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The Fate of Copernicus

The Telescope

Astronomical Discoveries

Experiment and Demonstration

Life on the Moon

Senses and Language

Inquisition and Abjuration

Exile in Siena

Without Resistance, All Matter Falls at Equal Rates 

Final Astronomical Observations

The Principle of Galilean Relativity




Galileo Galilei (1564-1642 CE) was born in Pisa . He had an early education in Latin and Greek that laid the foundation for his elegant and penetrating style of writing. He began to study medicine at the University of Pisa at age seventeen, but abandoned this for mathematics. He became professor of mathematics at Pisa in 1589 and was professor of mathematics at Padua between 1592 and 1610. During this period he used the newly invented telescope to observe the mountains on the moon and the rotation of the moons of Jupiter. In 1610 he published his findings in The Starry Messenger (Siderius Nuncius) He became mathematician and philosopher to the Grand Duke at Florence in 1611, remaining in this position until his imprisonment by the Inquisition in 1633.

In 1615 he went to Rome to promote the Copernican view of the solar system to high officials in the Church. He subsequently made enemies among the Jesuits and Dominicans by his forceful presentation of his views, not least by his argument for a philosophy that depended on verifiable observation rather than on the writings of "authorities" like Aristotle. He published the polemic The Assayer (Il Saggiatore) in 1623 in the form of a letter in which he puts forward his views on a variety of subjects including a form of atomic theory.

In 1629, Galileo’s exposition of the Copernican theory was published in the Dialogue Concerning Two World Systems. This was banned after five months. Physically ill and enfeebled by age, he was subsequently tried by the Inquisition, forced to recant his views under threat of torture, and sentenced to imprisonment. His daughter’s anguish at her father’s treatment caused her death.

In 1636 he produced his major scientific work on the physics of motion, in Dialogues Concerning Two New Sciences. He sent copies to friends who had the book printed. If his honor died at the hands of the Inquisition, then this was his resurrection. Like Ssi-ma Ch'ien, Grand Historian of China, and contrary to the example of Socrates, Galileo demonstrated that it is better not to die at the hands of dishonorable men but to endure so as to continue with work of great human value. 

Galileo presented his views in the form of dialogues among disputants. Where I have extracted quotations from these works, I have assumed that the character putting forward the view that prevails is in fact Galileo.

Albert Einstein wrote "Galileo. . . recognized that the effect of gravity on freely falling bodies manifests itself in a vertical acceleration of constant value; likewise that an unaccelerated horizontal motion can be superposed on this vertical accelerated motion. These discoveries contain essentially—at least qualitatively— the basis of the theory later formulated by Newton . . .It is difficult for us today to appreciate the imaginative power made manifest in the precise formulation of the concept of acceleration and in the recognition of its physical significance." Galileo's recognition that uniform motion was indistinguishable from a state of rest, so that such a state had no absolute meaning, formed the basis for the special theory of relativity. His insight that that all falling bodies accelerate at the same rate was a key contribution to the general theory of relativity.


The Fate of Copernicus

1     I have as yet read nothing beyond the preface of your book, from which, however, I catch a glimpse of your meaning, and feel great joy on meeting with so powerful an associate in the pursuit of truth, and, consequently, such a friend to truth itself; for it is deplorable that there should be so few who care about truth and who do not persist in a perverse mode of philosophizing. But as this is not the fit time for lamenting the melancholy condition of our times, but for congratulating you on your elegant discoveries in confirmation of the truth, I shall only add a promise to peruse your book dispassionately, and with the conviction that I shall find in it much to admire.

2     This I shall do the more willingly because many years ago I became a convert to the opinions of Copernicus, and by his theory have succeeded in explaining many phenomena which on the contrary hypothesis are altogether inexplicable. I have arranged many arguments and confutations of the opposite opinions, which, however, I have not yet dared to publish! fearing the fate of our master, Copernicus, who, although he has earned immortal fame among a few, yet by an infinite number (for so only can the number of fools be measured) is hissed and derided. If there were many such as you I would venture to publish my speculations, but since that is not so I shall take time to consider of it.

