Counter-Earth

The Counter-Earth is a hypothetical body of the Solar system hypothesized by the pre-Socratic Greek philosopher Philolaus (c. 470 – c. 385 BC) to support his non-geocentric cosmology, in which all objects in the universe revolve around an unseen "Central Fire" (distinct from the Sun which also revolves around it). The Greek word Antichthon (Greek: Ἀντίχθων) means "Counter-Earth."

In modern times a hypothetical planet always on the other side of the Sun from the Earth has been called a "Counter-Earth", and has been a recurring theme in fiction, science fiction and UFO claims.

An astronomical system positing that the Earth, Moon, Sun, planets and unseen "counter-earth" revolve around an unseen "Central Fire" was developed in the 5th century BC and attributed to the Pythagoreanphilosopher Philolaus. Philolaus' universe moved "the earth from the center of the cosmos", and provided the insight that "the apparent motion of the heavenly bodies" was (in large part) due to "the real motion of the observer"—i.e. Earth.

In Philolaus' system, the Earth did not rotate and its inhabited surface faced away from the Central Fire—possibly because it (the Earth) was flat. The revolution of the Earth around the Central Fire was not yearly but daily, while the Moon's revolution was monthly, and the sun's yearly. It was the Earth's speedy travel past the slower moving Sun that resulted in the appearance on Earth of the Sun rising and setting. Further from the Central Fire, the Planets' movement was slower still, and the outermost "sky" (i.e. stars) probably fixed.

Along with the Central Fire, the "mysterious" Counter-Earth (Antichthon) was the other heavenly body not visible from Earth. We know that Aristotle described it as "another Earth", from which Greek scholar George Burch infers that it must be similar in size, shape and constitution to Earth. Some (astronomer John Louis Emil Dreyer) think Philolaus had it following an orbit so that it was always located between Earth and the Central Fire, and a tale of Greek mythology may have placed it in that location to stop man from looking at the throne of Zeus directly. However, Burch argues Philolaus must have thought it orbited on the other side of the Fire from Earth. Since "counter" means "opposite", and opposite could only be in respect to the Central Fire, it follows that the Counter-Earth must be orbiting 180 degrees from Earth.

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