II
The considerable success of the Ptolemaic system
had brought high respectability to the intuitive concept
of a fixed earth, a doctrine strongly reinforced by the
influence of Aristotle, the dogma of the Church, and
the vanity of man.
Copernicus found the idea of a moving earth in the
writings of the ancients. In the dedication of his book,
De revolutionibus to Pope Paul III, he said that at
first he had thought it absurd. His greatness does not
reside primarily in his daring to take it seriously but
in his constructing a mathematically detailed system
capable of challenging the formidable geocentric sys-
tem expounded by Ptolemy. Like Ptolemy, he used
epicycles (Figure 1), and his system was far from sim-
ple. And though he held that the sun was fixed at the
center of the Universe, the pivotal point of the plane-
tary motions in his system was not the center of the
sun but an empty place that we may conveniently call
the center of the earth's orbit. In this sense the earth,
though relegated to the role of a planet, retained a
certain supremacy.
Dethronement of the earth was Kepler's doing, and
with it came beauty. The planets now moved in ellip-
tical orbits about a fixed sun at a common focus, their
speeds varying in orbit in such a way that the line
from the sun to a planet traces out equal areas in equal
times (Figure 2). No longer was there need for in-
tricate epicycles either for shape of orbit or speed in
orbit. Simplicity had taken their place.
In the drudgery of his lifelong search for laws of
planetary motion Kepler was sustained by a deep,
religious belief in the underlying harmony and beauty
of the heavens. Let us not forget, though, that a seeking
after beauty had motivated Copernicus, as it had the
founders of the Ptolemaic system, who believed, with
Plato, that uniform circular motion was the only one
worthy of the perfection of the heavens. These aes-
thetic aspects of their work and the work of Kepler
need to be stressed, for just such seemingly nonscientific
considerations will be playing a crucial role in the
developments waiting to be told, and we shall see that
science in its highest manifestations is more akin to
art than to the popular misconceptions concerning its
nature.
