Editing Counter-Earth

{{short description|Hypothetical planet on the other side of the Sun from Earth}}
[[File:Antichthon.svg|thumb|250px|right|Philolaus believed there was a "Counter-Earth" (''Antichthon'') orbiting the "Central Fire" (not labeled) that was not visible from Earth. The upper illustration depicts Earth at night while the lower one depicts Earth in the day.<ref name="DatEA">Source: ''Dante and the Early Astronomers'' by M. A. Orr, 1913.</ref>]]
The '''Counter-Earth''' is a [[:Category: Hypothetical bodies of the Solar System|hypothetical body of the Solar System]] that orbits on the other side of the [[Solar System]] from [[Earth]]. A Counter-Earth or '''''Antichthon''''' ({{langx|el|Ἀντίχθων}}) was hypothesized by the [[pre-Socratic philosophy|pre-Socratic]] [[Ancient Greece|Greek]] [[philosopher]] [[Philolaus]] ({{Circa|470|385 BC}}) to support his non-[[geocentric model|geocentric]] [[cosmology]], in which all objects in the universe revolve around a "Central Fire" (unseen from Earth and distinct from the [[Sun]] which also revolves around it).

In modern times a hypothetical planet always on the other side of the Sun from [[Earth]] has been called a "Counter-Earth",<ref name=Comins>{{cite book|last= Comins|first=Neil F.|title=What If the Earth Had Two Moons?: And Nine Other Thought-Provoking ...|year= 2010|publisher= macmillan|pages=171–172|url= https://books.google.com/books?id=TNqBI4QKZSAC&q=counter-earth&pg=PA171|isbn=9781429957939}}</ref> and has been a recurring theme in [[Unidentified flying object|UFO]] claims,<ref name=UFO/> as well as in fiction (particularly [[Fictional planets of the Solar System#Counter-Earth|science fiction]]).

==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>

==Modern era==
[[File:Gor-orbit-diagram.svg|thumb|Diagram of modern conception of the Counter-Earth, a planet in the same orbit as the Earth, but 180° out of phase]]
Philolaus's ideas were all eventually [[superseded scientific theory|superseded]] by the modern realization that a spherical Earth rotating on its own axis was one of several spherical planets following the laws of gravity and revolving around a much larger Sun. The idea of a Counter-Earth waned after the heliocentric model of the solar system became widely accepted from the 16th century. In the contemporary world, "Counter-Earth" usually refers to a hypothetical planet with an orbit as Burch described, on the other side of the "Central Fire"—i.e. the Sun. It cannot be seen from Earth, not because Earth faces away from the center,{{#tag:ref|Pythagoreans are thought to have believed that humans inhabited the side of earth facing away from the Central Fire and Counter Earth.<ref name=DatEA/>|group=Note}} but because the Sun's great size blocks its view. It has been a recurring motif in fiction—often serving as an [[allegory]] for the real Earth—and [[Unidentified flying object|UFO]] claims.<ref name=UFO>[https://books.google.com/books?id=BRHoKVTawnMC&dq=ufos+from+another+planet+behind+the+sun&pg=PA20 The Ufo Book of Lists]|  By STEPHEN J SPIGNESI, Stephen J. Spignesi|</ref> The 1968 ''[[Condon Committee|Scientific Study of Unidentified Flying Objects]]'' headed by Edward Condon at the [[University of Colorado]] even included a "Numerical Experiment on the Possible Existence of an 'Anti-Earth{{' "}} as an appendix.<ref name=DUNCOMBE/>

===Detectability===
[[File:Lagrange points2.svg|thumb|right|230px|A contour plot of the [[effective potential]] due to gravity and the [[centrifugal force]] of a two-body system in a rotating frame of reference. (In the context of this article the two bodies are the Sun—the yellow body—and Earth—the dark body between L1 and L2. The Counter-Earth would exist at L3.) The arrows indicate the gradients of the potential around the five [[Lagrange points]]—downhill toward them (<span style="color:red;">red</span>) or away from them (<span style="color:blue;">blue</span>). Counterintuitively, the L<sub>4</sub> and L<sub>5</sub> points are the [[Maxima and minima|high points]] of the potential. At the points themselves these forces are balanced.]]