                                    From a Letter to Kepler, August 4, 1597


The Telescope

3     You must know then that about two months ago a report was spread here that in Flanders a spy-glass [telescope] had been presented to Prince Maurice, so ingeniously constructed that it made the most distant objects appear quite near, so that a man could be seen quite plainly at a distance of 2 miles. This result seemed to me so extraordinary that it set me thinking. As it appeared to me that it depended upon the laws of perspective, I reflected on the manner of constructing it, and was at length so entirely successful that I made a telescope which far surpasses the report of the Flanders one. As the news had reached Venice that I had made such an instrument, six days ago I was summoned before their Highnesses, the Signoria, and exhibited it to them, to the astonishment of the whole senate. Many of the nobles and senators, although of a great age, mounted more than once to the top of the highest church tower in Venice, in order to see sails and shipping that were so far off that, without my spy-glass, it was two hours before they were seen steering full sail into the harbor; for the effect of my instrument is such that it makes an object 50 miles off appear as large as if it were only five.

                                    From a Letter to Landucci, August 29, 1609


Astronomical Discoveries

4     I am at present staying in Venice for the purpose of getting printed some observations which I have made on the celestial bodies by means of my spy-glass and which infinitely amaze me. Therefore do I give thanks to God, who has been pleased to make me the first observer of marvelous things unrevealed to bygone ages. I had already ascertained that the moon was a body very similar to the earth, and had shown our Serene Master, the Grand Duke, as much, but imperfectly, not then having such an excellent spy-glass as I now possess. This, besides showing the moon most clearly, has revealed to me a multitude of fixed stars never before seen, being more than ten times the number of those that can be seen by the unaided eye. Moreover, I have ascertained what has always been a matter of controversy among philosophers, namely, the nature of the Milky Way. But the greatest marvel of all is the discovery of four new planets. I have observed their motions proper to themselves and in relation to each other, and wherein they differ from the motions of the other planets. These new bodies move round another very great star [Jupiter], in the same way as Mercury and Venus, and, perhaps, the other known planets, move round the sun.

                                    From a Letter to Vinta, January 30, 1610

5     It is now not simply a case of one body (the moon) revolving around another body (the earth), while the two together make a revolution around the sun, as the Copernican doctrine teaches; but we have the case of four bodies or moons revolving round the planet Jupiter, as the moon does round the earth, while they all with Jupiter perform a grand revolution round the sun in a dozen years.

                                    From The Starry Messenger, 1610

6     You are the first and almost the only person, who, after a cursory investigation, has given entire credit to my statements.... We will not trouble ourselves about the abuse of the multitude, for against Jupiter even giants, to say nothing of pigmies, fight in vain. Let Jupiter stand in the heavens and let the sycophants bark at him as they will. . . . In Pisa, Florence, Bologna, Venice and Padua, many have seen the planets [moons of Jupiter], but all are silent on the subject and undecided; for the greater number recognize neither Jupiter nor Mars, and scarcely the moon, as a planet . . . What is to be done? Shall we side with Democritus or Heraclitus? I think, my Kepler, we will laugh at the extraordinary stupidity of the multitude. What do you say of the leading philosophers here to whom I have offered a thousand times of my own accord to show my studies, but who, with the lazy obstinacy of a serpent who has eaten his fill, have never consented to look at the planets, or moon, or telescope? Truly, just as serpents close their ears, so do men close their eyes to the light of truth. To such people philosophy is a kind of book—like the Aeneid or the Odyssey—where the truth is to be sought, not in the universe or in nature, but (I use their own words) by comparing texts!

    How you would laugh if you heard what things the first philosopher of the faculty at Pisa brought against me in the presence of the Grand Duke. He tried hard with logical arguments, as if with magical incantations, to tear down and argue the new planets out of heaven!

                                    From a letter to Kepler,19th August 1610

7     Venus rivals the appearances of the moon; for Venus being now arrived at that part of her orbit in which she is between the earth and the sun, and with only a part of her enlightened surface turned towards us, the telescope shows her in a crescent form, like the moon in a similar position.

