Editing Parsec (section)

== Usage and measurement ==
The parallax method is the fundamental calibration step for [[cosmic distance ladder|distance determination in astrophysics]]; however, the accuracy of ground-based [[telescope]] measurements of parallax angle is limited to about {{Val|0.01|u=arcsecond}}, and thus to stars no more than {{Val|100|u=pc}} distant.<ref>{{Cite web |title=Astronomy 162 |url=http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/distances.html |last=Pogge |first=Richard |publisher=Ohio State University}}</ref> This is because the Earth's atmosphere limits the sharpness of a star's image.{{cn|date=August 2022}} Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. Between 1989 and 1993, the ''[[Hipparcos]]'' satellite, launched by the [[European Space Agency]] (ESA), measured parallaxes for about {{Val|100000}} stars with an [[astrometry|astrometric]] precision of about {{Val|0.97|ul=mas}}, and obtained accurate measurements for stellar distances of stars up to {{Val|1000|u=pc}} away.<ref>{{Cite web |title=The Hipparcos Space Astrometry Mission |url=http://www.rssd.esa.int/index.php?project=HIPPARCOS |access-date=28 August 2007}}</ref><ref>{{Cite web |title=From Hipparchus to Hipparcos |url=http://wwwhip.obspm.fr/hipparcos/SandT/hip-SandT.html |last=Turon |first=Catherine}}</ref>

<!-- [[NASA]]'s [[Full-sky Astrometric Mapping Explorer|''FAME'' satellite]] was to have been launched in 2004, to measure parallaxes for about 40&nbsp;million stars with sufficient precision to measure stellar distances of up to 2000&nbsp;pc. However, the mission's funding was withdrawn by NASA in January 2002.<ref>[http://www.usno.navy.mil/FAME/news/ FAME news], 25 January 2002.</ref> -->
ESA's [[Gaia mission|''Gaia'' satellite]], which launched on 19 December 2013, is intended to measure one billion stellar distances to within {{Val|20|u=microarcsecond}}s, producing errors of 10% in measurements as far as the [[Galactic Center|Galactic Centre]], about {{Val|8000|u=pc}} away in the [[constellation]] of [[Sagittarius (constellation)|Sagittarius]].<ref>{{Cite web |title=GAIA |url=http://sci.esa.int/science-e/www/area/index.cfm?fareaid=26 |publisher=[[European Space Agency]]}}</ref>