Although Galileo seriously considered the priesthood as a young man, at his father's urging he instead enrolled in 1580 at the
University of Pisa for a medical degree. He was influenced by the lectures of
Girolamo Borro,
Domingo de Soto and
Francesco Buonamici of Florence. Up to this point, Galileo had deliberately been kept away from mathematics, since a physician earned a higher income than a mathematician. However, after accidentally attending a lecture on geometry, he talked his reluctant father into letting him study mathematics and
natural philosophy instead of medicine. Being inspired by the artistic tradition of the city and the works of the
Renaissance artists, Galileo acquired an
aesthetic mentality. While a young teacher at the Accademia, he began a lifelong friendship with the Florentine painter
Cigoli. In 1589, he was appointed to the chair of mathematics in Pisa. In 1591, his father died, and he was entrusted with the care of his younger brother
Michelagnolo. In 1592, he moved to the
University of Padua where he taught geometry,
mechanics, and astronomy until 1610. During this period, Galileo made significant discoveries in both pure
fundamental science as well as practical
applied science. His multiple interests included the study of
astrology, which at the time was a discipline tied to the studies of mathematics, astronomy and medicine. Additionally, Galileo engaged in practical
hydraulic engineering, obtaining a patent from the
Venetian Republic for a horse-powered
water pump in 1594.
Astronomy Kepler's supernova Tycho Brahe and others had observed the
supernova of 1572. Ottavio Brenzoni's letter of 15 January 1605 to Galileo brought the 1572 supernova and the less bright nova of 1601 to Galileo's notice. Galileo observed and discussed
Kepler's Supernova in 1604. Since these new stars displayed no detectable
diurnal parallax, Galileo concluded that they were distant stars, and, therefore, disproved the
Aristotelian belief in the immutability of the heavens.
Refracting telescope s at the
Museo Galileo, Florence, suspected to be Galilean telescopes (top: 1610–1630; bottom: 1609–1640) Perhaps based only on 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 3× magnification. He later made improved versions with up to about 30× magnification. With a
Galilean telescope, the observer could see magnified, upright images on the Earth—it was what is commonly known as a terrestrial telescope or a spyglass. He could also use it to observe the sky; for a time he was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, he demonstrated one of his early telescopes, with a magnification of about 8× or 9×, to
Venetian lawmakers. His telescopes were also a profitable sideline for Galileo, who sold them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a brief
treatise entitled
Sidereus Nuncius (
Starry Messenger).
Moon On 30 November 1609, Galileo aimed his telescope at the
Moon. While not being the first person to observe the Moon through a telescope (English mathematician
Thomas Harriot had done so four months before but only saw a "strange spottednesse"), Galileo was the first to deduce the cause of the uneven waning as light occlusion from lunar mountains and
craters. In his study, he also made topographical charts, estimating the heights of the mountains. The Moon was not what was long thought to have been a translucent and perfect sphere, as Aristotle claimed, and hardly the first "planet", an "eternal pearl to magnificently ascend into the heavenly empyrian", as put forth by
Dante. Galileo is sometimes credited with the discovery of the
lunar libration in latitude in 1632, although Thomas Harriot or
William Gilbert may have done so before. The painter Cigoli, a friend of Galileo, included a realistic depiction of the Moon in one of his paintings; he probably used his own telescope to make the observation. Galileo named the group of four the
Medicean stars, in honour of his future patron,
Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them
Galilean satellites in honour of their discoverer. These satellites were independently discovered by
Simon Marius on 8 January 1610 and are now called
Io,
Europa,
Ganymede, and
Callisto, the names given by Marius in his
Mundus Iovialis published in 1614. Galileo's observations of the satellites of Jupiter caused controversy in astronomy: a planet with smaller planets orbiting it did not conform to the principles of
Aristotelian cosmology, which held that all heavenly bodies should circle the Earth, and many astronomers and philosophers initially refused to believe that Galileo could have discovered such a thing. Compounding this problem, other astronomers had difficulty confirming Galileo's observations. When he demonstrated the telescope in Bologna, the attendees struggled to see the moons. One of them,
Martin Horky, noted that some fixed stars, such as
Spica Virginis, appeared double through the telescope. He took this as evidence that the instrument was deceptive when viewing the heavens, casting doubt on the existence of the moons.
