Galileo Galilei

Lived 1564 in Pisa (now in Italy) to 1642 in Arcetri (near Florence)

Notable Quotes:  

"[The universe] cannot be read until we have learnt the language and become familiar with the characters in which it is written. It is written in mathematical language, and the letters are triangles, circles and other geometrical figures, without which means it is humanly impossible to comprehend a single word."   Opere Il Saggiatore p. 171

1610 AD: Galileo publishes Sidereus Nuncius (Message from the stars) describing the astronomical discoveries he made with his telescopes. 

Biographical Info

MacTutor Biography of Galileo

Illustrated chronology of Galileo's Life at PBS site, Galileo's Battle for the Heavens

Also bio at

Activities and Links

PBS Site, Galileo's Battle for the Heavens - many activities, teaching notes, demonstrations, links, really the best site for younger learners.

Galileo's Daughter Website (Portions of the book by Dava Soble are online)

The Galileo Project - like the PBS site, lots there to browse, but written for higher level audience.

Galileo's Inclined Plane Experiment: While the Leaning Tower of Pisa experiment gets more attention, this one is as important (maybe more so) and can be duplicated. This site details how an experiment group replicated the experiment, down to proving that the water clock that some earlier scientists claimed could not have been as precise as Galileo's notes claim can in fact measure time to approximately one tenth of a second.

This site has excellent diagrams that help one explain the significance of this simple experiment and the conclusions of Galileo.

How did Galileo discover the mathematical properties of the path of a projectile? Another very good site with diagrams and pictures of Galileo's sketches.

Galileo and Einstein Physics

The Two Chief World Systems
Eyes on the Sky K-6 activities esp


History of Telescopes:

The Falling Tower of Pisa Galileo Games - nice site for younger kids:

Galileo's Experiments:

Interesting article linking Galileo's work to Renaissance Art

Musings on Galileo from the LivingMathForum list:

Galileo is an enigmatic guy from what I have been reading. You might be interested in this:

I wanted to highlight a quote from that article (which comes from a highly creditable source):

"Both theological and scientific arguments were put by all concerned. Galileo himself began to put forward theological as well as scientific arguments, including use of the Holy Scripture to support the Copernican theory, something he had strongly argued against earlier when he stated that the Holy Scripture was intended only for moral teaching, not to teach physics. Having now discovered arguments by Augustine in De Genesi ad Litteram to support his case, Galileo wrote the Letter to the Grand Duchess which vigorously attacked the followers of Aristotle. In this work, which he addressed to the Grand Duchess Christina of Lorraine, he used some of Augustine's arguments, which he developed much further, to argue strongly for a non-literal interpretation of Holy Scripture when the literal interpretation would contradict facts about the physical world proved by mathematical science.

Galileo certainly started with the assumption that the Holy Scriptures are true so there must be interpretations which agree with all scientifically proved theories. It is important to realise that Galileo was not opposing Christianity, quite the opposite in fact, for he felt that he was a devout Christian doing his very best to save Christianity from serious error. He points out that theologians cannot tell a mathematician what mathematics he must believe to be true (see for example [21]):-     "... there is a great difference between giving orders to a mathematician or a natural philosopher and giving them to a merchant or a lawyer; and that proved conclusions about natural and celestial phenomena cannot be changed with the same ease as opinions about what is or is not legitimate in a contract.

Shortly after publication of Dialogue Concerning the Two Chief Systems of the World - Ptolemaic and Copernican the Inquisition banned its sale and order Galileo to appear in Rome before them. Illness prevented Galileo from travelling to Rome until 1633. When he did so he was confronted with the alternative version of the ruling of 1616, which was an unsigned document. Galileo still had in his possession the certificate Bellarmine had signed and given him in 1616, although Bellarmine had died in 1621 so could not clarify the difficulty of the two versions. By the legal standards of today one would expect the alternative version stating that Galileo was forbidden to teach the Copernican theory to be overruled. Galileo's accusation at the trial which followed was exactly that he had breached the conditions of this unsigned alternative version. The truth of the Copernican theory was not an issue therefore; it was taken as a fact at the trial that this theory was false. This was logical, of course, since the judgement of 1616 had declared it totally false.


Found guilty, Galileo was condemned to lifelong imprisonment, but the sentence was carried out somewhat sympathetically and it amounted to housearrest rather than a prison sentence. He was able to return to his home but had to spend the rest of his life watched over by officers from the Inquisition.


