[6] 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.
[7] 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.
[6][11][12] However, Burch believes Aristotle was having a joke "at the expense of Pythagorean number theory,"[6] 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.
[6] 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—below the equator and the tropics, climes which he believed uninhabitable and impassably hot.
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.
[3] The 1968 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.
Even if the Sun blocked its view from Earth, a Counter-Earth would have gravitational influence (perturbation) upon the other planets, comets and man-made probes of the Solar System.
[16] 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.