A hypothetical counter-earth or planet on the opposite side of the sun from Earth, proposed by ancient Greek philosophers.
From Greek anti- (against, opposite) + chthon (earth). This term was coined by ancient Pythagorean philosophers who theorized an invisible planet opposite Earth to make celestial mechanics work mathematically, representing early speculative astronomy.
The Pythagoreans invented the antichthon around 450 BCE to explain why we always see the same side of the moon—they needed an invisible planet to balance cosmic numbers, showing how philosophy and early science created imaginary worlds before telescopes.
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