A Counter-Earth could still be detected from the Earth for a number of reasons. Even if the Sun blocked its view from Earth, a Counter-Earth would have gravitational influence ([[Perturbation (astronomy)|perturbation]]) upon the other planets, comets and man-made probes of the [[Solar System]].<ref name=DUNCOMBE>{{cite web|last=DUNCOMBE|first=R. L.|title=Appendix E. Report on Numerical Experiment on the Possible Existence of an "Anti-Earth"|url=http://files.ncas.org/condon/text/appndx-e.htm|work=1968|publisher=U.S. NAVAL OBSERVATORY|access-date=24 October 2013|quote=At the end of 112 years the perturbations induced by Clarion [a name for the Counter-Earth] in the motions of Venus, Earth, and Mars reached 1200", 3800", and 1660" respectively.}}</ref> Researchers have detected no such influence, and indeed space probes sent to [[Venus]], [[Mars]] and other places could not have successfully flown by or landed on their targets if a Counter-Earth existed, as the navigational calculations for their journeys did not take any putative Counter-Earth into account. Roughly speaking, anything larger than {{convert|100|mi|km}} in diameter should have been detected.<ref name=could>[http://www.popsci.com/scitech/article/2009-04/could-there-be-planet-hidden-opposite-side-our-sun Could There Be a Planet Hidden on the Opposite Side of our Sun? PopSci asks the scientist who has peered around it]| By Danny Freedman| PopSci | 20 April 2009</ref>

Any planetary-sized body 180 degrees from Earth should also have been visible to some space probes, such as NASA's [[STEREO]] [[coronagraph]] probes (two spacecraft launched into orbits around the Sun in 2006, one farther ahead of and one behind the Earth's orbit) which would have seen the Counter-Earth during the first half of 2007. The separation of the STEREO spacecraft from Earth would give them a view of the L3 point during the early phase of the mission.<ref name=could/>

A Counter-Earth would also eventually be visible from Earth because the gravitational forces of the other planets on it would make its own orbit unstable. [[Venus]] has 82% of the mass of Earth and would come within 0.3&nbsp;[[Astronomical unit|AU]] of the location of a Counter-Earth every 20 months, providing considerable gravitational pull that over the years would move its orbit into sight of observers on Earth.<ref name=DUNCOMBE2>{{cite web|last=DUNCOMBE|first=R. L.|title=Appendix E. Report on Numerical Experiment on the Possible Existence of an "Anti-Earth"|url=http://files.ncas.org/condon/text/appndx-e.htm|work=1968|publisher=U.S. NAVAL OBSERVATORY|access-date=24 October 2013|quote=The separation of [a Counter-Earth] from the line joining the Earth and the Sun shows a variation with increasing amplitude in time, the effect being most pronounced for the largest assumed mass. During the 112 years covered by the integration the separation becomes large enough in all cases that Clarion should have been directly observed, particularly at times of morning or evening twilight and during total solar eclipses.}}</ref>

