Editing Radio astronomy (section)

== History ==
[[File:JanskyatAntenna hi.tif|thumb|[[Karl Jansky]] and his rotating [[directional antenna]] (early 1930s) in [[Holmdel, New Jersey]], the world's first radio telescope, which was used to discover radio emissions from the [[Milky Way]]]]

Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources. In the 1860s, [[James Clerk Maxwell]]'s [[Maxwell's equations|equations]] had shown that [[electromagnetic radiation]] is associated with [[electricity]] and [[magnetism]], and could exist at any [[wavelength]]. Several attempts were made to detect radio emission from the [[Sun]], including an experiment by German astrophysicists [[Johannes Wilsing]] and [[Julius Scheiner]] in 1896 and a centimeter wave radiation apparatus set up by [[Oliver Lodge]] between 1897 and 1900. These attempts were unable to detect any emission due to technical limitations of the instruments. The discovery of the radio-reflecting [[ionosphere]] in 1902 led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, making them undetectable.<ref>{{cite web | url = http://www.nrao.edu/whatisra/hist_prehist.shtml | title = Pre-History of Radio Astronomy | author = F. Ghigo | publisher = [[National Radio Astronomy Observatory]] | access-date = 2010-04-09 | archive-date = 2020-06-15 | archive-url = https://web.archive.org/web/20200615213814/http://www.nrao.edu/whatisra/hist_prehist.shtml | url-status = live }}</ref>

[[Karl Jansky]] made the discovery of the first astronomical radio source [[Serendipity|serendipitously]] in the early 1930s. As a newly hired radio engineer with [[Bell Labs|Bell Telephone Laboratories]], he was assigned the task to investigate static that might interfere with [[short wave]] transatlantic voice transmissions. Using a large [[directional antenna]], Jansky noticed that his [[analog signal|analog]] pen-and-paper recording system kept recording a persistent repeating signal or "hiss" of unknown origin. Since the signal peaked about every 24 hours, Jansky first suspected the source of the interference was the [[Sun]] crossing the view of his directional antenna. Continued analysis, however, showed that the source was not following the 24-hour daily cycle of the Sun exactly but instead repeating on a cycle of 23 hours and 56 minutes. Jansky discussed the puzzling phenomena with his friend, astrophysicist Albert Melvin Skellett, who pointed out that the observed time between the signal peaks was the exact length of a [[sidereal time|sidereal day]]: the time it took for "fixed" astronomical objects, such as a star, to pass in front of the antenna every time the Earth rotated.<ref name="bookrags.com">{{cite book | url = http://www.bookrags.com/biography/karl-jansky-wsd/ | title = World of Scientific Discovery on Karl Jansky | access-date = 2010-04-09 | archive-date = 2012-01-21 | archive-url = https://web.archive.org/web/20120121112928/http://www.bookrags.com/biography/karl-jansky-wsd/ | url-status = live }}</ref> By comparing his observations with optical astronomical maps, Jansky eventually concluded that the radiation source peaked when his antenna was aimed at the densest part of the [[Milky Way]] in the [[constellation]] of [[Sagittarius (constellation)|Sagittarius]].<ref>{{cite journal | doi = 10.1038/132066a0 | author = Jansky, Karl G. |  title = Radio waves from outside the solar system | journal = Nature | volume = 132 | issue = 3323 | page = 66 | date = 1933 | bibcode = 1933Natur.132...66J| s2cid = 4063838 | doi-access = free }}</ref> 

