Department of History
University of California, Irvine
Instructor:    Dr. Barbara J. Becker

Lecture 8.  Argument and Agreement.

Galileo Galilei (1564-1642)
Notable Events in Galileo's Life
born at Pisa
enrolled at University of Pisa as medical student
abandoned medicine for mathematics
took teaching post at Pisa
began writing down his thoughts on motion
took position as professor of mathematics at University of Padua

earned money selling gadgets and serving as military consultant

served as mathematics tutor to Cosimo de' Medici
Cosimo became Grand Duke of Tuscany
published Sidereus Nuncius [Starry Messenger]
  -resigned post at Padua
  -took position of "Chief Mathematician and Philosopher" to Grand Duke Cosimo II
Revealing great, unusual, and remarkable spectacles, opening these to the consideration of every man,
and especially of philosophers and astronomers;

Gentleman of Florence
Professor of Mathematics in the University of Padua,


lately invented by him,

In the surface of the Moon, in innumerable Fixed Stars, in Nebulae,
and above all


swiftly revolving about Jupiter at differing distances and periods, and known to no one before the Author recently perceived them and decided that they should be named


To the Most Serene
Cosimo II de' Medici
Fourth Grand Duke of Tuscany

...scarce have the immortal graces of your spirit begun to shine on earth when in the heavens bright stars appear as tongues to tell and celebrate your exceeding virtues to all time.

Behold, then, four stars reserved to bear your famous name; bodies which belong not to the inconspicuous multitude of fixed stars, but to the bright ranks of the planets....

...Indeed, the Maker of the stars himself has seemed by clear indications to direct that I assign to these new planets Your Highness's famous name....

For just as these stars, like children worthy of their sire, never leave the side of Jupiter ... so clemency, kindness of heart, gentleness of manner, splendor of royal blood, nobility in public affairs, and excellency of authority and rule have all fixed their abode and habitation in Your Highness....

A New Role for Instruments:  Tools of Persuasion

Galileo's Telescope
Institute and Museum of the History of Science
Florence, Italy

  • Does the telescope help, or hinder our vision?
  • Does it extend, or merely distort our senses?
  • Aren't our eyes adequate to see all we are entitled to see?
  • Why would the natural world include things we can't see with our unaided eyes?
Galileo's Recorded Observations of the Medicean Stars
Compared with their Appearances for the same Dates
as Determined using a
Modern Computer Planetarium Program (Redshift)
Galileo's descriptions:
January 7, 1610

[T]wo stars were near [Jupiter] on the east and one on the west....  I was not the least concerned with their distances from Jupiter, for ... at first I believed them to be fixed stars....
January 8

[O]n the eighth ... I found a very different arrangement.  For all three little stars were to the west of Jupiter and closer to each other than the previous night....  I was aroused by the question of how Jupiter could be to the east of all the said fixed stars when the day before he had been to the west of them....
January 10

[O]n the tenth ... [o]nly two stars were near him, both to the east....

January 11

There were only two stars on the east, of which the middle one was three times as far from Jupiter than from the more eastern one....  I therefore arrived at the conclusion, entirely beyond doubt, that in the heavens there are three stars wandering around Jupiter like Venus and Mercury around the Sun....
January 13

[F]or the first time four little stars were seen by me....  They formed a very nearly straight line, but the middle star of the western ones was displaced a little to the north from the straight line....
January 15

[T]he four stars ... were all to the west and arranged very nearly in a straight line, except that the third one from Jupiter was raised a little bit to the north....  They were very brilliant and did not twinkle, as indeed was always the case....
January 17

On the seventeenth, 30 minutes after sunset, the configuration was thus.  There was only one star on the east, 3 minutes from Jupiter.   Likewise, one was 11 minutes from Jupiter to the west.  The eastern one appeared twice as large as the western one.   There were no more than these two....
January 17

...But after 4 hours ... on the eastern side a third began to emerge, which, I suspect, had earlier been united with the first one....
January 18

On the eighteenth ... [t]he eastern star was larger than the western one and 8 minutes distant from Jupiter, while the western one was 10 minutes from Jupiter.
January 19

There were three stars exactly on a straight line through Jupiter....  At this time I was uncertain whether between the eastern star and Jupiter there was a little star, very close to Jupiter, so that it almost touched him.
My First Observations of the New Planets (1610) 
by Thomas Hariot (c. 1560-1621)

What was the Copernican Revolution?

