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Galileo Galilei: 140 quotes89 likes

“Among all the great men who have philosophized about this remarkable effect, I am more astonished at Kepler than at any other. Despite his open and acute mind, and though he has at his fingertips the motions attributed to the earth, he nevertheless lent his ear and his assent to the moon's dominion over the waters, to occult properties, and to such puerilities.”

Galileo Galilei book Dialogue Concerning the Two Chief World Systems

In regard to Kepler's belief of the moon affecting the tides of the Earth, p. 328
Dialogue Concerning the Two Chief World Systems (1632)

“This [experimentation] is the custom—and properly so—in those sciences where mathematical demonstrations are applied to natural phenomena, as is seen in the case of perspective, astronomy, mechanics, music, and others where the principles, once established by well-chosen experiments, become the foundations of the entire superstructure.”

Galileo Galilei

Salviati, Third Day. Change of Position
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)

“Persisting in their original resolve to destroy me and everything mine by any means they can think of, these men are aware of my views in astronomy and philosophy. They know that as to the arrangement of the parts of the universe, I hold the sun to be situated motionless in the center of the revolution of the celestial orbs while the earth revolves about the sun. They know also that I support this position not only by refuting the arguments of Ptolemy and Aristotle, but by producing many counter-arguments; in particular, some which relate to physical effects whose causes can perhaps be assigned in no other way. In addition there are astronomical arguments derived from many things in my new celestial discoveries that plainly confute the Ptolemaic system while admirably agreeing with and confirming the contrary hypothesis.”

Galileo Galilei Letter to the Grand Duchess Christina

Variant translation: I hold that the Sun is located at the centre of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover … I confirm this view not only by refuting Ptolemy's and Aristotle's arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.
Letter to the Grand Duchess Christina (1615)

“Surely it is a great thing to increase the numerous host of fixed stars previously visible to the unaided vision, adding countless more which have never before been seen, exposing these plainly to the eye in numbers ten times exceeding the old and familiar stars.”

Galileo Galilei book Sidereus Nuncius

Translation by Stillman Drake in Discoveries and Opinions of Galileo (1957)
Sidereus Nuncius (Venice, 1609)

“Proposition I. Theorem I: When a projectile is carried in motion compounded from equable horizontal and from naturally accelerated downward [motions], it describes a semiparabolic line in its movement.”

Galileo Galilei

Author, Day Four, Stillman Drake translation (1974) p. 269
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)

“About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of the truly remarkable effect several experiences were related, to which some persons gave credence while others denied them. A few days later a report was confirmed to me in a letter from a noble Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to inquire into means by which I might arrive at the invention of a similar instrument. This I did shortly afterwards, my basis being the theory of refraction. First I prepared a tube of lead, at the ends I fitted two glass lenses, both plane on one side while on the other side one was spherically convex and the other concave. Then placing my eye near the concave lens I perceived objects satisfactorily large and near, for they appeared three times closer and nine times larger than when seen with the naked eye alone. Next I constructed another one, more accurate, which represented objects as enlarged more than sixty times. Finally, sparing neither labor nor expense, I succeeded in constructing for myself so excellent an instrument that objects seen by means of it appeared nearly one thousand times larger and over thirty times closer than when regarded with our natural vision.”

Galileo Galilei book Sidereus Nuncius

Translation by Stillman Drake in Discoveries and Opinions of Galileo (1957)
Sidereus Nuncius (Venice, 1609)

“I mentally conceive of some moveable [sphere] projected on a horizontal plane, all impediments being put aside. Now it is evident… that equable motion on this plane would be perpetual if the plane were of infinite extent, but if we assume it to be ended, and [situated] on high, the movable, driven to the end of this plane and going on further, adds on to its previous equable and indelible motion, that downward tendency which it has from its heaviness. Thus, there emerges a certain motion, compounded…”

Galileo Galilei

Author, Day Four, On the Motion of Projectiles, Stillman Drake translation (1974) p. 268
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)

“Revealing great, unusual, and remarkable spectacles, opening these to the consideration of every man, and especially of philosophers and astronomers; as observed by Galileo Galilei, Gentleman of Florence, Professor of Mathematics in the University of Padua, with the aid of a spyglass lately invented by him, in the surface of the Moon, in innumerable fixed stars, in nebulae, and above all in four planets swiftly revolving about Jupiter at differing distances and periods, and known to no one before the author recently perceived them and decided they should be named the Medicean Stars”

Galileo Galilei book Sidereus Nuncius

Translation by Stillman Drake in Discoveries and Opinions of Galileo (1957)
Sidereus Nuncius (Venice, 1609)

“And yet it moves.”

