points where Earth's axis of rotation intersects its surface. However, Earth's axis of rotation is subject to small "wobbles", so this definition is not adequate for precise work; the geographic coordinates of the South Pole are given as 90°S, since its longitude is geometrically undefined and irrelevant.
When a longitude is desired, it may be given as 0°. At the South Pole, all directions face north. For this reason, directions at the Pole are given relative to "grid north", which points northwards along the prime meridian. Along tight latitude circles, clockwise is east, counterclockwise is west, opposite to the North Pole; the Geographic South Pole is located on the continent of Antarctica. It sits atop a featureless, barren and icy plateau at an altitude of 2,835 metres above sea level, is located about 1,300 km from the nearest open sea at Bay of Whales; the ice is estimated to be about 2,700 metres thick at the Pole, so the land surface under the ice sheet is near sea level. The polar ice sheet is moving at a rate of 10 metres per year in a direction between 37° and 40° west of grid north, down towards the Weddell Sea. Therefore, the position of the station and other artificial features relative to the geographic pole shift over time; the Geographic South Pole is marked by a stake in the ice alongside a small sign.
The sign records the respective dates that Roald Amundsen and Robert F. Scott reached the Pole, followed by a short quotation from each man, gives the elevation as "9,301 FT.". A new marker stake is fabricated each year by staff at the site; the Ceremonial South Pole is an area set aside for photo opportunities at the South Pole Station. It is located some meters from the Geographic South Pole, consists of a metallic sphere on a short bamboo pole, surrounded by the flags of the original Antarctic Treaty signatory states. Amundsen's Tent: The tent was erected by the Norwegian expedition led by Roald Amundsen on its arrival on 14 December 1911, it is buried beneath the snow and ice in the vicinity of the Pole. It has been designated a Historic Site or Monument, following a proposal by Norway to the Antarctic Treaty Consultative Meeting; the precise location of the tent is unknown, but based on calculations of the rate of movement of the ice and the accumulation of snow, it is believed, as of 2010, to lie between 1.8 and 2.5 km from the Pole at a depth of 17 m below the present surface.
Argentine Flagpole: A flagpole erected at the South Geographical Pole in December 1965 by the First Argentine Overland Polar Expedition has been designated a Historic Site or Monument following a proposal by Argentina to the Antarctic Treaty Consultative Meeting. In 1820, several expeditions claimed to have been the first to have sighted Antarctica, with the first being the Russian expedition led by Fabian Gottlieb von Bellingshausen and Mikhail Lazarev; the first landing was just over a year when American Captain John Davis, a sealer, set foot on the ice. The basic geography of the Antarctic coastline was not understood until the mid-to-late 19th century. American naval officer Charles Wilkes claimed that Antarctica was a new continent, basing the claim on his exploration in 1839–40, while James Clark Ross, in his expedition of 1839–43, hoped that he might be able to sail all the way to the South Pole. British explorer Robert Falcon Scott on the Discovery Expedition of 1901–04 was the first to attempt to find a route from the Antarctic coastline to the South Pole.
Scott, accompanied by Ernest Shackleton and Edward Wilson, set out with the aim of travelling as far south as possible, on 31 December 1902, reached 82°16′ S. Shackleton returned to Antarctica as leader of the British Antarctic Expedition in a bid to reach the Pole. On 9 January 1909, with three companions, he reached 88°23' S – 112 miles from the Pole – before being forced to turn back; the first men to reach the Geographic South Pole were the Norwegian Roald Amundsen and his party on December 14, 1911. Amundsen named his camp Polheim and the entire plateau surrounding the Pole King Haakon VII Vidde in honour of King Haakon VII of Norway. Robert Falcon Scott returned to Antarctica with his second expedition, the Terra Nova Expedition unaware of Amundsen's secretive expedition. Scott and four other men reached the South Pole on January 17, 1912, thirty-four days after Amundsen. On the return trip and his four companions all died of starvation and extreme cold. In 1914 Ernest Shackleton's Imperial Trans-Antarctic Expedition set out with the goal of crossing Antarctica via the South Pole, but his ship, the Endurance, was frozen in pack ice and sank 1
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are recognized by the pair of grasping pedipalps and the narrow, segmented tail carried in a characteristic forward curve over the back, ending with a venomous stinger. Scorpions range in size from 9 mm / 0.3 in. to 23 cm / 9 in.. The evolutionary history of scorpions goes back to the Silurian period 430 million years ago, they have adapted to a wide range of environmental conditions, they can now be found on all continents except Antarctica. Scorpions number about 1,750 described species, with 13 extant families recognised to date; the taxonomy has undergone changes and is to change further, as genetic studies are bringing forth new information. All scorpions have a venomous sting, but the vast majority of the species do not represent a serious threat to humans, in most cases, healthy adults do not need any medical treatment after being stung. Only about 25 species are known to have venom capable of killing a human.
