Editing Definition of planet (section)

=== Satellites ===
{{see also|Satellite planet}}
[[File:Jupiter and the Galilean moons animation.gif|thumb|Motion of the Galilean moons seen from a telescope]]
When Copernicus placed Earth among the planets, he also placed the Moon in orbit around Earth, making the Moon the first [[natural satellite]] to be identified. When [[Galileo Galilei|Galileo]] discovered his four [[Galilean moons|satellites]] of Jupiter in 1610, they lent weight to Copernicus's argument, because if other planets could have satellites, then Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness."<ref>{{cite book|title=Siderius Nuncius|author=Galileo Galilei|publisher=University of Chicago Press|year=1989|page=26|others=Albert van Helden}}</ref> Similarly, [[Christiaan Huygens]], upon discovering Saturn's largest moon [[Titan (moon)|Titan]] in 1655, employed many terms to describe it, including "planeta" (planet), "stella" (star), "luna" (moon), and "satellite" (attendant), a word coined by [[Johannes Kepler]].<ref>{{cite web|title=Johannes Kepler: His Life, His Laws and Times|date=September 24, 2016|publisher=NASA|url=https://www.nasa.gov/kepler/education/johannes|accessdate=September 22, 2022|archive-date=June 24, 2021|archive-url=https://web.archive.org/web/20210624003856/https://www.nasa.gov/kepler/education/johannes/|url-status=dead}}</ref><ref>{{cite book|title=Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo|author= Christiani Hugenii (Christiaan Huygens)|publisher=Adriani Vlacq|year= 1659|pages= 1–50}}</ref> [[Giovanni Cassini]], in announcing his discovery of Saturn's moons [[Iapetus (moon)|Iapetus]] and [[Rhea (moon)|Rhea]] in 1671 and 1672, described them as ''Nouvelles Planetes autour de Saturne'' ("New planets around Saturn").<ref>{{cite book|title=Decouverte de deux Nouvelles Planetes autour de Saturne|author=Giovanni Cassini|publisher=Sabastien Mabre-Craniusy|year= 1673| pages=6–14}}</ref> However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons ([[Dione (moon)|Dione]] and [[Tethys (moon)|Tethys]]) in 1686, it referred to them strictly as "satellites", though sometimes Saturn as the "primary planet".<ref name="Cassini1686–1692">{{cite journal| doi = 10.1098/rstl.1686.0013| last = Cassini | first = G. D.| author-link = Giovanni Domenico Cassini| year = 1686–1692| title = An Extract of the Journal Des Scavans. Of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris| journal = Philosophical Transactions of the Royal Society of London| volume = 16| issue = 179–191| pages = 79–85| jstor = 101844| bibcode = 1686RSPT...16...79C| doi-access = free}}<!-- This journal became the Philosophical Transactions of the Royal Society of London in 1775. Are there any earlier publications? Two years seems a long time... --></ref> When William Herschel announced his discovery of two objects in orbit around Uranus in 1787 ([[Titania (moon)|Titania]] and [[Oberon (moon)|Oberon]]), he referred to them as "satellites" and "secondary planets".<ref>{{cite book|title=An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society|author=William Herschel|publisher=J. Nichols|year=1787|pages=1–4}}</ref> All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,<ref>See primary citations in [[Timeline of discovery of Solar System planets and their moons#References|Timeline of discovery of Solar System planets and their moons]]</ref> though the 1868 book "Smith's Illustrated Astronomy" referred to satellites as "secondary planets".<ref name="smith">{{cite book
 |first = Asa | last = Smith
 |year=1868
 |title=Smith's Illustrated Astronomy
 |publisher=Nichols & Hall
 |url=https://archive.org/details/smithsillustrat00smitgoog |page = [https://archive.org/details/smithsillustrat00smitgoog/page/n27 23] |quote = secondary planet Herschel. }}</ref>