The Renaissance set the stage for the astronomy of the sixteenth century by engendering interest in the physical world and its surroundings. By 1510 Leonardo da Vinci had developed many theories on the creation of the universe and the functioning of celestial bodies. In 1528, the French physician Jean Fernal made a calculation of the size of the Earth correct to one percent.

However, these accomplishments are far overshadowed by those of Nicolas Copernicus. Copernicus, a highly educated Pole, studied at university until the age of 30, excelling in classics, medicine, law, theology, and painting, as well as astronomy. He was not a practical astronomer, and only observed a handful of eclipses and oppositions of planets. Rather, he was a student of past observers and a theoretician. He studied the observed motions of heavenly bodies in relation to the accepted geocentric Aristotelian system, which placed the earth at the center of the solar system, with the sun and planets in orbit. Copernicus' observations led him to conclude that there was something wrong with the geocentric theory. He tested the hypothesis that the earth was in fact in orbit around the sun against the records of observation and found that this heliocentric theory was more feasible.

Copernicus' new scheme retained many of the aspects of the ancient theory. It still assumed that the universe was spherical and finite, ending at the range of the stars so distance that their movements could not be detected. It still assumed that the motion of all heavenly bodies was perfectly circular. Copernicus finished the description of his heliocentric theory in De Revolutionibus Orbium Coelestium (On the Revolutions of Celestial Bodies) in 1530, though it went unpublished until 1543, just months before his death.

The next great astronomer, Tycho Brahe, a Dane, differed from Copernicus in that he was, foremost, a practical astronomer who spent his time observing the heavens. For 21 years, from 1576-1597, Tycho worked out of a laboratory provided to him by the King of Denmark, systematically collecting observational data, which he used to test and revise astronomical theories. His records were far more extensive than any of his predecessors. Tycho set up a flawed, but fairly viable (by his observations), model of the solar system in which the Earth was central to the orbits of the moon and sun, and the sun was central to the orbits of the remaining planets. Mathematically, this system worked out identically to that suggested by Copernicus.

In 1584, Giordano Bruno, a renegade Italian monk, published three books explaining his philosophy. They are: The Ash-Wednesday Supper, On Cause, Principle, and Unity, and On the Infinite Universe and its Worlds. Bruno argued therein that not only does the Earth move, but so does the sun, and that there is no such thing as a point absolutely at rest in the universe. He argued further that the stars whose movements could not be detected were at vast, but varying distances from the solar system, and are the centers of remote solar systems. He claimed that it was folly to maintain that our planetary system was the center of the universe.

Bruno's conclusions were incompatible with the teachings of the Church, which maintained that the universe was finite and therefore separate from its creator. In 1600, Bruno was burned at the stake as a heretic. His work remained obscure during most of his career, but influenced some disciples, such as William Gilbert, whose work On the Magnet and Magnetic Bodies and Concerning that Great Magnet, the Earth, a New Physiology was published in 1600, and received high praise from future scientists.

Three basic conditions had to exist in order for the advancement of astronomy toward, and then beyond, the heliocentric theory. First, there had to be better astronomical tools, both physical and mathematical, and more accurate observations using these tools over a long period of time. Second, there had to be improved methods of mathematics for the interpretation of this collected data. Third, there had to be progress in the understanding of physics, and particularly motion. These conditions were evolving slowly during the sixteenth century, and as interest rose in astronomy at the opening of the seventeenth century, scientists concentrated much of their effort on the creation of these conditions.

Copernicus' teachings had very little immediate influence on contemporary thought. For nearly a century after the publication of De Revolutionibus, as it is often referred to, scholarly acknowledgement of Copernicus' work remained limited. Religion occupied the position of utmost importance during the period, and, having settled into the comfortable truths of the ancient thinkers and absorbed them as religious doctrine, by its nature the Church was hostile to revolutionary scientific advances. Bruno's theories on the universe heightened the Church's defensiveness, and though Copernicus' works were not banned, their effects were carefully monitored by the Church.

Both Tycho Brahe and Copernicus attempted to retain some of the major conventions of the Aristotelian system. Most prominently, both held to the theory that planetary orbits were perfect circles. Tycho's attempt to model the universe according to the ideal geometric form of the circle was one of the last attempts to retain this major stipulation of the Ptolemaic theory of the universe. Tycho's pupil, Johannes Kepler would continue this search for an ideal geometric scheme for the planetary system. However, following Tycho's era, astronomers began following a new paradigm: they gradually stopped attempting to fit the observed universe into an ideal structure and rather tried to observe the direct evidence of the nature of the heavens and glean from that the actual structure of the universe. Most were prepared to accept that this reality would not necessarily comply with an ideal form.

Bruno's theories, though not gleaned from any direct observation, were quickly recognized as antagonistic to the Church. Bruno's infinite universe differed greatly from the "created universe" that was the accepted truth of the Church. In an infinite universe, where were heaven and hell to reside? The alleged Creator of the "created universe" had to be separate from the creation in order to match up with Christian doctrine. Additionally, the Church maintained that the universe was centered on Earth and mankind, for it was mankind that had been granted the Divine Spirit. In this rejection of the teachings of the Church, Bruno's revolution was far greater than that of Copernicus. Many consider the publication of his three tracts in 1584 to be the true point of transition from medieval to modern science. It is important to note that his theories were not based on experimentation or observation. Rather, Bruno's contribution to science was a philosophy that opened the scientific mind to the possibility of new and strikingly different explanations of reality.