galileo and experimental science

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Galileo Galilei

By: History.com Editors

Updated: June 6, 2023 | Original: July 23, 2010

Galileo Galilei (1564-1642) is considered the father of modern science and made major contributions to the fields of physics, astronomy, cosmology, mathematics and philosophy. Galileo invented an improved telescope that let him observe and describe the moons of Jupiter, the rings of Saturn, the phases of Venus, sunspots and the rugged lunar surface. His flair for self-promotion earned him powerful friends among Italy’s ruling elite and enemies among the Catholic Church’s leaders. Galileo’s advocacy of a heliocentric universe brought him before religious authorities in 1616 and again in 1633, when he was forced to recant and placed under house arrest for the rest of his life.

Galileo’s Early Life, Education and Experiments

Galileo Galilei was born in Pisa in 1564, the first of six children of Vincenzo Galilei, a musician and scholar. In 1581 he entered the University of Pisa at age 16 to study medicine, but was soon sidetracked by mathematics. He left without finishing his degree. In 1583 he made his first important discovery, describing the rules that govern the motion of pendulums.

Did you know? After being forced during his trial to admit that the Earth was the stationary center of the universe, Galileo allegedly muttered, "Eppur si muove!" ("Yet it moves!" ). The first direct attribution of the quote to Galileo dates to 125 years after the trial, though it appears on a wall behind him in a 1634 Spanish painting commissioned by one of Galileo's friends.

From 1589 to 1610, Galileo was chair of mathematics at the universities of Pisa and then Padua. During those years he performed the experiments with falling bodies that made his most significant contribution to physics.

Galileo had three children with Marina Gamba, whom he never married: Two daughters, Virginia (Later “Sister Maria Celeste”) and Livia Galilei, and a son, Vincenzo Gamba. Despite his own later troubles with the Catholic Church, both of Galileo’s daughters became nuns in a convent near Florence.

Galileo, Telescopes and the Medici Court

In 1609 Galileo built his first telescope, improving upon a Dutch design. In January of 1610 he discovered four new “stars” orbiting Jupiter—the planet’s four largest moons. He quickly published a short treatise outlining his discoveries, “Siderius Nuncius” (“The Starry Messenger”), which also contained observations of the moon’s surface and descriptions of a multitude of new stars in the Milky Way. In an attempt to gain favor with the powerful grand duke of Tuscany, Cosimo II de Medici, he suggested Jupiter’s moons be called the “Medician Stars.”

“The Starry Messenger” made Galileo a celebrity in Italy. Cosimo II appointed him mathematician and philosopher to the Medicis , offering him a platform for proclaiming his theories and ridiculing his opponents.

Galileo’s observations contradicted the Aristotelian view of the universe, then widely accepted by both scientists and theologians. The moon’s rugged surface went against the idea of heavenly perfection, and the orbits of the Medician stars violated the geocentric notion that the heavens revolved around Earth.

Galileo Galilei’s Trial

In 1616 the Catholic Church placed Nicholas Copernicus ’s “De Revolutionibus,” the first modern scientific argument for a heliocentric (sun-centered) universe, on its index of banned books. Pope Paul V summoned Galileo to Rome and told him he could no longer support Copernicus publicly.

In 1632 Galileo published his “Dialogue Concerning the Two Chief World Systems,” which supposedly presented arguments for both sides of the heliocentrism debate. His attempt at balance fooled no one, and it especially didn’t help that his advocate for geocentrism was named “Simplicius.”

Galileo was summoned before the Roman Inquisition in 1633. At first he denied that he had advocated heliocentrism, but later he said he had only done so unintentionally. Galileo was convicted of “vehement suspicion of heresy” and under threat of torture forced to express sorrow and curse his errors.

Nearly 70 at the time of his trial, Galileo lived his last nine years under comfortable house arrest, writing a summary of his early motion experiments that became his final great scientific work. He died in Arcetri near Florence, Italy on January 8, 1642 at age 77 after suffering from heart palpitations and a fever.

What Was Galileo Famous For? 

Galileo’s laws of motion, made from his measurements that all bodies accelerate at the same rate regardless of their mass or size, paved the way for the codification of classical mechanics by Isaac Newton . Galileo’s heliocentrism (with modifications by Kepler ) soon became accepted scientific fact. His inventions, from compasses and balances to improved telescopes and microscopes, revolutionized astronomy and biology. Galilleo discovered craters and mountains on the moon, the phases of Venus, Jupiter’s moons and the stars of the Milky Way. His penchant for thoughtful and inventive experimentation pushed the scientific method toward its modern form.

In his conflict with the Church, Galileo was also largely vindicated. Enlightenment thinkers like Voltaire used tales of his trial (often in simplified and exaggerated form) to portray Galileo as a martyr for objectivity. Recent scholarship suggests Galileo’s actual trial and punishment were as much a matter of courtly intrigue and philosophical minutiae as of inherent tension between religion and science.

In 1744 Galileo’s “Dialogue” was removed from the Church’s list of banned books, and in the 20th century Popes Pius XII and John Paul II made official statements of regret for how the Church had treated Galileo.

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Galileo Galilei

Galileo Galilei (1564–1642) has always played a key role in any history of science, as well as many histories of philosophy. He is a—if not the —central figure of the Scientific Revolution of the seventeenth century. His work in physics (or “natural philosophy”), astronomy, and the methodology of science still evoke debate after more than 400 years. His role in promoting the Copernican theory and his travails and trials with the Roman Church are stories that still require re-telling. This article attempts to provide an overview of these aspects of Galileo’s life and work, but does so by focusing in a new way on his arguments concerning the nature of matter.

1. Brief Biography

2. introduction and background, 3. galileo’s scientific story, 4. galileo and the church, primary sources: galileo’s works, secondary sources, other internet resources, related entries.

Galileo was born in Pisa on February 15, 1564. By the time he died on January 8, 1642 (but for problems with the date, see Machamer 1998b, 24–25), he was as famous as any person in Europe. Moreover, when he was born there was no such thing as ‘science’; yet by the time he died, science was well on its way to becoming a discipline, and its concepts and method a complete philosophical system.

Galileo’s father Vincenzo, though of noble heritage, was a semi-itinerant court musician and composer of modest means, who also authored treatises on music theory; his mother, Giulia Ammannati, descended from Pisan cloth merchants. In 1572, they resettled the family in Florence. As a boy, Galileo was tutored privately and, for a time, by the monks at Vallombrosa, where he considered a religious vocation and may have started a novitiate. He returned home, however, and then enrolled for a medical degree at the University of Pisa in 1580. He never completed this degree, but instead studied mathematics, notably with Ostilio Ricci, a mathematics teacher attached to the Tuscan court and the Florentine Accademia del Disegno.

After leaving university, Galileo worked as a private mathematics tutor around Florence and Siena and cultivated the support of leading mathematicians. He visited Christoph Clavius, professor at the Jesuit Collegio Romano, and corresponded with the engineer Guildobaldo del Monte, Marchese of Urbino. In 1588, he applied and was turned down for a professorship in Bologna, but a year later, with the help of Clavius and del Monte, he was appointed lecturer in mathematics at Pisa. In 1592, he obtained, at a much higher salary, a chair of mathematics at the University of Padua, in the Venetian Republic. Galileo also supplemented his income by producing a calculating instrument of his own design (see Galilei 1606) and other devices in a household workshop, and by private tutoring and consulting on practical mathematics and engineering. During this period, he began a relationship with Marina Gamba, and their daughter Virginia was born in 1600. In 1601, they had another daughter, Livia, and a son, Vincenzo, in 1606.

In Padua, Galileo worked out much of the mechanics he would publish later in life, and which constitute his primary lasting contribution to physical science. However, these projects were interrupted in 1609, when Galileo heard about the recently invented spyglass, invented an improved telescope, and used it to make astounding celestial discoveries. He rushed these into print in Sidereus Nuncius ( Starry Messenger ), which appeared in March 1610 and launched Galileo onto the world stage. Among others, Johannes Kepler, Imperial Mathematician at Prague, lauded the work (Kepler 1610). Clavius and his colleagues at the Collegio Romano confirmed its results and threw a celebratory banquet when Galileo visited in 1611. During the same Roman sojourn, Galileo was admitted to what was perhaps the first scientific society, the Accademia dei Lincei; he would style himself “Lincean Academician” for the rest of his life. Some fascinating treatments of this period of Galileo’s life and motivations have recently appeared (Biagioli 2006; Reeves 2008; Wilding 2014).

Galileo also used the Starry Messenger to solicit patronage in his native Tuscany, naming the moons of Jupiter he had found the “Medicean” stars, in honor of the ruling Medici family. His negotiations were ultimately successful, and Galileo moved to Florence as “Chief Mathematician and Philosopher to the Grand Duke” and holder of a sinecure professorship at Pisa. His daughters moved with him and were shortly placed in the convent of Saint Matthew at Arcetri, near Florence. Vincenzo and his mother, Marina, were left behind in Venice.

Once a courtier, Galileo entered into several debates on scientific topics. In 1612, he published a Discourse on Floating Bodies , and in 1613, Letters on Sunspots , where he first openly expressed support for Copernican heliocentrism. In 1613–14, Galileo entered into discussions of Copernicanism through his student Benedetto Castelli, and wrote a Letter to Castelli defending the doctrine from theological objections. Meanwhile, it had become known that Copernicanism was under scrutiny by Church authorities. Galileo lectured and lobbied against its condemnation, expanding his Letter to Castelli into the widely circulated Letter to the Grand Duchess Christina in 1615 and travelling to Rome late that year. Nevertheless, in March 1616, Copernicus’s On the Revolutions of the Heavenly Orbs was suspended (i.e., temporarily censored), pending correction, by the Congregation of the Index of Prohibited Books. Galileo himself was called to an audience with Cardinal Robert Bellarmine, a leading theologian and member of the Roman Inquisition, who admonished him not to teach or defend Copernican theory. (The details of this episode are far from straightforward, and remain disputed even today. See Shea and Artigas 2003; Fantoli 2005.)

In 1623, Galileo published The Assayer , which deals with the nature of comets and argues they are sublunary phenomena. This book includes some of Galileo’s most famous methodological pronouncements, including the claim that the book of nature is written in the language of mathematics. It also contains passages suggestive of atomism, a heretical doctrine, for which the book was referred to the Inquisition, which dismissed the charge.

