Editing Libra (constellation)

{{Short description|Zodiac constellation in the southern celestial hemisphere}}
{{About|the astronomical constellation|the astrological sign|Libra (astrology)}}
{{Infobox constellation
| name = Libra
| abbreviation = Lib
| genitive = Librae
| pronounce = {{IPAc-en|ˈ|l|iː|b|r|ə}}, genitive {{IPAc-en|ˈ|l|iː|b|r|iː}} 
| symbolism = the [[weighing scale|balance]]
| RA = {{RA|15}}
| dec= {{DEC|−15}}
| family = [[Zodiac]]
| quadrant = SQ3
| areatotal = 538
| arearank = 29th
| numbermainstars = 4, 6
| numberbfstars = 56
| numberstarsplanets = 4
| numberbrightstars = 2
| numbernearbystars = 5
| brighteststarname = [[Beta Librae]] (β Lib)
| starmagnitude = 2.61
| neareststarname = [[Gliese 570]]
| stardistancely = 19.20
| stardistancepc = 5.89
| numbermessierobjects = 0
| meteorshowers = May Librids
| bordering = [[Serpens|Serpens Caput]]<br />[[Virgo (constellation)|Virgo]]<br />[[Hydra (constellation)|Hydra]]<br />[[Centaurus]] (corner)<br />[[Lupus (constellation)|Lupus]]<br />[[Scorpius]]<br />[[Ophiuchus]]
| latmax = [[65th parallel north|65]]
| latmin = [[South Pole|90]]
| month = June
| notes=}}
'''Libra''' {{IPAc-en|ˈ|l|iː|b|r|ə}} is a [[constellation]] of the [[zodiac]] and is located in the [[Southern celestial hemisphere]]. Its name is [[Latin]] for [[weighing scales]]. Its old astronomical symbol is [[Image:Libra symbol (fixed width).svg|20px]] (♎︎).  It is fairly faint, with no [[apparent magnitude|first magnitude]] [[star]]s, and lies between [[Virgo (constellation)|Virgo]] to the west and [[Scorpius]] to the east. [[Beta Librae]], also known as Zubeneschamali, is the brightest star in the constellation. Three star systems are known to have planets.

==Features==
===Stars===
{{See also|List of stars in Libra}}
[[Image:LibraCC.jpg|thumb|left|250px|The constellation Libra marked on a naked eye view.]]
Overall, there are 83 stars within the constellation's borders brighter than or equal to [[apparent magnitude]] 6.5.{{efn|1=Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.<ref>{{cite web|url=http://www.skyandtelescope.com/resources/darksky/3304011.html?page=1&c=y|title=The Bortle Dark-Sky Scale|last=Bortle|first=John E.|date=February 2001|work=[[Sky & Telescope]]|publisher=Sky Publishing Corporation|access-date=3 April 2015|archive-date=31 March 2014|archive-url=https://web.archive.org/web/20140331202746/http://www.skyandtelescope.com/resources/darksky/3304011.html?page=1&c=y|url-status=dead}}</ref>}}<ref name=tirionconst/>

The brightest stars in Libra form a [[quadrilateral|quadrangle]] that distinguishes it for the unaided observer. Traditionally, Alpha 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 (Alpha<sup>2</sup> Librae) is a blue-white star of magnitude 2.7 and the secondary (Alpha<sup>1</sup> Librae) is a white star of magnitude 5.2 and spectral type F3V that is 74.9 ± 0.7 light-years from Earth.  Its traditional name means "the southern claw". Zubeneschamali ([[Beta Librae]]) is the corresponding "northern claw" to Zubenelgenubi. The brightest star in Libra, Zubeneschamali is a green-tinged star of magnitude 2.6, 160 (or 185 ± 2) 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 magnitude 3.9, 152 light-years from Earth.{{sfn|Ridpath|Tirion|2001|pp=172-173}}

