II. Towards a sun-centered perspective 
A. The men of transition: The Renaissance Natural Philosophers 
COPERNICUS (1473-1543) 

A contemporary of Michaelangelo and an amateur astronomer, Copernicus was born in Poland and studied at the University of Cracow till 1494. In 1496 he goes to Italy to study law in Bologna and theology and medicine in Padua. In 1506 he went back to Poland and assumed the post of canon at the cathedral of Frauenburg, a clerical post he would hold for the rest of his life. He also set up an observatory.  

As a result of his observations, he came to realize that the actual positions of the planets did not match their positions predicted by the geocentric (ptolemaic) model, and that therefore, the Ptolemaic and Aristotelian systems were inadequate. This led him to change the geocentric system that had been evolved over millennia, and whose basics matched our everyday experience to a heliocentric (sun-centered) system. In his new system, it is no longer the earth, but the sun that is at the center of the universe, and the moon becomes a satellite of the earth. All six planets, (now including the earth) go around the sun. From the ancients he retains circular orbits. Unfortunately, these circular orbits did not prove to be better predictors of planetary orbits than those of Ptolemy.  

In 1530 he wrote a summary of his views in Commentariolus, a book that was well received by scholars and clergy alike. One supporter in particular was pope Clement VII. The full account of his work was finally published in De Revolutionibus Orbium Coelestium in 1543, and supposedly it was presented to him on his deathbed.  

While observations did play a part in Copernicus' rejection of the Ptolemaic model, it should be emphasized that the new system was based on philosophical and aesthetic motivations. His system, like those of the astronomers that came before him, had to fit what the naked eye saw. Copernicus' system also had to please the mind, to fit the basic axioms of physics, namely that all motion is uniform and circular. In his opinion, he evolved this new system to return order, simplicity and mathematical harmony to the world of astronomy. 

It must also be mentioned that Copernicus’ new cosmology was not challenged by the church. At that time, the church had no official astronomical dogma and, his works were commonly read in catholic universities. 

TYCHO BRAHE (1546-1601)

The son of a rich Danish nobleman, Brahe would pursue his passion for astronomical observation his entire life. In 1572 he observed a new star, a nova (actually an exploding star). According to Aristotle, novae happen between the Earth and the Moon. Careful observation showed Brahe that Aristotle was wrong and that this object was celestial, not sub-lunar in nature.  

In 1576 the Danish king, Fredrick II, granted Brahe an island in the North Sea (Hveen) where he constructed two observatories from where he would conduct careful observations and accumulate more astronomical facts than anyone before him. At Hveen, he observed several comets and was able to prove that comets are celestial objects, not atmospheric phenomena as Aristotle had claimed. Brahe noted that the paths of these comets clearly cut across the paths of the planets. If the heavens were made of nested crystalline spheres, as many ancient astronomers, including Aristotle, had thought, the comets would have collided with these spheres, therefore Aristotle, Ptolemy and the ancients were wrong.  

To Brahe, Copernicus was not right either. If the earth moved, Brahe should be able to detect parallax which he could not do. Thus in an attempt to reconcile the problems between his observations and the Ptolemaic and Copernican models he proposed his own system. Brahe's system was a modified geocentric model, with an unmoving earth at the center, and the other planets going around a sun that moved around the earth.  

In 1588, with the death of Fredrick II, and the lack of support from his successor, Christian IV, Brahe leaves Denmark to become Court Mathematician to the Emperor Rudolph II in Prague. There, his main task was to prepare and publish the Rudolphine tables, tables of astronomical positions. It is a task he himself would never complete, as he dies in 1601; but they would be completed 26 years after his death by his assistant Johannes Kepler.  

What makes Brahe so important in the history of astronomy is that he was a superb observational astronomer, undoubtedly the best before the use of telescopes, and that he kept detailed records of his observations for a span of nearly 40 years. It was left to his assistant, Johannes Kepler, who fell heir to Brahe’s data, to actually figure out the true motions of the solar system.