Counter-Earth

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 Pythagorean philosopher 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. Some (such as astronomer John Louis Emil Dreyer) have thought that Philolaus had it following an orbit such that it was always located between Earth and the Central Fire. However, Burch argues that Philolaus must have thought that 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. According to Aristotle—a critic of the Pythagoreans—the function of the Counter-Earth was to explain "eclipses of the moon and their frequency", which could not be explained by Earth alone blocking the light of the Sun if Earth did not revolve around the Sun. Aristotle suggested that it was also introduced "to raise the number of heavenly bodies around the central fire from nine to ten, which the Pythagoreans regarded as the perfect number". However, Burch believes that Aristotle was having a joke "at the expense of Pythagorean number theory", If there was a single Earth revolving at some distance from the center of space, then the universe's center of gravity, located in the Earth as its only dense body, would not coincide with its spatial center ... The universe, consequently, would be off center, so to speak—lopsided and asymmetric—a notion repugnant to any Greek, and doubly so to a Pythagorean. This could be corrected by another body with the same mass as Earth, orbiting the same central point but 180 degrees from Earth—the Counter-Earth. ==Modern era==

Philolaus's ideas were all eventually superseded by the modern realization that a spherical Earth rotating on its own axis was one of several spherical planets following the laws of gravity and revolving around a much larger Sun. The idea of a Counter-Earth waned after the heliocentric model of the solar system became widely accepted from the 16th century. In the contemporary world, "Counter-Earth" usually refers to a hypothetical planet with an orbit as Burch described, on the other side of the "Central Fire"—i.e. the Sun. It cannot be seen from Earth, not because Earth faces away from the center, but because the Sun's great size blocks its view. It has been a recurring motif in fiction—often serving as an allegory for the real Earth—and UFO claims. The 1968 Scientific Study of Unidentified Flying Objects headed by Edward Condon at the University of Colorado even included a "Numerical Experiment on the Possible Existence of an 'Anti-Earth as an appendix. Researchers have detected no such influence, and indeed space probes sent to Venus, Mars and other places could not have successfully flown by or landed on their targets if a Counter-Earth existed, as the navigational calculations for their journeys did not take any putative Counter-Earth into account. Roughly speaking, anything larger than in diameter should have been detected. Any planetary-sized body 180 degrees from Earth should also have been visible to some space probes, such as NASA's STEREO coronagraph probes (two spacecraft launched into orbits around the Sun in 2006, one farther ahead of and one behind the Earth's orbit) which would have seen the Counter-Earth during the first half of 2007. The separation of the STEREO spacecraft from Earth would give them a view of the L3 point during the early phase of the mission. In fiction Counter-Earth has made appearances in fiction since the late 1800s. and often employed as a vehicle for satire. Counter-Earth being inhabited by counterparts of the people of Earth is a recurring theme. ==See also==