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Natural Science - Year I

Unit 28: Galileo and the New Astronomy

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History Weblecture for Unit 28

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History Lecture for Unit 28: Galileo and the New Astronomy

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Lecture outline:

The Controversy over Copernicus

For nearly seventy years, astronomers in Europe used the Copernican theory, or not, according to their own lights. While some theologians raised questions about placing the sun at the center of the universe, most astronomers chose to follow Copernicus' calculations because in some instances, it gave better results that calculations made according to the Ptolemaic theory — but not always. From an astronomical position, neither theory could be conclusively supported by observational evidence.

At the beginning of the seventeenth century, this situation changed, and the Copernican theory became a battleground with religious, scientific, and political ramifications.

Galileo Galilei

Galileo is one of the most controversial figures in all of the history of science. As you read websites (including ours for this course!), be aware that the positions people take will influence how they present Galileo's story or interpret the events that led to his house arrest by the Roman Catholic authorities.

One area of Galileo's influence was in the methodology of science: he practically invented the experimental method, and he was one of the first to use detailed observations in a regular fashion to calculation the positions of moving bodies—whether they were lanterns on a pendulum, balls rolling down a wooden inclined plane, or the moons of the planet Jupiter.

Galileo's insistence on the reality of the helicentric model got him into serious trouble with the Catholic Church. Some of the problem was political—he offended many of the high church officials by his sarcastic criticism of some of their scientific methods and opinions. But one of their objections was solidly based on scientific observation. Galileo, like Copernicus and Kepler, still had no direct observational proof that the earth moved, and no physical cause for why any of the planets should orbit the sun.

Read the summary of Galileo's biography at the Catholic Encyclopedia website.

  • Where and how was Galileo educated? Where did he teach?
  • How did this improve his measurements?

Read the summary of Galileo's observations at the University of Tennessee Astronomy Course site.

    How do the following observations challenge Aristotle's claims about the nature of celestial objects?
  • Craters on the moon
  • Sunspots
  • The phases of Venus
  • The moons of Jupiter

Note: The physical reason was supplied by Newton, when he formulated the universal law of gravitation. But those who accepted the Copernican theory prior to about 1840 did so because it made the model simpler: the earth then followed the observable pattern of all the planets and the laws of motion and gravity. It wasn't until Bessel observed actually stellar parallax (see the science page) that there was proof for the earth's motion.

Galileo and the Catholic Church

The telescope observations made by Galileo which convinced him that Copernicus' theory of a heliocentric system of planets was correct. Galileo published his observations in 1610 in a work called The Sidereal Messenger. Since he had a very sarcastic way of dealing with those who opposed his views—primarily the Aristotelian scholars among the Catholic Church hierarchy—his work was not well received by them.

Confronted with the growing debate over Protestant interpretations of scripture, they viewed with alarm Galileo's claim that Copernicus was right. If so, then passages like the story in which the sun stopped moving and stood still in order to give Joshua daylight for long enough to win an important battle became subject to reinterpretation. [This bothered many leaders of the Protestant movement as well—Martin Luther was among the first of them to condemn Copernican theories as rank foolishness.] Rather than call a literal reading of certain Scriptures into question, the Holy Office of the Catholic Church placed Copernicus' book and one of Kepler's on a list of restricted works (but not Galileo's). The Copernican system could still be taught as an hypothesis (only a few passages in the de Revolutionibus had to be changed), but not as physical reality. Those who were knowledgeable scholars and understood the implications could still read the works in their entirety.

Galileo tried to convince the Holy Office that the Aristotelians were wrong. He secured permission to write a comparison of the Aristotelian and Copernican systems, which was granted. The work Galileo produced, On the Two Chief World Systems, was formatted as a debate between an Aristotelian, who presents a rather simple and superficial explanation of Aristotle's cosmology, and a Copernican who is able to marshall all of Galileo's considerable knowledge and wit in his defense of the heliocentric system.

When the book was published in 1632, the Aristotelians appealed to the Pope, saying that Galileo had been enjoined by the Roman Inquisition (the organization within the Catholic Church responsible for controlling heresy) to present an impartial argument, and that Galileo had disobeyed the order of the Inquisition. Galileo was summoned before the Inquisition, where he claimed he had violated no injunction, was tried, and condemned—but the records of the trial were sealed. Only recently have researchers been able to look at some of the papers filed in the case, and there is considerable debate over the authenticity of some of the documents from 1616 which were used in 1632 to show that Galileo had been ordered to stop teaching Copernicus' system. Though it is possible that the trial may have been manipulated by a few officials with motives that were more political than theological, the result was still bad for Galileo, who would up under house arrest, unable to publish anything further without review by scientists at the Papal court. This was actually milder than other sentences the court might have imposed.

Study/Discussion Questions:

Further Study/On Your Own