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Johannes Kepler

The Sun-Centered Universe

Becoming an Astronomer

Mysteries of the Cosmos

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.

On that day, while standing at the blackboard drawing a geometrical figure for his class, Kepler had an epiphany. He believed it was a divine inspiration. Kepler had drawn a triangle with a circle circumscribed around it, which meant that each of the triangle's corners touched the rim of the circle. Then he inscribed another circle inside the triangle, which meant that the center of each side of the triangle touched the inner circle.

When Kepler stepped back and looked at what he had drawn, he realized with a shock that the ratios of the two circles were the same as the ratios of the orbits of Saturn and Jupiter. And with that realization, inspiration struck. Jupiter and Saturn were the outermost planets of the solar system, and the triangle was the simplest polygon. Kepler wondered whether you could fit the orbits of the other planets around other geometric figures, and tried his best inscribing circles in squares and pentagons. But the planetary orbits refused to fit.

Then Kepler had a second epiphany. The solar system was three dimensional – so why would he think that its governing pattern would be found in two dimensional figures? Kepler turned to three dimensional objects, and found his answer in the five perfect solids. A perfect solid is a three dimensional figure, such as a cube, whose sides are all identical. Conveniently for Kepler, there are only five perfect solids: the tetrahedron (which has four triangular sides), cube (six square sides), octahedron (eight triangular sides), dodecahedron (twelve pentagonal sides), and icosahedron (twenty triangular sides). Each perfect solid can be inscribed in and circumscribed around a sphere.

Kepler believed that the orbits of the six known planets – Mercury, Venus, Earth, Mars, Jupiter, and Saturn – could be fit around the five regular solids. He had finally found his answer to his question of "why." The reason there were only six planets was because there were only five perfect solids; the spacing of the planets was determined by the spacing between the solids.

Kepler's new system was wrong. Kepler had made the incredible leap of asking the question "why" – but had come up with a completely wrong answer. However, Kepler would continue to cling to this system in some form for the rest of his life – he valued it far above all his other achievements. And perhaps he was right to do so. Though incorrect, his idea would launch him on a lifelong path of investigation and discovery. It would lead him to revolutionize astronomy and take his place as one of the fathers of the scientific revolution.

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