Christopher Clavius's observatory in Rome confirmed the observations and, although unsure how to interpret them, gave Galileo a hero's welcome when he visited the next year. Galileo continued to observe the satellites over the next eighteen months, and by mid-1611, he had obtained remarkably accurate estimates for their periodsa feat which
Johannes Kepler had believed impossible. Galileo saw a practical use for his discovery. Determining the east–west position of ships at sea required their clocks to be synchronized with clocks at the
prime meridian. Solving this
longitude problem had great importance to safe navigation and large prizes were established by Spain and later Holland for its solution. Since eclipses of the moons he discovered were relatively frequent and their times could be predicted with great accuracy, they could be used to set shipboard clocks and Galileo applied for the prizes. Observing the moons from a ship proved too difficult, but the method was used for land surveys, including the remapping of France.
Phases of Venus {{multiple image From September 1610, Galileo observed that
Venus exhibits
a full set of phases similar to
that of the Moon. The
heliocentric model of the
Solar System developed by
Nicolaus Copernicus predicted that all phases would be visible since the orbit of Venus around the
Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth-side of the Sun. In
Ptolemy's geocentric model, it was impossible for any of the planets' orbits to intersect the spherical shell carrying the Sun. Traditionally, the orbit of Venus was placed entirely on the near side of the Sun, where it could exhibit only crescent and new phases. It was also possible to place it entirely on the far side of the Sun, where it could exhibit only gibbous and full phases. After Galileo's telescopic observations of the crescent, gibbous and full phases of Venus, the Ptolemaic model became untenable. In the early 17th century, as a result of his discovery, the great majority of astronomers converted to one of the various geo-heliocentric planetary models, such as the
Tychonic,
Capellan and Extended Capellan models, each either with or without a daily rotating Earth. These all explained the phases of Venus without the 'refutation' of full heliocentrism's prediction of stellar parallax.
Saturn and Neptune In 1610, Galileo also observed the planet
Saturn, and at first mistook its rings for planets, thinking it was a three-bodied system. When he observed the planet later, Saturn's rings were directly oriented to Earth, causing him to think that two of the bodies had disappeared. The rings reappeared when he observed the planet in 1616, further confusing him. Galileo observed the planet
Neptune in 1612. It appears in his notebooks as one of many unremarkable dim stars. He did not realise that it was a planet, but he did note its motion relative to the stars before losing track of it.
Sunspots Galileo made naked-eye and telescopic studies of
sunspots. Their existence raised another difficulty with the unchanging perfection of the heavens as posited in orthodox Aristotelian celestial physics. An apparent annual variation in their trajectories, observed by
Francesco Sizzi and others in 1612–1613, also provided a powerful argument against both the Ptolemaic system and the geoheliocentric system of Tycho Brahe. A dispute over claimed priority in the discovery of sunspots, and in their interpretation, led Galileo to a long and bitter feud with the
Jesuit Christoph Scheiner. In the middle was
Mark Welser, to whom Scheiner had announced his discovery, and who asked Galileo for his opinion. Both of them were unaware of
Johannes Fabricius' earlier observation and publication of sunspots.
