from The Century Dictionary and Cyclopedia
- n. In Pythagorean astronomy, an imaginary invisible planet continually opposing the earth and eclipsing the central fire, round which it was supposed to revolve, in common with the earth, moon, sun, certain planets, and the fixed stars.
- n. (plural) The inhabitants of an opposite hemisphere.
The highest of these bodies is the outer crystal sphere of the heavens containing the fixed stars; next in their regular order, also affixed to spheres, were the five separate planets, then the sun, moon, earth, and, finally, the counter-earth, named Antichthon.
Ancient Greek philosophers were the first to come up with the idea of a Counter-Earth, an astronomical body forever hidden on the far side of the sun. 'Antichthon,' as they called it, has been described in fiction as an exact duplicate of Earth, a mirror image, or even a planet where time runs backward.
This comes from a Greek term meaning ‘counterearth, southern hemisphere.’