66 PART 1^ |^ EXPLORING THE SKY
Kepler’s evident deference was Tycho’s family, still powerful and
still intent on protecting Tycho’s reputation. Th ey even demanded
a share of the profi ts and the right to censor the book before
publication, though they changed nothing but a few words on
the title page and added an elaborate dedication to the
emperor.
Th e Rudolphine Tables was Kepler’s masterpiece. It could
predict the positions of the planets 10 to 100 times more accu-
rately than previous tables. Kepler’s tables were the precise model
of planetary motion that Copernicus had sought but failed to
fi nd. Th e accuracy of Th e Rudolphine Tables was strong evidence
that both Kepler’s laws of planetary motion and the Copernican
hypothesis for the place of Earth were correct. Copernicus would
have been pleased.
Kepler died in 1630. He had solved the problem of plane-
tary motion, and his Rudolphine Tables demonstrated his solu-
tion. Although he did not understand why the planets moved orwhy they followed ellipses, insights that had to wait half a cen-
tury for Isaac Newton, Kepler’s three laws worked. In science the
only test of a theory is, “Does it describe reality?” Kepler’s laws
have been used for almost four centuries as a true description of
orbital motion.SCIENTIFIC ARGUMENT
How was Kepler’s model with regular solids based on first
principles? How were his three laws based on evidence?
When he was younger, Kepler accepted Plato’s argument for the
perfection of the heavens. Furthermore, Kepler argued that the fi ve
regular solids were perfect geometrical fi gures and should be part
of the perfect heavens along with spheres. He then arranged the
fi ve regular solids to produce the approximate spacing among the
spheres that carried the planets in the Copernican model. Kepler’s
model was thus based on a fi rst principle—the perfection of the
heavens.The So-Called Scientifi c Method
Why is a theory much more than just a
guess? Scientists study nature by devising and
testing new hypotheses and then develop-
ing the successful ideas into theories and
laws that describe how nature works. A good
example is the connection between sour milk
and the spread of disease.
A scientist’s fi rst step in solving a natural
mystery is to propose a reasonable explanation
based on what is known so far. This proposal,
called a hypothesis, is a single assertion
or statement that must be tested through
observation and experimentation. From the
time of Aristotle philosophers believed that
food spoils as a result of the spontane-
ous generation of life—for example, mold
growing out of drying bread. French chemist
Louis Pasteur (1822–1895) hypothesized that
microorganisms were not spontaneously gener-
ated but were carried through the air. To test
his hypothesis he sealed an uncontaminated
nutrient broth in glass completely protecting
it from the microorganisms on dust particles
in the air. No mold grew, effectively disproving
spontaneous generation. Although others had
argued against spontaneous generation before
Pasteur, it was Pasteur’s meticulous testing of
his hypothesis through experimentation that
fi nally convinced the scientifi c community.
A theory generalizes the specifi c results of
well-confi rmed hypotheses to give a broader
description of nature, which can be applied toa wide variety of circumstances. For instance,
Pasteur’s specifi c hypothesis about mold grow-
ing in broth contributed to a broader theory
that disease is caused by microorganisms
transmitted from sick people to well people.
This theory, called the germ theory of disease,
is a cornerstone of modern medicine. It is a
Common Misconception that the
word “theory” means a tentative idea, a guess.
As you have just learned, scientists actually
use the word “theory” to mean an idea that is
widely applicable and confi rmed by abundant
evidence.
Sometimes, when a theory has been
refi ned, tested, and confi rmed so often that
scientists have great confi dence in it, it is
called a natural law. Natural laws are the most
fundamental principles of scientifi c knowledge.
Kepler’s laws of planetary motion are good
examples.
Confi dence is the key. In general, scientists
have more confi dence in a theory than in a
hypothesis and the most confi dence in a natu-
ral law. However, there is no precise distinc-
tion among a hypothesis, a theory, and a law,
and use of these terms is sometimes a matter
of tradition. For instance, some textbooks
refer to the Copernican “theory” of heliocen-
trism, but it had not been well tested when
Copernicus proposed it, and it is more rightly
called the Copernican hypothesis. At the
other extreme, Darwin’s “theory” of evolution,A fossil of a 500-million-year-old trilobite:
Darwin’s theory of evolution has been tested many
times and is universally accepted in the life
sciences, but by custom it is called Darwin’s
theory and not Darwin’s law.
(From the collection of John Coolidge III)4-2 Hypothesis, Theory, and Law
containing many hypotheses that have been
tested and confi rmed over and over for nearly
150 years, might more correctly be called a
natural law.