In some parts of the world with venomous species, human fatalities occur in areas with limited access to medical treatment. The word scorpion is thought to have originated in Middle English between 1175 and 1225 AD from Old French scorpion, or from Italian scorpione, both derived from the Latin scorpius, the romanization of the Greek word σκορπίος – skorpíos. Scorpions are found on all major land masses except Antarctica. Scorpions did not occur in Great Britain, Japan, South Korea, New Zealand and some of the islands in Oceania, but now have been accidentally introduced in some of these places by human trade and commerce; the greatest diversity of scorpions in the Northern Hemisphere is to be found in regions between the latitudes 23° N and 38° N. Above these latitudes, the diversity decreases with the northernmost natural occurrence of scorpions being the northern scorpion Paruroctonus boreus at Medicine Hat, Canada 50° N. Five colonies of scorpions have established themselves in Sheerness on the Isle of Sheppey in the United Kingdom.
This small population has been resident since the 1860s, having arrived with imported fruit from Africa. This scorpion species is small and harmless to humans. At just over 51 ° N, this marks the northernmost limit. Today, scorpions are found in every terrestrial habitat including: high-elevation mountains and intertidal zones, with the exception of boreal ecosystems such as: the tundra, high-altitude taiga, the permanently snow-clad tops of some mountains; as regards microhabitats, scorpions may be tree-living, rock-loving or sand-loving. Some species, such as Vaejovis janssi, are versatile and are found in every type of habitat in Baja California, while others occupy specialized niches such as Euscorpius carpathicus, endemic to the littoral zone of rivers in Romania. Thirteen families and about 1,750 described species and subspecies of scorpions are known. In addition, 111 described; this classification is based on that of Soleglad and Fet, which replaced the older, unpublished classification of Stockwell.
Additional taxonomic changes are from papers by Soleglad et al.. This classification covers extant taxa to the rank of family: Order ScorpionesInfraorder Orthosterni Pocock, 1911 Parvorder Pseudochactida Soleglad et Fet, 2003 Superfamily Pseudochactoidea Gromov, 1998 Family Pseudochactidae Gromov, 1998 Parvorder Buthida Soleglad et Fet, 2003 Superfamily Buthoidea C. L. Koch, 1837 Family Buthidae C. L. Koch, 1837 Family Microcharmidae Lourenço, 1996 Parvorder Chaerilida Soleglad et Fet, 2003 Superfamily Chaeriloidea Pocock, 1893 Family Chaerilidae Pocock, 1893 Parvorder Iurida Soleglad et Fet, 2003 Superfamily Chactoidea Pocock, 1893 Family Chactidae Pocock, 1893 Family Euscorpiidae Laurie, 1896 Family Superstitioniidae Stahnke, 1940 Family Vaejovidae Thorell, 1876 Superfamily Iuroidea Thorell, 1876 Family Caraboctonidae Kraepelin, 1905 Family Iuridae Thorell, 1876 Superfamily Scorpionoidea Latreille, 1802 Family Bothriuridae Simon, 1880 Family Hemiscorpiidae Pocock, 1893 Family Scorpionidae Latreille, 1802 Scorpions have been found in many fossil records, including marine Silurian and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber.
The oldest known scorpions lived around 430 million years ago in the Silurian period. Though once believed to have lived on the bottom of shallow tropical seas, early scorpions are now believed to have been terrestrial and to have washed into marine settings together with plant matter; these first scorpions were believed to have had gills instead of the present forms' book lungs, though this has subsequently been refuted. The oldest Gondwanan scorpions comprise the earliest known terrestrial animals from Gondwana. 111 fossil species of scorpion are known. Unusually for arachnids, there are more species of Palaeozoic scorpion than Mesozoic or Cenozoic ones. Fossil of ancestral scorpions had compound eyes, but as they adapted to a nocturnal lifestyle, their eyes became simplified; the eurypterids called "sea scorpions", were aquatic creatures that lived during the Palaeozoic era that share several physical traits with scorpions and may be related to them. Various species of Eurypterida could grow to be anywhere from 10 centimetres to 2.5 metres in length.
