Galileo Galilei

Galileo Galilei was born on 15 February 1565, in Pisa. He was the first of six children of Vincenzo Galilei, a famous lutenist, composer and music theorist, and Giulia Galilei. Galileo became an accomplished lutenist himself and learned early from his father a scepticism for established authority, the value of quantified experimentation, as well as the results expected from a combination of mathematics and experiment.

When Galileo was 8, the family moved to Florence, but he was left with Jacopo Borghini for two years. He was educated in the Vallombrosa Abbey.

Despite being a genuinely pious Catholic, Galileo fathered three children out of wedlock: Virginia (born in 1600), Livia (born 1601) and Vincenzo (born in 1606). Because of their illegitimate birth, Galileo considered his daughters unmarriageable: they were accepted by the convent of San Matteo in Arcetri, where they became nuns. Vincenzo was later legitimised as the legal heir.

Although Galileo considered being a priest when he was a young man, at his father's urging he instead enrolled at the University of Pisa for a medical degree. In 1581, while studying medicine, he noticed a swinging chandelier which swung in larger and smaller arcs. To him, it seemed that the chandelier took the same amount of time to swing back and forth, no matter how far it was swinging. When he returned home, he set up two pendulums of equal length and swung one with a large sweep and the other with a small sweep. He found that they kept time together. It was not until the work of Christiaan Huygens, almost one century later, that the tautochrone nature of a swinging pendulum was used to create an accurate time piece.
Up to this point, Galileo had deliberately been kept away from mathematics, since a physician earned a higher income. However, after accidentally attending a lecture on geometry, he talked his father into letting him study mathematics and natural philosophy, instead of medicine.

Galileo also studied disegno, a term encompassing fine art and, in 1588, obtained the position of instructor in the Accademia delle Arti del Disegno, in Florence, teaching perspective and chiaroscuro. In 1589, he was appointed to the chair of mathematics in Pisa. In 1592, he moved to the University of Padua, where he taught geometry, mechanics and astronomy until 1610. During this period, Galileo made several significant discoveries in both pure fundamental science as well as practical applied science.

Based only on uncertain descriptions of the first practical telescope, which Hans Lippershey tried to patent in the Netherlands in 1608, Galileo, in the following year, made a telescope with about 3x magnification. He later made improved versions with up to 30x magnification. With a Galilean telescope, one could see magnified upright images of the Earth; one could also use it to observe the sky. He published his initial telescopic astronomical observations in March 1610 in a brief treatise entitled Starry Messenger.

On 7 January 1610, Galileo observed with his telescope what he described as three fixed stars, totally invisible by their smallness, all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had been fixed stars. On 10 January, Galileo noted that one of them disappeared, an observation which he attributed to its being hidden behind the planet. Within a few days, he concluded that they orbited Jupiter - he had discovered three of Jupiter's four largest moons. Later, astronomers named them Galilean satellites, in honour of their discoverer. These satellites are now called Io, Europa, Ganymede and Callisto.

His observations of the moons of Jupiter caused a revolution in astronomy: a planet with smaller planets orbiting it did not conform to the Aristotelian cosmology. Many astronomers and philosophers refused to believe that Galileo could have discovered such a thing. His observations were confirmed by the observatory of Christopher Clavius, and he received a hero's welcome when he visited Rome in 1611.

Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without a true physical demonstration that the Sun does not circle the Earth but the Earth circles the Sun. Galileo considered his theory of tides provided the required evidence. This theory was so important to him, that he originally intended to entitle his Dialogue on the Two Chief World Views the Dialogue on the Ebb and Flow of the Sea. The reference to tides was removed from the title by order of the Inquisition. For Galileo, the tides were caused by the sloshing back and forth of the water in the seas as a point of the Earth sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616; his theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides. He also correctly accounted for the negligible tides halfway along the Adriatic Sea compared to those at the ends. However, his theory was a failure. If this theory was correct, there would only be one high tide per day. Galileo and his contemporaries were aware of this because there are two daily high tides at Venice instead of one, about 12 hours apart. Galileo dismissed this anomaly as the result of several secondary causes, including the shape of the sea, its depth and other factors. He also dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides.

In 1619, Galileo became embroiled in a controversy with Orazio Grassi, professor of mathematics at the Jesuit Collegio Romano. It began as a dispute over the nature of comets, but the time Galileo had published The Assayer in 1623, it became a much wider controversy over the nature of science itself.

Because The Assayer contains such wealth of Galileo's ideas on how science should be practiced, it has been referred to as his scientific manifesto. Early in 1619, Grassi had anonymously published a pamphlet, An Astronomical Disputation on the Three Comets of the Year 1618, which discussed the nature of a comet that appeared late in November 1618. Grassi concluded that the comet was a fiery body that had moved a segment of a great circle at a constant distance from the Earth, and since it moved in the sky more slowly than the moon, it must be farther away than the moon.

Grassi's arguments and conclusions were criticised in a subsequent article, Discourse on Comets, published under the name of one of Galileo's disciples although it had been largely written by Galileo himself. In its opening passage, the article gratuitously insulted the Jesuit Christopher Scheiner, and various uncomplimentary remarks about the professors of the Collegio Romano were scattered through the work. The Jesuits were offended, and Grassi soon replied with a polemical tract of his own: The Astronomical and Philosophical Balance. The Assayer was Galileo's devastating reply to the Astronomical Balance. It was greeted with wide acclaim, and particularly pleased the new Pope, Urban VIII, to whom it had been dedicated. Galileo's dispute with Grassi permanently alienated many of the Jesuits who had previously been sympathetic to his ideas.