===In fiction===
{{further|Fictional planets of the Solar System#Counter-Earth}}
Counter-Earth has made appearances in fiction since the late 1800s.<ref name="StanwayCounterweightWorlds" /> It is variously depicted as very similar to Earth or very different,<ref name="VisualEncyclopediaFutureAndAlternativeHistories">{{Cite book |title=[[The Visual Encyclopedia of Science Fiction]] |date=1977 |publisher=Harmony Books |isbn=0-517-53174-7 |editor-last=Ash |editor-first=Brian |editor-link=Brian Ash (bibliographer) |pages=122 |chapter=Future and Alternative Histories |oclc=2984418 |chapter-url=https://archive.org/details/visualencycloped00ashb/page/122/mode/2up}}</ref> and often employed as a vehicle for [[satire]].<ref name="StablefordPlanet">{{Cite book |last=Stableford |first=Brian |author-link=Brian Stableford |title=[[Science Fact and Science Fiction: An Encyclopedia]] |date=2006 |publisher=Taylor & Francis |isbn=978-0-415-97460-8 |pages=374–376 |language=en |chapter=Planet |chapter-url=https://books.google.com/books?id=uefwmdROKTAC&pg=PA375}}</ref> Counter-Earth being inhabited by counterparts of the people of Earth is a recurring theme.<ref name="StanwayCounterweightWorlds" /><ref name="SFECounterEarth" /> Variations on the concept also occasionally appear, including extrasolar equivalents, the [[Mars]] equivalent "Counter-Mars", and a planet hidden on the other side of the [[Moon]] rather than [[Sun]].<ref name="StanwayCounterweightWorlds">{{Cite web |last=Stanway |first=Elizabeth |author-link=<!-- No article at present (July 2024); Stanway is an astrophysicist at the University of Warwick who has been published in [[Foundation (journal)]], among others (see https://warwick.ac.uk/fac/sci/physics/research/astro/people/stanway/sciencefiction); Wikidata Q127710708 --> |date=2023-09-24 |title=Counterweight Worlds |url=https://warwick.ac.uk/fac/sci/physics/research/astro/people/stanway/sciencefiction/cosmicstories/counterweight_worlds/ |url-status=live |archive-url=https://web.archive.org/web/20240326225747/https://warwick.ac.uk/fac/sci/physics/research/astro/people/stanway/sciencefiction/cosmicstories/counterweight_worlds/ |archive-date=2024-03-26 |access-date=2024-03-27 |website=[[Warwick University]] |series=Cosmic Stories Blog}}</ref><ref name="SFECounterEarth">{{Cite encyclopedia |year=2022 |title=Counter-Earth |encyclopedia=[[The Encyclopedia of Science Fiction]] |url=https://sf-encyclopedia.com/entry/counter-earth |access-date=2023-04-10 |edition=4th |author1-last=Langford |author1-first=David |author1-link=David Langford |editor1-last=Clute |editor1-first=John |editor1-link=John Clute |editor2-last=Langford |editor2-first=David |editor2-link=David Langford |editor3-last=Sleight |editor3-first=Graham |editor3-link=Graham Sleight}}</ref><ref name="BloomMirrorUniversesAndReversedTimeUniverses">{{Cite book |last=Bloom |first=Steven D. |title=The Physics and Astronomy of Science Fiction: Understanding Interstellar Travel, Teleportation, Time Travel, Alien Life and Other Genre Fixtures |date=2016 |publisher=McFarland |isbn=978-0-7864-7053-2 |pages=104–106 |language=en |chapter=Mirror Universes and Reversed Time Universes |chapter-url=https://books.google.com/books?id=8NbIDAAAQBAJ&pg=PA104}}</ref><ref name="GernsbackYearsScienceFictionSolarSystem">{{Cite book |last1=Bleiler |first1=Everett Franklin |title=[[Science-fiction: The Gernsback Years : a Complete Coverage of the Genre Magazines ... from 1926 Through 1936]] |last2=Bleiler |first2=Richard |date=1998 |publisher=Kent State University Press |isbn=978-0-87338-604-3 |pages=539 |language=en |chapter=The Science-Fiction Solar System |author-link1=E. F. Bleiler |author-link2=Richard Bleiler |chapter-url=https://books.google.com/books?id=PbMdeizaCNcC&pg=PA539}}</ref>

==See also==
*[[Pythagorean astronomical system]]
*[[Globus Cassus]]
*[[Co-orbital moon]]
*[[Fictional planets of the Solar System]]
*[[Nemesis (hypothetical star)]]

==Notes==
{{reflist|group=Note}}

M.Admin. (2014, March 26). The Bizarre Greek Theory About The Solar System. KnowledgeNuts. https://knowledgenuts.com/2014/03/26/the-bizarre-greek-theory-about-the-solar-system/

==References==
{{reflist}}

==Further reading==
*''The Secret Teachings of All Ages: An Encyclopedic Outline of Masonic, Hermetic, Qabbalistic and Rosicrucian Symbolical Philosophy'', by Manly P. Hall, Philosophical Research Society Inc. {{ISBN|1-58542-250-9}}
*''Book of Earths'', by [[Edna Kenton]], Kessinger Publishing. {{ISBN|0-7661-2856-3}}

==External links==
*{{Commonscatinline|Counter-Earths}}

{{Greek astronomy|state=collapsed}}
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[[Category:Ancient Greek astronomy]]
[[Category:Early scientific cosmologies]]
[[Category:Pseudoastronomy]]
[[Category:Hypothetical bodies of the Solar System]]