Editing Sun (section)

== Observational history ==

=== Early understanding ===
{{See also|The Sun in culture}}
[[File:Solvognen DO-6865 2000.jpg|thumb|The [[Trundholm sun chariot]] pulled by a horse is a sculpture believed to be illustrating an important part of [[Nordic Bronze Age]] mythology.|alt=A sculpture of the sun in a chariot being pulled by a horse that has wheels instead of hoofs.]]
In many prehistoric and ancient cultures, the Sun was thought to be a [[solar deity]] or other [[supernatural]] entity.<ref name="e488">{{cite book |last=Hawthorn |first=Hannah |title=The Magick of Birthdays |publisher=Penguin |publication-place=New York |date=2022 |isbn=978-0-593-53854-8 |page=103}}</ref><ref name="t793">{{cite book |last=Singh |first=Madanjeet |title=The Sun |publisher=ABRAMS |publication-place=New York |date=1993 |isbn=978-0-8109-3838-0 |page=305}}</ref> In the early 1st millennium&nbsp;BC, [[Babylonian astronomy|Babylonian astronomers]] observed that the Sun's motion along the [[ecliptic]] is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an [[elliptic orbit]], moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.<ref>{{Cite book |title=Babylon to Voyager and beyond: a history of planetary astronomy |first=David |last=Leverington |publisher=[[Cambridge University Press]] |date=2003 |isbn=978-0-521-80840-8 |pages=6–7}}</ref>

One of the first people to offer a scientific or philosophical explanation for the Sun was the [[Ancient Greece|Greek]] philosopher [[Anaxagoras]]. He reasoned that it was a giant flaming ball of metal even larger than the land of the [[Peloponnese|Peloponnesus]] and that the Moon reflected the light of the Sun.<ref>{{Cite journal |last=Sider |first=D. |title=Anaxagoras on the Size of the Sun |jstor=269068 |journal=[[Classical Philology (journal)|Classical Philology]] |volume=68 |issue=2 |pages=128–129 |date=1973 |doi=10.1086/365951 |s2cid=161940013}}</ref> [[Eratosthenes]] estimated the distance between Earth and the Sun in the 3rd century&nbsp;BC as "of stadia [[myriad]]s 400 and 80000", the translation of which is ambiguous, implying either 4,080,000 [[Stadion (unit)|stadia]] (755,000&nbsp;km) or 804,000,000 stadia (148 to 153&nbsp;million kilometres or 0.99 to 1.02 AU); the latter value is correct to within a few per cent. In the 1st century&nbsp;AD, [[Ptolemy]] estimated the distance as 1,210 times [[Earth radius|the radius of Earth]], approximately {{convert|{{#expr:1.210*6.371round2}}|e6km|AU}}.<ref>{{Cite journal |last=Goldstein |first=B. R. |title=The Arabic Version of Ptolemy's Planetary Hypotheses |journal=Transactions of the American Philosophical Society |volume=57 |issue=4 |pages=9–12 |date=1967 |doi=10.2307/1006040 |jstor=1006040}}</ref>

The theory that the Sun is the centre around which the planets orbit was first proposed by the ancient Greek [[Aristarchus of Samos]] in the 3rd century&nbsp;BC,<ref>{{Cite journal |last=Stahl |first=William Harris |date=1945 |title=The Greek Heliocentric Theory and Its Abandonment |jstor=283344 |journal=Transactions and Proceedings of the American Philological Association |volume=76 |pages=321–332 |doi=10.2307/283344 |issn=0065-9711}}</ref> and later adopted by [[Seleucus of Seleucia]] (see [[Heliocentrism]]).<ref>{{cite book |last=Toomer |first=G. J. |chapter=Seleucus (5), of Seleuceia, astronomer |date=7 March 2016 |title=Oxford Research Encyclopedia of Classics |url=https://oxfordre.com/classics/view/10.1093/acrefore/9780199381135.001.0001/acrefore-9780199381135-e-5799 |access-date=27 May 2024 |publisher=Oxford University Press |doi=10.1093/acrefore/9780199381135.013.5799 |isbn=978-0-19-938113-5}}</ref> This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century by [[Nicolaus Copernicus]].<ref>{{Cite book |last1=Fraknoi |first1=Andrew |last2=Morrison |first2=David |last3=Wolff |first3=Sidney |date=9 March 2022 |chapter=2.4 The Birth of Modern Astronomy |title=Astronomy 2e |publisher=OpenStax |chapter-url=https://openstax.org/books/astronomy-2e/pages/2-4-the-birth-of-modern-astronomy |access-date=27 May 2024}}</ref>