[[Iota Librae]] is a complex multiple star, 377 light-years from Earth, with both optical and true binary components in it. The primary appears as a blue-white star of magnitude 4.5; it is a binary star indivisible in even the largest amateur instruments with a period of 23 years. 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; [[25 Librae]] is a star of magnitude 6.1, 219 light-years from Earth and visible in binoculars. [[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.{{sfn|Ridpath|Tirion|2001|pp=172-173}}

[[Delta Librae]] is an [[Algol variable|Algol-type]] [[eclipsing variable]] star, 304 lightyears from Earth. It has a period of 2 days, 8 hours; its minimum magnitude of 5.9 and its maximum magnitude is 4.9. [[FX Librae]], designated [[48 Librae]], is a [[shell star]] of magnitude 4.9. Shell stars, like [[Pleione (star)|Pleione]] and [[Gamma Cassiopeiae]], are [[blue supergiant]]s with irregular variations caused by their abnormally high speed of rotation. This ejects gas from the star's equator.{{sfn|Ridpath|Tirion|2001|pp=172-173}}

[[Sigma Librae]] (the proper name is Brachium<ref name="IAU-LSN">{{cite web | url=https://www.iau.org/public/themes/naming_stars/ | title=Naming Stars |publisher=IAU.org |access-date=8 August 2018}}</ref>) was formerly 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]].

==History and mythology==
[[File:Sidney Hall - Urania's Mirror - Libra.jpg|thumb|left|250px|Libra as depicted in ''[[Urania's Mirror]]'', a set of constellation cards published in London c.1825]]

Libra was known in [[Babylonian astronomy]] as ''MUL Zibanu'' (the "scales" or "balance"), or alternatively as the Claws of the Scorpion. The scales were held sacred to the sun god [[Shamash]], who was also the patron of truth and justice.<ref>''Babylonian Star-lore'' by Gavin White, Solaria Pubs, 2008, page 175</ref> Since these times, Libra has been associated with law, fairness and civility.

It was also seen as the Scorpion's Claws in [[ancient Greece]].{{sfn|Ridpath|Tirion|2001|pp=172-173}} This is the name used repeatedly by Aratus, which made it widespread in Hellenistic times: "few conspicuous stars the Claws can boast,
and their dim light mid brighter gems is lost"<ref>Aratus, ''The Phenomena and Diosemeia of Aratus by Aratus'', translated by John Lamb Phenomena,
 [https://en.wikisource.org/wiki/The_Phenomena_and_Diosemeia_of_Aratus/Phenomena 633]</ref> In Arabic ''zubānā'' means "scorpion's claws", and likely similarly in other Semitic languages: this resemblance of words may be why the Scorpion's claws became the Scales.{{Citation needed|date=November 2011}} Indeed, Zubenelgenubi and Zubeneschamali, the names of the constellation's two main stars, in Arabic mean "southern claw" and "northern claw" respectively. It has also been suggested that the scales are an allusion to the fact that when the sun entered this part of the [[ecliptic]] at the [[September equinox|autumnal equinox]], the days and nights are equal.<ref name="eb" /> 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 BC because of the [[precession of the equinoxes]].

In ancient Egypt the three brightest stars of Libra (α, β, and σ Librae) formed a constellation that was viewed as a boat.<ref>{{cite web|last1=Priskin|first1=Gyula|title=The Astral Myth of Osiris: the Decans of Taurus and Libra|url=http://www.enim-egyptologie.fr/index.php?page=enim-9&n=7|website=ENiM 9 (2016), 79-111.|access-date=22 September 2016}}</ref> Libra is not mentioned by [[Eudoxus of Cnidus|Eudoxus]] or [[Aratus]] as a constellation.<ref name=eb/> Libra is mentioned by [[Manetho]] (3rd century B.C.) and [[Geminus]] (1st century B.C.), and included by [[Ptolemy]] in his 48 [[Asterism (astronomy)|asterism]]s. Ptolemy catalogued 17 stars, [[Tycho Brahe]] 10, and [[Johannes Hevelius]] 20.<ref name=eb>{{EB1911|wstitle=Libra}}</ref> It only became a constellation in [[ancient Rome]], when it began to represent the scales held by [[Astraea (mythology)|Astraea]], the goddess of justice, associated with Virgo in the Greek mythology.{{sfn|Ridpath|Tirion|2001|pp=172-173}}