Also in 1623, Maffeo Barberini, Galileo’s supporter and friend, was elected Pope Urban VIII. Galileo felt empowered to begin work on his Dialogue Concerning the Two Chief World Systems . The “two systems” are the Ptolemaic and Copernican, and the text clearly, though not explicitly, favors the latter. Printing was completed in Florence by February 1632. Shortly afterwards, the Inquisition banned its sale, and Galileo was ordered to Rome for trial. In June 1633, Galileo was convicted of “vehement suspicion of heresy,” and a sentence of imprisonment was immediately commuted to perpetual house arrest. (There is more about these events and their implications in the final section of this article, Galileo and the Church .)

In 1634, while Galileo was confined to his villa in Arcetri, his beloved eldest daughter died (Sobel 1999). Around this time, he began work on his final book, Discourses and Mathematical Demonstrations Concerning Two New Sciences , based on the mechanics he had developed early in his career. The manuscript was smuggled out of Italy and published in Holland by the Elzeviers in 1638. Galileo died early in 1642, and due to his condemnation, his burial place was obscure until he was re-interred in 1737.

For detailed biographical material, the best and classic work dealing with Galileo’s scientific achievements is Stillman Drake’s Galileo at Work (1978). More recently, J. L. Heilbron has written a magnificent biography, Galileo (2010), that touches on all the multiple facets of his life.

From the seventeenth century onward, Galileo has been seen by many as the “hero” of modern science. He is renowned for his discoveries: he was the first to report telescopic observations of the mountains on the moon, the moons of Jupiter, the phases of Venus, and the rings of Saturn. He invented an early microscope and a predecessor to the thermometer. In mathematical physics—a discipline he helped create—he calculated the law of free fall, conceived of an inertial principle, determined the parabolic trajectory of projectiles, and advocated the relativity of motion. He is thought to be the first “real” experimental scientist, who dropped stones from towers and ships’ masts, and played with magnets, clocks, and pendulums (noting the isochrony of the latter). Much of his cultural stature also arises from his advocacy and popularization of Copernicanism and the resulting condemnation by the Catholic Inquisition, which has made him a purported “martyr” to the cause of rationality and enlightened modernity in the subsequent history of a supposed “warfare” between science and religion. This is no small set of accomplishments for one seventeenth-century Italian, who was the son of a court musician and who left the University of Pisa without a degree.

Momentous figures living in momentous times are full of interpretive fecundity, and Galileo has been the subject of manifold interpretations and much controversy. The use of Galileo’s work and the invocations of his name make a fascinating history (Segre 1991; Palmerino and Thijssen 2004; Finocchiaro 2005; Shea and Artigas 2006), but this is not our topic, which are the philosophical implications of his work.

Philosophically, Galileo has been used to exemplify many different themes, usually as a personification of whatever the writer wished to make the hallmark of the Scientific Revolution or of the nature of good science—whatever was good about the new science or science in general, it was Galileo who started it. One tradition of Galileo scholarship has divided Galileo’s work into three or four parts: (1) his physics, (2) his astronomy, and (3) his methodology, which might include his method of Biblical interpretation and/or his thoughts about the nature of proof or demonstration. In this tradition, typical treatments deal with his physical and astronomical discoveries and their background and/or who were Galileo’s predecessors. More philosophically, many ask how his mathematical practice relates to his natural philosophy. Was he a mathematical Platonist (Jardine 1976; Koyré 1978), an experimentalist (Settle 1967; Settle 1983; Settle 1992; Palmieri 2008), an Aristotelian emphasizing experience (Geymonat 1954), a precursor of modern positivist science (Drake 1978), or maybe an Archimedean (Machamer 1998a), who might have used a revised Scholastic method of proof (Wallace 1992; Miller 2018)? Or did he have no method and just fly like an eagle in the way that geniuses do (Feyerabend 1975)? Alongside these claims there have been attempts to place Galileo in an intellectual context that brings out the background to his achievements. Some have emphasized his debt to the artisan/engineer practical tradition (Rossi 1962; Valleriani 2010), others his mathematics (Giusti 1993; Feldhay 1998; Renn, et al. 2000; Palmieri 2001; Palmieri 2003; Peterson 2011; Palmerino 2016), his mixed (or subalternate) mathematics (Machamer 1978; Lennox 1986; Wallace 1992; Dear 1995; Machamer 1998a), his debt to atomism (Shea 1972; Redondi 1983), his use of Hellenistic and Medieval impetus theory (Moody 1951; Duhem 1954; Clagett 1959; Shapere 1974), or the idea that discoveries bring new data into science (Wootton 2015).

Still, almost everyone working in this tradition seems to think the three areas—physics, astronomy, and methodology—are somewhat distinct and represent different Galilean endeavors. More recent historical research has followed contemporary intellectual fashion and shifted foci, bringing new dimensions to our understanding of Galileo by studying his rhetoric (Finocchiaro 1980; Moss 1993; Feldhay 1998; Spranzi 2004), the power structures of his social milieu (Biagioli 1993; Biagioli 2006), his personal quest for acknowledgment (Shea and Artigas 2003), and more generally emphasizing the larger social and cultural history (Reeves 2008; Bucciantini, et al. 2015), in particular the court and papal culture in which Galileo functioned (Redondi 1983; Heilbron 2010).

In an intellectualist recidivist mode, this entry will outline his investigations in physics and astronomy and exhibit, in a new way, how these all cohered in a unified inquiry. In setting out this path, we shall show why, at the end of his life, Galileo felt compelled (in some sense of necessity) to write the Two New Sciences , which stands as a true completion of his overall project and is not just a reworking of his earlier research that he reverted to after his trial, when he was under house arrest and going blind. Particularly, we shall try to show why both of the two new sciences, especially the first, were so important—a topic not much treated except recently (Biener 2004; Raphael 2011). In passing, we shall touch on his methodology and his mathematics, and here refer you to some of the recent work by Palmieri (2001; 2003). At the end, we shall add some words about Galileo, the Catholic Church, and his trial.

The philosophical thread that runs through Galileo’s intellectual life is a strong and increasing desire to find a new conception of what constitutes natural philosophy and how natural philosophy ought to be pursued. Galileo signaled this goal clearly when he left Padua in 1610 to return to Florence and the court of the Medici. He asked for and received the designation ‘Philosopher’, in addition to ‘Mathematician’. This was not just a status-affirming request, but also a reflection of his programmatic aims. What Galileo accomplished by the end of his life was a reasonably articulated replacement for the traditional set of analytical concepts connected with the Aristotelian tradition of natural philosophy. He offered, in place of the Aristotelian categories, a set of mechanical archetypes that were accepted by most everyone who afterwards developed the “new sciences,” and which, in some form or another, became the hallmark of the new philosophy. His way of thinking became the way of the Scientific Revolution (and yes, there was such a revolution, pace Shapin 1996 and others; see the selections in Lindberg and Westman 1990; Osler 2000).

Some scholars might wish to describe what Galileo achieved in psychological terms, as an introduction of new mental models (Palmieri 2003) or a new model of intelligibility (Machamer 1994; Machamer, 1998a; Adams, et al. 2017). However phrased, Galileo’s main move was to dethrone the Aristotelian physical categories; namely, the one celestial element ( aether , or quintessence—i.e., “fifth element”) and the four terrestrial ones (fire, air, water, and earth), along with their respective motive natures (circular, and up and down). In their place, he left only one element, corporeal matter, whose properties and motions he described using the mathematics of proportional relations typified by the Archimedian simple machines—the balance, the inclined plane, and the lever—to which Galileo added the pendulum (Machamer 1998a; Machamer and Hepburn 2004; Palmieri 2008). In doing so, Galileo changed the acceptable way of talking about matter and its motion, and so ushered in the mechanical tradition that characterizes so much of modern science, even today. See Dijksterhuis 1961; Machamer, et al. 2000; Gaukroger 2006; Roux and Garber 2013.

As a way of understanding Galileo’s accomplishments, it is useful to see him as being interested in finding a unified theory of matter—a mathematical theory of the material stuff that constitutes the whole of the cosmos. Perhaps he did not realize that this was his grand project until the time he actually wrote the Two New Sciences in the mid-1630s. Despite working on problems of the nature of matter from 1590 onwards, he could not have written his final work much earlier than 1638; certainly not before the Starry Messenger of 1610, and probably not before the Dialogue Concerning the Two Chief World Systems of 1632. He had thought deeply about the nature of matter before 1610 and had tried to work out how best to describe matter, but before 1632, he did not have the theory and evidence he needed to support his claims about a unified, singular matter. The idea of unified matter theory had to wait for the establishment of principles of matter’s motion on a moving Earth. And this he did not accomplish until the Dialogue .

Galileo began his critique of Aristotle in a treatise he drafted around 1590, titled De Motu ( On Motion ). The first part of this manuscript deals with terrestrial matter and argues that Aristotle’s theory has it wrong. For Aristotle, the matter of the terrestrial realm within the sphere of the moon is of four elemental kinds—earth, water, air, and fire. These possess two formal principles that give rise to their natural motion: heaviness ( gravitas ; in earth and water) and lightness ( levitas ; in air and fire). Galileo, using an Archimedean model of floating bodies, and later the balance, argues that there is only one principle of motion—heaviness. Bodies move upward not because they have a natural lightness, he says, but because they are displaced or extruded by other heavier bodies moving downward. So on his view, heaviness is the cause of all natural terrestrial motion.

This move left Galileo with a problem: what is heaviness and how is it to be described? In De Motu , he argued that the moving arms of a balance could be used as a model for treating all problems of natural motion. In this model, heaviness is the proportionality of the weight of an object on one arm of the balance to the weight of another body on the other arm. In the context of floating bodies, heaviness is the weight of one body minus the weight of the medium. Galileo quickly realized these characterizations were insufficient, and so began to explore how heaviness might be related to specific gravities; i.e., the comparative weights of bodies having equal volume. He was trying to figure out the concept of heaviness that is characteristic of all matter. What he failed to work out—and this was probably the reason why he never published De Motu —was this positive characterization of heaviness. There seemed to be no way to find a standard measure of heaviness that would work across different substances. At this point, he did not have a useful replacement for Aristotelian gravitas .

A while later, in his 1600 manuscript version of Le Meccaniche ( On Mechanics ), Galileo introduced the concept of momento , a quasi force that applies to a body at a moment, and which is somehow proportional to weight or specific gravity (Galluzzi 1979). Still, he had no good way to measure or compare specific gravities of bodies of different kinds, and his notebooks during this early seventeenth-century period reflect his trying again and again to find a way to bring all matter under a single proportional measuring scale. He tried to study acceleration along an inclined plane and to find a way to think of what changes acceleration brings to momento . Yet the details and categories of how to properly treat weight and movement eluded him.

