Galileo was born on the 15th of February, 1564 in Pisa. His parents were Vincenzo Galilei and Guilia Ammannati. Vincenzo, who was born in Florence in 1520, was a teacher of music and a fine lute player. Guilia, who was born in Pescia, married Vincenzo in 1563 and they made their home in the countryside near Pisa. Galileo was their first child and spent his early years with his family in Pisa. In 1572, when Galileo was eight years old, his family returned to Florence, his father’s home town. However, Galileo remained in Pisa and lived for two years with Muzio Tided who was related to Galileo’s mother by marriage.
When he reached the age of ten, Galileo left Pisa to join his family in Florence and there he was tutored by Jacopo Borghini. Galileo began teaching private mathematics in Florence, and then during 1585-86 at Siena where he held a public appointment. During the summer of 1586 he wrote his first scientific book The Little Balance (La Balancitta) which described Archimedes’ method of finding the relative densities of substances using a balance. In the following year he traveled to Rome to visit Clavius who was professor of mathematics there.
A topic which was very popular with mathematicians at this time was centers of gravity and Galileo brought with him some results which he had discovered on this topic. But even though he impressed Clavius with his knowledge on various subjects, Galileo failed to gain a job to teach mathematics at the University of Bologna. When Fantoni left the chair of mathematics at the University of Pisa in 1589, Galileo was appointed to fill the post. Not only did he receive strong recommendations from Clavius, but he also had acquired an excellent reputation through his lectures at the Florence Academy in the previous year.
The young mathematician had rapidly acquired the reputation that was necessary to gain such a position, but there were still higher positions he could aim for. Galileo spent three years holding this post at the University of Pisa. During this time he wrote De Motu, a series of essays on the theory of motion which he never published. It is likely that he never published this material because he was less than satisfied with it, and this is fair for despite containing some important steps forward, it also contained some incorrect ideas.
Perhaps the most important new ideas which De Motu contains is that one can test theories by conducting experiments. Years later, in May 1609, Galileo received a letter from Paolo Sarpi telling him about a spyglass that a Dutchman had shown in Venice. Galileo wrote in the Starry Messenger in April 1610: 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 this truly remarkable effect several experiences were related, to which some persons believed while other denied them.
A few days later the report was confirmed by a letter I received from a Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to investigate means by which I might arrive at the invention of a similar instrument. This I did soon afterwards, my basis being the doctrine of refraction. From these reports, and using his own technical skills as a mathematician and as a craftsman, Galileo began to make a series of telescopes whose visual performance was much better than that of the Dutch instrument. His first telescope was made from available lenses and gave a magnification of about four times.
To improve on this Galileo learned how to grind and polish his own lenses, and by August 1609 he had an instrument with a magnification of around eight or nine. Galileo immediately saw the commercial and military applications of his telescope (which he called a perspicillum) for ships at sea. He kept Sarpi informed of his progress, and Sarpi arranged a demonstration for the Venetian Senate. They were very impressed and, in return for a large increase in his salary, Galileo gave the sole rights for the manufacture of telescopes to the Venetian Senate.
Galileo first turned his telescope on Saturn on July 25th, 1610, and it appeared as three bodies (his telescope was not good enough to show the rings but made them appear as lobes on either side of the planet). Continued observations were puzzling to Galileo as the bodies on either side of Saturn vanished when the ring system was edge on. Also in 1610, he discovered that when seen in the telescope, the planet Venus showed phases like those of the Moon, and therefore must orbit the Sun not the Earth.
This did not enable one to decide between the Copernican system, in which everything goes round the Sun, and that proposed by Tycho Brahe in which everything but the Earth (and Moon) goes round the Sun which in turn goes round the Earth. Other observations made by Galileo included the observation of sunspots. He reported these in Discourse on floating bodies which he published in 1612 and more fully in Letters on the sunspots which appeared in 1613. In 1616 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 argued strongly for a non-literal interpretation of Holy Scripture when the literal interpretation would contradict facts about the physical world proved by mathematical science. In this Galileo stated quite clearly that for him the Copernican theory is not just a mathematical calculating tool, but is a physical reality: 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.
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. It was a sad end for so great a man to die condemned of heresy. His will indicated that he wished to be buried beside his father in the family tomb in the Basilica of Santa Croce but his relatives feared that this would provoke opposition from the Church.
His body was concealed and only placed in a fine tomb in the church in 1737 by the civil authorities against the wishes of many in the Church. On 31 October 1992, 350 years after Galileo’s death, Pope John Paul II gave an address on behalf of the Catholic Church in which he admitted that errors had been made by the theological advisors in the case of Galileo. He declared the Galileo case closed, but he did not admit that the Church was wrong to convict Galileo on a charge of heresy because of his belief that the Earth rotates around the sun.
