Editing Counter-Earth (section)

==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Philolaus's ideas were all eventually [[superseded scientific theory|superseded]] by the modern realization that a spherical Earth rotating on its own axis was one of several spherical planets following the laws of gravity and revolving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>