                                    From a letter to Giuliano de Medici, January 1, 1611


Experiment and Demonstration

8     I think it would be the part of wisdom not to allow any one to apply passages of Scripture in such a way as to force them to support as true any conclusions concerning nature, the contrary of which may afterwards be revealed by the evidence of our senses, or by actual demonstration. Who will set bounds to man's understanding? Who can assure us that every thing that can be known in the world is known already? . . . I am inclined to think that Holy Scripture is intended to convince men of those truths which are necessary for their salvation, and which being far above man's understanding cannot be made credible by any learning, or by any other means than revelation. But that the same God who has endowed us with senses, reason, and understanding, does not permit us to use them, and desires to acquaint us in another way with such knowledge as we are in a position to acquire for ourselves by means of those faculties— that, it seems to me I am not bound to believe, especially concerning those sciences about which the Holy Scriptures contain only small fragments and varying explanations; and this is precisely the case with astronomy, of which there is so little that the planets are not all enumerated, only the sun and moon, and once or twice Venus under the name of Lucifer. This, therefore, being granted, I think that in discussing natural phenomena we ought not to begin with texts from Scripture, but with experiment and demonstration. . .

                                    From a letter to Castelli,, December 21, 1613

9 . . . to command professors of astronomy that they must themselves see to confuting their own observations and demonstrations is to ask the impossible, for it is not only to command them not to see what they do see, and not to understand what they do understand, but to seek for and to find the contrary. I would entreat these wise and prudent Fathers to consider diligently the difference between opinionative and demonstrative doctrines, to the end that they may assure themselves that it is not in the power of professors of demonstrative sciences to change their opinions at pleasure, and adopt first one side and then the other; that there is a great difference between ordering a mathematician, or a philosopher, as to what opinion to hold, and doing the same with a merchant, or a lawyer, since demonstrated conclusions touching things of nature and of the heavens cannot be changed with the same facility as opinions touching what is lawful, or not, in a contract, bargain, or bill of exchange.

                                    From a letter to the Dowager Grand Duchess Cristina, June, 1615

10     I cannot refrain from marveling that Sarsi [a pseudonym for the Jesuit Grassi] will persist in proving to me, by authorities, that which at any moment I can bring to the test of experiment. . . If Sarsi insists that I must believe, on Suidas's credit, that the Babylonians cooked eggs by swiftly whirling them in a sling, I will believe it; but I must say, that the cause of such an effect is very remote from that to which it is attributed, and to find the true cause I shall reason thus. If an effect does not follow with us which followed with others at another time, it is because, in our experiment, something is wanting which was the cause of the former success; and if only one thing is wanting to us, that one thing is the true cause. Now we have eggs, and slings, and strong men to whirl them, and yet they will not become cooked; no, if they were hot at first they more quickly become cold. And since nothing is wanting to us but to be Babylonians, it follows that being Babylonians is the true cause why the eggs became cooked, and not the friction of the air, which is what I wish to prove.

                                    From the dialogue in The Assayer (Il Saggiatore), 1623


11     Some years ago a salutary edict was promulgated at Rome, which, in order to obviate the perilous scandals of the present age, enjoined an opportune silence on the Pythagorean opinion of the earth's motion. Some were not wanting who rashly asserted that this decree originated, not in a judicious examination, but in ill-informed passion; and complaints were heard that counselors totally inexperienced in astronomical observations ought not to clip the wings of speculative minds by hasty prohibitions.

    My zeal could not keep silence when I heard these rash lamentations, and I thought it proper, as being fully informed with regard to that most prudent edict, to appear publicly as a witness of the actual truth. I happened at that time to be in Rome; I was admitted to the audiences, and enjoyed the approbation of the most eminent prelates of that Court; nor did the publication of the aforesaid decree occur without my receiving some prior intimation of it. Wherefore it is my intention in this present work to show to nations abroad that as much of this matter is known in Italy (and particularly in Rome) as foreign diligence can ever have discovered, and (collecting together all my own speculations on the Copernican system) to show them that the knowledge of all these preceded the Roman censures, and that from this country proceed not only dogmas for the salvation of the soul, but also ingenious discoveries for the gratification of the understanding.

