The Counter-Earth is a hypothetical body of the Solar system first hypothesized by the presocratic philosopher Philolaus to support his non-geocentric cosmology, in which all objects in the universe revolve around a Central Fire. The Greek word "Antichthon" means "Counter-Earth."
The need for a Counter-Earth
By 500 BC most contemporary philosophers considered the Earth to be spherical — there was obvious evidence for this from the behavior of objects near the horizon. This meant that all objects on the surface of the Earth had to be attracted to its center in some way, otherwise they would fall off. It also required other objects, such as the stars and planets, to float above the earth in relation to its center, otherwise they would presumably move rapidly away. This argument — reasonable in view of the data available at the time — resulted in a geocentric world view.
When the movements of celestial objects convinced Philolaus that the world must be not only turning on its own axis but revolving around a fixed point elsewhere in space, he was faced with the problem of explaining how a spherical world could move in this way without spilling everything on the surface into space. He came to the conclusion that the directions of up and down do not exist in space, except in that all things must fall towards the center of the universe, around which all things (including the Earth, Sun, and all the planets) must revolve. Our earth must be a practically flat world and the underside of our Earth must face this fiery, central point at all times, otherwise we would fall off.
This created a contradiction within the Pythagorean school of thought. Since planets, in their understanding, were composed of a fiery or ethereal matter having little or no density, they could quite easily rotate eccentric to the Earth without becoming off balance. However, the Earth was obviously made of the dense elements of Earth and Water. If there were a single Earth revolving at some distance from the center of space, the universe's center of balance would not coincide with its spatial center. Since this is the point towards which things fall, the earth must have a counter-balance of the same mass or the universe would be flung apart. This problem led Philolaus to develop the idea of a Counter-Earth, a second, flat Earth, identical but opposite to ours in every way. This conception of the solar system is outlined in the diagram at the right, with Counter-Earth referred to as Antichthon. The upper illustration depicts Earth at night while the lower one depicts Earth in the day. (In order to prevent confusion, the diagram fails to show that Earth and Counter-Earth are flat and point away from the Central Fire). It is likely that Philolaus believed that the whole orbit of Earth was composed of an ethereal sphere, with the Earth and Counter-Earth being local dense points on the surface.
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