However, they exhibit anatomical differences marking them off as a group distinct from t
Hook (hand tool)
A hook is a hand tool used for securing and moving loads. It consists of a round wooden handle with a strong metal hook about 8" long projecting at a right angle from the center of the handle; the appliance is held in a closed fist with the hook projecting between two fingers. This type of hook is used in many different industries, has many different names, it may be called a box hook, cargo hook, loading hook, docker's hook when used by longshoremen, a baling hook, bale hook, or hay hook in the agricultural industry. Other variants exist, such as in forestry, for moving logs, a type with a long shaft, used by city workers to remove manhole covers. Smaller hooks may be used in food processing and transport; the longshoreman's hook was used by longshoremen. Before the age of containerization, freight was moved on and off ships with extensive manual labor, the longshoreman's hook was the basic tool of the dockworker; the hook became an emblem of the longshoreman's profession in the same way that a hammer and anvil are associated with blacksmiths, or the pipe wrench with plumbers.
When longshoremen went on strike or retired, it was known as "hanging up the hook" or "slinging the hook", the newsletter for retired members of the International Longshore and Warehouse Union's Seattle Local is called The Rusty Hook. A longshoreman's hook was carried by hooking it through the belt; some cargo items are liable to be damaged if pulled at with a longshoreman’s hook: hence the "Use No Hooks" warning sign. A longshoreman's hook looks somewhat intimidating, as it was associated with strong tough dockworkers, it became a used weapon in crime fiction, similar to the ice pick. For example, in an episode of Alfred Hitchcock Presents entitled Shopping for Death, a character is murdered using a longshoreman's hook, it was sometimes used as a means of intimidation in real life as well. If he didn't like you he would pick you up with his hook." In the 1957 New York drama film Edge of the City, two longshoremen settle their dispute in a deadly baling hook fight. A hay hook is different in design from a longshoreman's hook, in that the shaft is longer.
It is used in hay bucking on farms to secure and move bales of hay, which are otherwise awkward to pick up manually. In gardening and agriculture, a variant with a long shaft is used to move large plants. A hook is placed in either side of the baled roots, allowing workers to carry or place the heavy load. Called a "Packhaken", "Hebehaken", or "Forsthaken" in German, this type is used in forestry to lift or move firewood. In Sweden, this tool, though different, is called a "timmerkrok", which translates as "timberhook", it is used by two people to move logs by hooking them in each end. Fishing gaff Prosthetic hook Pickaroon Cant hook Media related to Hooks at Wikimedia Commons Smithsonian Institution exhibit on the mechanization of the cargo shipping industry. Images of longshoreman's hooks
The Southern Hemisphere is the half of Earth, south of the Equator. It contains parts of five continents, four oceans and most of the Pacific Islands in Oceania, its surface is 80.9% water, compared with 60.7% water in the case of the Northern Hemisphere, it contains 32.7% of Earth's land. Owing to the tilt of Earth's rotation relative to the Sun and the ecliptic plane, summer is from December to March and winter is from June to September. September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox; the South Pole is in the center of the southern hemispherical region. Southern Hemisphere climates tend to be milder than those at similar latitudes in the Northern Hemisphere, except in the Antarctic, colder than the Arctic; this is because the Southern Hemisphere has more ocean and much less land. The differences are attributed to oceanic heat transfer and differing extents of greenhouse trapping. In the Southern Hemisphere the sun passes from east to west through the north, although north of the Tropic of Capricorn the mean sun can be directly overhead or due north at midday.
The Sun rotating through the north causes an apparent right-left trajectory through the sky unlike the left-right motion of the Sun when seen from the Northern Hemisphere as it passes through the southern sky. Sun-cast shadows turn anticlockwise throughout the day and sundials have the hours increasing in the anticlockwise direction. During solar eclipses viewed from a point to the south of the Tropic of Capricorn, the Moon moves from left to right on the disc of the Sun, while viewed from a point to the north of the Tropic of Cancer, the Moon moves from right to left during solar eclipses. Cyclones and tropical storms spin clockwise in the Southern Hemisphere due to the Coriolis effect; the southern temperate zone, a subsection of the Southern Hemisphere, is nearly all oceanic. This zone includes the southern tip of South Africa; the Sagittarius constellation that includes the galactic centre is a southern constellation and this, combined with clearer skies, makes for excellent viewing of the night sky from the Southern Hemisphere with brighter and more numerous stars.