A moving earth required a new physics.

A new physics required--

  • new instruments
  • new techniques
  • new rules for judging validity, and (?)....
  • Kepler?
  • Galileo??
Was Galileo a necessary ingredient of the Copernican Revolution?

Does the intellectual, political, social, economic climate of a time provide a "script" for individuals to play out?

If not Galileo, would someone else have stepped in to meet the challenges imposed by Copernicus' strange new heliocentric plan?

Galileo's Letter to the Grand Duchess Christina

The Grand Duchess Christina of Lorraine (1565-1637) was the granddaughter of Catherine de' Medici.  When her husband, Ferdinand de' Medici died in 1609, their eldest son, Cosimo (1590-1621)--whom Galileo had tutored in mathematics just a few years before--became the new Grand Duke of Tuscany.  In 1610, Galileo dedicated his new book, Sidereus Nuncius, to Cosimo II, and named the four newly discovered satellites of Jupiter the Medicean stars in honor of Cosimo and his three brothers.  Unfortunately, Cosimo became ill and died in 1621.  Cosimo's mother, Christina, and his widow, Maria Maddalena of Austria, served as regents for the new Grand Duke, young Ferdinand II (1610-1670).

Benedetto Castelli (1578-1643), a monk, was perhaps Galileo's favorite student.  In December 1613, Castelli, the newly appointed chair of mathematics at the University of Pisa, was invited to breakfast with the Tuscan court .  Here's how he described his experience in a letter (December 14, 1613) to Galileo:

Very Illustrious and Most Excellent Sir:

Thursday morning I had breakfast with our Lordships, and, when asked about school by the Grand Duke [Cosimo II], I gave him a detailed account of everything, and he seemed to be very satisfied.  He asked me whether I had a telescope, and I told him Yes and so began to relate the observation of the Medicean planets [four newly discovered moons of Jupiter] which I had made just the previous night.  Then Her Most Serene Ladyship [Cosimo's mother, the Grand Duchess Dowager Christina] inquired about their position and began saying to herself that they had better be real and not deceoptions of the instrument.  So their Highnesses asked Mr. Boscaglia, who answered that truly their existence could not be denied.  I used the occasion to add whatever I knew and could say about your wonderful invention and your proof of the motions of these planets....

As soon as I had come out of the palace, the porter of Her Most Serene Ladyship caught up with me and called me back.  However, before I say what followed, you must know that at the table Boscaglia had been whispering for a long time to the ear of Her Ladyship; he admitted as true all the celestial novelties you have discovered, but he said that the earth's motion was incredible and could not happen, especially since the Holy Scripture was clearly contrary to this claim.

Now, to get back to the story, I entered the chambers of Her Highness, where I found the Grand Duke, Her Ladyship, the Archduchess [Cosimo's wife, Maria Maddalena of Austria], Mr. Antonio, Don Paolo Giordano, and Boscaglia.  At this point, after some questions about my views, Her Ladyship began to argue against me by means of the Holy Scripture.  I first expressed the appropriate disclaimers, but then I began to play the theologian with such finesse and authority that you would have been especially pleased to hear....

The Grand Duke and the Archduchess were on my side and Don Paolo Giordano came to my defense with a very appropriate passage from the Holy Scripture.  Only Her Ladyship contradicted me, but in such a way that I thought she was doing it in order to hear me.  Mr. Boscaglia remained silent....

[A]s I was leaving ... [Don Antonio] explicitly uttered these words:  "Write Mr. Galileo about my having become acquainted with you and about what I said in the chambers of Her Highness."  I answered that I would report to  you about this beautiful opportunity I have had to serve you....

Because I have no more time, I kiss your hands and pray that you receive all good things from Heaven.

Pisa, 14 December 1613.

To you Very Illustrious and Most Excellent Sir.

Your Most Obliged Servant and Disciple,
Don Benedetto Castelli.

