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6. Introduction to the Galileo Case Study:

Pre-Copernican Theories

Reading Assignment

 

Koertge, Lecture VI

Optional Readings (See Study Materials Page in Introduction):

Koestler, The Sleepwalkers, Parts 1 and 2

Kuhn, The Copernican Revolution, Chapters 1-4

Langford, Galileo, Science and the Church, Chapters 1 and 2

Encyclopedia articles on Aristotle and Ptolemy

Objectives

-To learn the major arguments against the heliocentric system

-To appreciate the attractive features of Aristotelian physics and cosmology

-To become familiar with the basic facts which any astronomical theory must explain

-To understand the fundamentals of Ptolemaic astronomy


Key Terms

- Cosmology
- Natural vs. Violent Motions (in Aristotle's sense)
- Impetus
- Inertia
- Retrograde Motion
- Deferent
- Epicycle
- Rotation vs. Revolution

INTRODUCTORY REMARKS

The Semmelweis story was a fairly straightforward case of conjectures and refutations. Most of the hypotheses involved could be tested directly against observation. There were no serious Duhemian problems and no controversies about instrumentation or methodology.

In the Galileo case which we now begin, all sorts of complications arise--there are conflicts between what is best for astronomical theory and what makes sense as far as physics is concerned. There are problems about the -reliability of the telescope. And there are all sorts of philosophical and religious disputes.

We will approach this complex episode in a spiral fashion. You have already obtained a rough idea of what happened from the Brecht play. Chapter VI in the text provides a more detailed overview. The study guide will present an even closer analysis of the key issues.

A note about dates: For our purposes, the exact year in which Galileo made a certain telescopic discovery or was condemned by the Catholic Church is not very important. On the other hand, the order of events is very important. For example, was his theory of projectile motion published at the time he was condemned? Did Copernicus know about the phases of Venus? We cannot understand the conclusions people drew without knowing which information was available to them.

 

NOW READ LECTURE VI IN THE TEXT.

 

THE ARGUMENTS AGAINST THE MOTION OF THE EARTH

In Copernicus' theory the earth has two major motions, a daily rotation around its own axis and a yearly revolution around the sun. Neither idea was new. Aristarchus had described a heliocentric system in the third -century B.C. And Ptolemy went to some lengths to refute the-idea that the earth might be rotating. We will now look at various reasons people thought the earth did not move.

Common Sense Ideas About Motion

 

Suppose you were kidnapped and blindfolded. How could you tell whether your kidnappers were moving you or not?

There are various clues you could use: Does the wind whistle by your ears? Do you bounce up and down? Can you feel centrifugal force as you go over hills or around corners? And, if your blindfold slips a little, do you see the scenery whirling by? There seem to be many ways of feeling motion.

By none of these methods does the earth appear to be moving. We feel no 1000 m.p.h. wind from the east. We don't fly off the earth's surface like mud from a spinning wheel. We see the sun moving around us, not vice versa.

Common sense (which literally means that upon which the senses of all of us agree) clearly tells us that the earth is stationary.

 

Aristotelian Physics

 

Our common sense ideas about motion were neatly incorporated into Aristotle's theory of motion, which can be summarized as follows:

1. Motion is change of place.

2. All motions have a cause.

3. There are two kinds of motion, natural and violent.

4. Natural motions are a result of the essential nature of the

elements.

a.It is the nature of the heavy elements, earth and water, to go down, of the light elements, fire and air, to go up, and of the heavenly elements to go in circles.

b.The heavier an object made out of heavy elements is, the faster it goes down; likewise for light objects.

c. The closer an object gets to its natural place, the faster it goes.

 

5. Violent motions must be initiated and sustained by an external mover.

d.The greater the outside moving force, the faster the violent
motion.

e.The greater the resistance of the medium, the slower the
motion.

f.There is no violent motion in the heavens.

 

Aristotle's physics dictated a unique cosmology (structure for the universe): Earthy matter should be basically in the center, surrounded by water, then air, and then fire. Violent motions cause some circulations and upheavals (for example, volcanos throw rocks up in the air or wind can carry dust particles up and away from their natural place), but most objects-lie pretty much in their proper sphere. (The highest layer, that of fire, is only visible during thunderstorms.)

