Editing Sun (section)

=== Development of scientific understanding ===
[[File:Sun-bonatti.png|thumb|Sol, the Personification of the Sun, from a 1550 edition of [[Guido Bonatti]]'s ''{{lang|la|Liber astronomiae}}''|alt=A drawing of a man wearing a crown in a chariot, being pulled by horses.]]
Observations of sunspots were recorded by [[Chinese astronomers]] during the [[Han dynasty]] (202&nbsp;BC{{snd}}AD&nbsp;220), with records of their observations being maintained for centuries. [[Averroes]] also provided a description of sunspots in the 12th century.<ref>{{cite book |last=Ead |first=Hamed A. |title=Averroes As A Physician |publisher=[[University of Cairo]] |url=https://www.alchemywebsite.com/islam21.html |year=1998 |access-date=27 May 2024}}</ref> The invention of the telescope in the early 17th century permitted detailed observations of sunspots by [[Thomas Harriot]], [[Galileo Galilei]] and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.<ref>{{cite web |title=Galileo Galilei (1564–1642) |url=https://www.bbc.co.uk/history/historic_figures/galilei_galileo.shtml |publisher=BBC |access-date=22 March 2006 |archive-date=29 September 2018 |archive-url=https://web.archive.org/web/20180929134432/http://www.bbc.co.uk/history/historic_figures/galilei_galileo.shtml |url-status=live}}</ref>

[[Astronomy in the medieval Islamic world|Medieval Islamic astronomical contributions]] include [[al-Battani]]'s discovery that the direction of the Sun's [[apogee]] (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing.<ref>{{cite book |title=A short History of scientific ideas to 1900 |first=C. |last=Singer |publisher=Oxford University Press |year=1959 |page=151}}</ref> In modern heliocentric terms, this is caused by a gradual motion of the aphelion of the <em>Earth's</em> orbit. [[Ibn Yunus]] observed more than 10,000 entries for the Sun's position for many years using a large [[astrolabe]].<ref>{{cite book |chapter=The Arabian Science |first=C. |last=Ronan |pages=201–244 |title=The Cambridge Illustrated History of the World's Science |publisher=Cambridge University Press |year=1983}} at pp. 213–214.</ref>

The first reasonably accurate distance to the Sun was determined in 1684 by [[Giovanni Domenico Cassini]]. Knowing that direct measurements of the solar parallax were difficult, he chose to measure the Martian parallax. Having sent [[Jean Richer]] to [[Cayenne]], part of [[French Guiana]], for simultaneous measurements, Cassini in Paris determined the parallax of [[Mars]] when Mars was at its closest to Earth in 1672. Using the circumference distance between the two observations, Cassini calculated the Earth-Mars distance, then used [[Kepler's laws]] to determine the Earth-Sun distance. His value, about 10% smaller than modern values, was much larger than all previous estimates.<ref>{{Cite book |last=Rossi |first=Elisabetta |url=http://www.fedoabooks.unina.it/public/presses/1/17_Rossi_1.pdf |title=Unveiling the Size of the Universe: The first Accurate Measurement of the Earth-Sun Distance by Giovanni Domenico Cassini |date=2024 |publisher=FedOA – Federico II University Press |doi=10.6093/978-88-6887-277-9}}</ref>

From an observation of a [[transit of Venus]] in 1032, the Persian astronomer and polymath [[Avicenna|Ibn Sina]] concluded that Venus was closer to Earth than the Sun.<ref name=Goldstein>{{Cite journal |title=Theory and Observation in Medieval Astronomy |first=Bernard R. |last=Goldstein |journal=[[Isis (journal)|Isis]] |volume=63 |issue=1 |date=March 1972 |pages=39–47 [44] |doi=10.1086/350839 |bibcode=1972Isis...63...39G |s2cid=120700705}}</ref> In 1677, [[Edmond Halley]] observed a transit of Mercury across the Sun, leading him to realise that observations of the [[solar parallax]] of a planet (more ideally using the transit of Venus) could be used to [[Trigonometry|trigonometrically]] determine the distances between Earth, [[Venus]], and the Sun.<ref>{{Cite conference |last=Chapman |first=Allan |date=April 2005 |editor-last=Kurtz |editor-first=D. W. |title=Jeremiah Horrocks, William Crabtree, and the Lancashire observations of the transit of Venus of 1639 |conference=Transits of Venus: New Views of the Solar System and Galaxy, Proceedings of IAU Colloquium #196, held 7–11 June 2004 in Preston, U.K. |publisher=Cambridge University Press |publication-place=Cambridge |volume=2004 |pages=3–26 |bibcode=2005tvnv.conf....3C |doi=10.1017/S1743921305001225 |doi-access=free |journal=Proceedings of the International Astronomical Union}}</ref> Careful observations of the [[1769 transit of Venus observed from Tahiti|1769 transit of Venus]] allowed astronomers to calculate the average Earth–Sun distance as {{Convert|93726900|mi|km}}, only 0.8% greater than the modern value.<ref>{{Cite journal |last=Teets |first=Donald |date=December 2003 |title=Transits of Venus and the Astronomical Unit |url=http://www.maa.org/sites/default/files/pdf/pubs/mm_dec03-Venus.pdf |url-status=live |journal=Mathematics Magazine |volume=76 |pages=335–348 |doi=10.1080/0025570X.2003.11953207 |jstor=3654879 |s2cid=54867823 |archive-url=https://web.archive.org/web/20220203080207/https://www.maa.org/sites/default/files/pdf/pubs/mm_dec03-Venus.pdf |archive-date=3 February 2022 |access-date=3 April 2022 |number=5}}</ref>