Galileo may have suffered at the hands of the Church, but he remained totally committed the Christianity. He wrote near the end of his life (see for example [1]):- "I have two sources of perpetual comfort - first, that in my writings there cannot be found the faintest shadow of irreverence towards the Holy Church;and second, the testimony of my own conscience, which only I and God in heaven thoroughly know. And He knows that in this cause in which I suffer, though many might have spoken with more learning, none, not even the ancient Fathers, have spoken with more piety or with greater zeal for the Church than I.

Just my reading today of The Day the Universe Changed: How Galileo's Telescope Changed The Truth and Other Events in History That Dramatically Altered Our Understanding of the World puts many of these individuals in perspective. There was a very big revolution in thinking going on, and Galileo would have been much better off had he been born 100 years later. 


"God wrote the universe in the language of mathematics." - Galileo


From: Chris

> I think, from what little I've read, it was his personality that got him in trouble with some of the church officials more so than perhaps his position (there were Church astronomers who essentially agreed with Copernicus - who only published on his deathbed to avoid confronting the church<smile>).  Galileo could be pretty confrontational but that's not necessarily a bad thing.  It just gets one in trouble.  >


Julie:  Exactly, that quote from my first post shows that he felt mathematics and science belonged to mathematicians and scientists, not the church, and that belief in and of itself was heretical, even if Copernicus' model was coming into acceptance. Copernicus may have thought it, but didn't flaunt it. (The

Jesuits, Reston, page 273) :  "If Galileo had only known how to retain the favor of the fathers of this college, he would have stood in renown beforethe world; he would have been spared all his misfortunes, and could have written about everything, even about the motion of the earth."


There seems to be some differences as to how widely accepted Copernicus' ideas were, however:"Despite these problems, Pope Clement VII approved of a summary of Copernicus' workin 1530, and asked for a copy of the full work when it was available. this was not until 1543, the year Copernicus died. As Copernicus' new picture of the universe became more widely known, misgivings arose. The universe had after all been created for mankind, so why wasn't mankind at the center? An intellectual revolutionary called Giordano Bruno accepted Copernicus' view, and went further, claiming that the stars were spread through an infinite space, not just on an outer sphere, and there were infinitely many inhabited worlds. Bruno was burned at the stake in 1600. "


It could be dangerous to be bold in expressing unconventional ideas! From "Of course, Galileo's belief that his discoveries with the telescope strongly favored the Copernican world view meant he was headed for trouble with the Church. In fact, his Venetian friends warned him that it might be dangerous to leave the protection of the Venetian state. In 1611, Galileo went to Rome and met with the Jesuit astronomers. Probably he felt that if he could win them over, he would smooth his path in any future problems with the Church. Father Clavius, author of Gregorian Calendar and undisputed leader of Jesuit astronomy had a hard time believing

there were mountains on the moon, but he surrendered with good grace on looking through the telescope (Sant., pages 18, 20) One archbishop wrote (p 20): "Bellarmine asked the Jesuits for an opinion on Galileo, and the learned fathers sent the most favorable letter you can think of . " Bellarmine was chief theologian of the Church, and a Jesuit himself. Bellarmine wrote in a letter to A. Foscarini, 12 April 1615: "Third, I say that if there were a true demonstration that the sun is at the center of the world and the earth in the third heaven, and that the sun does not circle the earth but the earth circles the sun, then one would have to proceed with great care in explaining the Scriptures that appear contrary, and say rather that we do not understand them than that what is demonstrated is false. But I will not believe that there is such a demonstration, until

it is shown me."

Galileo's attitude was contemptuous, however, at times he was downright insulting, and you are right, that got him in trouble. But it seems it wasn't so much Copernicus' general view that was a problem as much as the "perfection" of heavenly bodies that the telescope now provided challenging views.

Galileo's Commandment: 2,500 Years of Great Science Writing printed three of Galileo's original writings. The first shows a great deal of humility (On Looking Through the Telescope), wonder, excitement at pure observation.  "Where Is The Center Of The Universe from The Two Chief World Systems (1632)".  He writes a dialogue between Simplicio, who defends the traditional wisdom of Aristotle; Salviati, who speaks for Galileo; and Sagredo, the intelligent layman whom the other two hope to convince. In this it is how his observations should be interpreted that is in dispute.When you see what he was capable of foreseeing, including Newtonian physics, well, I guess I can understand the contempt. It just was a CLM*.


* CLM - (career limiting move).


"The good Christian should beware of mathematicians and all those who make empty prophecies. The danger already exists that mathematicians have made a covenant with the devil to darken the spirit and confine man in the bonds of Hell."  -- St. Augustine