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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theory in De Revolutionibus Orbium Coelestium (On the Revolutions of Celestial Bodies) in 1530, though it went unpublished until 1543, just months before his death.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.
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