Editing Galilean moons (section)

=== Discovery ===
[[Image:Galileo.arp.300pix.jpg|thumb|upright|[[Galileo Galilei]], the discoverer of the four moons]]
As a result of [[Galilean telescope|improvements]] that [[Galileo Galilei]] made to the [[telescope]], with a magnifying capability of 20×,<ref>{{cite journal|first=Albert|last=Van Helden|title=The Telescope in the Seventeenth Century|journal=Isis|volume=65|issue=1|date=March 1974|pages=38–58|jstor=228880|doi=10.1086/351216|s2cid=224838258 }}</ref> he was able to see celestial bodies more distinctly than was previously possible. This allowed Galileo to observe in either December 1609 or January 1610 what came to be known as the Galilean moons.<ref name="Galileo89">{{Cite book |last1=Galilei |first1=Galileo |author-link1=Galileo Galilei |title=Sidereus Nuncius, or The Sidereal Messenger |title-link=Sidereus Nuncius |last2=Van Helden |first2=Albert |date=1989 |publisher=[[University of Chicago Press]] |isbn=978-0-226-27902-2 |location=Chicago |pages=14–16 |language=en, la}}</ref><ref>{{cite book|title=The Starry Messenger|last=Galilei|first=Galileo|date=1610|location=Venice|url=https://archive.org/details/starrymessengerb00sisp|quote=On the seventh day of January in this present date 1610....|isbn=978-0-374-37191-3|url-access=registration}}</ref>

On 7 January 1610, Galileo wrote a letter containing the first mention of Jupiter's moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these [[Celestial spheres|celestial orbs]] from 8 January to 2 March 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.<ref name=Galileo89/>

Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted [[Geocentric model|Ptolemaic world system]], which held that Earth was at the center of the universe and all other celestial bodies revolved around it.<ref>{{cite web|url=https://galileo.rice.edu/sci/observations/jupiter_satellites.html|title=Satellites of Jupiter|access-date=9 August 2007|work=The Galileo Project|publisher=[[Rice University]]|date=1995|url-status=live|archive-url=https://web.archive.org/web/20120211140650/https://galileo.rice.edu/sci/observations/jupiter_satellites.html|archive-date=11 February 2012}}</ref> Galileo's 13 March 1610, ''[[Sidereus Nuncius]]'' (''Starry Messenger''), which announced celestial observations through his telescope, does not explicitly mention [[Copernican heliocentrism]], a theory that placed the [[Sun]] at the center of the universe. Nevertheless, Galileo accepted the Copernican theory.<ref name=Galileo89/>

A Chinese historian of astronomy, [[Xi Zezong]], has claimed that a "small reddish star" observed near Jupiter in 364 BCE by Chinese astronomer [[Gan De]] may have been Ganymede. If true, this might predate Galileo's discovery by around two millennia.<ref>{{Cite journal |last=Ze-zong |first=Xi |date=June 1981 |title=The sighting of Jupiter's satellite by Gan De 2000 years before Galileo |journal=Chinese Astronomy and Astrophysics |language=en |volume=5 |issue=2 |pages=242–243 |doi=10.1016/0275-1062(81)90039-4|bibcode=1981ChA&A...5..242X }}</ref>

The observations of [[Simon Marius]] are another noted example of observation, and he later reported observing the moons in 1609.<ref name="solarviews">{{Cite web |title=The Discovery of the Galilean Satellites |url=https://solarviews.com/eng/galdisc.htm |url-status=live |archive-url=https://web.archive.org/web/20191119090237/http://solarviews.com/eng/galdisc.htm |archive-date=2019-11-19 |access-date=2019-11-17 |website=solarviews.com}}</ref> However, because he did not publish these findings until after Galileo, there is a degree of uncertainty around his records.<ref name="solarviews" />