Forests in the Southern Hemisphere have special features which set them apart from those in the Northern Hemisphere. Both Chile and Australia share, for example, unique beech species or Nothofagus, New Zealand has members of the related genera Lophozonia and Fuscospora; the eucalyptus is native to Australia but is now planted in Southern Africa and Latin America for pulp production and biofuel uses. 800 million humans live in the Southern Hemisphere, representing only 10–12% of the total global human population of 7.3 billion. Of those 800 million people, 200 million live in Brazil, the largest country by land area in the Southern Hemisphere, while 141 million live on the island of Java, the most populous island in the world; the most populous nation in the Southern Hemisphere is Indonesia, with 261 million people. Portuguese is the most spoken language in the Southern Hemisphere, followed by Javanese; the largest metropolitan areas in the Southern Hemisphere are São Paulo, Buenos Aires, Rio de Janeiro and Sydney.
The most important financial and commercial centers in the Southern Hemisphere are São Paulo, where the Bovespa Index is headquartered, along with Sydney, home to the Australian Securities Exchange, home to the Johannesburg Stock Exchange and Buenos Aires, headquarters of the Buenos Aires Stock Exchange, the oldest stock market in the Southern Hemisphere. Among the most developed nations in the Southern Hemisphere are Australia, with a nominal GDP per capita of US$51,850 and a Human Development Index of 0.939, the second highest in the world as of 2016. New Zealand is well developed, with a nominal GDP per capita of US$38,385 and a Human Development Index of 0.915, putting it at #13 in the world in 2016. The least developed nations in the Southern Hemisphere cluster in Africa and Oceania, with Burundi and Mozambique at the lowest ends of the Human Development Index, at 0.404 and 0.418 respectively. The nominal GDP per capitas of these two countries don't go above US$550 per capita, a tiny fraction of the incomes enjoyed by Australians and New Zealanders.
The most widespread religions in the Southern Hemisphere are Christianity in South America, southern Africa and Australia/New Zealand, followed by Islam in most of the islands of Indonesia and in parts of southeastern Africa, Hinduism, concentrated on the island of Bali and neighboring islands. The oldest continuously inhabited city in the Southern Hemisphere is Bogor, in western Java, founded in 669 CE. Ancient texts from the Hindu kingdoms prevalent in the area definitively record 669 CE as the year when Bogor was founded. However, there is some evidence that Zanzibar, an ancient port with around 200,000 inhabitants on
Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point above the earth in question. When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system. An old term, right ascension refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle, it contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle. Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction on an equator. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is located in the constellation Pisces.
Right ascension is measured continuously in a full circle from that alignment of Earth and Sun in space, that equinox, the measurement increasing towards the east. As seen from Earth, objects noted to have 12h RA are longest visible at the March equinox. On those dates at midnight, such objects will reach their highest point. How high depends on their declination. Any units of angular measure could have been chosen for right ascension, but it is customarily measured in hours and seconds, with 24h being equivalent to a full circle. Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the Earth rotates; the line which passes through the highest point in the sky, called the meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360°, 1/24 of a circle is measured as 1h of right ascension, or 15°. A full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86400s, or 24 × 60 = 1440m, or 24h.
Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 1h 30m 00s is at its meridian a star with RA = 20h 00m 00s will be on the/at its meridian 18.5 sidereal hours later. Sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Measured in degrees, it is the complement of right ascension with respect to 24h, it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earth's axis rotates westward about the poles of the ecliptic, completing one cycle in about 26,000 years; this movement, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, astronomers specify them with reference to a particular year, known as an epoch.
Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. Right ascension for "fixed stars" near the ecliptic and equator increases by about 3.05 seconds per year on average, or 5.1 minutes per century, but for fixed stars further from the ecliptic the rate of change can be anything from negative infinity to positive infinity. The right ascension of Polaris is increasing quickly; the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. The used standard epoch is J2000.0, January 1, 2000 at 12:00 TT. The prefix "J" indicates. Prior to J2000.0, astronomers used the successive Besselian epochs B1875.0, B1900.0, B1950.0. The concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC, but Hipparchus and his successors made their star catalogs in ecliptic coordinates, the use of RA was limited to special cases.