We see from this period that Galileo’s law of free fall arises out of this struggle to find the proper categories for his new science of matter and motion. Galileo accepted, probably as early as the 1594 draft of Le Meccaniche , that natural motions might be accelerated. Particularly in the cases of the pendulum, the inclined plane, free fall, and projectile motion, Galileo must have observed that the speeds of bodies increase as they move downwards and, perhaps, do so naturally. But that accelerated motion is properly measured against time is an idea he realized only later, chiefly through his failure to find any satisfactory dependence on place and specific gravity. Also at this time, he began to think about percussive force. For many years, he thought that the correct science of these phenomena should describe how bodies change according to where they are on their paths. Specifically, it seemed that height is crucial. Percussive force is directly related to height and the motion of the pendulum seems to involve equilibrium with respect to the height of the bob (and time also, but isochrony did not lead directly to a recognition of time’s importance).

One of Galileo’s problems was that the Archimedian simple machines he was using as his models of intelligibility, especially the balance, are not easily conceived of in a dynamic way (but see Machamer and Woody 1994). Since they generally work by establishing static equilibrium, time is not a feature of their action one would normally attend to. In discussing a balance, for instance, one does not normally think about how fast an arm of the balance descends, nor how fast a body on the opposite arm is rising (though Galileo does in his Postils to Rocco circa 1634–45; see Palmieri 2005). The converse is also true. It is difficult to model dynamic phenomena that involve rates of change as balance arms moving upwards or downwards because of differential weights. So it was that Galileo’s puzzle about how to describe time and the force of percussion (the force of body’s impact) would remain unsolved. Throughout his life, he could not find systematic relations among specific gravities, heights of fall, and percussive forces. Even while the Two New Sciences was going to press in 1638, Galileo was laboring on an additional “Fifth Day” (not published until 1718) that presciently explored the concept of the force of percussion, which would become, after his death, one of the most fecund ways to think about matter and its motion.

In the period 1603–9, Galileo experimented with inclined planes and, most importantly, pendulums. These studies again exhibited to Galileo that acceleration and, therefore, time is a crucial variable. Moreover, the isochrony of the pendulum—the period depends only on the length of the cord, regardless of the weight of the bob—went some way towards showing that time is a possible term in the equilibrium (or ratio) that needs to be made explicit to represent motion. It also shows that, in at least one case, time can displace weight as a crucial variable.

The law of free fall—i.e., a body in free fall from rest traverses a distance proportional to the square of the time elapsed—was discovered by Galileo through the inclined plane experiments (Drake 1999, v. 2). At first, Galileo attempted to represent this phenomenon with a velocity-distance relation, and the equivalent mean proportional relation. His later and correct definition of natural acceleration dependent on time was an insight gained through recognizing the physical significance of that mean proportional relation (Machamer and Hepburn 2004; for a different analysis of Galileo’s discovery of free fall see Renn, et al. 2000). Yet Galileo would not publish anything making time central to his analysis of motion until 1638, in the Two New Sciences .

In 1609, Galileo began his work with the telescope. There are many ways to describe Galileo’s findings, and many interpreters have taken this to be an interlude irrelevant to his physics. However, they are remarkable insofar as they are his start at dismantling the celestial-terrestrial distinction entrenched in Aristotelian cosmology (Feyerabend 1975). Perhaps the most unequivocal case of this is when he analogizes the mountains on the moon to mountains in Bohemia in the Starry Messenger . Also crucial was his discovery of the four moons circling Jupiter, which lent credence to the Copernican system since it meant that a planet-moon arrangement was not unique to the Earth. The abandonment of the dichotomy between heavens and earth implied that all matter, whether celestial or terrestrial, is of the same kind. Further, if there is only one kind of matter, there can be only one kind of natural motion—one kind of motion that this matter has by nature. So it has to be that one law of motion will hold throughout the terrestrial and celestial realms. This is a far stronger claim than he had made in 1590, which concerned only the terrestrial elements.

A few years later, in his Letters on Sunspots (1613), Galileo offered new telescopic evidence that supported the Copernican theory. But these observations also served as additional reasons for dissolving the celestial-terrestrial distinction. One was that the sun is not an immutable aetherial sphere, but has changing spots ( maculae ) on its surface. Another was that the sun rotates circularly around its axis, like the Earth. A third was the discovery that Venus undergoes a full sequence of phases (like the moon), which entails that Venus revolves around the sun, and suggests that the Earth is likewise a celestial body moving around the sun. Certainly the phases of Venus contradicted the Ptolemaic ordering of the planets.

Later, in 1623, Galileo argued for a quite mistaken material thesis. In The Assayer , he tried to show that comets are sublunary phenomena and that their properties could be explained by optical refraction. While this work stands as a masterpiece of scientific rhetoric, it is somewhat strange that Galileo should have argued against the superlunary nature of comets, which the great Danish astronomer Tycho Brahe had demonstrated earlier.

Yet even with all these developments, Galileo still needed to work out general principles concerning the nature of motion for this newly unified matter. In this respect, Galileo differed from Ptolemy (at least of the Almagest ), Copernicus, or even Tycho Brahe, who treated their planetary systems—be they Earth- or sun-centered—merely as models of the planets’ observed motions; that is, as mathematical conceits for calculating observable positions. For Galileo, by contrast, Copernicanism was also a commitment to a physically realizable cosmography. Consequently, he needed to work out, at least qualitatively, a way of thinking about the actual motions of matter. He had to devise (or shall we say, discover) principles of local motion that would fit a central sun, planets moving around that sun, a whirling Earth, and everything on it.

This he did by introducing two new principles. In Day One of his Dialogue Concerning the Two Chief World Systems (1632), Galileo argues that matter will move naturally along circular trajectories, neither speeding up nor slowing down. Then, in Day Two, he introduces his version of the famous principle of the relativity of observed motion. This latter holds that observers cannot detect uniform motions they share with objects under observation; only differential motion can be seen. Of course, neither of these principles was entirely original with Galileo. They had predecessors. But no one needed them for the reasons that he did, namely that they were necessitated by a unified cosmological matter.

One key effect of these principles is that the diurnal terrestrial rotation asserted by the Copernican system is unobservable. The Earth and all the objects on it naturally move in circles around the Earth’s axis once a day, but since human observers share this motion, it cannot be detected. We only notice departures from shared rotation, such as bodies falling or rising. Consequently, “all experiments practicable upon the Earth are insufficient measures” for proving its stability or its mobility, “since they are indifferently adaptable to an Earth in motion or at rest” (Galilei 1967, 6). This blunted standard objections to Copernicanism on the grounds that there is no evidence of terrestrial motion.

Having dispelled these arguments against the Copernican system, Galileo then dramatically argues in its favor. In Day Three of the Dialogue . Salviati, Galileo’s avatar, has Simplicio, the ever-astounded Aristotelian, make use of astronomical observations, especially the facts that Venus has phases and that Venus and Mercury are never far from the sun, to construct a diagram of the planetary positions. The resulting diagram neatly corresponds to the Copernican model. Then in Day Four, Galileo offers a supposed proof of Copernican theory on the basis of the tides, asserting that they result from the combination of the Earth’s diurnal rotation and its annual motion around the sun.

In the Dialogue , things are more complicated than we have just sketched. Galileo, as noted, argues for circular natural motion. Yet he also introduces, in places, an intrinsic tendency for rectilinear motion. For example, Galileo recognizes that a stone whirled circularly in a sling would fly off along the rectilinear tangent if released (Galilei 1967, 189–94; see Hooper 1998). Further, in Day Four, when he is giving his mechanical explanation of the tides, he nuances his matter theory by attributing to water an additional power of retaining an impetus for motion such that it can generate a reciprocal movement once it is sloshed against a side of a basin. This was not Galileo’s first dealing with water. We saw it first in the De Motu around 1590, where Galileo discusses submerged and floating bodies, but he learned much more in his dispute over floating bodies (which produced the Discourse on Floating Bodies in 1612). In fact, a large part of that debate turned on the exact nature of water as matter, and what kind of mathematical proportionality could be used to correctly describe it and bodies moving in it (see Palmieri 1998; 2004).

The final chapter of Galileo’s scientific story came in 1638, with the publication of the Two New Sciences . The second science, discussed in the last two Days, deals with the principles of local motion and has been much commented upon in the Galilean literature. But the first science, discussed in the first two Days, has been misunderstood and infrequently discussed. It has misleadingly been called the science of the strength of materials, and so seems to have found a place in history of engineering, since such a course is still taught today. However, this science is not about the strength of materials per se . It is Galileo’s attempt to provide a mathematical science of his unified matter (see Machamer 1998a; Biener 2004; Machamer and Hepburn 2004). Galileo realizes that, before he can work out a science of the motion of matter, he must have some way of showing that the nature of matter may be mathematically characterized. Both the mathematical nature of matter and the mathematical principles of motion he believes belong to the science of “mechanics,” which is the name he gives for this new way of philosophizing.

So it is in Day One that Galileo begins to discuss how to describe mathematically (or geometrically) the causes of the breaking of beams. But this requires a way to reconcile mathematical description with the physical constitution of material bodies. In this vein, Galileo rejects using finite atoms as a basis for physical discussion, since they are not representable by continuously divisible mathematical magnitudes. Instead, he treats matter as composed of infinitely many indivisible—which is to say, mathematical—points. This allows him to give mathematical accounts for various properties of matter. Among these are the density of matter, its coherence in material bodies, and the properties of the resisting media in which bodies move. The Second Day lays out the mathematical principles concerning how bodies break. Galileo does all this by reducing the problems of matter to problems of how a lever and a balance function, which renders them mathematically tractable via the law of the lever. He had begun this back in 1590, though this time he believes he is getting it right, showing mathematically how bits of matter solidify and stick together, and how they break into bits.

The First Day also contains Galileo’s account of the acceleration of falling bodies, and the argument that they fall equally fast in a vacuum, whatever their weight. This discussion contains the famous thought experiment refuting the Aristotelian theory of fall, according to which the speed of a body’s fall is proportional to its weight. In this “short and conclusive” argument, Galileo supposes that two bodies, one heavier than the other, are suddenly conjoined in the midst of falling. On the one hand, if Aristotle is correct, the faster fall of the heavier body will be retarded by the slower motion of the lighter body, so that the conjoined body will fall slower than the original heavy body. And yet, the conjoined body is heavier than either original body, so it should also fall faster. Hence, there is a contradiction in the Aristotelian position (Gendler 1998; Palmieri 2005; Brown and Fehige 2019).

Galileo’s second new science, in Days Three and Four of the Two New Sciences, deals with the mathematical description of local motion and the laws governing it. This is now the motion of all matter, not just sublunary stuff, and the treatment takes the categories of time and acceleration as basic. Here is where Galileo enunciates his law of free fall, the parabolic path of projectiles, and other physical “discoveries” that would lay the foundation for modern physics (Drake 1999, v. 2).

