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.

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. 

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....

• History 60 Home Page

• De Motu [On Motion] (c. 1590) by Galileo Galilei (1564-1642);

• Dialogue on the Two Chief World Systems (1632) by Galileo; and

• Principles of Philosophy (1644) by René Descartes (1596-1650).