When Nicolaus Copernicus published his On the Revolutions of the Heavenly Spheres in 1543, the scientific community was forced to choose sides. There were the existing model put forth by Ptolemy that had held up for over 1000 years and the new conclusions reached by Copernicus. The two theories had deep implications about the organization of the universe that sharply divided scientists in the 1500’s. It is important to note that at the time these theories were being discussed, there was not enough information to unequivocally confirm or deny either theory. Astronomers were merely trying to find the best model to fit their observations, just like scientists of today.

These systems were consistent on a few issues, the most relevant of which being that the Sun is the primary source of light in the solar system and that the orbits of celestial bodies are circular.

However, the geocentric and heliocentric models differed on several issues beyond just which celestial body lies at the center of the solar system. The geocentric model places the Earth at the center of the universe while the Sun is the central object in the heliocentric model. Both have different explanations for the apparent retrograde motion of planets like Mars. This phenomenon occurs when Mars appears to move backwards in its position in the sky over the course of a few weeks. Ptolemy and his contemporaries explained this phenomenon with the use of epicycles. These are smaller cycles that occur along a planet’s larger orbit that would cause the planet to appear to move backwards in its position for a short time. This was an effective enough explanation for the phenomenon of retrograde motion at the time. However, as astronomers began to track the orbits of planets under this presumption, they found that orbits rapidly became convoluted and undesirable. There was also no explanation for what force would cause the planet to deviate from its primary orbit and was one of the major reasons that the heliocentric theory was favored eatly on. The Copernican explanation, and true reason, for  the apparent retrograde motion of Mars is due to the fact that its orbit is longer than Earth’s. As Earth revolves around the Sun and “passes” Mars on the same side of the Sun, Mars appears to move backwards from West to East over the course of a view weeks. This happens about once every two years.

One of the most decisive arguments for the heliocentric theory came from the great Italian astronomer, Galileo Galilei, a few decades after Copernicus. His experimentation with telescopes allowed him to view the universe from a new perspective. When he turned his lens to Venus, he found something fascinating: the planet experienced phases very similar to those of the Moon. Under the geocentric model, theoretically we should not be able to see more of Venus’ surface than a First or Third Quarter phase because Venus always lies between the Sun and the Earth. However, this conflicted with Galileo’s observations which fit much more neatly under the heliocentric model. This was the first conclusive, or rather non-theoretical, evidence of one system being favored over another and was the beginning of the end for Ptolemy’s understanding of the universe.

What Are the Possible Phases of Venus?
Heliocentric Model Geocentric Model
[NMSU, N. Vogt]

It was still a long time until heliocentrism became as widely accepted as it is today. Many people continued to reject the theory for various reasons. Some people could not understand the physics of how the Earth could be moving through space, yet everything on Earth remained still. The answer is the same reason why we are still in our cars once we reach a constant speed, however the physics behind this would not be organized until Newton in the 17th century. Most, even some of the foremost scientists like Tycho Brahe, objected to the theory for religious reasons. The Catholic Church and other major religious organizations labeled the model and its supporters as heretical for disputing the teachings of the Bible, having historically supported the geocentric model.




Sources: http://astronomy.nmsu.edu/geas/lectures/lecture11/slide02.html



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