                                    From A Dialogue Concerning the Two Chief World Systems, 1631


Life on the Moon

12     A few days ago, when paying my respects to the illustrious Cardinal Muti, a discussion arose on the inequalities of the moon's surface. Signor Alessandro Capoano, in order to disprove the fact, argued that if the lunar outer surface be unequal and mountainous, one may say as a consequence that, since Nature has made our earth mountainous for the benefit of plants and animals beneficial to man, so on the moon there must be other plants and other animals beneficial to other intellectual creatures. Such a consequence, he said, being most false, therefore the fact from which it is drawn must also be false, therefore lunar mountains do not exist!

    To this I replied: As to the inequalities of the moon's surface we have only to look through a telescope to be convinced of their existence; as to the 'consequences,' I said, they are not only not necessary but absolutely false and impossible, for I was in a position to prove that neither men, nor animals, nor plants as on this earth, nor anything at all like them can exist on the moon. I said then, and I say now, that I do not believe that the body of the moon is composed of earth and water, and wanting these two elements we must necessarily conclude that it wants all the other things which without these elements cannot exist or subsist. I added further: even allowing that the matter of the moon may be like that of the earth (a most improbable supposition), still not one of those things which the earth produces can exist on the moon, since to their production other things besides earth and water are necessary—namely, the sun—the greatest agent in Nature—and the resulting vicissitudes of heat and cold, and of day and night.

    Now, such vicissitudes are on the moon very different from those on the earth. In the latter case, to produce a diversity of seasons, the sun rises and falls more than 47° (in passing from one tropic to the other); in the former case the variation is only 5 on each side of the ecliptic. While, therefore, on the earth the sun in every 24 hours illuminates all parts of its surface, each half of the moon is alternately in sunshine and darkness for 15 continuous days of 24 hours. Now, if our plants and animals were exposed to ardent sunshine every month for 360 consecutive hours, and then for a similar time were plunged in cold and darkness, they could not possibly preserve themselves, much less produce and multiply. We must, therefore, conclude that what would be impossible on our earth under the circumstances we have supposed to exist, must be impossible on the moon where those conditions do exist.

                                        From a Letter to Giacomo Muti, February 28, 1616


Senses and Language

13     I have now only to fulfill my promise of declaring my opinions on the proposition that motion is the cause of heat, and to explain in what manner it appears to me that it may be true. But I must first make some remarks on that which we call heat, since I strongly suspect that a notion of it prevails which is very remote from the truth; for it is believed that there is a true accident, affection, or quality, really inherent in the substance by which we feel ourselves heated. This much I have to say, that as soon as I form a conception of a material or corporeal substance, I simultaneously feel the necessity of conceiving that it has its boundaries, and is of some shape or other; that, relatively to others, it is great or small; that it is in this or that place, in this or that time; that it is in motion, or at rest; that it touches, or does not touch another body; that it is unique, rare, or common. I can not, by any act of the imagination, disconnect it from these qualities; but I do not find myself absolutely compelled to apprehend it as necessarily accompanied by such conditions as that it must be white or red, bitter or sweet, sonorous or silent, smelling sweetly or disagreeably. And if the senses had not pointed out these qualities, it is probable that language and imagination alone could never have arrived at them.

    Therefore, I am inclined to think that these tastes, smells, colors, etc., with regard to the object in which they appear to reside, are nothing more than mere names, and exist only in the sensitive body; insomuch that when the living creature is removed all these qualities are carried off and annihilated; although we have imposed particular names upon them (different from those other and real accidents), and would happily persuade ourselves that they truly and in fact exist. But I do not believe that there exists anything in external bodies for exciting tastes, smells, and sounds, but size, shape, quantity, and motion, swift or slow; and if ears, tongues, and noses were removed, I am of opinion that shape, quantity, and motion would remain, but there would be an end of smells, tastes, and sounds, which, abstractedly from the living creature, I take to be mere words.

                                                From The Assayer (Il Saggiatore), 1623

14     I write in Italian because I wish every one to be able to read what I say. I see young men brought together indiscriminately to study to become physicians, philosophers, etc., who although furnished, as Ruzzante might say, with a decent set of brains, yet are unable to understand things written in gibberish, assume that in crabbed folios there must be some grand hocus pocus of logic and philosophy much too high up for them to jump at. I want such people to know that as Nature has given eyes to them—just as well as to philosophers—for the purpose of seeing her works, so she has given them brains for examining and understanding them.