Milky Way and stars Galileo observed the
Milky Way, previously believed to be
nebulous, and found it to be a multitude of stars packed so densely that they appeared from Earth to be clouds. He located many other stars too distant to be visible to the naked eye. He observed the double star
Mizar in
Ursa Major in 1617. In the
Starry Messenger, Galileo reported that stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope, and contrasted them to planets, which the telescope revealed to be discs. But shortly thereafter, in his
Letters on Sunspots, he reported that the telescope revealed the shapes of both stars and planets to be "quite round". From that point forward, he continued to report that telescopes showed the roundness of stars, and that stars seen through the telescope measured a few seconds of arc in diameter. He also devised a method for measuring the apparent size of a star without a telescope. As described in his
Dialogue Concerning the Two Chief World Systems, his method was to hang a thin rope in his line of sight to the star and measure the maximum distance from which it would wholly obscure the star. From his measurements of this distance and of the width of the rope, he could calculate the angle subtended by the star at his viewing point. In his
Dialogue, he reported that he had found the apparent diameter of a star of
first magnitude to be no more than 5
arcseconds, and that of one of sixth magnitude to be about 5/6 arcseconds. Like most astronomers of his day, Galileo did not recognise that the apparent sizes of stars that he measured were spurious, caused by diffraction and atmospheric distortion, and did not represent the true sizes of stars. However, Galileo's values were much smaller than previous estimates of the apparent sizes of the brightest stars, such as those made by Brahe, and enabled Galileo to counter anti-Copernican arguments such as those made by Tycho that these stars would have to be absurdly large for their annual
parallaxes to be undetectable. Other astronomers such as Simon Marius,
Giovanni Battista Riccioli, and
Martinus Hortensius made similar measurements of stars, and Marius and Riccioli concluded the smaller sizes were not small enough to answer Tycho's argument.
Theory of tides 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 the
tides to provide such evidence. This theory was so important to him that his
Dialogue Concerning the Two Chief World Systems was originally entitled 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 water in the seas as a point on the Earth's surface 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, addressed to
Cardinal Orsini. His theory gave insight into the importance of the shapes of ocean basins in the size and timing of tides; it accounted, for instance, for the negligible tides halfway along the
Adriatic Sea compared to those at the ends. Galileo's theory, however, fails to explain the observed phenomena of tides. It implies only one high tide per day, and in his 1616 account, he claimed that this occurred in the Atlantic. He attributed the two daily high tides seen at
Venice and other places to secondary causes, including the shape of the sea, its depth, and other factors. However, tides occur twice-daily in the Atlantic and most seas. Upon learning this, Galileo put forth his theory in the
Dialogue without referencing the Atlantic or other locations with once-daily tides, leaving the daily tides question unsolved. He also dismissed the idea,
known from antiquity and by his contemporary Johannes Kepler, that the
Moon caused the tides, which is the basis of modern theories.
Controversy over comets and The Assayer In 1619, Galileo became embroiled in a controversy with Father
Orazio Grassi, professor of mathematics at the Jesuit
Collegio Romano. It began as a dispute over the nature of comets, but by the time Galileo had published
The Assayer (
Il Saggiatore) in 1623, his last salvo in the dispute, it had become a much wider controversy over the very nature of science itself. The title page of the book describes Galileo as a philosopher and "Matematico Primario" of the Grand Duke of Tuscany. Because
The Assayer contains such a wealth of Galileo's ideas on how science should be practised, it has been referred to as his scientific manifesto. Early in 1619, Father 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 had appeared late in November of the previous year. Grassi concluded that the comet was a fiery body that had moved along 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, a Florentine lawyer named
Mario Guiducci, although it had been largely written by Galileo himself. Galileo and Guiducci offered no definitive theory of their own on the nature of comets, although they did present some tentative conjectures that are now known to be mistaken. (The correct approach to the study of comets had been proposed at the time by Tycho Brahe.) In its opening passage, Galileo and Guiducci's
Discourse gratuitously insulted the Jesuit
Christoph Scheiner, and various uncomplimentary remarks about the professors of the
Collegio Romano were scattered throughout the work. The Jesuits were offended, and Grassi soon replied with a
polemical tract of his own,
The Astronomical and Philosophical Balance, under the pseudonym Lothario Sarsio Sigensano, purporting to be one of his own pupils.
The Assayer was Galileo's devastating reply to the
Astronomical Balance. It has been widely recognized as a masterpiece of polemical literature, in which "Sarsi's" arguments are subjected to withering scorn. It was greeted with wide acclaim and particularly pleased the new pope,
Urban VIII, to whom it had been dedicated. In Rome, in the previous decade, Barberini, the future Urban VIII, had come down on the side of Galileo and the
Lincean Academy. Galileo's dispute with Grassi permanently alienated many Jesuits, and Galileo and his friends were convinced that they were responsible for bringing about his later condemnation, although supporting evidence for this is not conclusive.
