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The Sun-Centered Universe

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In 1543, twenty-eight years before Kepler's birth, Copernicus published the landmark astronomical text De Revolutionibus, or On the Revolutions of the Heavenly Spheres. The standard story about Copernicus's achievement is that by the sixteenth century, the Ptolemaic system had gotten too complicated and inaccurate to bear. In a stroke of genius, Copernicus moved the sun to the center of the universe creating a new system of brilliant simplicity and inarguable accuracy. Despite the attempts of the Catholic Church to drown out Copernican arguments, Ptolemy's system was soon overthrown. The Copernican system is thus heralded as a prime example of the triumph of a new, modern scientific era.

The story is true only in part. Copernicus did revolutionize astronomy by introducing a heliocentric system. But the concept of a sun-centered universe was not brand new and, in fact, had occurred to many of the ancient philosophers. Despite popular belief, Copernicus did not drastically simplify the Ptolemaic system. What we now think of as the Copernican system – six planets traveling around in the sun in simple, circular orbits and no epicycles – was only made possible by Kepler's later refinements. In fact, Copernicus's new heliocentric universe contained almost as many epicycles as the old system. Copernicus was just as devoted as his colleagues to the concept of uniform circular motion, and was willing to introduce as many mathematical devices as was necessary to simulate it. The Copernican system was no less complicated than the Ptolemaic system, nor was it any more accurate. Each of the systems yielded predictions that were accurate enough for the astronomers and navigators of the time. Copernicus's achievement was undeniably remarkable. But almost as remarkable was the ability of a few astronomers to grasp the truth of the heliocentric system, even though there was little evidence to recommend it.

Kepler was one of those insightful few. At a time when the Ptolemaic system still ruled in the European universities and the public mind, when other astronomers refused to publicly support Copernicus for fear of ridicule, Kepler was an unabashed Copernican. Although he had no technical evidence supporting one system over the other, he remained certain that the sun was at the center of the universe. While historians can never be sure exactly Kepler latched on to the heliocentric view so quickly and so firmly, most believe that he was attracted to it by the same combination of physical intuition and mystical theorizing that guided him throughout his professional career.

Kepler learned of the Copernican system at the University of Tueringen, from his first mentor, the professor Michael Maestlin. Maestlin publicly supported the Ptolemaic system – he had even written an astronomy textbook based on Ptolemy. However, in the safety of his own classroom, Maestlin was a full- fledged Copernican, and Kepler soon followed suit. Kepler would soon become the first well-known astronomer to support the Copernican system. At the same time, he would recreate that system in a much more physically and mathematically accurate form. What we now think of as the Copernican conception of the universe is actually Kepler's system.

Once at Gratz, Kepler focused on studying and refining Copernican astronomy. He accepted the Copernican construction of the universe, but one all-encompassing question remained: why were the planets arranged the way they were? More specifically, he wondered why there were only six planets (as was thought at the time), why they moved at the speed they did, and why they were spaced as they were. These were revolutionary questions. Before Kepler, no one had thought to wonder about why the universe was constructed in a certain way. For millennia, astronomers had devoted themselves to describing the way the planets moved, rather than questioning why that movement occurred. In the centuries before Kepler, astronomy had been purely mathematical. Kepler was the first major astronomer of the modern age to introduce questions of physics into the study of the stars.

A deeply devout man, Kepler was convinced that God had created an orderly universe, and his first major pursuit was figuring out what God's intentions might have been. Kepler played with the numbers for months, searching fruitlessly for a pattern. Finally on July 9, 1595, he found one.

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