Galileo composed a lengthy response (December 21, 1613) to Castelli.  In it, he expressed his concern that theologians, ignorant of basic astronomical principles, risked compounding their errors through misinterpretation of Holy Scripture and misapplication of its authority to problems in natural philosophy:

Very Reverend Father and My Most Respectable Sir: 

Yesterday Mr. Niccolò Arrighetti came to visit me and told me about you....

[My greatest] pleasure was to hear him relate the arguments which through the great kindness of their Most Serene Highness [Cosimo II and his wife], you had the occasion of advancing at their table and then of continuing in the chambers of the Most Serene Ladyship [the Grand Duchess Christina]....  What greater fortune can you wish than to see their Highnesses themselves enjoying discussing with you, putting forth doubts, listening to your solutions, and finally remaining satisfied with your answers? 

After Mr. Arrighetti related the details you had mentioned, they gave me the occasion to go back to examine some general questions about he use of the Holy Scripture in disputes involving physical conclusions and some particular other ones about Joshua's passage, which was presented in opposition to the earth's motion and sun's stability by the Grand Duchess Dowager with some support by the Most Serene Archduchess. 

In regard to the first general point of the Most Serene Ladyship, it seems to me very prudent of her to propose and of you to concede and to agree that the Holy Scripture can never lie or err, and that its declarations are absolutely and inviolably true.  I should have added only that, though the Scripture cannot err, nevertheless some of its interpreters and expositors can sometimes err in various ways....  

Thus, given that in many places the Scripture is not only capable but necessarily in need of interpretations different from the apparent meaning of the words, it seems to me that in disputes about natural phenomena it should be reserved to the last place.  For the Holy Scripture and nature both equally derive from the divine Word, the former as the dictation of the Holy Spirit, the latter as the most obedient executrix of God's commands; moreover, in order to adapt itself to the understanding of all people, it was appropriate for the Scripture to say many things which are different from absolute truth, in appearance and in regard to the meaning of the words; on the other hand, nature is inexorable and immutable, and she does not care at all whether or not her recondite reasons and modes of operations are revealed to human understanding, and so she never transgresses the terms of the laws imposed on her; therefore, whatever sensory experience places before our eyes or necessary demonstrations prove to us concerning natural effects should not in any way be called into question on account of scriptural passages whose words appear to have a different meaning, since not every statement of the Scripture is bound to obligations as severely as each effect of nature.... 

Given this, and moreover it being obvious that two truths can never contradict each other, the task of wise interpreters is to strive to find the true meanings of scriptural passages agreeing with those physical conclusions of which we are already certain and sure from clear sensory experience or from necessary demonstrations....  

I should think it would be prudent not to allow anyone to oblige scriptural passages to have to maintain the truth of any physical conclusions whose contrary could ever be proved to us by the senses and demonstrative and necessary reasons.  Who wants to fix a limit for the human mind?  Who wants to assert that everything which is knowable in the world is already known?...

I should believe that the authority of the Holy Writ has merely the aim of persuading men of those articles and propositions which are necessary for their salvation and surpass all human reason, and so could not become credible through some other science or any other means except the mouth of the Holy Spirit itself.  However, I do not think it necessary to believe that the same God who has furnished us with senses, language, and intellect would want to bypass their use and give us by other means the information we can obtain with them.  This applies especially to those sciences about which one can read only very small phrases and scattered conclusions in the Scripture, as is particularly the case for astronomy, of which it contains such a small portion that one does not even find in it the names of all the planets; but if the first sacred writers had been thinking of persuading the people about the arrangement and the movements of the heavenly bodies, they would not have treated of them so sparsely, which is to say almost nothing in comparison to the infinity of very lofty and admirable conclusions contained in such a science. 

So you see, if I am not mistaken, how disorderly is the procedure of those who in disputes about natural phenomena that do not directly involve the Faith give first place to scriptural passages, which they quite often misunderstand anyway....

I now come to examining the specific passage of Joshua.... 