With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do, to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive is used with an equatorial mount to cancel out the Earth's rotation; as the equatorial mount became adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could be pointed at objects with known right ascension and declination by the use of setting circles; the first star catalog to use right ascen
Libra is a constellation of the zodiac. Its name is Latin for weighing scales, its symbol is, it is faint, with no first magnitude stars, lies between Virgo to the west and Scorpius to the east. Beta Librae known as Zubeneschamali, is the brightest star in the constellation. Three star systems are known to have planets. Libra was known in Babylonian astronomy as MUL Zibanu, or alternatively as the Claws of the Scorpion; the scales were held sacred to the sun god Shamash, the patron of truth and justice. It was seen as the Scorpion's Claws in ancient Greece. Since these times, Libra has been associated with law and civility. In Arabic zubānā means "scorpion's claws", similarly in other Semitic languages: this resemblance of words may be why the Scorpion's claws became the Scales, it has been suggested that the scales are an allusion to the fact that when the sun entered this part of the ecliptic at the autumnal equinox, the days and nights are equal. Libra's status as the location of the equinox earned the equinox the name "First Point of Libra", though this location ceased to coincide with the constellation in 730 because of the precession of the equinoxes.
In ancient Egypt the three brightest stars of Libra formed a constellation, viewed as a boat. Libra is a constellation not mentioned by Aratus. Libra is mentioned by Manetho and Geminus, included by Ptolemy in his 48 asterisms. Ptolemy catalogued 17 stars, Tycho Brahe 10, Johannes Hevelius 20, it only became a constellation in ancient Rome, when it began to represent the scales held by Astraea, the goddess of justice, associated with Virgo in the Greek mythology. Libra is bordered by the head of Serpens to the north, Virgo to the northwest, Hydra to the southwest, the corner of Centaurus to the southwest, Lupus to the south, Scorpius to the east and Ophiuchus to the northeast. Covering 538.1 square degrees and 1.304% of the night sky, it ranks 29th of the 88 constellations in size. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is'Lib'; the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 12 segments.
In the equatorial coordinate system, the right ascension coordinates of these borders lie between 14h 22m 08.08s and 16h 02m 17.23s, while the declination coordinates are between −0.47° and −30.00°. The whole constellation is visible to observers south of latitude 60°N. Overall, there are 83 stars within the constellation's borders brighter than or equal to apparent magnitude 6.5. The brightest stars in Libra form a quadrangle. Traditionally and Beta Librae are considered to represent the scales' balance beam, while Gamma and Sigma are the weighing pans. Alpha Librae, called Zubenelgenubi, is a multiple star system divisible into two stars when seen through binoculars, The primary is a blue-white star of magnitude 2.7 and the secondary is a white star of magnitude 5.2 and spectral type F3V, 74.9 ± 0.7 light-years from Earth. Its traditional name means "the southern claw". Zubeneschamali is the corresponding "northern claw" to Zubenelgenubi; the brightest star in Libra, it is a green-tinged star of 160 light-years from Earth.
Gamma Librae is called Zubenelakrab, which means "the scorpion's claw", completing the suite of names referring to Libra's archaic status. It is an orange giant of 152 light-years from Earth. Iota Librae is a complex multiple star, 377 light-years from Earth, with both optical and true binary components; the primary appears as a blue-white star of magnitude 4.5. The secondary, visible in small telescopes as a star of magnitude 9.4, is a binary with two components, magnitudes 10 and 11. There is an optical companion to Iota Librae. Mu Librae is a binary star divisible in medium-aperture amateur telescopes, 235 light-years from Earth; the primary is of magnitude 5.7 and the secondary is of magnitude 6.8. Delta Librae is an Algol-type eclipsing 304 lightyears from Earth, it has a period of 8 hours. FX Librae, designated 48 Librae, is a shell star of magnitude 4.9. Shell stars, like Pleione and Gamma Cassiopeiae, are blue supergiants with irregular variations caused by their abnormally high speed of rotation.
This ejects gas from the star's equator. Sigma Librae was known as Gamma Scorpii despite being well inside the boundaries of Libra, it was not redesignated as Sigma Librae until 1851 by Benjamin A. Gould. Libra is home to the Gliese 581 planetary system, which consists of the star Gliese 581, three confirmed planets, two unconfirmed planets. Both Gliese 581d, Gliese 581g are debatably the most promising candidates for life, although Gliese 581g's existences has been disputed and has not been confirmed or agreed on in the scientific community. Gliese 581c is considered to be the first Earth-like extrasolar planet to be found within its parent star's habitable zone. Gliese 581e is the smallest mass exoplanet orbiting a normal star found to date All of these exoplanets are of significance for establishing the likelihood of life outside of the Solar System; the family of candidate habitable planets was extended in late September 2010 to include exoplanets around red dwarf stars because of Gliese 581g, a tidally lock