[[File:BBSO full-disk H-alpha 2002-07-26 153931 color.png|thumb|left|Sun as seen in Hydrogen-alpha light|alt=A photograph of the sun]]
In 1666, [[Isaac Newton]] observed the Sun's light using a [[Prism (optics)|prism]], and showed that it is made up of light of many colours.<ref>{{cite news |title=Sir Isaac Newton (1643–1727) |publisher=BBC Teach |url=https://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml |access-date=22 March 2006 |archive-date=10 March 2015 |archive-url=https://web.archive.org/web/20150310093436/http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml |url-status=live}}</ref> In 1800, [[William Herschel]] discovered [[infrared]] radiation beyond the red part of the solar spectrum.<ref>{{cite web |title=Herschel Discovers Infrared Light |url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html |publisher=Cool Cosmos |access-date=22 March 2006 |url-status=dead |archive-url=https://web.archive.org/web/20120225094516/http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html |archive-date=25 February 2012}}</ref> The 19th century saw advancement in spectroscopic studies of the Sun; [[Joseph von Fraunhofer]] recorded more than 600 [[absorption lines]] in the spectrum, the strongest of which are still often referred to as ''[[Fraunhofer lines]]''. The 20th century brought about several specialised systems for observing the Sun, especially at different narrowband wavelengths, such as those using Calcium-H (396.9&nbsp;nm), Calcium-K (393.37&nbsp;nm) and [[Hydrogen-alpha]] (656.46&nbsp;nm) [[Astronomical filter|filtering]].<ref>{{cite book |chapter=Instruments for observing the Corona |first=Gudrun |last=Wolfschmidt |title=Instruments of Science, An Historical Encyclopedia |year=1998 |pages=147–148 |isbn=9780815315612 |publisher=Science Museum, London, and National Museum of American History, Smithsonian Institution |editor1-first=Deborah Jean |editor1-last=Warner |editor2-first=Robert |editor2-last=Bud |chapter-url=https://books.google.com/books?id=1AsFdUxOwu8C&pg=PA148}}</ref>

During early studies of the [[optical spectrum]] of the photosphere, some absorption lines were found that did not correspond to any [[chemical element]]s then known on Earth. In 1868, [[Norman Lockyer]] hypothesised that these absorption lines were caused by a new element that he dubbed ''helium'', after the Greek Sun god [[Helios]]. Twenty-five years later, helium was isolated on Earth.<ref name="Lockyer">{{Cite web |last=Parnel |first=C. |title=Discovery of Helium |url=http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html |url-status=live |archive-url=https://web.archive.org/web/20151107043457/http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html |archive-date=7 November 2015 |access-date=22 March 2006 |publisher=University of St Andrews}}</ref>

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. [[Lord Kelvin]] suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat.<ref name=kelvin>{{Cite journal |last=Thomson |first=W. |title=On the Age of the Sun's Heat |url=http://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html |journal=[[Macmillan's Magazine]] |date=1862 |volume=5 |pages=388–393 |access-date=25 August 2006 |archive-date=25 September 2006 |archive-url=https://web.archive.org/web/20060925190954/http://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html |url-status=live}}</ref> Kelvin and [[Hermann von Helmholtz]] then proposed a [[Kelvin–Helmholtz mechanism|gravitational contraction]] mechanism to explain the energy output, but the resulting age estimate was only 20&nbsp;million years, well short of the time span of at least 300&nbsp;million years suggested by some geological discoveries of that time.<ref name=kelvin /><ref>{{cite journal |year=2000 |title=Kelvin's age of the Earth paradox revisited |journal=[[Journal of Geophysical Research]] |volume=105 |issue=B6 |pages=13155–13158 |bibcode=2000JGR...10513155S |doi=10.1029/2000JB900028 |last1=Stacey |first1=Frank D. |doi-access=free}}</ref><!-- In XIX century, before discovery of radionuclear dating, there was no reason to suggest that Earth exists for as long as 4 billion years. --> In 1890, Lockyer proposed a meteoritic hypothesis for the formation and evolution of the Sun.<ref>{{Cite journal |last=Lockyer |first=J. N. |title=The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems |journal=London and New York |year=1890 |bibcode=1890mhsr.book.....L}}</ref>