Galileo Galilei

Or "still it moves" is a comment he is alleged to have made in regard to the Earth after his recantation before the Inquisition. Giuseppe Baretti was apparently the first person to record the story. Noted as a misattribution in Paul F. Boller, John George, They Never Said It: A Book of Fake Quotes, Misquotes, and Misleading Attributions (1990), p. 30.
Disputed

“It now remains that we find the amount of time of descent through the channel. This we shall obtain from the marvelous property of the pendulum, which is that it makes all its vibrations, large or small, in equal times. This requires, once and for all, that two or three or four patient and curious friends, having noted a fixed star that stands against some fixed marker, taking a pendulum of any length, shall go counting its vibrations during the whole time of return of the fixed star to its original point, and this will be the number of vibrations in 24 hours. From the number of these we can find the number of vibrations of any other pendulums, longer or shorter, at will, so that if for example those counted by us in 24 hours were 234,567, then taking another shorter pendulum with which one counts 800 vibrations while another counts 150 of the longer pendulum, we already have, by the golden rule, the number of vibrations for the whole time of 24 hours; and if we want to know the time of descent through the channel, we can easily find not only the minutes, seconds, and sixtieths of seconds, but beyond that as we please. It is true that we can pass a more exact measure by having observed the flow of water through a thin passage, for by collecting this and having weighed what passes in one minute, for example, then by weighing what passes in the time of descent through the channel we can find the most exact measure and quantity of this time, especially by making use of a balance so precise as to weigh one sixtieth of a grain.”

Galileo Galilei

Letter to Giovanni Battista Baliani (1639)

“I am certainly interested in a tribunal in which, for having used my reason, I was deemed little less than a heretic. Who knows but men will reduce me from the profession of a philosopher to that of historian of the Inquisition! But they behave to me in order that I may become the ignoramus and the fool of Italy…”

Galileo Galilei

Source: Letter to Fr. Vincenzo Renieri (c. 1633), p. 244

“If I shall have sufficient strength to improve and amplify what was written and published by me up to now about motion by adding some little speculations, and in particular those relating to the force of percussion, in the investigation of which I have consumed hundreds and thousands of hours, and finally reduced this to very easy explanation, so that people can understand it in less than half an hour of time.”

Galileo Galilei

Letter to Giovanni Battista Baliani (1639)

“It seems to me proper to adorn the Author's thought here with its conformity to a conception of Plato's regarding the determination of the various speeds of equable motion in the celestial motions of revolution. …he said that God, after having created the movable celestial bodies, in order to assign to them those speeds with which they must be moved perpetually in equable circular motion, made them depart from rest and move through determinate spaces in that natural straight motion in which we sensibly see our moveables to be moved from the state of rest, successively accelerating. And he added that these having been made to gain that degree [of speed] which it pleased God that they should maintain forever, He turned their straight motion into circulation, the only kind [of motion] that is suitable to be conserved equably, turning always without retreat from or approach toward any pre-established goal desired by them. The conception is truly worthy of Plato, and it is to be more esteemed to the extent that its foundations, of which Plato remained silent, but which were discovered by our Author in removing their poetical mask or semblance, show it the guise of a true story.”

Galileo Galilei

I. Bernard Cohen's thesis: Galileo believed only circular (not straight line) motion may be conserved (perpetual), see The New Birth of Physics (1960).
Sagredo, Day Four, Stillman Drake translation (1974) pp.283-284
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)

“I tell you that if natural bodies have it from Nature to be moved by any movement, this can only be circular motion, nor is it possible that Nature has given to any of its integral bodies a propensity to be moved by straight motion. I have many confirmations of this proposition, but for the present one alone suffices, which is this. I suppose the parts of the universe to be in the best arrangement, so that none is out of its place, which is to say that Nature and God have perfectly arranged their structure. This being so, it is impossible for those parts to have it from Nature to be moved in straight, or in other than circular motion, because what moves straight changes place, and if it changes place naturally, then it was at first in a place preternatural to it, which goes against the supposition. Therefore, if the parts of the world are well ordered, straight motion is superfluous and not natural, and they can only have it when some body is forcibly removed from its natural place, to which it would then return by a straight line, for thus it appears that a part of the earth does [move] when separated from its whole. I said "it appears to us," because I am not against thinking that not even for such an effect does Nature make use of straight line motion.”

