Editing Jacobus Kapteyn

{{for|the Dutch-Ghanaian minister|Jacobus Capitein}}
{{Use dmy dates|date=November 2017}}
{{Infobox scientist
|honorific_prefix   = [[Professor]]
|name=Jacobus Kapteyn
|honorific_suffix   = {{postnominals|country=GBR|size=100%|FRS|FRSE}}
|image=Jacobus Kapteyn.jpg
|caption   = Jacobus Kapteyn. Painting by [[Jan Veth]] (1921).
|fields      = [[Astronomy]]
|known_for   = discovery of evidence for [[Galaxy rotation curve|galactic rotation]]
|birth_name  = Jacobus Cornelius Kapteyn
|birth_date  = {{Birth date|1851|1|19|df=y}}
|birth_place = [[Barneveld (town)|Barneveld]], [[Netherlands]]
|death_date  = {{Death date and age|1922|6|18|1851|1|19|df=y}}
|death_place = [[Amsterdam]], [[Netherlands]]
|alma_mater  = [[Utrecht University|University of Utrecht]]
|awards      = [[Bruce Medal]] 1913
}}
[[File:Jacobus Kapteyn00.jpg|thumb|Jacobus Kapteyn on the occasion of his 40th anniversary as professor in [[Groningen (city)|Groningen]]. Sir [[David Gill (astronomer)|David Gill]] in background. Painting by [[Jan Veth]].]]
'''Jacobus Cornelius Kapteyn''' (19 January 1851 – 18 June 1922) was a Dutch [[astronomy|astronomer]].  He carried out extensive studies of the [[Milky Way]]. He found that the apparent movement of stars was not randomly distributed but had two preferential directions: the two star streams. This discovery was later reinterpreted as evidence for [[Galaxy rotation curve|galactic rotation]]. Kapteyn also suggested that these stellar velocities could be used to find the amount of non-luminous matter in the galaxy,<ref>Kapteyn, Jacobus Cornelius (1922). "First attempt at a theory of the arrangement and motion of the sidereal system". Astrophysical Journal. 55: 302–327. Bibcode:1922ApJ....55..302K. doi:10.1086/142670. In concurrence with his contemporaries, he used the term "dark matter" to designate this non-lumininous matter, which was thought by many to be dust and gas. "It is incidentally suggested that when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect."</ref> which his student, [[Jan Oort]], measured in 1932, referring to it as "invisible matter". 

== Biography ==
Kapteyn was born in [[Barneveld (town)|Barneveld]] to Gerrit J. and Elisabeth C. (née Koomans) Kapteyn,<ref>{{cite book |title=The Biographical Encyclopedia of Astronomers |last=Hockey |first=Thomas |date=2009 |publisher=[[Springer Science+Business Media]] |isbn=978-0-387-31022-0 |accessdate=August 22, 2012 |url=http://www.springerreference.com/docs/html/chapterdbid/58744.html}}</ref><ref>{{cite book |title=Jacobus Cornelius Kapteyn, Born Investigator of the Heavens |last=van der Kruit |first=Pieter |series=Astrophysics and Space Science Library |date=2015 |volume=416 |publisher=[[Springer Science+Business Media]] |doi=10.1007/978-3-319-10876-6 |isbn=978-3-319-10876-6 |url=https://dx.doi.org/10.1007/978-3-319-10876-6}}</ref> and went to the [[Utrecht University|University of Utrecht]] to study [[mathematics]] and [[physics]] in 1868. In 1875, after having finished his [[thesis]], he worked for three years at the [[Leiden Observatory]], before becoming the first [[Professor]] of Astronomy and Theoretical [[Mechanics]] at the [[University of Groningen]], where he remained until his [[retirement]] in 1921. In 1888 he became a member of the [[Royal Netherlands Academy of Arts and Sciences]].<ref>{{cite web|author= |url=http://www.dwc.knaw.nl/biografie/pmknaw/?pagetype=authorDetail&aId=PE00001201 |title=Jacob Cornelius Kapteyn (1851 - 1922) |publisher=[[Royal Netherlands Academy of Arts and Sciences]] |date= |accessdate=22 July 2015}}</ref>

Between 1896 and 1900, lacking an observatory, he volunteered to measure [[photographic plate]]s taken by [[David Gill (astronomer)|David Gill]], who was conducting a photographic survey of [[southern hemisphere]] stars at the [[Royal Observatory at the Cape of Good Hope]]. The results of this collaboration was the publication of ''[[Cape Photographic Durchmusterung]]'', a catalog listing positions and magnitudes for 454,875 [[star]]s in the [[Southern Hemisphere]].

