During the Age of Galileo, people believed in the existence of only one truth. This guiding principle would prove to be a problem when the Copernican theory rose to challenge the Ptolemaic theory as the true model of the universe. The two rival theories were contradictory; either the earth was at the center of the universe or it wasn’t. The task at hand was to decide which theory was the true one, and this is when the scientific stalemate between the two theories began.
The scientific stalemate that Cardinal Bellarmine referred to when he wrote his letter to Foscarini in 1615 was due to the inability of anyone to prove the superiority of either the Copernican or Ptolemaic/Aristotelian theory to the other. Both theories of the universe, although “saving the appearances” made by astronomers over the years, offered a different explanation of celestial mechanics. The Aristotelian theory held that the earth is motionless at the center of the universe, and that the sun, planets, and stars revolve daily around it. It was the most easily understood model, agreeing with simple observations such as the sun, moon, planets, and stars apparently racing across the sky daily. The Copernican theory argued that the sun lay at the center of the universe, and the earth and planets revolve around the sun. Both theories made predictions agreeing, with great accuracy, to observation. However, only one theory could be the correct one.
Two important points of disagreement that existed between the Ptolemaic and Copernican astronomers before Galileo were the poor structure of the Copernican model compared to the Ptolemaic, and the lack of any experimental evidence that could both support the Copernican model and argue against the Ptolemaic. These two points were a major cause of the stalemate, and until Galileo became involved, neither of these points could be decided upon.
A major factor in the stalemate was that neither theory was more simple or complex than the other, making it difficult for people to decide which theory was the correct one. Cohen hints to the reason for this indecisiveness: “A comparison of the two figures representing the Ptolemaic and Copernican systems does not show that one was in any obvious way ‘simpler’ than the other” (45).
The Copernican theory did explain everything the Ptolemaic theory did, and additionally, it gained support from observations made by Galileo. The observation that Jupiter had four moons suggested that if Jupiter could move in an orbit and “not lose four of its moons, why could not the earth move without losing a single moon” (Cohen 72)? Also, since Venus was discovered to have phases, this proved that “Venus shines by reflected light, and not by a light of its own [as stated in Ptolemaic theory]…Here was another point of similarity between the planets and the earth, another weakening of the ancient philosophical barrier between earth and ‘heavenly’ objects” (Cohen 72).
However, there were drawbacks to the...