Editing Counter-Earth (section)

==Greek Pythagorean universe==
{{further|Pythagorean astronomical system}}
An [[Cosmology#Historical cosmologies|astronomical system]] positing that the Earth, Moon, Sun, planets and unseen "counter-earth" revolve around an unseen "Central Fire" was developed in the 5th century BC and attributed to the [[Pythagoreanism|Pythagorean]]<ref>[http://www.drury.edu/ess/History/Ancient/pythagoreans1.html History of the Pythagorean school] {{Webarchive|url=https://web.archive.org/web/20020612102818/http://www.drury.edu/Ess/History/Ancient/pythagoreans1.html |date=2002-06-12 }} hosted by [[Drury University]], [[Springfield, Missouri]]</ref> [[Ancient Greek philosophy|philosopher]] [[Philolaus]]. Philolaus' universe moved "the earth from the center of the cosmos,"<ref name=stanford>[http://plato.stanford.edu/entries/philolaus/ Philolaus], Stanford Encyclopedia of Philosophy, [[Carl Huffman]].</ref> and provided the insight that "the apparent motion of the heavenly bodies" was (in large part) due to "the real motion of the observer"—i.e. Earth.<ref name=burch/>

In Philolaus' system, the Earth and Counter-Earth revolved around the unseen Central Fire every 24 hours, while the Moon's revolution was monthly, and the Sun's yearly. It was the Earth's speedy travel past the slower moving Sun that resulted in the appearance on Earth of the Sun rising and setting. Further from the Central Fire, the Planets' movement was slower still, and the outermost "sky" (i.e. stars) probably fixed.<ref name=stanford/>

===Counter-Earth<!--Linked from 'Pythagorean astronomical system'-->===
Along with the Central Fire, the "mysterious"<ref name=stanford/> Counter-Earth (''Antichthon'') was the other heavenly body not visible from Earth. [[Aristotle]] described it as "another Earth," from which Greek scholar George Burch infers that it must be similar in size, shape and constitution to Earth.<ref>Burch, 1954, p.285</ref> Some (such as astronomer [[John Louis Emil Dreyer]]) have thought that Philolaus had it following an orbit such that it was always located between Earth and the Central Fire.<ref name=Dreyer>{{cite book|last=Dreyer|first=John Louis Emil|title=History of the planetary systems from Thales to Kepler|year=1906|page=42|quote=To complete the number ten, Philolaus created the antichthon, or counter-earth. This tenth planet is always invisible to us, because it is between us and the central fire and always keeps pace with the Earth.|url=https://books.google.com/books?id=OgRAAQAAIAAJ&q=counter-earth+Antichthon+mysterious&pg=PA42}}</ref> However, Burch argues that Philolaus must have thought that it orbited on the other side of the Fire from Earth. Since "counter" means "opposite," and opposite could only be in respect to the Central Fire, it follows that the Counter-Earth must be orbiting 180 degrees from Earth.<ref>Burch, 1954, p.280</ref>

According to Aristotle—a critic of the Pythagoreans—the function of the Counter-Earth was to explain "eclipses of the moon and their frequency",<ref name=heath>{{cite book|last=Heath|first=Thomas|title=A History of Greek Mathematics, Volume 1|year=1981|publisher=Dover|page=165|url=https://books.google.com/books?id=drnY3Vjix3kC&q=counter-earth&pg=PA164|isbn=9780486240732}}</ref> which could not be explained by Earth alone blocking the light of the Sun if Earth did not revolve around the Sun. Aristotle suggested that it was also introduced "to raise the number of heavenly bodies around the central fire from nine to ten, which the Pythagoreans regarded as the perfect number".<ref name=burch>[https://www.jstor.org/stable/301675?seq=3&Search=yes&searchText=Bosworth.&searchText=Counter-Earth.&searchText=George&searchText=Burch%2C&list=hide&searchUri=%2Faction%2FdoBasicSearch%3FQuery%3DBurch%252C%2BGeorge%2BBosworth.%2BThe%2BCounter-Earth.%2B%26Search%3DSearch%26gw%3Djtx%26prq%3DBurch%252C%2BGeorge%2BBosworth.%2BThe%2BCounter-Earth.%2BOsirus%252C%2Bvol.%2B11.%2BSaint%2BCatherines%2BPress%252C%2B1954.%2Bp.%2B267-294%26hp%3D25%26acc%3Don%26aori%3Da%26wc%3Don%26fc%3Doff&prevSearch=&item=1&ttl=1&returnArticleService=showFullText&resultsServiceName=null Burch, George Bosworth. The Counter-Earth]. ''Osirus'', vol. 11. Saint Catherines Press, 1954. p. 267-294</ref><ref>Arist., Metaph. 986a8–12. quoted in [http://plato.stanford.edu/entries/philolaus/#AstSys Philolaus], Stanford Encyclopedia of Philosophy, Carl Huffman.</ref><ref>{{cite web|title=Greek cosmology, The Pythagoreans|url=http://physics.ucr.edu/~wudka/Physics7/Notes_www/node32.html|publisher=University of California, Riverside|quote=The importance of pure numbers is central to the Pythagorean view of the world. A point was associated with 1, a line with 2 a surface with 3 and a solid with 4. Their sum, 10, was sacred and omnipotent.|access-date=2013-10-20|archive-url=https://web.archive.org/web/20180706094459/http://physics.ucr.edu/~wudka/Physics7/Notes_www/node32.html|archive-date=2018-07-06|url-status=dead}}</ref>

However, Burch believes Aristotle was having a joke "at the expense of Pythagorean number theory,"<ref name=burch/> and that the true purpose of the Counter-Earth was to "balance" Philolaus' cosmos—balance being needed because without a counter there would be only one dense, massive object in the system—Earth. Although his system had both the Earth and the Planets orbiting a single point, the [[Greek astronomy|ancient Greeks]] did not consider Earth a "planet." In the time before [[Galileo Galilei|Galileo]] could observe from his telescope that planets were spheres like Earth, they were thought to be different from stars only in brightness and in their motion, and like stars composed of a fiery or ethereal matter having little or no density. However, the Earth was obviously made of the dense [[classical element|elements]] of earth and water. According to Burch,
<blockquote>If there was a single Earth revolving at some distance from the center of space, then the universe's center of gravity, located in the Earth as its only dense body, would not coincide with its spatial center ... The universe, consequently, would be off center, so to speak—lopsided and asymmetric—a notion repugnant to any Greek, and doubly so to a Pythagorean.<ref>Burch, 1954, p.286-7</ref></blockquote> 
This could be corrected by another body with the same mass as Earth, orbiting the same central point but 180 degrees from Earth—the Counter-Earth.<ref name="burch"/>

===Later===
In the 1st century AD, after the idea of a spherical Earth had gained more general acceptance, [[Pomponius Mela]], a Latin cosmographer, developed an updated version of the idea, wherein a [[spherical Earth]] must have a more or less balanced distribution of land and water, even though all known continents were in the northern hemisphere. Mela drew a map which postulated a continental landmass in the unknown, southern half of Earth—the [[Antipodes#Historical significance|antipodes]]—below the equator and the tropics, [[clime]]s which he believed uninhabitable and impassably hot. Mela ascribed the name "[[Antichthones]]" to the inhabitants of this continent.<ref>Pomponius Mela. ''de Chorographia''.</ref>