In ancient astronomy, a hypothetical planet thought to exist on the opposite side of the sun from Earth.
From 'counter-' + 'earth' (from Old English 'eorthe'). The Pythagoreans proposed counterearth to make the number of celestial bodies equal ten, a number they considered sacred.
The ancient Greeks invented counterearth as a philosophical necessity because they thought ten was a perfect number—it's a stunning example of how metaphysics once shaped astronomy. We wanted the universe to fit our math, not the other way around.
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