In the early 17th century, three primary beliefs, or models on planetary movement existed. The three competing theories were Ptolemaic, which was that the universe consisted of a set of nested spheres. He developed a system for accurately predicting the past and future positions of the planets. The second theory was that of Tycho Brahe, whose system combined the mathematical benefits of the Copernican system with the philosophical elements of the Ptolemaic system. The Tychonian system considered the Earth to be the center of the circle, but that the five known planets at the time circled the sun. The Copernican theory is that all of the planets, including the Earth, rotate around the sun. This analysis will support the thesis that Galileo was justified in supporting Copernicus’ heliocentric theory, using the methods and knowledge available at the time.

Ptolemy lived in Rome around 100AD. His model of the solar system was based on the models produced by the Greek astronomers who came before him. This theory considered the Earth to be stationary and all of the other planets to be in motion. One of his most basic theories is that the heavenly bodies moved in a uniform manner. He also distinguished between celestial matter and earthly matter. His system allowed him to predict the exact positions of the heavenly bodies, both in the past and in the future. His theories went unchallenged until the late 16th century. The stationary Earth model was the accepted theory for hundreds of years.

Tycho Brahe lived from 1546 to 1601. His theories included the theory of uniform circular motion, which was developed after he discovered a new star in Cassiopeia. He challenged Ptolemy’s theory that there was a difference between celestial matter and earthly matter. He believed that they were the same. He agree with the concentric system of Ptolemy, but disagreed that the spheres were hard. His model of the solar system agreed that the Earth was stationary and that the other planets and the sun revolved around the Earth. However, he also believed that Mercury, Venus, Mars, Jupiter, and Saturn revolved around the sun. His greatest contribution to astronomy, aside from his model of the solar system was that he believed the advancement of the study required more precise observations than had been conducted in the past. Tycho’s theories was that the Earth was the center for some celestial motions, but not all of them.

Nicholaus Copernicus was born in 1473 and died in 1543, shortly after publishing his controversial book introducing he heliocentric theory. The heliocentric view is that the Earth traveled around the sun, instead of all of the heavenly bodies traveling around the Earth. The widely held theory that the Sun and other planets revolve around the Earth was held as an absolute truth by the church, basing its beliefs on a single story in the Bible where the sun was made to stand still in order to lengthen the day. Copernicus’ theories were felt to be a direct violation of a prohibition against promoting material that directly contradicted Biblical teachings. The Church feared that these theories would undermine their authority. Shortly after Copernicus died, Galileo took on the task of proving Copernicus’ theories using modern instrumentation to support his claims. Until 1608, all observations and theories were based on data produced without the use of a telescope. He was later tried by the Inquisition and forced to recant his claims, although he never wholeheartedly rejected them in his mind. He was only acting out of fear for his life.

The question asked in this analysis is to what extent Galileo was justified in supporting the ideals proposed by Copernicus. The prevailing accepted status quo was based on the Ptolemic and Tychonian models where the Earth was stationary and the other heavenly bodies moved around it. This ideal had gone unchallenged for over 1,000 years. It was supported and accepted as the only allowable model by the Church. In order to examine whether Galileo’s support of the Copernican heliocentric model is justified, one must examine the merits of the Copernican model and the motives for its development, within historical context.

First, it is important to understand that Copernicus was not the first to develop the heliocentric model. In his early writings, he cites the work of Aristarchus of Samos, an astronomer who lived around 200 B.C. This early model claimed that the Earth spun on its axis and that the Earth revolved around the sun. This early theory was developed purely on observations of the movements of heavenly bodies. The theory was rejected based on several observational questions. The first was that it was supposed that if the Earth were spinning, then everything on it would go flying off. The second was that if the Earth were spinning, then the spinning Earth would outpace the birds in the sky, leaving them behind. The third reason for rejecting this model is that there was not parallax effect. It is now known that a parallax effect is observed, but it was too small to detect with the naked eye or the instruments available at the time of Copernicus. Copernicus felt that through the use of modern instruments, Samos’ theories were relevant.

General rejection of the Copernican model, and consequently, the rejection of Galileo’s support of it, stemmed from three main arguments. The first is that this theory went against traditional views that were held for over 1,000 years. The second is that there is not observational data to support it, the lack of observed parallax effect is the main argument for this point. The third is that it contradicts the literal interpretation of the Bible at the time. These reasons were not logical, but based on popular cultural ideas. Galileo could not allow himself as a scientist to allow what he saw as incorrect ideas to go unchallenged, simply because it was unpopular and went against common beliefs to do so. The main reasons presented to prevent an exploration of the ideals were not based on observation and science, but on the ideals of popular culture. Any ideal that was contradictory to consensus on scriptural meanings was considered heretical.

An examination of the evidence that was available to Galileo was the main reason for his adoption and support of the Copernican model. When one examines the main pieces of evidence available to Galileo at the time, his support for these ideas becomes more logical. Galileo knew of observations based on a falling body that supported rotation of the Earth. When an object is dropped from a tall building, it does not strike the earth directly below the point of release. It strikes it slightly to the east. This suggests that the Earth has moved. This evidence supports Copernicus’ model of the Earth’s rotation.

At the time of Galileo’s trial, telescopes were not good enough to detect the parallax. The Focault Pendulum would not be invented for another 200 years. Galileo had to rely primarily on observation and the mathematical merits of Copernicus’ model as the basis for his defense of it. The Church made several corrections to the work of Copernicus that made the theories more questionable than the original text. For instance, Copernicus postulated that because there were no objections to his arguments about the motion of the Earth, they should be accepted as correct. The Church corrected this to read as if they were still only a theory, putting the entire model in quesiton. Regardless of the alteration of the original text by the Church, Galileo saw ample reason to consider Copernicus’s model worth exploring further.

The Church did not ban Galileo’s writing based on the scientific merits of the work. It was banned because it violated mandates against publishing anything that went against the ideals of the Church. Galileo might have accepted a more scientific challenge to his work more readily than one that he considered to be based on religious, rather than logical argument. Galileo and Copernicus did not have the luxury of modern scientific evidence to support their claims. They had to rely on observation and theory. Galileo’s greatest reason for supporting Copernicus’ model of the motions of the planets is that the observations could be used to make accurate mathematical predictions. Galileo’s strength was as a mathematician. The ability to use the observations made by Copernicus to make future predictions provided Galileo with the drive to explore these theories further, rather than to dismiss them. Galileo surmised that the predictive accuracy of the models demonstrated that there was something scientifically worthwhile behind them. He made it is work to find out more about planetary physics and about the reasons for the observed and predicted events. The ability to support the model mathematically, even in the absence of hard observational evidence was the driving force behind Galileo’s support of Copernican theories.

Galileo’s trial was not about whether the heliocentric model that was proposed, nor the merits or fallacies of it. The trial focused on whether Galileo had broken a contract with Bellarmino over the content of the book. It was never about whether the reasoning in the book was justified. The discoveries that support the heliocentric model would develop over the course of the next 200 years. The instrumentation needed to further these theories were not available at the time of Copernicus. Copernicus worked with simple instruments, the same basic instruments that had been used by sailors for centuries. Galileo began his work without the use of a telescope. It’s invention would allow him to come closer to proving many elements of the model, but the first telescopes were not sensitive enough to serve as definitive proof of the principles. The main support for Copernicus’ theories stems from its ability to be supported in the ability to make mathematical predictions. Had Copernicus’ theories not produced predicable mathematic results, Galileo would have been more likely to discredit them. Galileo say the potential in marrying astronomy and mathematics.