In the projected Fifth Day, Galileo would have treated the power of moving matter to act by impact, which he calls the force of percussion. Ultimately, Galileo was unable to give mathematical principles of this kind of interaction, but this problem subsequently became an important locus of interest. René Descartes, probably following Isaac Beeckman, would eventually convert the problem into the task of finding equilibrium between the forces conserved by colliding bodies. Descartes’s own mathematical treatment was mistaken, but correct principles were given in 1668–9 by Christiaan Huygens, John Wallis, and Christopher Wren.

The sketch above provides the basis for understanding Galileo’s career. He offered a new science of matter, a new physical cosmography, and a new science of local motion. In all these, he used a mathematical mode of description based upon, though somewhat changed from, the proportional geometry of Book VI of Euclid’s Elements and of Archimedes (for details on the changes, see Palmieri 2001).

It is in this way that Galileo developed the categories of the mechanical new science, the science of matter and motion. His new categories utilized some of the basic principles of traditional mechanics, to which he added the category of time and so emphasized acceleration. But throughout, he was working out the details about the nature of matter so that it could be understood as uniform and universal, and treated in a way that allowed for coherent discussion of the principles of motion. It was due to Galileo that a unified matter became accepted and its nature became one of the problems for the new science that followed. After him, matter really mattered.

No account of Galileo’s importance to philosophy can be complete if it does not discuss the Galileo Affair—the sequence of interactions with the Church that resulted in Galileo’s condemnation. The end of the affair is simply stated. In late 1632, in the aftermath of the publication of the Dialogue Concerning the Two Chief World Systems , Galileo was ordered to appear in Rome to be examined by the Congregation of the Holy Office; i.e., the Inquisition. In January 1633, a very ill Galileo made an arduous journey to Rome. From April, Galileo was called four times to hearings; the last was on June 21. The next day, June 22, 1633, Galileo was taken to the church of Santa Maria sopra Minerva, and ordered to kneel while his condemnation was read. He was declared guilty of “vehement suspicion of heresy,” and made to recite and sign a formal abjuration:

I have been judged vehemently suspect of heresy, that is, of having held and believed that the sun in the center of the universe and immoveable, and that the Earth is not at the center of same, and that it does move. Wishing however, to remove from the minds of your Eminences and all faithful Christians this vehement suspicion reasonably conceived against me, I abjure with a sincere heart and unfeigned faith, I curse and detest the said errors and heresies, and generally all and every error, heresy, and sect contrary to the Holy Catholic Church. (Quoted in Shea and Artigas 2003, 194)

Tradition, but not historical fact, holds that, after abjuring, Galileo mumbled, “ Eppur si muove (and yet it moves).” He was sentenced to “formal imprisonment at the pleasure of the Inquisition,” but this was commuted to house arrest, first in the residence of the Archbishop of Siena, and then, from December 1633, at his villa in Arcetri. When he later finished his last book, the Two New Sciences (which does not mention Copernicanism at all), it had to be printed in Holland, and Galileo professed amazement at how it could have been published.

The details and interpretations of these proceedings have long been debated, and it seems that each year we learn more about what actually happened. One point of controversy is the legitimacy of the charges against Galileo, both in terms of their content and the judicial procedure. Galileo was charged with teaching and defending the Copernican doctrine that holds the sun is at the center of the universe and the Earth moves. The status of this doctrine was cloudy. In 1616, an internal commission of the Inquisition had determined that it was heretical, but this was not publicly proclaimed. Instead, Copernicus’s book had been placed on the Index of Prohibited Books —the list of books Catholics were forbidden to read without special permission—with the status “suspended until corrected.” Even more confusingly, the requisite corrections had appeared in 1620, but the book nevertheless remained on the Index until 1835. In fact, the Church’s first public pronouncement that Copernicanism is a heresy appears in Galileo’s condemnation.

Galileo’s own status was also problematic. In 1616, at the same time that the Inquisition was evaluating Copernicanism, they were also investigating Galileo personally—a separate proceeding of which Galileo himself was not likely aware. The outcome was Bellarmine’s admonition not to “defend or hold” the Copernican doctrine. This “charitable admonition” may (or may not) have been followed by a “formal injunction” “not to hold, teach, or defend it in any way whatever, either orally or in writing.” When the records of this disposition of the 1616 case were discovered in 1633, it made Galileo appear guilty of recidivism, having violated the Inquisition’s injunction by publishing the Dialogue .

To confound issues further, the case against Galileo transpired in a fraught political context. Galileo was a creature of the powerful Medici and a personal friend of Pope Urban VIII, connections that significantly modulated developments (Biagioli 1993). There were also pressures stemming from the Counter Reformation, the Thirty Years War, and resulting tensions within Urban’s papacy (McMullin 2005; Miller 2008). It has even been argued (Redondi 1983), that the charge of Copernicanism was the effect of a plea bargain meant to cover up Galileo’s genuinely heretical atomism, though this latter hypothesis has not found much support.

The legitimacy of the underlying condemnation of Copernicus on theological and rational grounds is even more problematic. Galileo had addressed this problem in 1615, when he wrote his Letter to Castelli and then the Letter to the Grand Duchess Christina . In these texts, Galileo argues that there are two truths: one derived from Scripture, the other from the created natural world. Since both are expressions of the divine will, they cannot contradict one another. However, Scripture and Creation both require interpretation in order to glean the truths they contain—Scripture because it is a historical document written for common people, and thus accommodated to their understanding so as to lead them towards true religion; Creation because the divine act must be distilled from sense experience through scientific enquiry. While the truths are necessarily compatible, biblical and natural interpretations can go awry, and are subject to correction.

Much philosophical controversy, before and after Galileo’s time, revolves around this doctrine of the two truths and their seeming incompatibility. Which of course, leads us immediately to such questions as: “What is truth?” and “How is truth known or shown?”

Cardinal Bellarmine was willing to countenance scientific truth if it could be proven or demonstrated (McMullin 1998). But Bellarmine held that the planetary theories of Ptolemy and Copernicus (and presumably Tycho Brahe) are only mathematical hypotheses; since they are just calculating devices, they are not susceptible to physical proof. This is a sort of instrumentalist, anti-realist position (Machamer 1976; Duhem 1985). There are any number of ways to argue for some sort of instrumentalism. Duhem (1985) himself argued that science is not metaphysics, and so only deals with useful conjectures that enable us to systematize phenomena. Subtler versions of this position, without an Aquinian metaphysical bias, have been argued subsequently and more fully by Van Fraassen (1980) and others. Less sweepingly, it can reasonably be argued that both Ptolemy and Copernicus’s theories were primarily mathematical, and that Galileo was defending not Copernicus’s theory per se , but the physical realization of it. In fact, it might be better to say the Copernican theory that Galileo was constructing was a physical realization of a simplified version of Copernicus’s theory, which dispensed with many of the technical details (eccentrics, epicycles, Tusi couples and the like). Galileo would be led to such a view by his concern with matter theory, which minimized the kinds of motion ascribed uniformly to all bodies. Of course, when put this way, we are faced with the question of what constitutes identity conditions for a theory. Still, there is clearly a way in which Galileo’s Copernicanism is not Copernicus’s.

The other aspect of all this that has been hotly debated is what constitutes proof or demonstration of a scientific claim. Galileo believed he had a proof of terrestrial motion. This argument concerning the cause of the tides is contained in On the Ebb and Flow of the Tides , a manuscript he composed in 1616 while Copernicanism was under the Inquisition’s scrutiny, and the main thrust of which appears in Day Four of the Dialogue Concerning the Two Chief World Systems .

In the first place, Galileo restricts the possible class of causes of the tides to mechanical interactions, and so rules out Kepler’s attribution of the cause to the moon. How could the moon cause the tides to ebb and flow without any connection to the seas? Such an explanation would be an invocation of magic or occult powers. Thus, for Galileo, the only conceivable (or maybe plausible) physical cause for the regular reciprocation of the tides is the combination of the diurnal and annual motions of the Earth. Briefly, as the Earth rotates around its axis, some parts of its surface are moving along with the annual revolution around the sun and some parts are moving in the contrary direction. (In the same way that a point near the top of a car’s wheel is rotating in the same direction as the car is moving, while a point near the ground is rotating toward the rear.) In the frame of the fixed stars, this creates accelerations and retardations of the Earth’s surface, and since the terrestrial waters are not attached to the surface, they slosh back and forth as their basins speed up and slow down. Hence the tides. Moreover, since the Earth’s diurnal and annual rotations are regular, so are the tidal periods. Local differences in tidal flows are due to the differences in the physical conformations of the basins in which they occur (for background and more detail, see Palmieri 1998). Albeit mistaken, Galileo’s commitment to mechanically intelligible causation makes this is a plausible argument. One can see why Galileo thinks he has some sort of proof for the motion of the Earth, and therefore for Copernicanism.

Yet one can also see why Bellarmine and the instrumentalists would not have been impressed. First, they did not accept Galileo’s restriction of possible causes to mechanically intelligible causes. Second, Galileo’s explanation is not precise; it does not account for many details of tidal motion. Most significantly, the motion of the Earth’s surface varies over twelve hours, not the six-hour cycle of the tides. Third, the argument does not touch upon the central position of the sun or arrangement of the planets as calculated by Copernicus. So at best, Galileo’s argument is an inference to the best partial explanation from a limited set of features of Copernicus’s theory. Meanwhile, there were compelling considerations about the size of celestial bodies that weighed against the Copernican cosmology, stemming from a lack of understanding of the telescope’s optics (Graney 2015).

Nevertheless, when the tidal argument is added to the earlier telescopic observations that show the improbabilities of the older celestial picture—the fact that Venus has phases like the moon and so must revolve around the sun; the principle of the relativity of perceived motion which neutralizes the physical arguments against a moving Earth; and so on—it was enough for Galileo to believe that he had the necessary proof to convince the doubters. Unfortunately, it was not until after Galileo’s death and the acceptance of a unified material cosmology, utilizing the presuppositions about matter and motion that were published in the Two New Sciences , that people were ready for such proofs. But this could occur only after Galileo had changed the acceptable parameters for gaining knowledge and theorizing about the world.

To read many of the documents of Galileo’s trial, see Finocchiaro 1989; Mayer 2012. To understand the long, tortuous, and fascinating aftermath of the Galileo affair see Finocchiaro 2005; and for Pope John Paul II’s 1992 rehabilitation of Galileo, see Coyne 2005.