==The constellation==
[[File:Libra constellation map.svg|thumb|Libra constellation map]]
[[File:NGC 5897GCL01a.jpg|right|thumb|NGC 5897]]
[[File:NGC5885 - SDSS DR14.jpg|right|thumb|NGC 5885]]
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&nbsp;square degrees and 1.304% of the night sky, it ranks 29th of the 88 constellations in size.<ref name=tirionconst>{{cite web| url=http://www.ianridpath.com/constellations2.html | title=Constellations: Lacerta–Vulpecula | work= Star Tales |author=Ridpath, Ian|publisher=Self-published | access-date= 3 April 2016| author-link=Ian Ridpath }}</ref> The three-letter abbreviation for the constellation, as adopted by the [[International Astronomical Union]] in 1922, is "Lib".<ref name="pa30_469">{{cite journal | last=Russell | first=Henry Norris |author-link=Henry Norris Russell | title=The New International Symbols for the Constellations | journal=[[Popular Astronomy (US magazine)|Popular Astronomy]] | volume=30 | page=469 | bibcode=1922PA.....30..469R | date=1922}}</ref> The official constellation boundaries, as set by [[Eugène Joseph Delporte|Eugène Delporte]] in 1930, are defined by a polygon of 12 segments (''illustrated in infobox''). In the [[equatorial coordinate system]], the [[right ascension]] coordinates of these borders lie between {{RA|14|22|08.08}} and {{RA|16|02|17.23}}, while the [[declination]] coordinates are between −0.47° and −30.00°.<ref name="boundary">{{Cite journal | title=Libra, Constellation Boundary | journal=The Constellations | publisher=[[International Astronomical Union]] | url=https://www.iau.org/public/themes/constellations/#lib| access-date=3 April 2016}}</ref> The whole constellation is visible to observers south of latitude 60°N.{{efn|1=While parts of the constellation technically rise above the horizon to observers between 60°N and 89°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.<ref name=tirionconst/>}}

===Planetary systems===
Libra is home to the [[Gliese 581]] planetary system, which consists of the star Gliese 581 and three confirmed planets. This system gained attention in the late 2000s and early 2010s as the subject of some of the earliest claims of potentially habitable exoplanets,<ref>{{cite web|url=http://www.space.com/scienceastronomy/070424_hab_exoplanet.html|title=Major Discovery: New Planet Could Harbor Water and Life|website=space.com|date=24 April 2007 }}</ref><ref>{{Citation |last=Shiga |first=David |url=https://www.newscientist.com/article/dn19519-found-first-rocky-exoplanet-that-could-host-life.html |title=Found: first rocky exoplanet that could host life |publisher=[[New Scientist]]|date=September 29, 2010|access-date=September 30, 2010}}</ref> but it is now known that [[Gliese 581c]] is too hot to be potentially habitable, and the planet candidates [[Gliese 581d]] and [[Gliese 581g|g]] likely do not exist.<ref name="Robertson2014">{{cite journal |last1=Robertson |first1=Paul |author2-link=Suvrath Mahadevan |last2=Mahadevan |first2=Suvrath |last3=Endl |first3=Michael |last4=Roy |first4=Arpita |title=Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581 |journal=[[Science (journal)|Science]] |date=3 July 2014 |doi=10.1126/science.1253253 |pmid=24993348 |arxiv=1407.1049 |bibcode=2014Sci...345..440R |volume=345 |issue=6195 |pages=440–444 |citeseerx=10.1.1.767.2071 |s2cid=206556796 }}</ref> At the time of its discovery in 2009, [[Gliese 581e]] was the smallest mass [[exoplanet]] known orbiting a normal star.<ref name="mayor">{{cite journal |doi=10.1051/0004-6361/200912172 |bibcode=2009A&A...507..487M |journal=[[Astronomy and Astrophysics]] |volume=507 |issue=1 |year=2009 |pages=487–494 |last1=Mayor |first1=Michel |last2=Bonfils |first2=Xavier |last3=Forveille |first3=Thierry |last4=Delfosse |first4=Xavier |last5=Udry |first5=Stéphane |last6=Bertaux |first6=Jean-Loup |last7=Beust |first7=Hervé |last8=Bouchy |first8=François |last9=Lovis |first9=Christophe |last10=Pepe |first10=Francesco |last11=Perrier |first11=Christian |last12=Queloz |first12=Didier |last13=Santos |first13=Nuno C. |title=The HARPS search for southern extra-solar planets, XVIII. An Earth-mass planet in the GJ 581 planetary system |arxiv=0906.2780 |display-authors=3 |url=http://obswww.unige.ch/~udry/Gl581_preprint.pdf |s2cid=2983930 |url-status=dead |archive-url=https://web.archive.org/web/20090521052641/http://obswww.unige.ch/~udry/Gl581_preprint.pdf |archive-date=21 May 2009 }}</ref>

