58 PART 1^ |^ EXPLORING THE SKY
linked with Christian teachings. According to the Aristotelian
universe, the most perfect region was in the heavens and the
most imperfect at Earth’s center. Th is classical geocentric uni-
verse matched the commonly held Christian geometry of heaven
and hell, so anyone who criticized the Ptolemaic model was not
only questioning Aristotle’s geometry but also indirectly chal-
lenging belief in heaven and hell.
For this reason, Copernicus probably found it diffi cult at
fi rst to consider alternatives to the Ptolemaic universe. Th roughout
his life, he was associated with the Catholic Church, which had
adopted many of Aristotle’s ideas. His uncle was an important
bishop in Poland, and, through his uncle’s infl uence, Copernicus
was appointed a canon at the cathedral in Frauenberg at the
unusually young age of 24. (A canon was not a priest but a
Church administrator.) Th is gave Copernicus an income,
although he continued his studies at the universities in Italy.
When he left the universities, he joined his uncle and served as
his secretary and personal physician until his uncle died in 1512.
At that point, Copernicus moved into quarters adjoining the
cathedral in Frauenburg, where he served as canon for the rest of
his life.
His close connection with the Church notwithstanding,
Copernicus began to consider an alternative to the Ptolemaic
universe, probably while he was still at university. Sometime
before 1514, he wrote an essay proposing a model of a heliocen-
tric universe in which the sun, not Earth, was the center of that
universe. To explain the daily and annual cycles of the sky, he
proposed that Earth rotates on its axis and revolves around the
sun. He distributed this commentary in handwritten form, with-
out a title, and in some cases anonymously, to friends and astro-
nomical correspondents. He may have been cautious out of
modesty, or out of respect for the Church, or out of fear that his
revolutionary ideas would be attacked unfairly. After all, the
place of Earth was a controversial theological subject. Although
this early essay discusses every major aspect of his later work, it
did not include observations and calculations. His ideas needed
supporting evidence, so he began gathering observations and
making detailed calculations to be published as a book that
would demonstrate the truth of his revolutionary idea.De Revolutionibus
Copernicus worked on his book De Revolutionibus Orbium
Coelestium (Th e Revolutions of the Celestial Spheres) over a period
of many years and was essentially fi nished by about 1529; yet he
hesitated to publish it even though other astronomers already
knew of his theories. Even Church offi cials, concerned about the
reform of the calendar, sought his advice and looked forward to
the publication of his book.
One reason he hesitated was that he knew that the idea of a
heliocentric universe would be highly controversial. Th is was a
time of rebellion in the Church—Martin Luther (1483–1546)SCIENTIFIC ARGUMENT
How did the astronomy of Hipparchus and Ptolemy violate the
principles of the early Greek philosophers Plato and Aristotle?
Today, scientifi c arguments depend on evidence and theory; but, in
classical times, they started from fi rst principles. Hipparchus and
Ptolemy lived very late in the history of classical astronomy, and
they concentrated more on the mathematical problems and less on
philosophical principles. They replaced the perfect spheres of Plato
with nested circles in the form of epicycles and deferents. Earth
was moved slightly away from the center of the deferent, so their
models of the universe were not exactly geocentric, and the epi-
cycles moved uniformly only as seen from the equant. The celestial
motions were no longer precisely uniform, and the principles of
geocentrism and uniform circular motion were weakened.
The work of Hipparchus and Ptolemy led eventually to a new
understanding of the heavens, but fi rst astronomers had to aban-
don uniform circular motion. Construct an argument in the classical
style based on fi rst principles to answer the following: Why did
Plato argue for uniform circular motion?The Copernican
RevolutionYou would not have expected Nicolaus Copernicus to trig-
ger a revolution in astronomy and science. He was born in 1473
to a merchant family in Poland. Orphaned at the age of 10, he
was raised by his uncle, an important bishop, who sent him to
the University of Cracow and then to the best universities in
Italy. Th ere he studied law and medicine before pursuing a life-
long career as an important administrator in the Church.
Nevertheless, he had a passion for astronomy (■ Figure 4-8).
The Copernican
Model
If you had sat beside Copernicus
in his astronomy classes, you
would have studied the Ptolemaic
universe. Th e central location
of Earth was widely accepted,
and everyone knew that the
heavens moved in uniform cir-
cular motion. For most scholars,
questioning these principles was
not an option because, over the
course of centuries, Aristotle’s
proposed geometry had become4-2
■ Figure 4-8
Nicolaus Copernicus (1473–1543) pursued a lifetime career in the Church,
but he was also a talented mathematician and astronomer. His work trig-
gered a revolution in human thought. These stamps were issued in 1973 to
mark the 500th anniversary of his birth.