=== 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>

=== Solar space missions ===
{{See also|Solar observatory|List of heliophysics missions}}
[[File:Pioneer-6-9.jpg|thumb|[[Pioneer 6, 7, 8, and 9|''Pioneer 6'', ''7'', ''8'', and ''9'']]]]
The first satellites designed for long term observation of the Sun from interplanetary space were [[Pioneer 6, 7, 8, and 9|''Pioneer 6'', ''7'', ''8'', and ''9'']], which were launched by NASA between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. ''Pioneer 9'' operated for a particularly long time, transmitting data until May 1983.<ref>{{cite web |last=Wade |first=M. |title=Pioneer 6-7-8-9-E |url=http://www.astronautix.com/craft/pio6789e.htm |date=2008 |publisher=[[Encyclopedia Astronautica]] |access-date=22 March 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060422075141/http://www.astronautix.com/craft/pio6789e.htm |archive-date=22 April 2006 }}</ref><ref>{{cite web |title=Solar System Exploration: Missions: By Target: Our Solar System: Past: Pioneer 9 |url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Pioneer_09 |publisher=[[NASA]] |access-date=30 October 2010 |quote=NASA maintained contact with Pioneer 9 until May 1983 |url-status=dead |archive-url=https://web.archive.org/web/20120402205810/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Pioneer_09 |archive-date=2 April 2012 }}</ref>

In the 1970s, two [[Helios (spacecraft)|''Helios'' spacecraft]] and the Skylab [[Apollo Telescope Mount]] provided scientists with significant new data on solar wind and the solar corona. The ''Helios 1'' and ''2'' probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion.<ref name=Burlaga2001 /> The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station.<ref name=Dwivedi2006 /> Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona.<ref name=Dwivedi2006 /> Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of [[coronal hole]]s, now known to be intimately associated with the solar wind.<ref name=Burlaga2001>{{Cite journal |last=Burlaga |first=L. F. |title=Magnetic Fields and plasmas in the inner heliosphere: Helios results |year=2001 |journal=Planetary and Space Science |volume=49 |issue=14–15 |pages=1619–1627 |doi=10.1016/S0032-0633(01)00098-8 |bibcode=2001P&SS...49.1619B |url=https://zenodo.org/record/1259695 |access-date=25 August 2019 |archive-date=13 July 2020 |archive-url=https://web.archive.org/web/20200713051926/https://zenodo.org/record/1259695 |url-status=live}}</ref>

[[File:Smm.jpg|thumb|left|Drawing of a [[Solar Maximum Mission]] probe|alt=See caption]]

In 1980, the [[Solar Maximum Mission]] probes were launched by NASA. This spacecraft was designed to observe gamma rays, [[X-ray]]s and [[ultraviolet]] radiation from solar flares during a time of high solar activity and solar luminosity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984, [[Space Shuttle Challenger|Space Shuttle ''Challenger'']] mission [[STS-41-C]] retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before [[Atmospheric reentry|re-entering]] Earth's atmosphere in June 1989.<ref>{{cite web |last=Burkepile |first=C. J. |title=Solar Maximum Mission Overview |url=http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html |date=1998 |access-date=22 March 2006 |archive-url=https://web.archive.org/web/20060405183758/http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html |archive-date=5 April 2006}}</ref>

