Though he never completed his degree at Pisa, he nonetheless was called upon to teach at the universities of Pisa and Padua, to serve as mathematician and philosopher in Florence, and embark on a lecture trail that involved teaching and research in mathematics, physics, astronomy, and mechanics.

Galileo was a flambouyant figure, not averse to publicity, the kind of person that stories easily get built around.  One of the more famous ones is his experiment from atop the leaning tower of Pisa from which he dropped weights of a different size to prove that weight made no difference in the speed of their fall – in dramatic refutation of what every person since Aristotle had assumed to be the case.

It was this same flambouyancy, or sense of the dramatic, that got him in trouble with the Catholic authorities.  In his work in astronomy he was tampering with the long-standing world view of Christian civilization – and he was quite aware of the fact. Working with a description of a Dutch telescope, he built a model (claiming  for himself the invention of the concept!) which he used ably in exploring the heavens.

In a short book published in 1610, Messenger of the Stars, he declared that contrary to the teachings of Aristotle, neither the sun nor the moon were perfect spheres.  He detected the solar flares on the surface of the sun, and noted that the surface of the moon was pocked by craters and showed mountains and valleys – much like the earth's.  That is to say, the heavenly realm was not some place of perfection in divine distinction to the imperfection of the earth – as Aristotle had taught.  The realm of the heavens and the realm of the earth were much alike.  He also noticed moons around Jupiter, suggesting that the earth with its moon functioned much like other planetary objects in orbit around the sun.

Evidence of the sun rather than the earth being the center of our heavenly system was further confirmed by Galileo's discovery several years later that Venus evidenced changing light-shadow phases similar to the moon's – and that the cause of these phases or changes was undisputedly the movement of Venus around the sun.

Then in 1616 he wrote an open Letter to the Grand Duchess Christina in which he cautioned against taking certain biblical passages as being literally true (such as the sun standing still so that Joshua could finish a successful battle against the Canaanites).  In defense of scripture, he suggested that the meaning of such passages was to be found at a level much deeper, more allegorical, than at their face-value testimony.

In the context of the hot theological debates taking during the Catholic-Protestant religious wars, and in the zeal of the Church to stamp out further heresy, Galileo's attempt to move scripture from a literal to an allegorical truth-basis was not well received by the Italian authorities.  His friend, Cardinal Belarmine, warned Galileo to interpret his own astronomical findings more as allegory than as literal truth!

For a while Galileo backed down.  But in 1632 he published his Dialogue Concerning the Two Chief World Systems: Ptolemaic and Copernican.  In this he clearly attacked the old Ptolemaic world-view that Western culture had been built on since ancient times--the comforting idea that the earth was the center of all creation.  He amassed vast data to prove that Copernicus was correct in asserting that the sun, not the earth, was the center of the scheme of things.  No matter how uncomfortable that might make us feel about the earth being dethroned as the center of all things, this was simply the truth.

Immediately the Inquisition banned the book.  However it was not ready to condemn Galileo himself – and thus church authorities undertook efforts to get him to come quietly under compliance with the religious priorities of the day.  Galileo wrote a brief apology for the extremity of his work, but could not bring himself to denounce his work.  This appeared as simple intransigence to the authorities and Galileo was placed under arrest – much to his naive surprise.  In the end his sentence was reduced from imprisonment to house arrest – for the rest of his life.  And thus did he live out his days.

Nonetheless he was able to get a final work of his, Concerning Two New Sciences, published in the Protestant Netherlands in 1638.  Here he laid out clearly the underlying assumptions of his work:  that the universe is a composite of matter operating quite predictably in accordance with certain laws of motion.  There was no real distinction between how those laws worked on the earth or in the starry realm.

Four years later, still under house arrest, Galileo died blind – but not forgotten.  His works, however injudicious politically, had left their permanent mark on Western history.  There would be no going back to the old Ptolemaic world view.