The medieval philosophers amended Aristotle's theory of physics (because of the problems of projectile motion) to say that violent motions could be sustained
without an external mover--the object could store up impetus which would keep it going for a short time. However, it left his cosmology intact.

On neither account was it possible that the earth moved. For the earth, any sort of circular motion (rotation or revolution) would be violent; there-fore, it would have to be both initiated and then sustained by an external mover over centuries. No such outside moving force existed.

 

Pre-Ptolemaic Astronomy

 

Everyone is familiar with the basic phenomena which any theory of motion must explain: stones fall down, heavy wagons are harder to move than empty ones, a ball thrown at a 45* angle makes a smooth arc back to the earth, and so on.

In a pastoral society the basic facts about the movements of the heavenly bodies which any theory of astronomy must account for were just as familiar. Let us quickly summarize the most important ones:

 

Shared Diurnal Motions

 

1.The sun rises daily in the east and sets in the west.

2.Likewise for the moon.

3.The stars (including the planets) also move across the sky and
appear to move in circles around the North Star. (This is true only in the northern hemisphere, but the people we will be talking about did not know the southern sky.)

 

The observations listed above could all be accounted for if we imagine that the heavenly bodies are embedded in a big sphere which rotates about the earth every 24 hours, However, we must add refinements to our model to account for the following:

"Proper" or Individualized Motions of the Heavenly Bodies

 

4.The moon goes through its cycle of phases every month.

5.The sun goes through a yearly cycle, rising higher in the
summer. And just before dawn different constellations are found near the horizon at different times of the year. The sun appears to move clear around the zodiac in a year's time.

6.The planets (wandering stars) also move slowly through the zodiac.
Mercury and Venus complete their cycle in 365 days. Jupiter and Saturn each have longer "years."

 

To account for the so-called proper motions of the seven principal heavenly bodies, a model of concentric spheres was proposed. This was the model accepted by nearly all ancient astronomers, including Ptolemy.

TheBasic Ptolemaic System without epicycles is found in Kuhn, Figure 16, p. 53

The "fixed" stars (i.e., all of them except the planets) rotated around in a fixed formation every 24 hours. The seven special celestial bodies shared this diurnal motion but also moved slowly against it. (Remember the ants or, if you like, imagine a merry-go-round operator slowly walking backwards between the rapidly moving horses.)

The moon's phases were explained just as we would today, in terms of the relative positions of the sun, moon and earth. To account for the sun's seasonal behavior, the sun's proper motion (along what was called the ecliptic) was said to be at an angle to the earth's equator--but this need not concern us here.

The concentric spheres model was well-known in Aristotle's time, but there was one puzzling set of data which did not fit in at all:

7. Retrograde motions of the planets.

I have already mentioned that the planets move slowly through the zodiac. For example, Mars might be in Aries on April lst and move eastward into Taurus by June Ist. However, then it might double back into Aries by August Ist! This so-called retrograde motion (retro means backwards; grade can mean direction, as in gradient) seemed to violate Aristotle's theory. Celestial bodies were supposed to move in circles--there were not supposed to be violent motions in the heavens!

For an Example of Mars' Retrograde motion see Kuhn Figure 15, p. 48.

For over three centuries astronomers tried to solve what Plato, Aristotle's teacher, called the "problem of the planets." Could one compound uniform circular motions in such a way as to "save the phenomena"? Could one propose a model of the universe, using only circles which would explain why the planets appeared to move backwards?

 

Ptolemy's Solution

 

As you saw in the text, Ptolemy finally solved the problem by introducing epicycles. If we imagine the motion of a planet to be composed of two circular motions, as on the left (opposite page), its resultant motion would appear as an the right:

 

Ptolemy's Theory of Epicycles is illustrated in Kuhn, Figure 19, p. 61.

 

Ptolemy's solution was a brilliant achievement. There were still lots of details left to fill in of course--exactly how' big would the epicycle have to be and how fast should it be moving in order to account precisely for the position of the planet at every minute of the year? But everyone believed that the basic structure of the universe was now understood.

 

Study Suggestions

 

1.As usual, learn the meanings and spellings of the key words.

2.Prepare a chronological outline of the key events in this story. For example, list "invention of epicycles" and put down an approximate date.