Jansky announced his discovery at a meeting in Washington, D.C., in April 1933 and the field of radio astronomy was born.<ref name="aas">{{cite web |last1=Hirshfeld |first1=Alan |title=Karl Jansky and the Discovery of Cosmic Radio Waves |url=https://aas.org/posts/news/2018/07/month-astronomical-history-1 |publisher=American Astronomical Society |access-date=21 September 2021 |date=2018 |quote=In April 1933, closing in on nearly two years of study, Jansky read his breakthrough paper, “Electrical Disturbances Apparently of Extraterrestrial Origin,” before a meeting of the International Scientific Radio Union in Washington, DC. The strongest of the extraterrestrial waves, he found, emanate from a region in Sagittarius centered around right ascension 18 hours and declination — 20 degrees — in other words, from the direction of the galactic center. Jansky’s discovery made the front page of the New York Times on 5 May 1933, and the field of radio astronomy was born. |archive-date=29 September 2021 |archive-url=https://web.archive.org/web/20210929185905/https://aas.org/posts/news/2018/07/month-astronomical-history-1 |url-status=live }}</ref> In October 1933, his discovery was published in a journal article entitled "Electrical disturbances apparently of extraterrestrial origin" in the ''[[Proceedings of the Institute of Radio Engineers]]''.<ref>{{cite journal |last=Jansky |first=Karl Guthe |author-link=Karl Guthe Jansky |date=October 1933 |title=Electrical disturbances apparently of extraterrestrial origin |journal=Proc. IRE |volume=21 |issue=10 |page=1387 |doi=10.1109/JRPROC.1933.227458 }} Reprinted 65 years later as {{cite journal |last=Jansky |first=Karl Guthe |author-link=Karl Guthe Jansky |title=Electrical disturbances apparently of extraterrestrial origin |doi=10.1109/JPROC.1998.681378 |journal=[[Proc. IEEE]] |volume=86 |issue=7 |date=July 1998 |pages=1510–1515|s2cid=47549559 }} along with an explanatory preface by W.A. Imbriale, {{doi-inline|10.1109/JPROC.1998.681377|Introduction To "Electrical Disturbances Apparently Of Extraterrestrial Origin"}}.</ref> Jansky concluded that since the Sun (and therefore other stars) were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy, in particular, by "thermal agitation of charged particles."<ref name="bookrags.com"/><ref>{{cite journal |last=Jansky |first=Karl Guthe |author-link=Karl Guthe Jansky |date=October 1935 |title=A note on the source of interstellar interference |journal=Proc. IRE |volume=23 |issue=10 |page=1158|doi=10.1109/JRPROC.1935.227275 |s2cid=51632813 }}</ref> (Jansky's peak radio source, one of the brightest in the sky, was designated [[Sagittarius A]] in the 1950s and was later hypothesized to be emitted by [[electrons]] in a strong magnetic field. Current thinking is that these are ions in orbit around a massive [[black hole]] at the center of the galaxy at a point now designated as Sagittarius A*. The asterisk indicates that the particles at Sagittarius A are ionized.)<ref>{{cite book |title=Relativity, Astrophysics and Cosmology: Volume 1 |last=Belusević |first=R. |year=2008 |url=https://books.google.com/books?id=WeICTHIxP2MC&pg=PA163|page=163 |publisher=Wiley-VCH |isbn=978-3-527-40764-4}}</ref><ref>{{Cite book |last=Kambič |first=B. |title=Viewing the Constellations with Binoculars |date=6 October 2009 |url=https://books.google.com/books?id=3vxLNPNHOcwC&pg=PA131|pages=131–133 |publisher=[[Springer (publisher)|Springer]] |isbn=978-0-387-85355-0}}</ref><ref>{{cite journal | last1 = Gillessen | first1 = S. | last2 = Eisenhauer | first2 = F. | last3 = Trippe | first3 = S. |display-authors=etal  | year = 2009 | title = Monitoring Stellar Orbits around the Massive Black Hole in the Galactic Center | journal = The Astrophysical Journal | volume = 692 | issue = 2| pages = 1075–1109 | doi = 10.1088/0004-637X/692/2/1075 | arxiv=0810.4674 | bibcode=2009ApJ...692.1075G| s2cid = 1431308 }}</ref><ref>{{cite journal | last1 = Brown | first1 = R.L. | year = 1982 | title = Precessing jets in Sagittarius A – Gas dynamics in the central parsec of the galaxy | bibcode =  1982ApJ...262..110B| journal = Astrophysical Journal | volume = 262 | pages = 110–119 | doi = 10.1086/160401| doi-access = free }}</ref>

After 1935, Jansky wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of [[flux density]], the [[jansky]] (Jy), after him.<ref name="aps">{{cite web |title=This Month in Physics History May 5, 1933: The New York Times Covers Discovery of Cosmic Radio Waves |url=https://www.aps.org/publications/apsnews/201505/physicshistory.cfm |website=aps.org |publisher=American Physical Society (May 2015) Volume 24, Number 5 |access-date=21 September 2021 |quote=Jansky died in 1950 at the age of 44, the result of a massive stroke stemming from his kidney disease. When that first 1933 paper was reprinted in Proceedings of the IEEE in 1984, the editors noted that Jansky’s work would mostly likely have won a Nobel prize, had the scientist not died so young. Today the “jansky” is the unit of measurement for radio wave intensity (flux density). |archive-date=14 September 2021 |archive-url=https://web.archive.org/web/20210914000424/https://www.aps.org/publications/apsnews/201505/physicshistory.cfm |url-status=live }}</ref>

