CHAPTER 4 | THE ORIGIN OF MODERN ASTRONOMY 61
SCIENTIFIC ARGUMENT
Why would you say the Copernican hypothesis was correct but
the model was inaccurate?
To build this argument, you must distinguish carefully between a
hypothesis and a model. The Copernican hypothesis was that the
sun and not Earth was the center of the universe. Given the limited
knowledge of the Renaissance astronomers about distant stars and
galaxies, that hypothesis was correct.
The Copernican model, however, included not only the helio-
centric hypothesis but also uniform circular motion. The model is
inaccurate because the planets don’t really follow circular orbits,
and the small epicycles that Copernicus added to his model never
quite reproduced the motions of the planets.
Now build a new argument. The Copernican hypothesis won
converts because it is elegant and can explain retrograde motion.
How does its explanation of retrograde motion work, and how
is it more elegant than the Ptolemaic explanation?not just the adoption of a new idea but a total change in the way
astronomers, and, in fact, all of humanity, thought about the
place of the Earth (How Do We Know? 4-1).
Th ere are probably a number of reasons why the Copernican
hypothesis gradually won support, including the revolutionary
temper of the times, but the most important factor may have
been the elegance of the idea. Placing the sun at the center of the
universe produced a symmetry among the motions of the planets
that is pleasing to the eye as well as to the intellect (Figure 4-10b).
In the Ptolemaic model, Mercury and Venus were treated diff er-
ently from the rest of the planets; their epicycles had to remain
centered on the Earth–sun line. In the Copernican model, all of
the planets were treated the same. Th ey all followed orbits that
circled the sun at the center. Furthermore, their speed depended
in an orderly way on their distance from the sun, with those
closest moving fastest.
Th e most astonishing consequence of the Copernican
hypothesis was not what it said about the sun but what it said
about Earth. By placing the sun at the center, Copernicus made
Earth into a planet, moving along an orbit like the other planets.
By making Earth a planet, Copernicus revolutionized humanity’s
view of its place in the universe and triggered a controversy that
would eventually bring the astronomer Galileo Galilei before the
Inquisition. Th is controversy over the apparent confl ict between
scientifi c knowledge and philosophical and theological ideals
continues even today.The So-Called Scientifi c Method
How do scientifi c revolutions occur? You
might think from what you know of the
scientifi c method that science grinds forward
steadily as new theories are tested against
evidence and accepted or rejected. In fact,
science sometimes leaps forward in scientifi c
revolutions. The Copernican Revolution is
often cited as the perfect example; in a few
decades, astronomers rejected the 2000-year-
old geocentric model and adopted the helio-
centric model. Why does that happen? It’s all
because scientists are human.
The American philosopher of science
Thomas Kuhn has referred to a commonly
accepted set of scientifi c ideas and assump-
tions as a scientifi c paradigm. The pre-
Copernican astronomers shared a geocentric
paradigm that included uniform circular
motion, geocentrism, and the perfection of the
heavens. Although they were intelligent, they
were prisoners of that paradigm. A scientifi cparadigm is powerful because it shapes your
perceptions. It determines what you judge to
be important questions and what you judge
to be signifi cant evidence. Consequently, the
ancient astronomers could not recognize how
their geocentric paradigms limited what they
understood.
You will see here how the work of
Copernicus, Galileo, and Kepler overthrew the
geocentric paradigm. Scientifi c revolutions
occur when the defi ciencies of the old paradigm
build up until fi nally a scientist has the insight
to think “outside the box.” Pointing out the
failings of the old ideas and proposing a new
paradigm with supporting evidence is like pok-
ing a hole in a dam; suddenly the pressure is
released, and the old paradigm is swept away.
Scientifi c revolutions are exciting because
they give you a dramatic new understanding of
nature, but they are also times of confl ict as
new insights sweep away old ideas. The ancients believed the stars were attached
to a starry sphere. (NOAO and Nigel Sharp)4-1 Scientifi c Revolutions
Planetary Motion
The Copernican hypothesis solved the problem of the place of
Earth, but it didn’t explain planetary motion. If planets don’t
move in uniform circular motion, how do they move? Th e puzzle
of planetary motion was solved during the century following the
death of Copernicus through the work of two men. One com-
piled the observations, and the other did the analysis.4-3