Let us then assume and concede to the opponent that the words of the sacred text should be taken precisely in their literal meaning, namely that in answer to Joshua's prayers God made the sun stop and lengthened the day, so that as a result he achieved victory; but I request that the same rule should apply to both, so that the opponent should not pretend to tie me and to leave himself free to change or modify the meanings of the words.  Given this, I say that this passage shows clearly the falsity and impossibility of the Aristotelian and Ptolemaic world system, and on the other hand agrees very well with the Copernican one.

I first ask the opponent whether he knows with how many motions the sun moves.  If he knows, he must answer that it moves with two motions, namely with the annual motion from west to east and with the diurnal motion in the opposite direction from east to west. 

Then, secondly, I ask him whether these two motions, so different and almost contrary to each other, belong to the sun and are its own to an equal extent.  The answer must be No, but that only one is specifically its own, namely the annual motion, whereas the other is not but belongs to the highest heaven, I mean the Prime Mobile; the latter carries along with it the sun as well as the other planets and the stellar sphere, forcing them to make a revolution around the earth in twenty-four hours, with a motion, as I said, almost contrary to their own natural motion. 

Coming to the third question, I ask him with which of these two motions the sun produces night and day, that is, whether with its own motion or else with that of the Prime Mobile.  The answer must be that night and day are effects of the motion of the Prime Mobile and that what depends on the sun's own motion is not night or day but the various seasons and the year itself. 

Now, if the day derives not from the sun's motion but from that of the Prime Mobile, who does not see that to lengthen the day one must stop the Prime Mobile and not the sun?  Indeed, is there anyone who understands these first elements of astronomy and does not know that, if God had stopped the sun's motion, He would have cut and shortened the day instead of lengthening it?  For, the sun's motion being contrary to the diurnal turning, the more the sun moves toward the east the more its progression toward the west is slowed down, whereas by its motion being diminished or annihilated the sun would set that much sooner; this phenomenon is observed in the moon, whose diurnal revolutions are slower than those of the sun inasmuch as its own motion is faster than that of the sun.  It follows that it is absolutely impossible to stop the sun and lengthen the day in the system of Ptolemy and Aristotle, and therefore either the motions must not be arranged as Ptolemy says or we must modify the meaning of the words of the Scripture; we would have to claim that, when it says that God stopped the sun, it meant to say that He stopped the Prime Mobile, and that is said the contrary of what it would have said if speaking to educated men in order to adapt itself to the capacity of those who are barely able to understand the rising and setting of the sun. 

Add to this that it is not believable that God would stop only the sun, letting the other spheres proceed; for He would have unnecessarily altered and upset all the order, appearances, and arrangements of the other stars in relation to the sun, and would have greatly disturbed the whole system of nature.  On the other hand, it is believable that He would stop the whole system of celestial spheres, which could then together return to their operations without any confusion or change after the period of intervening rest. 

However, we have already agreed not to change the meaning of the words in the text; therefore it is necessary to resort to another arrangement of the parts of the world, and to see whether the literal meaning of the words flows directly and without obstacle from its point of view.  This is in fact what we see happening. 

For I have discovered and conclusively demonstrated that the solar globe turns on itself, completing an entire rotation in about one lunar month, in exactly the same direction as all the other heavenly revolutions; moreover, it is very probable and reasonable that, as the chief instrument and minister of nature and almost the heart of the world, the sun gives not only light (as it obviously does) but also motion to all the planets that revolve around it; hence, if in conformity with Copernicus's position the diurnal motion is attributed to the earth, anyone can see that it sufficed stopping the sun to stop the whole system, and thus to lengthen the period of the diurnal illumination without altering in any way the rest of the mutual relationships of the planets; and that is exactly how the words of the sacred text sound.  Here then is the manner in which by stopping the sun one can lengthen the day on the earth, without introducing any confusion among the parts of the world and without altering the words of the Scripture. 

I have written much more than is appropriate in the view of my slight illness.  So I end by reminding you that I am at your service, and I kiss your hands and pray the Lord to give you happy holidays and all you desire. 

Florence, 21 December 1613 
To Your Very Reverend Paternity. 

Your Most Affectionate Servant, 
Galileo Galilei. 