Not until 1904 was a documented solution offered. [[Ernest Rutherford]] suggested that the Sun's output could be maintained by an internal source of heat, and suggested [[radioactive decay]] as the source.<ref>{{cite web |last=Darden |first=L. |title=The Nature of Scientific Inquiry |url=http://www.philosophy.umd.edu/Faculty/LDarden/sciinq/ |year=1998 |access-date=25 August 2006 |archive-date=17 August 2012 |archive-url=https://web.archive.org/web/20120817040843/http://www.philosophy.umd.edu/Faculty/LDarden/sciinq/ |url-status=live}}</ref> However, it would be [[Albert Einstein]] who would provide the essential clue to the source of the Sun's energy output with his [[mass–energy equivalence]] relation {{nowrap|''E'' {{=}} ''mc''<sup>2</sup>}}.<ref>{{Cite book |last=Hawking |first=S. W. |author-link=Stephen Hawking |date=2001 |title=The Universe in a Nutshell |publisher=Bantam |isbn=978-0-553-80202-3 |page=12 |url=https://books.google.com/books?id=0CO2iwfzRJkC&pg=PA12}}</ref> In 1920, Sir [[Arthur Eddington]] proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass.<ref>{{cite web |title=Studying the stars, testing relativity: Sir Arthur Eddington |url=http://www.esa.int/esaSC/SEMDYPXO4HD_index_0.html |website=Space Science |publisher=[[European Space Agency]] |date=2005 |access-date=1 August 2007 |archive-date=20 October 2012 |archive-url=https://web.archive.org/web/20121020174459/http://www.esa.int/esaSC/SEMDYPXO4HD_index_0.html |url-status=live}}</ref> The preponderance of hydrogen in the Sun was confirmed in 1925 by [[Cecilia Payne-Gaposchkin|Cecilia Payne]] using the ionisation theory developed by [[Meghnad Saha]]. The theoretical concept of fusion was developed in the 1930s by the astrophysicists [[Subrahmanyan Chandrasekhar]] and [[Hans Bethe]]. Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.<ref name="Bethe">{{Cite journal |last1=Bethe |first1=H. |title=On the Formation of Deuterons by Proton Combination |journal=[[Physical Review]] |volume=54 |issue=10 |page=862 |date=1938 |doi=10.1103/PhysRev.54.862.2 |last2=Critchfield |first2=C. |bibcode=1938PhRv...54Q.862B}}</ref><ref name="Bethe2">{{Cite journal |last=Bethe |first=H. |title=Energy Production in Stars |journal=[[Physical Review]] |volume=55 |issue=1 |pages=434–456 |date=1939 |doi=10.1103/PhysRev.55.434 |pmid=17835673 |bibcode=1939PhRv...55..434B |s2cid=36146598 |doi-access=free}}</ref> In 1957, [[Margaret Burbidge]], [[Geoffrey Burbidge]], [[William Alfred Fowler|William Fowler]] and [[Fred Hoyle]] showed that most of the elements in the universe have been [[Nucleosynthesis|synthesised]] by nuclear reactions inside stars, some like the Sun.<ref>{{Cite journal |first1=E. M. |last1=Burbidge |first2=G. R. |last2=Burbidge |first3=W. A. |last3=Fowler |first4=F. |last4=Hoyle |title=Synthesis of the Elements in Stars |journal=[[Reviews of Modern Physics]] |volume=29 |issue=4 |pages=547–650 |year=1957 |doi=10.1103/RevModPhys.29.547 |bibcode=1957RvMP...29..547B |url=https://authors.library.caltech.edu/45747/1/BURrmp57.pdf |doi-access=free |access-date=12 April 2020 |archive-date=23 July 2018 |archive-url=https://web.archive.org/web/20180723054833/https://authors.library.caltech.edu/45747/1/BURrmp57.pdf |url-status=live}}</ref>