The main body of Galileo’s work is collected in:

• Favaro, Antonio (ed.), 1890–1909, Le Opere di Galileo Galilei , Edizione Nazionale, 20 vols., Florence: Barbera; reprinted 1929–1939 and 1964–1966. [ available online ]

English translations:

• Fermi, Laura, and Bernardini, Gilberto (trans.) in Fermi, Laura, and Bernardini, Gilberto, 1961, Galileo and the Scientific Revolution , New York: Basic Books; reprinted 1965, New York: Fawcett; and 2003 and 2013, Mineola, NY: Dover.
• Drabkin, I. E. (trans.) in Galilei, Galileo, 1960, On Motion and On Mechanics, Madison: University of Wisconsin Press.
• Fredette, Raymond (trans.), 2000, De Motu Antiquiora , Berlin: Max Planck Institute for the History of Science. [available online]
• Drake, Stillman (trans.) in Galilei, Galileo, 1960, On Motion and On Mechanics, Madison: University of Wisconsin Press.
• Drake, Stillman (trans.), 1978, Operations of the Geometric and Military Compass , Washington, D.C.: Smithsonian Institution.
• Van Helden, Albert (trans.), 1989, Sidereus Nuncius, or The Sidereal Messenger , Chicago: University of Chicago Press; 2nd edition, 2016; reprinted, with facsimile of Library of Congress’s first edition and expository essays, in De Simone, Daniel, and John W. Hessler (eds.), 2013, The Starry Messenger: From Doubt to Astonishment , Washington, D.C.: Library of Congress/Levenger Press.
• Barker, Peter (trans.), 2004, Sidereus Nuncius , Oklahoma City: Byzantium Press.
• Shea, William R. (trans.), 2009, Galileo’s Sidereus Nuncius, or A Sidereal Message , Sagamore Beach, MA: Science History Publications; 2nd revised printing, 2012.
• Drake, Stillman (trans.) in Drake, Stillman, 1984, Cause, Experiment, and Science , Chicago: Chicago University Press.
• Reeves, Eileen, and Van Helden, Albert (trans.) in Galilei, Galileo, and Scheiner, Christoph 2010, On Sunspots , Chicago: University of Chicago Press.
• Drake, Stillman (trans.), in Galilei, Galileo, Grassi, Horatio, Guiducci, Mario, and Kepler, Johannes, 1960, The Controversy on the Comets of 1618, Philadelphia: University of Pennsylvania Press.
• Drake, Stillman (trans.), 1967, Dialogue Concerning the Two Chief World Systems , Berkeley: University of California Press; reprinted 2001, New York: The Modern Library.
• Crew, Henry, and de Salvio, Alfonso (trans.), 1954, Dialogues Concerning Two New Sciences , New York: Dover Publications. This inferior translation, first published in 1914, has been reprinted numerous times and is widely available.
• Drake, Stillman (trans.), 1974, [ Discourses on the ] Two New Sciences , Madison: University of Wisconsin Press; 2nd edition, 1989, reprinted 2000, Toronto: Wall and Emerson.

Collections of primary sources in English:

• Drake, Stillman (ed.), 1957, The Discoveries and Opinions of Galileo , New York: Anchor Books.
• Finocchiaro, Maurice A. (ed.), 1989, The Galileo Affair: A Documentary History, Berkeley: University of California Press.
• Finocchiaro, Maurice A. (ed.), 2008, The Essential Galileo , Indianapolis: Hackett.
• Shea, William R., and Davie, Mark (ed.), 2012, Galileo: Selected Writings , Oxford: Oxford University Press.
• Adams, Marcus P., Zvi Biener, Uljana Feest, and Jacqueline A. Sullivan (eds.), 2017, Eppur si Muove: Doing History and Philosophy of Science with Peter Machamer , Dordrecht: Springer.
• Biagioli, Mario, 1990, “Galileo’s System of Patronage,” History of Science , 28 (1): 1–62.
• –––, 1993, Galileo, Courtier: The Practice of Science in the Culture of Absolutism , Chicago: University of Chicago Press.
• –––, 2006, Galileo’s Instruments of Credit: Telescopes, Images, Secrecy , Chicago: University of Chicago Press.
• Biener, Zvi, 2004, “Galileo’s First New Science: The Science of Matter,” Perspectives on Science , 12 (3): 262–287.
• Brown, James Robert, and Yiftach Fehige, 2019, “Thought Experiments,” The Stanford Encyclopedia of Philosophy (Winter 2019 Edition), E. N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/win2019/entries/thought-experiment/>.
• Bucciantini, Massimo, Michele Camerota, and Franco Giudice, 2015, Galileo’s Telescope: A European Story , C. Bolton (trans.), Cambridge, MA: Harvard University Press.
• Carugo, Adriano, and Alistair C. Crombie, 1983, “The Jesuits and Galileo’s Ideas of Science and Nature,” Annali dell’Istituto e Museo di Storia della Scienza di Firenze , 8 (2): 3–68.
• Clagett, Marshall, 1959, The Science of Mechanics in the Middle Ages , Madison: University of Wisconsin Press.
• Coyne, George V., 2005, “The Church’s Most Recent Attempt to Dispel the Galileo Myth,” in E. McMullin (ed.), The Church and Galileo , Notre Dame, IN: University of Notre Dame Press, pp. 340–359.
• Crombie, Alistair C., 1975, “Sources of Galileo’s Early Natural Philosophy,” in M. L. R. Bonelli and W. R. Shea (eds.), Reason, Experiment, and Mysticism in the Scientific Revolution , New York: Science History Publications, pp. 157–175.
• Dear, Peter, 1995, Discipline and Experience: The Mathematical Way in the Scientific Revolution , Chicago: University of Chicago Press.
• Dijksterhuis, E. J., 1961, The Mechanization of the World Picture , C. Dikshoorn (trans.), Oxford: Clarendon Press.
• Drake, Stillman, 1976, Galileo Against the Philosophers , Los Angeles: Zeitlin & Ver Brugge.
• –––, 1978, Galileo at Work , Chicago: University of Chicago Press.
• –––, 1999, Essays on Galileo and the History and Philosophy of Science , N. M. Swerdlow and T. H. Levere (eds.), 3 vols, Toronto: University of Toronto Press.
• Duhem, Pierre, 1954, Le Systeme du monde , 6 vols, Paris: Hermann.
• –––, 1985, To Save the Phenomena: An Essay on the Idea of Physical Theory from Plato to Galileo , A. Roger (trans.), Chicago: University of Chicago Press.
• Fantoli, Annibale, 2005, “The Disputed Injunction and Its Role in Galileo’s Trial,” in E. McMullin (ed.), The Church and Galileo , Notre Dame, IN: University of Notre Dame Press, pp. 117–149.
• Feldhay, Rivka, 1995, Galileo and the Church: Political Inquisition or Critical Dialogue? , Cambridge: Cambridge University Press.
• –––, 1998, “The Use and Abuse of Mathematical Entities,” in P. Machamer (ed.), The Cambridge Companion to Galileo , Cambridge: Cambridge University Press, pp. 80–145.
• Feyerabend, Paul, 1975, Against Method , London and New York: Verso and Humanities Press.
• Finocchiaro, Maurice A., 1980, Galileo and the Art of Reasoning , Dordrecht: D. Reidel.
• –––, 2005, Retrying Galileo, 1633–1992 , Berkeley: University of California Press.
• Galluzzi, Paolo, 1979, Momento: Studi Galileiani , Rome: Edizioni dell’Ateneo e Bizzarri.
• Gattei, Stefano, 2019, On the Life of Galileo: Viviani’s Historical Account and Other Early Biographies , Princeton: Princeton University Press.
• Gaukroger, Stephen, 2006, The Emergence of a Scientific Culture: Science and the Shaping of Modernity, 1210–1685 , Oxford: Oxford University Press.
• Gendler, Tamar Szabó, 1998, “Galileo and the Indispensability of Scientific Thought Experiment,” British Journal for the Philosophy of Science , 49 (3): 397–424.
• Geymonat, Ludovico, 1954, Galileo: A Biography and Inquiry into his Philosophy of Science , S. Drake (trans.), New York: McGraw Hill.
• Giusti, Enrico, 1993, Euclides Reformatus. La Teoria delle Proporzioni nella Scuola Galileiana , Turin: Bottati-Boringhieri.
• Graney, Christopher M., 2015, Setting Aside All Authority: Giovanni Battista Riccioli and the Science against Copernicus in the Age of Galileo , Notre Dame, IN: University of Notre Dame Press.
• Heilbron, John L., 2010, Galileo , Oxford: Oxford University Press.
• Hooper, Wallace, 1998, “Inertial Problems in Galileo’s Preinertial Framework,” in P. Machamer (ed.), Cambridge Companion to Galileo , Cambridge: Cambridge University Press, pp. 146–174.
• Jardine, Nicholas, 1976, “Galileo’s Road to Truth and the Demonstrative Regress,” Studies in History and Philosophy of Science , 7 (4): 277–318.
• Kepler, Johannes, 1610, Dissertation cum Nuncio Sidereo , Prague; translated as Kepler’s Conversation with Galileo’s Sidereal Messenger , E. Rosen (trans.), New York: Johnson Reprint Corporation, 1965.
• Koyré, Alexandre, 1966, Études Galiléennes , Paris: Hermann; translated as Galileo Studies , J. Mepham (trans.), Atlantic Highlands, N.J.: Humanities Press, 1978.
• Lennox, James G., 1986, “Aristotle, Galileo and the ‘Mixed Sciences’,” in W. A. Wallace (ed.), Reinterpreting Galileo , Washington, D.C.: Catholic University of America Press, pp. 29–52.
• Lindberg, David C., and Robert S. Westman (eds.), 1990, Reappraisals of the Scientific Revolution , Cambridge: Cambridge University Press.
• Machamer, Peter, 1976, “Fictionalism and Realism in 16th Century Astronomy,” in R. S. Westman (ed.), The Copernican Achievement , Berkeley: University of California Press, pp. 346–353.
• –––, 1978, “Galileo and the Causes,” in R. Butts and J. C. Pitt (eds.), New Perspectives on Galileo , Dordrecht: Kluwer, pp. 161–180.
• –––, 1991, “The Person-Centered Rhetoric of the Seventeenth Century,” in M. Pera and W. R. Shea (eds.), Persuading Science: The Art of Scientific Rhetoric , Canton, MA: Science History Publications, pp. 143–156.
• –––, 1998a, “Galileo’s Machines, His Mathematics, and His Experiments,” in P. Machamer (ed.), The Cambridge Companion to Galileo , Cambridge: Cambridge University Press, pp. 53–79.
• –––, 1998b, “Introduction,” in P. Machamer (ed.), Cambridge Companion to Galileo , Cambridge: Cambridge University Press, pp. 1–26.
• –––, 1999, “Galileo and the Rhetoric of Relativity,” Science and Education , 8 (2): 111–120; reprinted in E. Gianetto, F. Bevilacqua, and M. R. Matthews (eds.), 2001, Science Education and Culture: The Contribution of History and Philosophy of Science , Dordrecht: Kluwer, pp. 31–40.
• Machamer, Peter, Lindley Darden, and Carl Craver, 2000, “Thinking about Mechanisms,” Philosophy of Science , 67: 1–25.
• Machamer, Peter, and Brian Hepburn, 2004, “Galileo and the Pendulum: Latching on to Time,” Science and Education , 13: 333–347; reprinted in M. R. Matthews, C. F. Gauld, and A. Stinner (eds.), 2005, The Pendulum: Scientific, Historical, Philosophical and Educational Perspectives , Dordrecht: Springer, pp. 99–113.
• Machamer, Peter, and Andrea Woody, 1994, “A Model of Intelligibility in Science: Using Galileo’s Balance as a Model for Understanding the Motion of Bodies,” Science and Education , 3 (3): 215–244.
• Mayer, Thomas F. (ed.), 2012, The Trial of Galileo 1612–1633 , North York, Ontario: University of Toronto Press.
• McMullin, Ernan (ed.), 1967, Galileo, Man of Science , New York: Basic Books.
• –––, 1998, “Galileo on Science and Scripture,” in P. Machamer (ed.), Cambridge Companion to Galileo , Cambridge: Cambridge University Press, pp. 271–347.
• ––– (ed.), 2005, The Church and Galileo: Religion and Science , Notre Dame: University of Notre Dame Press.
• Miller, David Marshall, 2008, “The Thirty Years War and the Galileo Affair,” History of Science , 46: 49–74.
• –––, 2018, “Regressus and Empiricism in the Controversy about Galileo’s Lunar Observations,” Perspectives on Science , 26 (3): 293–324.
• Moody, Ernest A., 1951, “Galileo and Avempace: The Dynamics of the Leaning Tower Experiment,” Journal of the History of Ideas , 12: 163–193, 375–422.
• Moss, Jean Dietz, 1993, Novelties in the Heavens , Chicago: University of Chicago Press.
• Osler, Margaret, 2000, Rethinking the Scientific Revolution , Cambridge: Cambridge University Press.
• Palmerino, Carla Rita, 2016, “Reading the Book of Nature: The Ontological and Epistemological Underpinnings of Galileo’s Mathematical Realism,” in G. Gorham, B. Hill, E. Slowik, and C. K. Waters (eds.), The Language of Nature: Reassessing the Mathematization of Natural Philosophy the Seventeenth Century , Minneapolis: University of Minnesota Press, pp. 29–50.
• Palmerino, Carla Rita, and Johannes M. M. H. Thijssen (eds.), 2004, The Reception of the Galilean Science of Motion in Seventeenth-Century Europe , Dordrecht: Springer.
• Palmieri, Paolo, 1998, “Re-examining Galileo’s Theory of Tides,” Archive for History of Exact Sciences , 53: 223–375.
• –––, 2001, “The Obscurity of the Equimultiples: Clavius’ and Galileo’s Foundational Studies of Euclid’s Theory of Proportions,” Archive for the History of the Exact Sciences , 55 (6): 555–597.
• –––, 2003, “Mental Models in Galileo’s Early Mathematization of Nature,” Studies in History and Philosophy of Science Part A , 34: 229–264.
• –––, 2004, “The Cognitive Development of Galileo’s Theory of Buoyancy,” Archive for the History of the Exact Sciences , 59: 189–222.
• –––, 2005, “‘Spuntar lo scoglio piu duro’: Did Galileo Ever Think the Most Beautiful Thought Experiment in the History of Science?,” Studies in History and Philosophy of Science Part A , 36 (2): 223–240.
• –––, 2008, Reenacting Galileo’s Experiments: Rediscovering the Techniques of Seventeenth-Century Science , Lewiston, NY: Edwin Mellen Press.
• Peterson, Mark A., 2011, Galileo’s Muse: Renaissance Mathematics and the Arts , Cambridge, MA: Harvard University Press.
• Pitt, Joseph C., 1992, Galileo, Human Knowledge, and the Book of Nature: Method Replaces Metaphysics , Dordrecht: Springer.
• Raphael, Renee, 2011, “Making Sense of Day 1 of the Two New Sciences: Galileo’s Aristotelian-Inspired Agenda and His Jesuit Readers,” Studies in History and Philosophy of Science Part A , 42: 479–491.
• Redondi, Pietro, 1983, Galileo Eretico , Turin: Einaudi; translated as Galileo Heretic , R. Rosenthal (trans.), Princeton: Princeton University Press, 1987.
• Reeves, Eileen, 2008, Galileo’s Glassworks: The Telescope and the Mirror , Cambridge, MA: Harvard University Press.
• Renn, Jürgen, Peter Damerow, and Simone Rieger, 2000, “Hunting the White Elephant: When and How did Galileo Discover the Law of Fall?,” Science in Context , 13 (3-4): 299–419; reprinted in J. Renn (ed.), 2005, Galileo in Context , Cambridge: Cambridge University Press, pp. 29–149.
• Rossi, Paolo, 1962, I Filosofi e le Macchine , Milan: Feltrinelli.
• Roux, Sophie, and Daniel Garber (eds.), 2013, The Mechanization of Natural Philosophy , New York: Springer.
• Segre, Michael, 1991, In the Wake of Galileo , New Brunswick, NJ: Rutgers University Press.
• –––, 1998, “The Never-Ending Galileo Story,” in P. Machamer (ed.), The Cambridge Companion to Galileo , Cambridge: Cambridge University Press, pp. 388–416.
• Settle, Thomas, 1967, “Galileo’s Use of Experiment as a Tool of Investigation,” in E. McMullin (ed.), Galileo, Man of Science , New York: Basic Books, pp. 315–337.
• –––, 1983, “Galileo and Early Experimentation,” in R. Aris, H. T. Davis, and R. H. Stuewer (eds.), Springs of Scientific Creativity: Essays on Founders of Modern Science , Minneapolis: University of Minnesota Press, pp. 3–20.
• –––, 1992, “Experimental Research and Galilean Mechanics,” in M. B. Ceolin (ed.), Galileo Scientist: His Years at Padua and Venice , Padua: Istituto Nazionale di Fisica Nucleare, pp. 39–57.
• Shapere, Dudley, 1974, Galileo: A Philosophical Study , Chicago: University of Chicago Press.
• Shapin, Steven, 1996, The Scientific Revolution , Chicago: University of Chicago Press.
• Shea, William R., 1972, Galileo’s Intellectual Revolution , New York: Science History Publications.
• Shea, William R., and Mariano Artigas, 2003, Galileo in Rome: The Rise and Fall of a Troublesome Genius , Oxford: Oxford University Press.
• –––, 2006, Galileo Observed: Science and the Politics of Belief , Sagamore Beach, MA: Science History Publications.
• Sobel, Dava, 1999, Galileo’s Daughter , New York: Walker and Company.
• Spranzi, Marta, 2004, Galilee. Le Dialogue sur les deux grands systemes du monde: Rhetorique, dialectique et démonstration , Paris: Presses Universitaires de France.
• –––, 2004, “Galileo and the Mountains of the Moon: Analogical Reasoning, Models and Metaphors in Scientific Discovery,” Journal of Cognition and Culture 4 (3): 451–483.
• Valleriani, Matteo, 2010, Galileo Engineer , Dordrecht: Springer.
• Van Fraassen, Bas C., 1980, The Scientific Image , Oxford: Clarendon Press.
• Wallace, William A., 1984, Galileo and His Sources: The Heritage of the Collegio Romano in Galileo’s Science , Princeton: Princeton University Press.
• –––, 1992, Galileo’s Logic of Discovery and Proof , Dordrecht: Kluwer Academic.
• Westman, Robert (ed.), 1976, The Copernican Achievement , Berkeley: University of California Press.
• Wilding, Nick, 2014, Galileo’s Idol: Gianfrancesco Sagredo and the Politics of Knowledge , Chicago: University of Chicago Press.
• Wisan, W. L., 1974, “The New Science of Motion: A Study of Galileo’s De motu locali ,” Archive for History of Exact Sciences , 13 (2-3): 103–306.
• Wootton, David, 2010, Galileo: Watcher of the Skies , New Haven, CT: Yale University Press.
• –––, 2015, The Invention of Science: A New History of the Scientific Revolution , New York: Harper Collins.

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Galileo Galilei’s Notes on Motion , Joint project of the Biblioteca Nazionale Centrale, Florence; Museo Galileo, Florence; Max Planck Institute for the History of Science, Berlin.
• The Galileo Project , maintained by Albert Van Helden; contains Dava Sobel’s translations of all 124 letters from Suor Maria Celeste to Galileo in the sequence in which they were written.
• Museo Galileo , The Institute and Museum of the History of Science, Florence, Italy.

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Galileo Galilei

Galileo Galilei (1564-1642) was an Italian mathematician, physicist, astronomer, and natural philosopher. He created a superior telescope with which he made new observations of the night sky, notably that the surface of the Moon has mountains, that Jupiter has four satellite moons, and that the sunspots of the Sun, under careful observation, reveal that it is a moving sphere.

Besides astronomy, Galileo conducted many other scientific experiments over his long lifetime as he was greatly interested in physics. Testing age-old theories and coming up with new ones after meticulous experimentation, the scientist fell foul of the Catholic Church for questioning the accepted Ptolemaic view of the universe. Found guilty of heresy in a trial in 1633, Galileo was obliged to live his final years under house arrest at his villa in Tuscany. His discoveries and, above all, his approach to experimentation and testing hypotheses made Galileo an influential figure in the Scientific Revolution .

Galileo Galilei was born in Pisa, Italy , on 15 February 1564. His family belonged to the minor nobility but was rather down on its luck. Galileo inherited an interest in science from his father, Vincenzo Galilei (c. 1520-1591), who wrote treatises based on his practical experiments in musical science. Vincenzo might have earned acclaim in music , but he earned money as a cloth merchant, the family of his wife and Galileo's mother Giulia being in that trade . From 1581, Galileo studied medicine at the University of Pisa, but it was the mathematics part of the course (then part of a traditional education in medicine) that appealed to him most. So much so that Galileo left Pisa without graduating and took up a post as a mathematics teacher in Florence. Galileo was keen to make his mark , and his private studies resulted in his first contribution to the ever-growing knowledge of the Scientific Revolution. Galileo studied the action of pendulums and formed his theory of constant motion. Such was Galileo's expertise in mathematics that he was awarded a position in that field at the University of Pisa in 1589; three years later, he was made the professor of mathematics at the University of Padua.

