CHAPTER 4 | THE ORIGIN OF MODERN ASTRONOMY 71
Of all of Galileo’s telescopic observations, the moons of Jupiter
caused the most debate, but the craters on the moon and the
phases of Venus were also critical evidence. Build an argument
to discuss that evidence. How did craters on the moon and the
phases of Venus argue against the Ptolemaic model?Modern Astronomy
The science known as modern astronomy began during the
99 years between the deaths of Copernicus and Galileo (1543 to
1642). It was an age of transition that marked the change from
the Ptolemaic model of the universe to the Copernican model
with the attendant controversy over the place of the Earth. But
that same period also marked a transition in the nature of astron-
omy in particular and of science in general, a transition illus-
trated in the solution of the puzzle of planetary motion. Th e
puzzle was not solved by philosophical arguments about the
perfection of the heavens or by debate over the meaning of scrip-
ture. It was solved by precise observation and careful computa-
tion, techniques that are the foundation of modern science.
Th e discoveries made by Kepler and Galileo found acceptance
in the 1600s because the world was in transition. Astronomy was
not the only thing changing during this period. Th e Renaissance is
commonly taken to be the period between 1300 and 1600, and
these 99 years of astronomical history lie at the culmination of the
reawakening of learning in all fi elds (■ Figure 4-21). Ships were
sailing to new lands and encountering new cultures. Th e world was
open to new ideas and new observations. Martin Luther reformed
the Christian religion, and other philosophers and scholars
rethought their areas of human knowledge. Had Copernicus not
published his hypothesis, someone else would have suggested that
the universe is heliocentric. History was ready to shed the Ptolemaic
system.
Th is period marks the beginning of the modern scientifi c
method. Beginning with Copernicus, scientists such as Tycho,
Kepler, and Galileo depended more and more on evidence, obser-
vation, and measurement rather than on fi rst principles. Th is, too,
is coupled to the Renaissance and its advances in metalworking
and lens making. At the time of Copernicus, no astronomer had
looked through a telescope, because one could not be made. By
1642, not only telescopes but also other sensitive measuring
instruments had transformed science into something new and
precise. As you can imagine, scientists were excited by these dis-
coveries, and they founded scientifi c societies that increased the
exchange of observations and hypotheses and stimulated more
and better work. Th e most important advance, however, was the
application of mathematics to scientifi c questions. Kepler’s work
demonstrated the power of mathematical analysis; and, as the
quality of these numerical techniques improved, the progress of
science accelerated. Th is story of the birth of modern astronomy
is actually the story of the birth of modern science as well.4-5
SCIENTIFIC ARGUMENT
How were Galileo’s observations of the moons of Jupiter evidence
against the Ptolemaic model?
Scientifi c arguments are based on evidence, and reasoning from evi-
dence was Galileo’s fundamental way of knowing about the heavens.
Galileo presented his arguments in the form of evidence and con-
clusions, and the moons of Jupiter were key evidence. Ptolemaic
astronomers argued that Earth could not move or it would lose its
moon, but even in the Ptolemaic universe Jupiter moved, and the
telescope showed that it had moons and kept them. Evidently, Earth
could move and not leave its moon behind. Furthermore, moons cir-
cling Jupiter did not fi t the classical belief that all motion was cen-
tered on Earth. Obviously there could be other centers of motion.
Finally, the orbital periods of the moons were related to their dis-
tance from Jupiter, just as the orbital periods of the planets were,
in the Copernican system, related to their distance from the sun.
This similarity suggested that the sun rules its harmonious family of
planets just as Jupiter rules its harmonious family of moons.■ Figure 4-20
Although he did not invent it, Galileo will always be associated with the
telescope because it was the source of the observational evidence he used
to try to understand the universe. By depending on observation of reality
instead of fi rst principles of philosophy and theology, Galileo led the way
to the invention of modern science as a way to know about the natural
world.