In 1897, as part of the above work, he discovered [[Kapteyn's Star]]. It had the highest [[proper motion]] of any star known until the discovery of [[Barnard's Star]] in 1916.

In 1904, studying the [[proper motion]]s of stars, Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of the Milky Way Galaxy, which ultimately led to the finding of [[Galaxy rotation curve|galactic rotation]] by [[Bertil Lindblad]] and [[Jan Oort]].

In 1906, Kapteyn launched a plan for a major study of the distribution of stars in the Galaxy, using counts of stars in different directions. The plan involved measuring the [[apparent magnitude]], [[spectral type]], [[radial velocity]], and [[proper motion]] of stars in 206 zones. This enormous project was the first coordinated statistical analysis in astronomy and involved the cooperation of over forty different observatories.

He was awarded the [[James Craig Watson Medal]] in 1913. Kapteyn later retired in 1921 at the age of seventy, but on the request of his former student and director of [[Leiden Observatory]] [[Willem de Sitter]], Kapteyn went back to Leiden to assist in upgrading the observatory to contemporary astronomical standards.

His life's work, ''First attempt at a theory of the arrangement and motion of the sidereal system'', was published in 1922, and described a lens-shaped [[island universe]] of which the density decreased away from the center, now known as the ''[[Kapteyn Universe]]''. In his model the Galaxy was thought to be 40,000 [[light year]]s in size, the [[Sun]] being relatively close (2,000 light years) to its center. The model was valid at high [[Galactic coordinate system|galactic latitude]]s but failed in the [[galactic plane]] because of the lack of knowledge of [[interstellar medium|interstellar]] absorption.

It was only after Kapteyn's death, in [[Amsterdam]], that [[Robert Trumpler]] determined that the amount of [[interstellar reddening]] was actually much greater than had been assumed. This discovery increased the estimate of the galaxy's size to 100,000 light years, with the Sun replaced to a distance of 30,000 light years from the [[Galactic Center]].

The astronomy institute of the [[University of Groningen]] is named after Kapteyn. A street in the city of Groningen is also named after Kapteyn: the J.C. Kapteynlaan. And the [[Isaac Newton Group of Telescopes]] on [[La Palma]] in the [[Canary Islands]] named the [[Jacobus Kapteyn Telescope]] (JKT) after him.

His daughter Henrietta (1881-1956) married astronomer [[Ejnar Hertzsprung]].

==Honours==
{{commons|Jacobus Cornelius Kapteyn}}
'''Awards'''
*[[Gold Medal of the Royal Astronomical Society]] (1902)
*[[James Craig Watson Medal]] (1913)
*[[Bruce Medal]] (1913)

'''Named after him'''
*[[Kapteyn (crater)]] on the [[Moon]]
*[[Asteroid]] [[818 Kapteynia]]
*[[Kapteyn's Star]]
*[[Kapteyn Astronomical Institute]] at the University of Groningen
*[[Jacobus Kapteyn Telescope]] (JKT) at La Palma, one of the [[Canary Islands]]
*[[Kapteyn Package]], Astronomical package for Python

==References==
{{reflist}}

==External links==
*[https://web.archive.org/web/20080202023546/http://www.rug.nl/sterrenkunde/onderzoek/geschiedenis?lang=en History of the Kapteyn Institute]

{{Authority control}}

{{DEFAULTSORT:Kapteyn, Jacobus}}
[[Category:1851 births]]
[[Category:1922 deaths]]
[[Category:People from Barneveld]]
[[Category:Dutch astronomers]]
[[Category:Utrecht University alumni]]
[[Category:Academic staff of the University of Groningen]]
[[Category:Recipients of the Bruce Medal]]
[[Category:Recipients of the Gold Medal of the Royal Astronomical Society]]
[[Category:Foreign members of the Royal Society]]
[[Category:Members of the Royal Netherlands Academy of Arts and Sciences]]
[[Category:Honorary Fellows of the Royal Society of Edinburgh]]
[[Category:Corresponding members of the Saint Petersburg Academy of Sciences]]