It was at Padua that Galileo began a life-long friendship with the philosopher Cesare Cremonini (1550-1631). Galileo also met Marina Gamba in Padua, and although they never married, they had three children together: Virginia (b. 1600), Livia (b. 1601), and Vincenzio (b. 1606). Galileo was never far from financial distress, and he supplemented his meagre lecturer's income with private lessons and the occasional detailed horoscope. In 1613, when his daughters reached their teens, Galileo, unable to live openly with his mistress, entered Virginia and Livia into a nunnery outside Florence (both became nuns when reaching maturity). Galileo supported his daughters in the nunnery, buying better rooms and supplying them with food grown on his own estate to supplement the rather meagre standard fare of the nunnery. Virginia, by then known as Maria Celeste, was a great help to her father in his old age.

From Theory to Practice: A New Science

Mathematics was crucial to Galileo's understanding of the universe, as here he explains in his The Assayer of 1623:

One cannot understand it unless one first learns to understand the language and recognize the characters in which it is written. It is written in mathematical language and the characters are triangles, circles and other geometrical figures; without these means it is humanly impossible to understand a word of it; without these there is only clueless scrabbling around in a dark labyrinth . (Wootton, 163).

But Galileo was an all-round thinker interested in any discipline of thought that would provide the answers to the problems he wished to solve. His first biographer, Vincenzo Viviani, notes the following (paraphrased by Heilbron):

[He] could compete with the best lunatists in Tuscany, advise painters and poets on matter of artistic taste, and recite vast stretches of Petrarch , Dante , and Ariosto by heart. But his great strength, Galileo said when negotiating for a post at the Medici court in 1610, was philosophy , on which he had spent more years of study than he had months on mathematics. (v)

In short, Galileo "was no more (or less!) a mathematician than he was a musician, artist, writer, philosopher, or gadgeteer…Galileo would have done well in any of several professions" ( ibid ).

It was in the 1590s that Galileo began to move away from pure mathematical studies towards experimentation, although the story he dropped cannonballs from the Leaning Tower of Pisa is apocryphal. Rejecting the old Aristotelian understanding of physics, Galileo studied such subjects as uniform acceleration, inertia, and mechanics in his private workshop. He discovered all sorts of startling physical facts, such as a falling object has the same rate of acceleration regardless of its weight (which means, if air resistance is removed, two falling objects, though of different weight, will hit the ground at the same time), that a projectile follows a parabola in its path from one spot to another, and that a body moving on a perfectly smooth surface rolls on at a constant speed and, in a vacuum, would do so indefinitely. This latter point was used by Galileo to explain an age-old problem. If the Earth revolves around the Sun, then what causes it to move? Galileo demonstrated that, assuming God had started it off at the Creation, no continuous force was needed.

Galileo became deeply interested in astronomy and, from 1597, began an enduring correspondence with that other great thinker and astronomer Johannes Kepler (1571-1630). These two men would find the physical evidence to confirm the controversial theories of Nicolaus Copernicus (1473-1543) and finally bury the outdated ones of Ptolemy (c. 100 to c. 170). Copernicus believed the Earth revolved around the Sun, while Ptolemy believed the Sun revolved around the Earth (a view favoured by the Church). Galileo rejected the traditional working methods of the medieval astronomer, which was to create meticulous charts and tables using complex mathematics, and instead focussed his telescope on direct observation and discovery. In this sense, "Galileo fundamentally changed the notion of what astronomy was about" (Burns, 63).

Galileo's Telescope

The first telescope was invented in the Netherlands around 1608, perhaps by Hans Lippershey (c. 1570 to c. 1619). The simple idea of using a convex and concave lens at either end of a tube soon spread around Europe , and it reached Galileo's ears within a year or two. Galileo built his own version using superb lenses, which he ground himself (although he would not tell anyone exactly how). Going through several prototypes, Galileo arrived at a telescope with a magnification of 33 diameters, far more powerful than any in possession of his contemporaries. Galileo's telescope, what he called his occhiale ('eyeglass'), had two lenses set at either end of a lead tube around 60 cm (24 in) long. It was so powerful and well-made that other scientists had trouble believing what Galileo claimed to see through it since their own telescopes failed to spot what the Italian could see. Galileo even invented a pair of binoculars, but the idea did not catch on. Other gadgets Galileo invented early versions of include the thermometer (actually a thermoscope), a hydrostatic balance, and a compass (what we today would call a military compass or sector). It was the telescope, though, which revolutionised thought in the 17th century.

Galileo used his new telescope to study the heavens in tremendous detail, publishing the fruit of his research in Sidereus Nuncius ( The Starry Messenger ) in 1610. Galileo was able to observe the Moon and note that its surface seemed similar to Earth's with mountains and valleys, suggesting it was not, as many had previously thought, made of some entirely different matter. Galileo spotted for the first time the four largest moons of Jupiter (we now know there are more), studied the composition of the Milky Way, and identified the phases of Venus , which proved that it orbits the Sun. Galileo built theories on what he saw, such as the movement of Jupiter's moons must mean they orbit Jupiter (and not some other body like the Sun). He believed (correctly) that just as we can see the shine of the Moon, so on the Moon one should be able to see the shine of the Earth, that is the reflected light of the Sun. These new discoveries made Galileo as famous as Christopher Columbus (1451-1506), the discoverer of the New World, with whom, as the discoverer of a new Cosmos, Galileo was frequently compared.

Galileo's most important discovery, though, was not the details of the Moon or Jupiter's satellites but his observation of the sunspots on the Sun, using his telescope. The sunspots had been noted in antiquity, but Galileo could now, using filters, see things nobody had ever seen before. Galileo described what he saw:

The dark spots seen in the solar disk by means of the telescope are not at all distant from its surface, but are either contiguous to it or separated by an interval so small as to be quite imperceptible…They vary in duration from one or two days to thirty or forty. For the most part they are of most irregular shape, and their shapes continually change, some quickly and violently, others more slowly and moderately…Besides all these disordered movements they have in common a general uniform motion across the face of the sun in parallel lines. From special characteristics of this motion one may learn that the sun is absolutely spherical, that it rotates from west to east and around its own centre, carries the spots along with it in parallel circles, and completes an entire revolution in about a lunar month. (Fermi, 57).

In order to secure a position at the court of Cosimo II de' Medici, Grand Duke of Tuscany (r. 1609-1621), Galileo cleverly named the moons of Jupiter he had discovered the 'Medicean stars' in honour of the Medici family. Sure enough, in 1610, Galileo was appointed the duke's official mathematician and natural philosopher (the latter title now allowed Galileo to present theories on the place of Earth in the universe, something a lowly mathematician could not do). In 1611, Galileo was admitted into the prestigious scientific society in Rome called the Academia dei Lincei. In 1612, Galileo's Discourse on Floating Bodies attacked Aristotelian natural philosophy. In 1613, Galileo presented his pro-Copernicus theories in Letters on Sunspots , a work that landed the scientist into serious trouble.

The Trial of Galileo

Ptolemy had presented the theory that Earth was the centre of the universe with everything revolving around it. The Christian Church liked this idea since it put humanity at the centre of things. Copernicus presented his theory that it was the Sun which was at the centre and Earth and other planets revolved around it. This the Catholic Church, in particular, did not like. When Galileo sided with Copernicus, whose work was put on the Catholic Church's Index of Forbidden Books in 1616, he opened himself to the possibility of formal censure for heresy. Galileo was not denying the existence of God but, perhaps crucially, he had made many personal enemies over the years besides institutional ones. There was, for example, a notable feud with the Jesuit astronomer Christoph Scheiner (1573-1650). Galileo, it seems, had a particular skill for rubbing up people the wrong way (this was one reason why he left Pisa for Padua back in 1592). His delight in poking fun at the beliefs of others and his skill at philosophical discussion where he pulled such beliefs apart made him as few friends as Socrates had done in 5th-century BCE Athens . On the other side, Galileo was also good at gaining friends and supporters since he was "a forceful publicist of his own ideas and a superb communicator of technical ideas" (Henry, 29). In short, Galileo was a sticky problem to handle for the Church.

Most astronomers were not actually interested in challenging religious orthodoxy and did not view their new discoveries using telescopes and other instruments as necessarily challenging a universe created as described in the Bible . Galileo considered theology and natural philosophy as entirely different subjects. What he was doing was showing that the physical world on Earth was entirely related in terms of matter and physical laws to what could be seen in the heavens. This went against the traditional Aristotelian view. In the end, Galileo's writings were not banned by the Church, but he was taken to one side and privately admonished by Cardinal Robert Bellarmine (1542-1621). Galileo was by now a public figure, especially since he wrote his works in Italian rather than the more audience-restricted Latin most other great thinkers used. Galileo's works were also translated into several other languages shortly after publication. In a meeting on 26 February 1616, Galileo was encouraged not to pursue his pro-Copernicus theories, which appeared to contradict the Bible. This he did, for a while, but the Copernicus view of the universe was now becoming more and more widely accepted following the work of other astronomers. There was, too, some middle ground, with Tycho Brahe (1546-1601) famously endorsing a compromise view that the Sun orbited Earth and the other planets orbited the Sun. In short, the problem of what revolved around what was not going away, no matter how much the Church wanted to brush the investigations of the astronomers under the ecclesiastical carpet of accepted doctrine.

In 1632, Galileo wrote his Dialogue on the Two Chief Systems of the World . Here, he has two great thinkers, one pro-Ptolemy and the other pro-Copernicus, argue the matter of which bodies revolve around what in our galaxy (and by now, Galileo was convinced what we could see through a telescope was only a galaxy and not the entire universe). There is a third character, a neutral thinker who is ultimately persuaded to accept the Copernicus model. Tellingly, the pro-Ptolemy philosopher is called Simplicio (suspiciously like 'simpleton'), and the other, really Galileo himself, is called Salviati (hinting at salvation through correct knowledge). The Dialogue was a step too far for the Church, and Galileo was accused of heresy. He was hauled before a panel for trial in 1633. Found guilty, Galileo had to desist from promoting pro-Copernicus theories, and he was obliged to stay under house arrest in his home in Florence for the remainder of his life. He also had to recite the Penitential Psalms once every week for the next three years, a minor but no doubt annoying punishment for a man who so valued his time.

Galileo might have become an enemy of Catholicism, but his case has certain unique features, not least the long line of enemies the scientist had created who now took their opportunity for revenge. As the historian J. Henry notes, "The Galileo affair should not be taken as a general indicator of relations between science and religion in the early modern period" (86).