Controversy over heliocentrism 's 1857 painting
Galileo facing the Roman Inquisition At the time of Galileo's conflict with the Church, Europe was convulsed by the
Wars of religion and the
Counter-Reformation. The majority of educated people subscribed to the
Aristotelian geocentric view that the Earth is the
centre of the Universe and the orbits of all heavenly bodies, or Tycho Brahe's new system blending geocentrism with heliocentrism. Galileo's writings on heliocentrism faced both religious and scientific objections. Religious opposition arose from biblical passages implying the fixed nature of the Earth. Scientific opposition came from Brahe, who argued that heliocentrism would imply an annual stellar parallax, though none was observed at the time. Aristarchus and Copernicus had correctly postulated that parallax was negligible because the stars were so distant. However, Brahe countered that since stars
appear to have measurable angular size, if the stars were that distant, they would have to be far larger than the Sun or even the orbit of the Earth. It would not be until much later that astronomers realized the apparent magnitudes of stars were caused by an optical phenomenon called the
airy disk, and were functions of their brightness rather than true physical size (see
the history of magnitude). Galileo defended heliocentrism based on
his astronomical observations of 1609. In 1611, the same year Galileo's telescopic discoveries were acknowledged by Jesuit members of the Collegio Romano, a commission of cardinals began investigating Galileo, inquiring if he had been involved in the trial of
Cesare Cremonini, who had taught alongside Galileo at the University of Padua and had been charged for heresy. These inquiries marked the first time Galileo's name was mentioned by the Roman Inquisition. In December 1613, the Grand Duchess
Christina of Florence 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 an eight page
letter to Castelli in which he argued that heliocentrism was actually not contrary to biblical texts and that the Bible was an authority on faith and morals, not science. This letter was not published but circulated widely. Two years later, Galileo wrote a
letter to Christina that expanded his arguments to forty pages. , 1635 By 1615, Galileo's writings on heliocentrism had been submitted to the
Roman Inquisition by Father
Niccolò Lorini, who claimed 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. Lorini specifically cited Galileo's letter to Castelli. Galileo went to Rome to defend himself and his ideas. At the start of 1616,
Francesco Ingoli initiated a debate with Galileo, sending him an essay disputing the Copernican system. Galileo later stated that he believed this essay to have been instrumental in the action against Copernicanism that followed. Ingoli may have been commissioned by the Inquisition to write an expert opinion on the controversy, with the essay providing the basis for the Inquisition's actions. The essay focused on eighteen physical and mathematical arguments against heliocentrism. It borrowed primarily from Tycho Brahe's arguments, notably that heliocentrism would require the stars as they appeared to be much larger than the Sun. The essay also included four theological arguments, but Ingoli suggested Galileo focus on the physical and mathematical arguments, and he did not mention Galileo's biblical ideas. In February 1616, an Inquisitorial commission declared heliocentrism to be "foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture". The Inquisition found that the idea of the Earth's movement "receives the same judgement in philosophy and... in regard to theological truth, it is at least erroneous in faith".
Pope Paul V instructed Cardinal Bellarmine to deliver this finding to Galileo, and to order him to abandon heliocentrism. On 26 February, Galileo was called to Bellarmine's residence and ordered "to abandon completely... the opinion that the sun stands still at the centre of the world and the Earth moves, and henceforth not to hold, teach, or defend it in any way whatever, either orally or in writing." The decree of the
Congregation of the Index banned Copernicus's
De Revolutionibus and other heliocentric works until correction. For the next decade, Galileo stayed well away from the controversy. He revived his project of writing a book on the subject, encouraged by the election of Cardinal Maffeo
Barberini as
Pope Urban VIII in 1623. Barberini was a friend and admirer of Galileo and had opposed the admonition of Galileo in 1616. Galileo's resulting book,
Dialogue Concerning the Two Chief World Systems, was published in 1632, with formal authorization from the Inquisition and papal permission. Earlier, Pope Urban VIII had personally asked Galileo to give arguments for and against heliocentrism in the book and to be careful not to advocate heliocentrism. Whether unknowingly or deliberately, Simplicio, the defender of the Aristotelian geocentric view in
Dialogue Concerning the Two Chief World Systems, was often caught in his own errors and sometimes came across as a fool. Indeed, although Galileo states in the preface of his book that the character is named after a famous Aristotelian philosopher (
Simplicius in Latin, "Simplicio" in Italian), the name "Simplicio" in Italian also has the connotation of "simpleton". This portrayal of Simplicio made
Dialogue Concerning the Two Chief World Systems appear as a polemic against Aristotelian geocentrism in defence of the Copernican theory. Most historians agree Galileo had not intended to satirize and was genuinely surprised by the reaction to his book. However, the Pope did not take the perceived public disrespect lightly, nor the Copernican advocacy.