                                     From A Dialogue Concerning the Two Chief World Systems, 1631


Inquisition and Abjuration

15     Many years ago when the stir about Copernicus was beginning, I wrote a letter [to the Grand-Duchess Cristina], in which, supported by the authority of numerous Fathers of the Church, I showed what an abuse it was to appeal so much to Holy Scripture in questions of natural science. As soon as I am in less trouble I will send you a copy. I say in less trouble, because I am just now going to Rome, whither I have been summoned by the Holy Office, which has already prohibited the circulation of my Dialogues. I hear from well-informed persons that the Jesuit Fathers have insinuated in the highest quarters that my book is more execrable and injurious to the Church than the writings of Luther and Calvin . . .

                                    From a Letter to Elia Diodati, January 15, 1633

16     That my Dialogues recently published should find adversaries was not to be doubted, as your Eminence no doubt foresaw. It is in general the lot of all opinions which in one way or another run counter to the accepted doctrines. From the reception of my other works I expected as much, but what I did not expect was that the hate of one or two of my enemies (furious at seeing the luster of their works tarnished by mine) would be able so to influence my superiors as to make them believe that my works are unworthy of the light of day, and should be stifled. The prohibition of the printing and sale of my Dialogues has been a cruel blow to me, but I am consoled by the knowledge of the extreme purity of my conscience, and by the feeling that I shall have no difficulty in justifying my intentions.

    I will not conceal from your Eminence that the injunction to present myself without delay before the tribunal of the Holy Office has afflicted me profoundly. It is impossible to think without bitterness that the fruits of my labors and studies for so many years (which gave to my name in the scientific world a certain eclat) should now be branded as criminal. All this depresses me to such an extent as to make me curse the time I have devoted to these labors—yes, I regret having given to the world so much of my results. I feel even the desire to suppress, to destroy for ever, to commit to the flames, what remains in my hands. Thus I should satisfy the burning hate of my enemies.

    These are some of the thoughts which afflict me, and increase the burden of my seventy years; they aggravate my numerous physical sufferings, and cause me persistent insomnia. When to these is added a journey, rendered more painful and dangerous by sundry causes, I am almost certain that I shall not reach the end alive. The desire to live, common to all men, makes me implore the intercession of your Eminence, encouraged thereto by the kindness of heart which distinguishes you, and of which I as well as others have been the recipient. I beg you, then, to represent to the Holy Father my present pitiable situation . . .

    Whether it be necessary to receive my justification in writing, or by viva voce, I would point out that there are here in Florence the Inquisitor, the Archbishop, and other learned functionaries of the Church, who would be able, it seems to me, to decide graver causes than mine, and before whom I am ready to appear. It is hardly likely that in a book, which has been carefully examined by the censors, with full power to omit, to add here, to correct there, there should still remain errors so grave that their correction or the punishment due to them, should be beyond the power of the local authorities.

                            From a Letter to Cardinal Barberini ( the Pope's nephew), October 13, 1632


17 . . . because I have been enjoined, by this Holy Office, altogether to abandon the false opinion which maintains that the sun is the center and immovable, and forbidden to hold, defend, or teach, the said false doctrine in any manner; and because, after it had been signified to me that the said doctrine is repugnant to the Holy Scripture, I have written and printed a book, in which I treat of the same condemned doctrine, and adduce reasons with great force in support of the same, without giving any solution, and therefore have been judged grievously suspected of heresy; that is to say, that I held and believed that the sun is the center of the world and immovable, and that the earth is not the center and movable, I am willing to remove from the minds of your Eminences, and of every Catholic Christian, this vehement suspicion rightly entertained towards me, therefore, with a sincere heart and unfeigned faith, I abjure, curse, and detest the said errors and heresies, and generally every other error and sect contrary to the said Holy Church; and I swear that I will never more in future say, or assert anything, verbally nor in writing, which may give rise to a similar suspicion of me; but that if I shall know any heretic, or any one suspected of heresy, I will denounce him to this Holy Office, or to the Inquisitor and Ordinary of the place in which I may be.

