Antichthon

Part of speech: noun

Definitions

1. A hypothetical counter-Earth, believed to exist on the opposite side of the Sun from our planet
2. an ancient philosophical concept suggesting a celestial body that mirrors Earth's orbit
3. a theoretical location posited in ancient cosmology, representing an unseen world parallel to our own

Etymology: In ancient Greek cosmology, the "antichthon" was a hypothetical body placed on the far side of the central fire, directly opposite the known Earth, so that it stayed permanently hidden from people living on our side. The name is a plain compound: "antí" meaning "against" or "opposite," and "khthōn," meaning "earth" or "land." Literally, it means "counter-earth." The idea was most closely associated with the Pythagoreans, who held that the cosmos was organised around a central fire rather than around the Earth. The counter-earth was needed to make the number of heavenly bodies come out to a sacred total, and it also explained why the central fire was never seen from our side. Later writers, including Aristotle, argued against the notion, though the concept remained a point of debate in Greek astronomy.