In the Christian world prior to Galileo's conflict with the church, the majority of educated people believed in either the Aristotelian view that the Earth was the centre of the universe and that all heavenly bodies revolved around the Earth, or the Tychonic system, that blended geocentrism with heliocentrism. Nevertheless, after the death of Copernicus and before Galileo, heliocentrism was relatively uncontroversial.

Opposition to heliocentrism and Galileo's writings combined religious and scientific objections and were fuelled by political events. Scientific opposition came from Tycho Brahe and others, and arose from the fact that, if heliocentrism was true, an annual stellar parallax should be observed, though none was (Copernicus had correctly postulated that parallax was negligible because the stars were so distant). Religious opposition arose from the Bible, which includes texts stating that: the world is firmly established, it cannot be moved, the Lord set the earth on its foundations; it can never be moved and and the sun rises and sets and returns to its place.

Galileo defended heliocentrism based on his astronomical observations of 1609. In December 1613, Grand Duchess Christina of Tuscany confronted one of Galileo's friends and followers, Benedetto Castelli, with biblical objections to the motion of the Earth. Prompted by this incident, Galileo wrote a letter to Castelli in which he argued that heliocentrism was actually not contrary to biblical beliefs, and that the Bible was an authority on faith and morals, not on science.

By 1615, Galileo's writings on heliocentrism had been submitted to the Roman Inquisition, with claims that Galileo and his followers were attempting to reinterpret the Bible, which was seen as a violation of the Council of Trent and looked dangerously like Protestantism. Galileo went to Rome to defend himself and his Copernican and biblical ideas. At the start of 1616, Monsignor Francesco Ingoli initiated a debate with Galileo, sending him an essay disputing the Copernican system. Galileo later said that he believed this essay to have been instrumental in the action against Copernicanism that followed. The essay focused on 18 physical and mathematical arguments against heliocentrism, borrowed primarily from the arguments of Tycho Brahe; it also included 4 theological arguments, but Ingoli suggested that Galileo focus on the physical and mathematical arguments. In February 1616, an Inquisitorial commission declared heliocentrism to be foolish and absurd in philosophy, and formally heretic since it explicitly contradicts in many places the sense of Holy Scripture.

Pope Paul V instructed Cardinal Bellarmine to deliver this finding to Galileo, and to order him to abandon the opinion that heliocentrism was physically true. On 26 February, Galileo was ordered ...to abandon completely...the opinion that the sun stands still at the centre of the universe and that the earth moves, and henceforth not to hold, teach, or defend it in anyway whatever, either orally or in writing.

For the next decade, Galileo stayed well and away from the controversy. He revived his project of writing a book on the subject, encouraged by Pope Urban VIII in 1623; the Pope was a friend and admirer of Galileo, and had opposed the condemnation in 1616. The resulting book, Dialogue Concerning the Two Chief World Views, was published in 1632, with formal authorisation from the Inquisition and papal permission. Urban VIII had personally asked Galileo to give arguments for and against heliocentrism, and to be careful not to advocate heliocentrism; he also requested that his own views on the matter be included in Galileo's book. Only the latter was fulfilled.

Galileo had alienated one of his biggest and most powerful allies, the Pope, and was called to Rome to defend his writings in September 1632. He finally arrived in February 1633 and was brought before inquisitor Vincenzo Maculani to be charged. Throughout his trial, Galileo steadfastly maintained that since 1616 he had faithfully kept his promise not to hold any condemned options, and initially, he denied defending them. However, he was eventually persuaded to admit that, contrary to his true intention, a reader of his Dialogue could well have obtained the impression that it was intended to be a defence of Copernicanism. In view of Galileo's rather implausible denial that he never held Copernican ideas after 1616 or ever intended to defend them in the Dialogue, his final interrogation concluded with his being threatened with torture if he did not tell the truth. He maintained his denial.

The sentence of the Inquisition was delivered on 22 June. It was in three essential parts: he was found vehemently suspect of heresy and was required to abjure, curse and detest his opinions; he was sentenced to formal imprisonment at the pleasure of the Inquisition; and his Dialogue was banned and the publication of any of his works (including future works) was forbidden. According to popular legend, after recanting his theory that the Earth moved around the Sun, Galileo allegedly muttered the rebellious phrase: And yet it moves.

After a period with the friendly Archbishop of Sienna, Galileo was allowed to return to his villa in Arcetri in 1634, where he spent the remainder of his life under house arrest. Galileo was ordered to read the seven penitential psalms once a week for the next three years. However, his daughter Maria Celeste (previously named Virginia) relieved him of the burden after securing ecclesiastical permission to take it upon herself.

It was while he was under house arrest that he dedicated his time to one of his finest works, Two New Sciences. Here, Galileo summarised the work he had done some 40 years earlier, on the two new sciences now called kinematics and strength of materials. As a result of his work, Galileo is often called the father of modern physics. He went completely blind in 1638 and was suffering from a painful hernia and insomnia.

Galileo Galilei continued to receive visitors until 1642 when, after suffering from heart palpitations, he died on 8 January 1642, aged 77.

Galileo Galilei

Justus Sustermans

c. 1636

(Galileo Galilei's scientific career included many more achievements. However, I selected just a few...otherwise, this text would be 3 km long!)