[[File:Grote Antenna Wheaton.gif|left|thumb|[[Grote Reber]]'s Antenna at [[Wheaton, Illinois]], world's first parabolic radio telescope]]
[[Grote Reber]] was inspired by Jansky's work, and built a parabolic radio telescope 9m in diameter in his backyard in 1937. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies.<ref>{{cite web | url = http://www.nrao.edu/whatisra/hist_reber.shtml | title = Grote Reber | access-date = 2010-04-09 | archive-date = 2020-08-07 | archive-url = https://web.archive.org/web/20200807095454/https://www.nrao.edu/whatisra/hist_reber.shtml | url-status = live }}</ref> On February 27, 1942, [[James Stanley Hey]], a [[British Army]] research officer, made the first detection of radio waves emitted by the Sun.<ref>{{cite book |last=Hey |first=J.S. |title=Radio Universe |edition=2nd |publisher=[[Pergamon Press]] |year=1975 |isbn= 978-0080187617 }}</ref> Later that year, [[George Clark Southworth]],<ref>{{cite journal |last=Southworth |first=G.C. |year=1945 |title=Microwave radiation from the Sun |journal=Journal of the Franklin Institute |volume=239 |issue=4 |pages=285–297 |doi=10.1016/0016-0032(45)90163-3}}</ref> at [[Bell Labs]] like Jansky, also detected radiowaves from the Sun. Both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first.<ref>{{cite journal|last1=Kellerman|first1=K. I.|title=Grote Reber's Observations on Cosmic Static|journal=Astrophysical Journal|date= 1999|volume=525C|page=371 |bibcode=1999ApJ...525C.371K}}</ref> Several other people independently discovered solar radio waves, including [[E. Schott]] in [[Denmark]]<ref>{{cite journal |last=Schott |first=E. |year=1947 |title=175 MHz-Strahlung der Sonne |journal=Physikalische Blätter |volume=3 |issue=5 |pages=159–160 |doi=10.1002/phbl.19470030508 |language=de|doi-access=free }}</ref> and [[Elizabeth Alexander (scientist)|Elizabeth Alexander]] working on [[Norfolk Island]].<ref>{{cite book |last1=Alexander |first1=F.E.S. |date=1945 |title=Long Wave Solar Radiation |publisher=[[Department of Scientific and Industrial Research (New Zealand)|Department of Scientific and Industrial Research]], Radio Development Laboratory }}</ref><ref>{{cite book |last1=Alexander |first1=F.E.S. |date=1945 |title=Report of the Investigation of the "Norfolk Island Effect" |publisher=[[Department of Scientific and Industrial Research (New Zealand)|Department of Scientific and Industrial Research]], Radio Development Laboratory |bibcode=1945rdlr.book.....A }}</ref><ref>{{cite journal |last1=Alexander |first1=F.E.S. |date=1946 |title=The Sun's radio energy |journal=Radio & Electronics |volume=1 |issue=1 |pages=16–17}} (see [http://nlnzcat.natlib.govt.nz/vwebv/holdingsInfo?bibId=405978 ''R&E'' holdings at NLNZ] {{Webarchive|url=https://archive.today/20160723215554/http://nlnzcat.natlib.govt.nz/vwebv/holdingsInfo?bibId=405978 |date=2016-07-23 }}.)</ref><ref name=Orchiston>{{cite book |last=Orchiston |first=W. |year=2005 |chapter=Dr Elizabeth Alexander: First Female Radio Astronomer |title=The New Astronomy: Opening the Electromagnetic Window and Expanding Our View of Planet Earth |series=Astrophysics and Space Science Library |volume=334 |pages=71–92 |doi=10.1007/1-4020-3724-4_5 |isbn=978-1-4020-3723-8}}</ref>

[[File:Chart Showing Radio Signal of First Identified Pulsar.jpg|thumb|upright|Chart on which [[Jocelyn Bell Burnell]] first recognised evidence of a [[pulsar]], in 1967 (exhibited at [[Cambridge University Library]]) ]]
At [[Cambridge University]], where ionospheric research had taken place during [[World War II]], [[J. A. Ratcliffe]] along with other members of the [[Telecommunications Research Establishment]] that had carried out wartime research into [[radar]], created a radiophysics group at the university where radio wave emissions from the Sun were observed and studied.
This early research soon branched out into the observation of other celestial radio sources and interferometry techniques were pioneered to isolate the angular source of the detected emissions. [[Martin Ryle]] and [[Antony Hewish]] at the [[Cavendish Astrophysics Group]] developed the technique of Earth-rotation [[aperture synthesis]]. The radio astronomy group in Cambridge went on to found the [[Mullard Radio Astronomy Observatory]] near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the [[Titan (1963 computer)|Titan]]) became capable of handling the computationally intensive [[Fourier transform]] inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5&nbsp;km' effective aperture using the One-Mile and Ryle telescopes, respectively. They used the [[Cambridge Interferometer]] to map the radio sky, producing the [[Second Cambridge Catalogue of Radio Sources|Second]] (2C) and [[Third Cambridge Catalogue of Radio Sources|Third]] (3C) Cambridge Catalogues of Radio Sources.<ref>{{cite web|url=http://www.phy.cam.ac.uk/history/years/radioast.php|archive-url=https://web.archive.org/web/20131110022209/http://www.phy.cam.ac.uk/history/years/radioast.php|archive-date=2013-11-10|title=Radio Astronomy|publisher=Cambridge University: Department of Physics}}</ref>