Editing Parsec (section)

{{short description|Unit of length in astronomy}}
{{Other uses}}
{{Use dmy dates|date=May 2020}}
{{Infobox unit
| image    = Stellarparallax parsec1.svg
| image_size = 200px
| caption  = A parsec is the distance from the Sun to an [[astronomical object]] that has a [[parallax]] angle of one [[Minute and second of arc#Symbols and abbreviations|arcsecond]] (not to scale)
| standard = astronomical units
| quantity = [[length]]/[[distance]]
| symbol   = pc
| units1   = [[metric system|metric]] ([[International System of Units|SI]]) units
| inunits1 = {{convert|1|pc|m|disp=out|sigfig=5|lk=on}} <br />{{nbsp|3}}≈{{convert|1|pc|Pm|disp=out|sigfig=2|abbr=off|lk=on}}
| units2   = [[Imperial units|imperial]]&nbsp;&nbsp;[[United States customary units|US]]&nbsp;units
| inunits2 = {{convert|1|pc|mi|disp=out|sigfig=5|lk=on}}
| units3   = [[Astronomical system of units|astronomical units]]
| inunits3 = {{convert|1|pc|au|disp=out|sigfig=6|lk=on}}<br />{{nbsp|3}}{{convert|1|pc|ly|disp=out|sigfig=6|lk=on}}
}}

The '''parsec''' (symbol: '''pc''') is a [[unit of length]] used to measure the large distances to [[astronomical object]]s outside the [[Solar System]], approximately equal to {{convert|1|pc|ly|2|abbr=off|lk=out|disp=out}} or {{convert|1|pc|AU|0|abbr=off|lk=out|disp=out}} (AU), i.e. {{convert|30.9|e12km|e12mi|abbr=off|lk=on}}.{{efn|name=trillion|One trillion here is [[long and short scales|short scale]], ie. 10<sup>12</sup> (one million million, or billion in long scale).}} The parsec unit is obtained by the use of [[parallax]] and [[trigonometry]], and is defined as the distance at which 1 AU [[subtended angle|subtends]] an angle of one [[arcsecond]]<ref>{{Cite web |title=Cosmic Distance Scales – The Milky Way |url=https://heasarc.gsfc.nasa.gov/docs/cosmic/milkyway_info.html |access-date=24 September 2014}}</ref> ({{sfrac|3600}} of a [[degree (angle)|degree]]). The nearest star, [[Proxima Centauri]], is about {{convert|1.3|pc|ly|abbr=off}} from the [[Sun]]: from that distance, the gap between the Earth and the Sun spans slightly less than one arcsecond.<ref>{{Cite conference |last=Benedict |first=G.&nbsp;F. |display-authors=etal |title=Astrometric Stability and Precision of Fine Guidance Sensor #3: The Parallax and Proper Motion of Proxima Centauri | url = http://clyde.as.utexas.edu/SpAstNEW/Papers_in_pdf/%7BBen93%7DEarlyProx.pdf |pages=380–384 |access-date=11 July 2007 |book-title=Proceedings of the HST Calibration Workshop}}</ref> Most [[Naked-eye stars|stars visible to the naked eye]] are within a few hundred parsecs of the Sun, with the most distant at a few thousand parsecs, and the [[Andromeda Galaxy]] at over 700,000 parsecs.<ref>{{cite web |title=Farthest Stars |url=https://stardate.org/radio/program/2021-05-15 |website=[[StarDate]] |publisher=[[University of Texas at Austin]] |access-date=5 September 2021 |date=15 May 2021}}</ref>

The word ''parsec'' is a shortened form of ''a distance corresponding to a parallax of one second'', coined by the British astronomer [[Herbert Hall Turner]] in 1913.<ref name="dyson">{{Cite journal |last=Dyson |first=F.&nbsp;W. |author-link=Frank Watson Dyson |date=March 1913 |title= The distribution in space of the stars in Carrington's Circumpolar Catalogue |journal= [[Monthly Notices of the Royal Astronomical Society]] |volume=73 |issue=5 |page=342 <!-- the whole article is at pp.=334–345 but single page in the source that supports the content" has preference. Note that both OUP.com and Harvard.edu PDFs are truncated at p. 342 --> | bibcode=1913MNRAS..73..334D |doi=10.1093/mnras/73.5.334 |doi-access=free | quote= [''paragraph 14, page 342''] Taking the unit of distance R* to be that corresponding to a parallax of 1″·0 [… Footnote:] <br> * There is need for a name for this unit of distance. Mr. [[Carl Charlier|Charlier]] has suggested [[Sirius|Sirio]]meter, but if the violence to the Greek language can be overlooked, the word ''Astron'' might be adopted. Professor [[Herbert Hall Turner|Turner]] suggests ''Parsec'', which may be taken as an abbreviated form of "a distance corresponding to a parallax of one second".}}</ref> The unit was introduced to simplify the calculation of astronomical distances from raw observational data. Partly for this reason, it is the unit preferred in [[astronomy]] and [[astrophysics]], though in [[popular science]] texts and common usage the [[light-year]] remains prominent. Although parsecs are used for the shorter distances within the [[Milky Way]], multiples of parsecs are required for the larger scales in the universe, including [[kilo-|kilo]]<nowiki/>parsecs (kpc) for the more distant objects within and around the Milky Way, [[Mega-|mega]]<nowiki/>parsecs (Mpc) for mid-distance galaxies, and [[giga-|giga]]<nowiki/>parsecs (Gpc) for many [[quasar]]s and the most distant galaxies.

In August 2015, the [[International Astronomical Union]] (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent [[bolometric magnitude]] scale, mentioned an existing explicit definition of the parsec as exactly {{sfrac|{{Val|648000}}|{{pi}}}}&nbsp;au, or approximately {{Val|30856775814913673|}}<!-- if absurdly many digits are needed, let the full listing correspond to rounded meters -->&nbsp;metres, given the IAU 2012 exact definition of the astronomical unit in metres. This corresponds to the small-angle definition of the parsec found in many astronomical references.<ref>{{Cite book |title=Allen's Astrophysical Quantities |date=2000 |publisher=AIP Press / Springer |isbn=978-0387987460 |editor-last=Cox |editor-first=Arthur N. |edition=4th |location=New York |bibcode=2000asqu.book.....C}}</ref><ref>{{Cite book |last1=Binney |first1=James |title=Galactic Dynamics |last2=Tremaine |first2=Scott |date=2008 |publisher=Princeton University Press |isbn=978-0-691-13026-2 |edition=2nd |location=Princeton, NJ |bibcode=2008gady.book.....B}}</ref>