The Geocentric View of the Solar System
Geocentrism is a disproved theory that places the Earth at the center of the Universe with every other heavenly body orbit around it. The theory was first introduced in the 6th century B.C by Anaximander, but did not become widely popular until the 2nd century A.D. When Ptolemy introduced his theories. Today the theory is only studied as an obsolete belief; however, there are some religions that use a modified version to support their theological beliefs.
The geocentric model can be found in Greek philosophy before Socrates. Anaximander proposed that the Earth was shaped like a cylinder and was at the center of everything. The Sun, Moon, and planets were holes in invisible wheels surrounding the Earth. He believed that humans could see concealed fire through those holes. The Pythagorean school thought that the Earth was a sphere, but not at the center of the universe. They believed that it was in motion around an unseen fire. These views were eventually combined, so most educated Greeks from the 4th century BC on thought that the Earth was a sphere at the center of the universe. The 4th century B.C. Brought two influential Greek philosophers who wrote about the geocentric model(Plato and Aristotle). According to Plato, the Earth was a stationary sphere at the center of the universe. He believed that the stars and planets were carried around the Earth on sphere arranged in the following order: Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, then the fixed stars. Eudoxus developed a mathematical explanation of the planets’ motion based on Plato’s work stating that everything in the heavens can be explained with uniform circular motion. Aristotle expanded Eudoxus’ system. In the Aristotelian system, the spherical Earth is at the center of the universe. All heavenly bodies are attached to 56 concentric spheres which rotate around the Earth . The Moon is on the innermost sphere, so it touches the realm of Earth, contaminating it and causing the dark spots(macula) and giving it the ability to go through phases.
The Ptolemaic system holds that each planet is moved by two or more spheres: one of which is the deferent. He described the deferent as a circle centered on a point halfway between the equant and the Earth. Another sphere, the epicycle, is embedded in the deferent. The planet is embedded in the epicycle sphere. The deferent rotates around the Earth while the epicycle rotates within the deferent, causing the planet to move closer to and farther from Earth during its orbit. The epicycle could cause a planet to slow down, stop, and move backward (retrograde motion). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun. That was meant to explain why they were always near in the sky. The Ptolemaic order of spheres from Earth outward is: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, the fixed stars, and the sphere of Prime Mover. By using an equant, Ptolemy claimed motion was uniform and circular. The end result was an unwieldy system by modern standards. Each planet required an epicycle revolving on a deferent which offset by an equant that was different for each planet. The plus side was that it predicted various celestial motions, including the beginnings and ends of retrograde motion, fairly well for the period.
Today it is hard to separate geocentrism from the Ptolemaic system. Even though many of his theories have been disproved, Ptolemy is one of the greatest contributors to to modern astronomy.
We have written many articles about Geocentrism for Universe Today. Here’s an article about the Geocentric Model, and here’s an article about the difference between the Geocentric and Heliocentric Model.
If you’d like more info on Geocentrism, check out an article about the Geocentric Theory, and here’s a link to an article about the center of the galaxy.
We’ve also recorded a series of episodes of Astronomy Cast about the center of the universe. Listen here, Episode 77: Where is the Centre of the Universe.
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