===Deep-sky objects===
Libra is home to one bright [[globular cluster]], [[NGC 5897]]. It is a loose cluster, 50,000 light-years from Earth; it is fairly large and has an integrated magnitude of 9.{{sfn|Ridpath|Tirion|2001|pp=172-173}} [[IC 1059]] is a [[galaxy]] in the constellation Libra.<ref>[http://cseligman.com/text/atlas/ic10a.htm#ic1059 csligman.com]</ref>

==Astrology==
{{Main|Libra (astrology)}}
{{As of|2002}}, the Sun appears in the constellation Libra from October 31 to November 22. In [[tropical astrology]], the Sun is considered to be in the sign [[Libra (astrology)|Libra]] from the northern autumnal equinox (c. September 23) to on or about October 23, and in [[sidereal astrology]], from October 16 to November 15.

==Namesakes==
* [[USS Libra (AKA-12)|''Libra'' (AKA-12)]] was a [[United States]] navy ship named after the constellation.
* [[Tropical Storm Tembin]] - Four tropical cyclones in the western Pacific have been given its Japanese name.

==See also==
* [[Constellation family]]
* [[Former constellations]]
* [[Libra (Chinese astronomy)]]
* [[Lists of stars by constellation]]

==Notes==
{{Notelist}}

==References==
{{Reflist}}

== Sources ==
* {{citation |title = Stars and Planets Guide |last1 = Ridpath |first1 = Ian |last2 = Tirion |first2 = Wil |date = 2001 |publisher = Princeton University Press |isbn = 0-691-08913-2}}
* Ian Ridpath and Wil Tirion (2007). ''Stars and Planets Guide'', Collins, London. {{ISBN|978-0-00-725120-9}}. Princeton University Press, Princeton. {{ISBN|978-0-691-13556-4}}.

== External links ==
{{Commons and category|Libra|Libra (constellation)}}
* [http://www.allthesky.com/constellations/libra/ The Deep Photographic Guide to the Constellations: Libra]
* [http://wikisky.org/?object=Libra&zoom=2 Libra constellation]
* [http://www.ianridpath.com/startales/libra.html Ian Ridpath's Star Tales – Libra]
* [https://iconographic.warburg.sas.ac.uk/category/vpc-taxonomy-017065 Warburg Institute Iconographic Database (medieval and early modern images of Libra)]

{{Stars of Libra}}
{{Constellations}}
{{Zodiac}}
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[[Category:Libra (constellation)| ]]
[[Category:Constellations]]
[[Category:Southern constellations]]
[[Category:Constellations listed by Ptolemy]]