An imagined extra planet of our own solar system, supposedly sharing Earth's orbit but always concealed from observation by its position on the far side of the Sun (that is, at or near the L3 Lagrange Point of the Earth-Sun system). The concept is ancient: Pythagoras proposed both the world, which he called Antichthon, and the mechanism of its concealment in the fifth century BCE. Although Counter-Earth – or Vulcan, as it is often named – would not in fact remain hidden owing to Newtonian perturbations of its orbit by the other planets, it has a superficial plausibility as an alternate Earth which does not require the explanatory apparatus of a Parallel World and some Imaginary-Science means of travelling there. An early sf instance is D L Stump's From World to World (1896; exp vt The Love of Meltha Laone; Or, Beyond the Sun 1913), where Counter-Earth proves to be an advanced Utopia. Paul Capon's Antigeos trilogy beginning with The Other Side of the Sun (1950) is a more recent example. The Gor series by John Norman – opening with Tarnsman of Gor (1966) – uses Counter-Earth as a setting for idiosyncratic Planetary Romance; so, less controversially, does Richard A Lupoff's Countersolar! (1987). The Cybermen, recurring Cyborg villains in Doctor Who (1963-current), supposedly originate from a former Counter-Earth called Mondas.
The notion of Earth's twin planet has frequently been developed into more or less literal twinning, as in Planetoid 127 (4 September-23 October 1924 The Mechanical Boy; as title story of coll 1929; 1986) by Edgar Wallace – where stock exchange movements on "Vulcan" conveniently anticipate Earth's by three days – and in Split Image (1955) by Reed de Rouen. Earthly characters often have doubles or counterparts on such Counter-Earths: complications of this nature are central to the film Doppelganger (1969). The "Terra" of the later film The Stranger (1973; vt Stranded in Space) is almost indistinguishable from Earth, and the Counter-Earth of Ben Barzman's Out of this World (1960; vt Twinkle, Twinkle, Little Star 1960; vt Echo X 1962) is essentially an Alternate-History setting in which World War Two did not happen.
By way of variation, Paul Ernst proposed a miniature, roughly Moon-sized Counter-Earth forever (impossibly) hidden behind our satellite in "The World Behind the Moon" (April 1931 Astounding). Another such planet is placed behind the Moon by its Alien inhabitants in W J Passingham's identically titled "The World Behind the Moon" (22 October 1938-28 June 1939 Modern Wonder). Malcolm MacCloud's A Gift of Mirrorvax (1981) sets its equivalents of Earth and Counter-Earth – here called Vax and Mirrorvax – in another solar system.
A Fantastic Voyage to a Counter-Earth is implied by the title of Lord Dunsany's Club Story "On the Other Side of the Sun" (in The Fourth Book of Jorkens, coll 1947), in which raconteur Jorkens puts money on being able to prove he has visited the titular location. He wins, since six months previously the entire Earth ... [DRL]
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