                                From Galileo’s Abjuration before the Inquisition,

                                under threat of torture and death, June 22, 1633


Exile in Siena

18     According to the sentence pronounced on me by the Holy Office, I was condemned to imprisonment during the pleasure of his Holiness, who was pleased, however, to assign the palace and gardens of the Grand Duke near the Trinita dei Monti as my place of imprisonment. As this was in June of last year, and I had been given to understand that if I asked for a full pardon after the lapse of that and the following month I should receive it, I asked, meanwhile, to avoid having to spend the whole summer and, perhaps, part of the autumn there, to be allowed, on account of the climate, to go to Siena, where the Archbishop's house was assigned to me as a residence. I stayed there five months, when this durance was exchanged for banishment to this little villa, a mile from Florence, with a strict injunction not to go to the city, and neither to receive the visits of many friends at once, nor to invite any.

    Here, then, I was living, keeping perfectly quiet, and paying frequent visits to a neighboring convent where two daughters of mine were living as nuns. I was very fond of them, especially of the elder who possessed extraordinary mental gifts, combined with rare goodness of heart; and she was very much attached to me. During my absence, which she considered very perilous for me, she fell into a profound melancholy which undermined her health, and she was at last attacked by a violent dysentery of which she died after six days' illness, just thirty-three years of age, leaving me in the deepest grief.

    From this and other circumstances which it would take too long to describe, it will be seen that the fury of my powerful persecutors continually increases. They have, at length, chosen to reveal themselves to me. Thus, about two months ago when a dear friend of mine at Rome was speaking of my affairs to Father Cristoforo Griemberger, mathematician at the Collegio Romano, this Jesuit uttered the following precise words: " If Galileo had only known how to retain the favor of the fathers of this college he would have stood in renown before the world, he would have been spared all his misfortunes, and could have written what he pleased about everything—even about the motion of the earth."

From this you will see, honored Sir, that it is not this opinion or that which has brought and still brings about my calamities, but my being in disgrace with the Jesuits

    . . . Berigard and Chiaramonti, professors at Pisa, have written long works against me—the latter in his own defense; the former against his wish, as he says, but at the instigation of one who may be useful to him! A certain Jesuit father has printed at Rome that the opinion of the motion of the earth is of all heresies the most abominable, the most pernicious, the most scandalous; and that one may maintain in professorial chairs, in society, in public discussions, and in books, any and every argument against the principal articles of faith, against the immortality of the soul, against the creation, against the Incarnation, against everything, with one exception only—the dogma of the immobility of the earth!

                                    From a Letter to Elia Diodati, July 25, 1634

19     I have said, my Lord, that I hope for no alleviation, and this is because I have committed no wrong. If I had erred I might hope to obtain grace and pardon, since the transgressions of the subject are the means by which the prince finds occasion for the exercise of mercy and indulgence. Wherefore, when a man is wrongly condemned to punishment, it becomes necessary for his judges to use the greater severity in order to cover their own misapplication of the law . . .

    Could all the frauds, the calumnies, the stratagems, the deceits, which were made use of at Rome eighteen years ago for the purpose of imposing on the supreme authority—could all these, I say, be brought to light, their only effect would be to enhance the purity and uprightness of my intentions.

    But you, having read my works, will have seen how they justify my assertion of sincerity, and you will have understood the true cause for which, under the mask of religion, I have been persecuted, and which now continually assails me and crosses my path, so that no help can come to me from without; nor can I undertake my own defense, all the Inquisitors having received express orders neither to allow the reprinting of my published works, nor to grant a license for any fresh work I may wish to publish. Thus I am not only reduced to silence towards those who strive to distort my opinions, and so to make my ignorance (as they call it) manifest, but I must also bear the insults, the contempt; and the bitter taunts of men more ignorant than myself, without being able to utter a word in my own defense.