In 1615, Galileo wrote a lengthier and more detailed version of this letter which he addressed to the Grand Duchess Christina herself.  His letter to Castelli, meanwhile, was privately circulated.  But it eventually came to the attention of some of Galileo's enemies.  In February 1615, a Dominican friar,  Niccolò Lorini, sent a copy of Galileo's letter to Rome as evidence of his heresy, prompting the authorities of the Inquisition to call Galileo in for questioning and admonishment.

Notable Events in Galileo's Life (cont'd)
Galileo wrote letter to the Grand Duchess Christina
Copernican theory condemned by Church

Galileo told to stop teaching it

Biblical Passages Used to Affirm the Geocentric System
Joshua 10:12-14

12.  Then spake Joshua to the Lord in the day when the Lord delivered up to the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gideon; and thou, Moon, in the valley of Ajalon.

13.  And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies.  Is not this written in the book of Jasher?  So the sun stood still in the midst of heaven, and hasted not to go down about a whole day.

14. And there was no day like that before it or after it, that the Lord hearkened unto the voice of a man:  for the Lord fought for Israel.


Ecclesiastes 1:5-9

5.  The sun also riseth, and the sun goeth down; and hasteth to his place where he arose.

6.  The wind goeth toward the south, and turneth about until the north; it whirleth about continually, and the wind returneth again according to his circuits.

7.  All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again...

9.  The thing that hath been, it is that which shall be; and that which is done is that which shall be done:  and there is no new thing under the sun.

How Does the Sun "Move"?


Every day, according to Ptolemy, the Sun appears to be carried around the Earth from East to West by the sphere of the fixed stars.

Like a stationary passenger whirling around on a merry-go-round or carried down by an escalator, the setting Sun appears to follow a path parallel to the celestial equator.

But, in Ptolemy's system the Sun also moves on its own -- every day it slowly shifts its position from West to East against the background stars in its yearly trip around the ecliptic.

Depiction of the sun's "proper motion" along the ecliptic over a period of several days.  Like a person walking up a down escalator, the Sun appears to plod from west to east along the ecliptic (Circle of the Different) against the persistent daily motion (Circle of the Same) of celestial objects from east to west.

According to Galileo:

[I]t is obvious that if the sun should cease its own proper motion, the day would become shorter, and not longer ....

The way to lengthen the day would be to speed up the sun's proper motion; and to cause the sun to remain above the horizon for some time in one place ... it would be necessary to [accelerate] the customary speed of the sun about three hundred sixty times....

[Imagine you are walking up a down escalator.  The constant downward motion of the escalator represents the daily relentless motion of the heavens -- the Circle of the Same. Your walking up the escalator represents the motion of the sun along the ecliptic -- the Circle of the Different.  If you were told to stop walking, would you arrive at the bottom of the escalator (sunset) sooner or later than usual?  If you were to start running up the escalator, how would that affect the time of your arrival at the bottom of the escalator?]

[I]f Joshua had intended his words to be taken in their pure and proper sense, he would have ordered the sun to accelerate its own motion....  But since his words were to be heard by people who very likely knew nothing of any celestial motions beyond the great general movement from east to west, he stooped to their capacity and spoke according to their understanding....

[Theologians] are forced to interpret the words in the midst of the heavens a little knottily, saying that this means no more than that the sun stood still while it was ... above our horizon.  [U]nless I am mistaken we may avoid this and all other knots if, in agreement with the Copernican system, we place the sun in the "midst" -- that is, in the center -- of the celestial orbs of planetary rotations....

Notable Events in Galileo's Life (cont'd)
Cardinal Maffeo Barberini became Pope Urban VIII
Galileo began writing a defense of Copernican theory (future Dialogue...)
published Dialogue Concerning the Two Chief World Systems; written in colloquial Italian, not Latin

Galileo ordered to stand trial

placed under house arrest
Dialogue... translated into Latin and English
letter to Grand Duchess Christina published in Italian and Latin
Discourses and Demonstrations Concerning  Two New Sciences published at Leyden -- a handbook for those who wish to learn the language and methods of the new physics

Go to:
  • Dialogue on the Two Chief World Systems (1632) by Galileo; and
Weekly Readings
Lecture Notes