Launched in 1991, Japan's [[Yohkoh]] (''Sunbeam'') satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.<ref>{{cite press release |title=Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere |url=http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html |publisher=[[Japan Aerospace Exploration Agency]] |date=13 September 2005 |access-date=22 March 2006 |archive-date=10 August 2013 |archive-url=https://web.archive.org/web/20130810150641/http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html |url-status=dead}}</ref>

The [[Solar and Heliospheric Observatory]], jointly built by the [[European Space Agency]] and NASA, was launched on 2 December 1995.<ref name=Dwivedi2006 /> Originally intended to serve a two-year mission,<ref>{{cite web |url=https://www.universetoday.com/138664/22-years-of-the-sun-from-soho/ |title=22 Years of the Sun from SOHO |website=Universe Today |access-date=31 May 2024 |date=26 February 2018 |first=Evan |last=Gough}}</ref> SOHO remains in operation as of 2024.<ref>{{cite web |url=https://www.universetoday.com/166353/someone-just-found-sohos-5000th-comet/ |title=Someone Just Found SOHO's 5,000th Comet |first=Nancy |last=Atkinson |date=28 March 2024 |access-date=31 May 2024 |website=Universe Today}}</ref> Situated at the [[Lagrangian point]] between Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch.<ref name=Dwivedi2006 /> Besides its direct solar observation, SOHO has enabled the discovery of a large number of [[comet]]s, mostly tiny [[sungrazing comet]]s that incinerate as they pass the Sun.<ref>{{cite web |title=Sungrazing Comets |url=https://sungrazer.nrl.navy.mil/ |publisher=[[Large Angle and Spectrometric Coronagraph|LASCO]] ([[US Naval Research Laboratory]]) |date=13 March 2015 |access-date=19 March 2009 |archive-date=25 May 2015 |archive-url=https://web.archive.org/web/20150525060147/http://sungrazer.nrl.navy.mil/ |url-status=live}}</ref>

[[File:The Ulysses spacecraft undergoes testing at the vacuum spin-balancing facility in ESTEC.jpg|thumb|[[Ulysses (spacecraft)|''Ulysses'' spacecraft]] testing at the vacuum spin-balancing facility|alt=A photograph of Ulysses spacecraft]]

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The [[Ulysses (spacecraft)|''Ulysses'' probe]] was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" into an orbit that would take it far above the plane of the ecliptic. Once ''Ulysses'' was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750&nbsp;km/s, which was slower than expected, and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays.<ref>{{cite web |author=[[Jet Propulsion Laboratory|JPL]]/[[California Institute of Technology|CALTECH]] |title=Ulysses: Primary Mission Results |url=http://ulysses.jpl.nasa.gov/science/mission_primary.html |publisher=NASA |year=2005 |access-date=22 March 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060106150819/http://ulysses.jpl.nasa.gov/science/mission_primary.html |archive-date=6 January 2006}}</ref>

Elemental abundances in the photosphere are well known from [[astronomical spectroscopy|spectroscopic]] studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, ''[[Genesis (spacecraft)|Genesis]]'', was designed to allow astronomers to directly measure the composition of solar material.<ref>{{Cite journal |last1=Calaway |first1=M. J. |title=Genesis capturing the Sun: Solar wind irradiation at Lagrange 1 |journal=[[Nuclear Instruments and Methods in Physics Research B]] |volume=267 |issue=7 |pages=1101–1108 |date=2009 |doi=10.1016/j.nimb.2009.01.132 |last2=Stansbery |first2=Eileen K. |last3=Keller |first3=Lindsay P. |bibcode=2009NIMPB.267.1101C |url=https://zenodo.org/record/1259269 |access-date=13 July 2019 |archive-date=11 May 2020 |archive-url=https://web.archive.org/web/20200511052700/https://zenodo.org/record/1259269 |url-status=live}}</ref>