What is the center of our solar system? Is it the earth, or the sun? At one time the geocentric or earth-centered model of the universe as espoused by Ptolemy reigned supreme. Then came Copernicus and his heliocentric or sun-centered model. What are the differences?
The Logical Choice
Simply put, a geocentric model1 positions earth (Greek: ge) as the center of our solar system (once thought to be the center of the universe), whereas a heliocentric model2 places the sun (Greek: helios) as the center. These two opposing models or theories were the source of much contention, in the scientific, the religious and the political arenas.
Imagine many hundreds of years ago, looking up to see the sun arcing across the sky. At night, the stars and planets do the same, giving credence to the belief that all heavenly bodies move, while the earth remains at rest. It makes sense; the earthly observer senses no motion. There was no evidence to the contrary.
Imagine someone sitting in a favorite rocker on his front porch. Traffic in the road in front of his house is moving, each of the vehicles at its own speed. Then he notices that occasionally some of them reverse direction, backing up a short distance, then resuming forward motion. In fact not just one, but a number of them are doing the same thing.
In astronomy, this is called retrograde motion, and it happens a lot. The planets in our solar system move west-to-east then change direction to move east-to-west a short while, then revert to west-to-east motion. This peculiarity should have alerted early scientific minds, and indeed, it did. However based on limited data, rather than abandon it, they sought to “explain the anomalous behavior" within the context of the geocentric model.
The Catholic Church
The Bible indicates God created the heavens and the earth, and then Man in the Garden of Eden. To some this meant God views the earth as the center of the universe. Anyone saying differently was considered by the Catholic Church to be committing serious sin, meriting severe punishment. This religious influence significantly hampered consideration of a sun-centered solar system. Galileo faced the Inquisition for his belief in the heliotropic model of the solar system. He recanted and lived, but suffered imprisonment for the rest of his life.
“Explaining" Retrograde Motion
Although it’s an unwritten maxim that the simpler a theory is the more likely it is to be true, usually that which is more complex comes to mind first. In an effort at saving the geocentric model, theorists grabbed at a straw mathematicians call the “epicycle." An epicycle is a curve (in this case an orbit) that can be produced from a circle on a circle.
Visualize a moderately large plastic circle that freely rotates clockwise about its center. Now imagine another much smaller plastic circle that attaches near the edge of the larger circle that rotates counter-clockwise. A point near the edge of the smaller circle, if followed as it moves, creates an epicycle. A demonstration of this is listed in the references below, which include an Ohio State University video.
Proponents of the geocentric theory held that if the earth really revolves about the sun, stellar parallax3 should be observed, which it was not. In fact, such parallax does occur, but it could not be detected due to the tremendous distances involved plus the lack of technology required to observe it. It was not until the 1830’s that stellar parallax actually was observed.
Early Heliocentric Theory Advocate
It was not simplicity alone that led to the demise of the geocentric theory. The scientist Galileo, had it brought to his attention that if Venus rotates about the sun it should appear from an earthly perspective in phases. Galileo with his newly developed telescope was able to see that indeed it does.
Galileo penned a famous document, “Dialogue Concerning the Two Chief World Systems" in 1632. Unfortunately, rather than immediately paving the way for scientific advancement, this document led to Galileo’s 1633 conviction of heresy by the Catholic Church.
Celestial Mechanics & Afterward
The death knell of the geocentric theory came with Isaac Newton and his development of the laws of Celestial Mechanics, in the 1700’s. It was not until 1992 that the Catholic Church admitted it was wrong in its judgement against Galileo.
Footnotes & References
1 The geocentric model is associated with 2nd century Roman astronomer Claudius Ptolemy.
2 The heliocentric model is associated with the Polish astronomer Nicolas Copernicus.
3 Stellar parallax is the apparent shift of stars that should be observed as the earth revolves about the sun. To learn the answer to the question, Do stars actually move, please see BrightHub author R.C. Davison's enjoyable article, "Stellar Motion: Do Stars Really Move?".
UCLA Division of Astronomy and Astrophysics - “Solar System Cosmology"
University of Tennessee at Knoxville - “The Universe of Aristotle and Ptolemy"