A counter-Earth is a planet always opposite the Earth in its orbit around the Sun - or, as first conceived by Greek philosopher Philolaus, in its orbit around the central fire, around which the Sun also orbited.
Did any scifi works feature a counter-Earth prior to John Norman's Gor in the 1960s?
Everett F. Bleiler's monumental Science-Fiction: The Early years, "A Full Description of More Than 3,000 Science-fiction Stories from Earliest Times to the Appearance of the Genre Magazines in 1930", has a very detailed Motif Index. Of the four stories listed under the heading "The counterearth (A planet in earth's orbit forever invisible, on the other side of the sun)", the earliest is From World to World, an 1896 novel by D(avid) L(eroy) Stump. Here is an excerpt from Bleiler's review:
An eccentric novel that is in some ways a preparation for the more elaborate The Love of Meltha Laone.
The story line: Chris Asbury, who has reasoned out that there should be a counterearth on the other side of the sun, builds a small spaceship, flies out away from the earth's gravity, and awaits the coming of the other planet. Everything moves according to plan, and Chris is soon exploring the counterearth with the Laone family and becomes engaged to Meltha, the daughter of the house. Deciding to remain on the other world with Meltha, Chris sends his manuscript back to earth by an automatic device that can be used for further communications.
More significant than the romance, which is handled briefly and in passing, is the description of the other world. It is almost identical to earth in general features, and some principle of rigid parallel evolution must be at work, since the people are completely human and speak a language close enough to English to be understood.
The author stresses social and economic circumstances, which are obviously greatly influenced by Bellamism, although no reference is made to Bellamy. The country is socialist in that the state technically owns all the land and buildings, but the holder can bequeath, in a limited way, buildings and lands as long as they are used. Money has been abolished, and goods are placed in warehouses, whence they are ordered through what amount to catalogue stores. Telephone orders are possible, and extensive pneumatic tubes convey merchandise and other matters directly to individual homes.
The 1969 movie Journey to the Far Side of the Sun, AKA Doppelgänger, was the first time I heard of the idea.
from the IMDB entry:
A planet is discovered in the same orbit as Earth's but is located on the exact opposite side of the sun, making it not visible from Earth. The European Space Exploration Council decide to send American astronaut Glenn Ross and British scientist John Kane via spaceship to explore the other planet. After a disastrous crash-landing Ross awakes to learn that Kane lies near death and that they apparently have returned to Earth, as evidenced by the presence of the Council director and his staff. Released to the custody of his wife, he soon learns things are not as they seem.
The Earliest counter-Earth in American radio was probably Krypton. Krypton was and is usually described as an exoplanet in another star system.
But the radio program The Adventures of Superman (1940-1951) described Krypton as a counter-Earth beginning on February 1940. In the first few episodes Krypton was destroyed and Superman journeyed to Earth.
https://en.wikipedia.org/wiki/The_Adventures_of_Superman_(radio)1
In the American TV series The Adventures of Superman (1952-1958) the first episode showed the destruction of Krypton and Superman travelling to Earth. The narrator describes Krypton as being "millions of miles" from Earth, which is literally consistent with it being a counter-Earth.
https://en.wikipedia.org/wiki/Krypton_(comics)2
If Krypton was not depicted as a counter-Earth the first in an American science fiction TV program might have been Olympus in the animated Sport Billy, produced by Filmation in the USA but first broadcast in Europe in 1980 before being shown in the USA in 1982.
https://en.wikipedia.org/wiki/Sport_Billy3
The first counter-Earth in American comic strips was probably Terra in Twin Earths which rain as a daily strip from June 16, 1952 to May 25, 1963 and a Sunday strip from March 1, 1953 to December 28, 1958.
https://en.wikipedia.org/wiki/Twin_Earths4
The first counter-Earth in an American novel I could find was in Planetoid 127 by Edgar Wallace in 1924 (but user 14111 seems to have found an 1896 example as in his accepted answer). John Norman could have read about a counter-Earth in Ben Barzman's 1960 novel Out of This World, also known as Twinkle Twinkle Little Star, also known as Echo X.
https://www.cthreepo.com/book-reviews/echo-x-by-ben-barzman-1960/5
http://galacticjourney.org/dec-29-1960-out-of-this-world-ben-barzmans-twinkle-twinkle-little-star/6
