Editing Stellar parallax (section)

=== 19th and 20th centuries ===
[[File:Koenigsberg helio.jpg|thumb|right|Bessel's heliometer]]
[[File: Bamberg Heliometer No 3253.jpg|thumb|The split lens of the Bamberg Heliometer (late 19th century)]]

Measurement of annual parallax was the first reliable way to determine the distances to the closest stars. In the second quarter of the 19th century, technological progress reached to the level which provided sufficient accuracy and precision for stellar parallax measurements. [[Giuseppe Calandrelli]] noted stellar parallax in 1805-6 and came up with a 4-second value for the star [[Vega]] which was a gross overestimate.<ref>{{cite book | author=Gore, J.E. | url=https://archive.org/details/studiesinastrono00gorerich/page/42/mode/1up | title=Studies in astronomy |publisher=Chatto & Windus |year=1904 |place=London |page=42}}</ref> The first successful stellar parallax measurements were done by [[Thomas Henderson (astronomer)|Thomas Henderson]] in [[Cape Town]], [[South Africa]], in 1832–1833, where he measured parallax of one of the closest stars, [[Alpha Centauri]].<ref name=Henderson1839>{{Cite journal|last=Henderson|first=Thomas|date=1839|title=On the Parallax of α Centauri|journal=Monthly Notices of the Royal Astronomical Society|volume=4|issue=19 |pages=168–170|doi=10.1093/mnras/4.19.168 |bibcode=1839MNRAS...4..168H|doi-access=free}}</ref><ref name=Henderson1840>{{Cite journal|last=Henderson|first=Thomas|date=1840|title=On the Parallax of α Centauri|journal=Memoirs of Royal Astronomical Society|volume=11|pages=61–68|bibcode=1840MmRAS..11...61H}}</ref> Between 1835 and 1836, astronomer [[Friedrich Georg Wilhelm von Struve]] at the [[University of Tartu Old Observatory|Dorpat university observatory]] measured the distance of [[Vega]], publishing his results in 1837.<ref name=Struve1837>{{Cite journal|last=von Struve|first=Friedrich Georg Wilhelm|date=1837|title=Stellarum duplicium et multiplicium mensurae micrometricae per magnum Fraunhoferi tubum annis a 1824 ad 1837 in specula Dorpatensi institutae|journal=Astronomische Nachrichten|volume=14|issue=16 |pages=249–252|doi=10.1002/asna.18370141609 |bibcode=1837AN.....14..249S|url=https://zenodo.org/record/1817390 }}</ref> [[Friedrich Bessel]], a friend of Struve, carried out an intense observational campaign in 1837–1838 at [[Koenigsberg Observatory]] for the star [[61 Cygni|61&nbsp;Cygni]] using a [[heliometer]], and published his results in 1838.<ref name=ZG44>{{harvnb|Zeilik|Gregory|1998 | page=44}}.</ref><ref>Bessel, FW, "[http://www.ari.uni-heidelberg.de/gaia/documents/bessel-1838/index.html Bestimmung der Entfernung des 61sten Sterns des Schwans] {{webarchive|url=https://web.archive.org/web/20070624220502/http://www.ari.uni-heidelberg.de/gaia/documents/bessel-1838/index.html |date=2007-06-24 }}" (1838) ''[[Astronomische Nachrichten]]'', vol.&nbsp;16, pp.&nbsp;65–96.</ref> Henderson published his results in 1839, after returning from South Africa.

Those three results, two of which were measured with the best instruments at the time (Fraunhofer great refractor used by Struve and Fraunhofer heliometer by Bessel) were the first ones in history to establish the reliable distance scale to the stars.<ref name=Reid2022>{{Cite journal|last=Reid|first=Mark|date=2020|title=The first stellar parallaxes revisited|journal=Astronomische Nachrichten|volume=341|issue=9 |pages=860–869|doi=10.1002/asna.202013833 |arxiv=2009.11913 |bibcode=2020AN....341..860R|s2cid=221949223 }}</ref>

A large heliometer was installed at [[Kuffner Observatory]] (In Vienna) in 1896, and was used for measuring the distance to other stars by trigonometric parallax.<ref name="Kuffner Observatory">{{cite journal|last=Habison|first=Peter|date=1998|title=Astrometry and early astrophysics at Kuffner Observatory in the late 19th century|journal=Acta Historica Astronomiae|volume=3|pages=93–94|bibcode=1998AcHA....3...93H|issn=0003-2670}}</ref> By 1910 it had computed 16 parallax distances to other stars, out of only 108 total known to science at that time.<ref name="Kuffner Observatory" />
[[File:EB1911 - Heliometer - Fig. 10.jpg|thumb|500px|left|Diagram of a heliometer from the 1911 ''Encyclopædia Britannica'', which would be a view looking towards the split lens of a heliometer]]
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Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the [[filar micrometer]].  [[Astrograph]]s using astronomical [[photographic plate]]s sped the process in the early 20th century.  Automated plate-measuring machines<ref>[http://cdsweb.cern.ch/record/1107461 CERN paper on plate measuring machine] USNO StarScan</ref> and more sophisticated computer technology of the 1960s allowed more efficient compilation of [[star catalogue]]s. In the 1980s, [[charge-coupled device]]s (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond.{{citation needed|date=August 2020}}

Stellar parallax remains the standard for calibrating other measurement methods (see [[Cosmic distance ladder]]). Accurate calculations of distance based on stellar parallax require a measurement of the distance from Earth to the Sun, now known to exquisite accuracy based on [[radar]] reflection off the surfaces of planets.<ref>{{harvnb|Zeilik|Gregory|1998|loc=§&nbsp;22-3}}.</ref>