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.