Galileo Galilei

A note on this statement is included by Stillman Drake in his Galileo at Work, His Scientific Biography (1981): Galileo adhered to this position in his Dialogue at least as to the "integral bodies of the universe." by which he meant stars and planets, here called "parts of the universe." But he did not attempt to explain the planetary motions on any mechanical basis, nor does this argument from "best arrangement" have any bearing on inertial motion, which to Galileo was indifference to motion and rest and not a tendency to move, either circularly or straight.
Letter to Francesco Ingoli (1624)

“I would say here something that was heard from an ecclesiastic of the most eminent degree [probably Caesar Baronius]: "The intention of the Holy Ghost is to teach us how one goes to heaven, not how heaven goes."”

Galileo Galilei Letter to the Grand Duchess Christina

Variant translation: I would say here something that was heard from an ecclesiastic of the most eminent degree: "That the intention of the Holy Spirit is to teach us how one goes to heaven, not how the heavens go."
Letter to the Grand Duchess Christina (1615)

“Mathematics is the key and door to the sciences.”

Galileo Galilei

As quoted in Building Fluency Through Practice and Performance (2008) by Timothy Rasinski and Lorraine Griffith, p. 64, but in fact a quotation by Roger Bacon: Et harum scientiarum porta et clavis est Mathematica, "And of these sciences the door and key is mathematics", from Bacon's Opus Majus (1267) https://books.google.co.uk/books?id=UfqcGd8NOFsC&pg=PA97&lpg=PA97&dq=%22porta+et+clavis%22+opus+majus&source=bl&ots=nGgt2Lhxqe&sig=88kIPB5EAKAKtm0APk6J5OrS1D0&hl=en&sa=X&ved=0ahUKEwiU36D2gIbLAhVBWBQKHSW9CKgQ6AEINDAE#v=onepage&q=%22porta%20et%20clavis%22%20opus%20majus&f=false. <br class="br">Attributed

“Whence do you have it that the terrestrial globe is so heavy? For my part, either I do not know what heaviness is, or the terrestrial globe is neither heavy nor light, as likewise all other globes of the universe. Heaviness to me (and I believe to Nature) is that innate tendency by which a body resists being moved from its natural place and by which, when forcibly removed therefrom, it spontaneously returns there. Thus a bucketful of water raised on high and set free, returns to the sea; but who will say that the same water remains heavy in the sea, when being set free there, does not move?”

Galileo Galilei

Letter to Francesco Ingoli (1624)

“I have been in my bed for five weeks, oppressed with weakness and other infirmities from which my age, seventy four years, permits me not to hope release. Added to this (proh dolor! [O misery! ]) the sight of my right eye — that eye whose labors (dare I say it) have had such glorious results — is for ever lost. That of the left, which was and is imperfect, is rendered null by continual weeping.”

Galileo Galilei

Letter to Élie Diodati (4 July 1637), as translated in The Private Life of Galileo : Compiled primarily from his correspondence and that of his eldest daughter, Sister Maria Celeste (1870) http://books.google.com/books?id=ixUCAAAAYAAJ by Mary Allan-Olney, p. 278 <br class="br">Other quotes

“Well, since paradoxes are at hand, let us see how it might be demonstrated that in a finite continuous extension it is not impossible for infinitely many voids to be found.”

Galileo Galilei

Salviati, First Day, Stillman Drake translation (1974)
Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)

“After the publication of my dialogues, I was summoned to Rome by the Congregation of the holy Office, where, being arrived on the 10th of February 1633, I was subjected to the infinite clemency of that tribunal, and of the Sovereign Pontiff, Urban the Eighth; who, notwithstanding, thought me deserving of his esteem.”

Galileo Galilei

Source: Letter to Fr. Vincenzo Renieri (c. 1633), pp. 145–146