                                    From a Letter to Peiresc, February 21, 1635


Without Resistance, All Matter Falls at Equal Rates

20     And I would not have you do as some are accustomed, who fasten upon some saying of mine that may want a hair's breadth of the truth, and under this hair seek to hide another man's blunder as big as a cable. Aristotle says that an iron ball weighing 100 lbs. will fall through a space of 100 yards while a weight of one pound is falling through a space of one yard. I say they will reach the ground together. They find the greater weight to anticipate the lesser by two inches, and under these two inches they seek to hide Aristotle's 99 yards! . . .

    We have found the difference of velocities in moving objects of different weights to be more and more as the media are more and more resisting Thus, in a medium of quicksilver, gold does not only sink to the bottom more swiftly than lead, but it is the only thing that will sink at all—all other metals and stones move upwards in it and float on its surface. On the other hand, between balls of gold, lead, brass, or any other heavy matter, the inequality of their motion in the air is almost wholly indiscernible; so that, indeed, a ball of gold falling from a height of I00 yards in the end of its fall does not outstrip one of wax by four inches.

    This being so, I have thought that if the resistance of the media be wholly taken away, all matter would descend with equal velocity . . .

A heavy body has by nature an intrinsic force or principle of moving towards the common center of heavy things [the center of the earth] with a motion continually accelerated, in such manner that in equal times there are always equal additions of velocity. This is to be understood as holding true only when all accidental and external impediments are removed, amongst which is one that we cannot obviate, namely, the resistance of the medium. This opposes itself more or less according as it opens slowly or speedily to make way for the moving body, which, being continually accelerated, encounters a continually increasing resistance in the medium, until at last the velocity reaches that degree and the resistance that power that they balance each other. All further acceleration is then prevented, and the moving object continues for ever after with a uniform and equable motion.

                                        From Dialogues on the Two New Sciences, 1636


Final Astronomical Observations

21     I have observed a most marvelous appearance on the surface of the moon. Though she has been looked at such millions of times by such millions of men, I do not find that any have observed the slightest alteration in her surface, believing that exactly the same side has always been presented to our eyes. Now I find that such is not the case, but that she changes her aspect, as one who, having his full face turned towards us, should move it sideways, first to the right and then to the left; or should raise or then lower it; or, lastly, should incline it first to the right shoulder then to the left. All these changes I see in the moon; and the large anciently-known spots which are seen on her face will help to make evident the truth of what I say. Add to these a further marvel, which is that these three mutations have their several periods—the first daily, the second monthly, the third yearly.

                                    From a Letter to Micanzio, November 7, 1637


22     If Pieroni may be mistaken in supposing that he had observed a stellar parallax of a few seconds [proving that the earth moves], those others may be still more mistaken who assert that the visible firmament never varies, not even one or two seconds, for such an exact observation is utterly impossible, partly from the insufficiency of astronomical instruments, and partly from the refraction of light.

                                    From a letter to Rinuccini, March 29, 1641


The Principle of Galilean Relativity


23   Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals. . .With the ship standing still, observe carefully how the. little animals fly with equal speed to all sides of the cabin. . . and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. 

    When you have observed all these things carefully (though there is no doubt that when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. In jumping, you will pass on the floor the same spaces as before, nor will you make larger jumps toward the stern than toward the prow even though the ship is moving quite rapidly, despite the fact that during the time that you are in the air the floor under you will be going in a direction opposite to your jump. In throwing something to your companion, you will need no more force to get it to him whether he is in the direction of the bow or the stern, with yourself situated opposite. . . the butterflies and flies will continue their flights indifferently toward every side, nor will it ever happen that they are concentrated toward the stern, as if tired out from keeping up with the course of the ship. . . 

    The cause of all these correspondences of effects is the fact that the ship's motion is common to all the things contained in it, and to the air also. [Uniform motion is indistinguishable from a state of rest. We do not feel the earth's motion around the sun.]



1-22  Adapted from Galileo: His Life and Work, by J. J. Fahie. John Murray, London, 1903.

23     Adapted from Galileo:Dialogue Concerning the Two Chief World Systems, translated by Stillman Drake. University of California Press, Berkeley, California, 1967. This book also contains Einstein's remarks on Galileo.

Galileo's works in Latin and Italian are online at La Biblioteca telematica

A condensed version of Dialogue Concerning the Two Chief World Systems, translated by Stillman Drake, is available at the Calendars through the Ages site