Dava Sobel argues that prior to Galileo's 1633 trial and judgement for heresy, Pope Urban VIII had been accused of weakness in defending the church, and became preoccupied with court intrigue and problems of state, fearing even for his own life. In this context, Sobel argues that Urban felt betrayed by Galileo's
Dialogues, and court insiders and enemies of Galileo exploited this sentiment. Mario Livio places the Galileo affair in the context of modern science and politics, making a parallel with contemporary science denial. Galileo had alienated his most powerful supporter, 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 of the condemned opinions, and initially he denied even defending them. However, he was eventually persuaded to admit that, contrary to his declared intention, a reader of his
Dialogue could well get the impression that it was a defence of Copernicanism. In view of Galileo's rather implausible denial that he had ever held Copernican ideas after 1616 or ever intended to defend them in the
Dialogue, his final interrogation, in July 1633, concluded with the threat of
torture if he did not tell the truth, but he maintained his denial despite the threat. The sentence of the Inquisition was delivered on 22 June. It was in three essential parts: • Galileo was found "vehemently suspect of heresy" (though he was never formally charged with heresy, relieving him from corporal punishment), for having held the opinions that the Sun lies motionless at the centre of the universe, that the Earth is not at its centre and moves, and that one may hold and defend an opinion as probable after it has been declared contrary to Holy Scripture. He was required to "
abjure, curse and detest" those opinions. • He was sentenced to formal imprisonment at the pleasure of the Inquisition. On the following day, this was commuted to house arrest, under which he remained for the rest of his life. • His offending
Dialogue was banned; and in an action not announced at the trial, publication of any of his works was forbidden, including any he might write in the future. ''; not legible in this image) scratched on the wall of his prison cell. The attribution and narrative surrounding the painting have since been contested. According to popular legend, after recanting his theory that the Earth moved around the Sun, Galileo muttered the rebellious phrase "
And yet it moves". The earliest known written account of the legend dates to a century after his death. Supporting the legend was a claim that a 1640s painting by the Spanish painter
Bartolomé Esteban Murillo or an artist of his school, in which the words were hidden until restoration work in 1911, depicts an imprisoned Galileo apparently gazing at the words "E pur si muove" written on the wall of his dungeon. Based on the painting,
Stillman Drake wrote "there is no doubt now that the famous words were already attributed to Galileo before his death". However, an intensive investigation by astrophysicist
Mario Livio concludes that the supposed Murillo painting is most probably much more recent, a copy of an 1837 Flemish painting by Roman-Eugene Van Maldeghem. After a period with the friendly
Ascanio Piccolomini (Archbishop of
Siena), Galileo was allowed to return to his villa at
Arcetri near Florence in 1634, where he spent part of his life under house arrest. He was ordered to read the
Seven Penitential Psalms once a week for the next three years. However, his daughter Maria Celeste relieved him of the burden after securing
ecclesiastical permission to take it upon herself. While under house arrest, Galileo dedicated his time to one of his finest works,
Two New Sciences, a major reason Albert Einstein called Galileo the "father of modern physics". Here he summarised work he had done some forty years earlier, on the two sciences now called
kinematics and
strength of materials. It was published in Holland to avoid Catholic censorship. Galileo went completely blind in 1638 and developed a painful
hernia and
insomnia, and he was permitted to travel to Florence for medical advice. == Scientific contributions ==