Death & Legacy

Galileo spent his remaining time designing a pendulum clock, and he wrote a summary of all his work in physics in Discourse on Two New Sciences , completed in 1638, but, because of his trial and punishment, published in Leiden in the Netherlands. Galileo eventually lost his eyesight (endlessly peering through lenses might have been responsible for this), and he suffered from arthritis. The tranquillity of his forced retirement was only broken by occasional visits from outsiders such as the poet John Milton.

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Galileo still pressed on with his studies, especially the pendulum and the attempt to find a working navigational aid for mariners, but just as his visual world had diminished from the stars to the hand before his face, the end was closing in. "I do not stop with my speculations, although with considerable damage to my health, since along with my other troubles they deprive me of sleep, which increases my melancholy at night" (Heilbron, 348). Galileo died on 8 January 1642; he was 77 years old. His remains were buried in the Church of Sante Croce in Florence.

Other thinkers came along and built upon and very often corrected the ideas that Galileo had presented. Johannes Kepler created a new model of the universe where the planets moved in elliptical orbits, not in perfect circles as Galileo had thought. Isaac Newton (1642-1727) discovered the force of gravity, and this explained phenomena that had puzzled Galileo, such as how the planets rotate, maintain their satellite moons, and move at different speeds depending on their distance from the Sun. Galileo, though, had made a much more lasting contribution to world knowledge than any specific discovery or theory. Galileo had uniquely combined the theory of mathematics, the observations of natural philosophy, and the use of repeated experiments to test hypotheses. As a consequence, he created a new and more rigorous methodology of inquiry that became the standard approach adopted by all other serious thinkers during the Scientific Revolution, a period when science relentlessly sought out new and definitive answers to questions humanity had been posing for millennia.

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Bibliography

• Burns, William E. The Scientific Revolution in Global Perspective. Oxford University Press, 2015.
• Burns, William E. The Scientific Revolution. ABC-CLIO, 2001.
• Fermi, Laura & Bernardini, Gilberto. Galileo and the Scientific Revolution. Dover Publications, 2013.
• Heilbron, John L. Galileo. Oxford University Press, 2012.
• Henry. The Scientific Revolution and the Origins of Modern Science . Red Globe Press, 2008.
• Jardine, Lisa. Ingenious Pursuits. Anchor, 2000.
• Rundle, D. (ed). The Hutchinson Encyclopedia of the Renaissance. Helicon, 2023.
• Wootton, David. The Invention of Science. Harper, 2015.

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Galileo Galilei: Biography, inventions & other facts

Galileo revolutionized our understanding of the universe

Galileo's Experiments

Galileo's telescope, copernican system, galileo quotes.

Italian astronomer Galileo Galilei provided a number of scientific insights that laid the foundation for future scientists. His investigation of the laws of motion and improvements on the telescope helped further the understanding of the world and universe around him. Both led him to question the current belief of the time — that all things revolved around the Earth. 

[See also our overview of Famous Astronomers and great scientists from many fields who have contributed to the rich history of discoveries in astronomy .]

The Ancient Greek philosopher, Aristotle, taught that heavier objects fall faster than lighter ones, a belief still held in Galileo's lifetime. But Galileo wasn't convinced. Experimenting with balls of different sizes and weights, he rolled them down ramps with various inclinations. His experiments revealed that all of the balls boasted the same acceleration independent of their mass - although some modern physicists remain determined to prove him wrong . He also demonstrated that objects thrown in the air travel along a parabola.

At the same time, Galileo worked with pendulums. In his life, accurate timekeeping was virtually nonexistent. Galileo observed, however, that the steady motion of a pendulum could improve this. In 1602, he determined that the time it takes a pendulum to swing back and forth does not depend on the arc of the swing. Near the end of his lifetime, Galileo designed the first pendulum clock .

Galileo is often incorrectly credited with the creation of a telescope. ( Hans Lippershey applied for the first patent in 1608, but others may have beaten him to the actual invention.) Instead, he significantly improved upon them. In 1609, he first learned of the existence of the spyglass, which excited him. He began to experiment with telescope-making , going so far as to grind and polish his own lenses. His telescope allowed him to see with a magnification of eight or nine times. In comparison, spyglasses of the day only provided a magnification of three. 

It wasn't long before Galileo turned his telescope to the heavens. He was the first to see craters on the moon, he discovered sunspots, and he tracked the phases of Venus. The rings of Saturn puzzled him, appearing as lobes and vanishing when they were edge-on — but he saw them, which was more than can be said of his contemporaries. 

Of all of his telescope discoveries, he is perhaps most known for his discovery of the four most massive moons of Jupiter, now known as the Galilean moons: Io , Ganymede , Europa and Callisto . When NASA sent a mission to Jupiter in the 1990s, it was called Galileo in honor of the famed astronomer.

In his book " Sidereus Nuncius " ("Starry Messenger"), published in 1610, Galileo wrote:

"On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavens through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . . When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night."

"I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter."

Galileo may also have made the first recorded studies of the planet Neptune, though he didn't recognize it as a planet. While observing Jupiter's moons in 1612 and 1613, he recorded a nearby star whose position is not found in any modern catalogues.

"It has been known for several decades that this unknown star was actually the planet Neptune," University of Melbourne physicist David Jamieson told Space.com . "Computer simulations show the precision of his observations revealing that Neptune would have looked just like a faint star almost exactly where Galileo observed it."

In Galileo's lifetime, all celestial bodies were thought to orbit the Earth. Supported by the Catholic Church, teaching opposite of this system was declared heresy in 1615.

Galileo, however, did not agree. His research — including his observations of the phases of Venus and the fact that Jupiter boasted moons that didn't orbit Earth — supported the Copernican system, which (correctly) stated that the Earth and other planets circle the sun.

In 1616, he was summoned to Rome and warned not to teach or write about this controversial theory. But in 1632, believing that he could write on the subject if he treated it as a mathematical proposition, he published work on the Copernican system. He was found guilty of heresy , and was placed under house arrest for the remaining nine years of his life.

Today, Galileo is finally recognized for his groundbreaking discoveries, for which he is remembered as the "father of modern science".

Related: 'Galileo Project' will search for evidence of extraterrestrial life from the technology it leaves behind

"And yet it moves."

"I have never met a man so ignorant that I couldn't learn something from him."

"I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use."

"You cannot teach a man anything, you can only help him find it within himself."

"It is a beautiful and delightful sight to behold the body of the Moon."

"Wine is sunlight, held together by water."

— Find other quotes at GoodReads.com .

Additional resources

• Rice University: The Galileo Project
• JPL: Galileo Mission to Jupiter
• Stanford University Solar Center: Galileo’s Discoveries

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Galileo Galilei

Galileo Galilei (February 15, 1564 – January 8, 1642) was an Italian physicist , astronomer, and philosopher , whose career coincided with that of Johannes Kepler . His work constitutes a significant break from that of Aristotle and medieval philosophers and scientists (who were then referred to as "natural philosophers"). He has therefore been called the “father of modern astronomy ,” the “father of modern physics,” and also the “father of science .” Galileo’s achievements include improvements to the telescope , various astronomical observations, and initial formulation of the first and second laws of motion. He is best remembered for his effective support for Copernicanism , as he solidified the scientific revolution that shifted the paradigm of Ptolemaic geocentric cosmology to the Copernican heliocentric view. His experimental approach is widely considered complementary to the writings of Francis Bacon in establishing the modern scientific method.

• 1 Family and early career
• 2 Experimental science
• 3 Astronomy
• 5 Mathematics
• 6 Technology
• 7 Accusations against Galileo of scientific errors and misconduct
• 8 Controversy between Galileo and the Church
• 9 Named after Galileo
• 10 Galileo's writings
• 11 Writings on Galileo
• 12 References
• 13 External links

Galileo came into conflict with the Roman Catholic Church of his day because of the Church's endorsement of geocentric cosmology and opposition to the heliocentric view. That conflict is almost universally taken to be a major example of the ongoing friction between religion and science, or between religious authorities and their dogma, on the one hand, and scientific methods of inquiry, on the other. Although the Church won the immediate battle with Galileo, it lost the war. Nearly 350 years after Galileo's death, Pope John Paul II publicly acknowledged that Galileo had been correct.

Family and early career

Galileo Galilei was born in Pisa, in the Tuscan region of Italy , on February 15, 1564. He was the son of Vincenzo Galilei, a mathematician and musician born in Florence in 1520, and Giulia Ammannati, born in Pescia. They married in 1563, and Galileo was their first child. Although a devout Catholic , Galileo fathered three children—two daughters and a son—with Marina Gamba out of wedlock. Because of their illegitimate birth, both girls were sent to the convent of San Matteo in Arcetri at early ages.

• Virginia (1600 – 1634) took the name Maria Celeste upon entering a convent. Galileo's eldest child, she was the most beloved and inherited her father's sharp mind. She died on April 2, 1634. She is buried with Galileo at the Basilica di Santa Croce di Firenze.
• Livia (b. 1601) took the name Suor Arcangela. She was sickly for most of her life at the convent.
• Vincenzio (b. 1606) was later legitimized and married Sestilia Bocchineri.

Galileo was home-schooled at a very young age. He then attended the University of Pisa but was forced to cease his studies there for financial reasons. He was, however, offered a position on its faculty in 1589 and taught mathematics. Soon after, he moved to the University of Padua and served on its faculty teaching geometry , mechanics , and astronomy until 1610. During this time, he explored science and made many landmark discoveries.

Experimental science

Galileo occupies a high position in the pantheon of scientific investigators because of his pioneering use of quantitative experiments in which he analyzed the results mathematically. There was no tradition of such an approach in European science at that time. William Gilbert, the great experimentalist who immediately preceded Galileo, did not use a quantitative approach. Galileo's father, however, had performed experiments in which he discovered what might be the oldest known nonlinear relation in physics, between the tension and pitch of a stretched string.

The popular notion of Galileo inventing the telescope is inaccurate, but he was one of the first people to use the telescope to observe the sky, and for a time he was one of very few who could make a telescope good enough for that purpose. Based on sketchy descriptions of telescopes invented in the Netherlands in 1608, Galileo made an instrument with about 8-power magnification and then made improved models up to about 20-power. On August 25, 1609, he demonstrated his first telescope to Venetian lawmakers. His work on the device made for a profitable sideline with merchants who found it useful for their shipping businesses. He published his initial telescopic astronomical observations in March 1610, in a short treatise entitled Sidereus Nuncius (Sidereal Messenger) .

On January 7, 1610, Galileo discovered three of Jupiter 's four largest moons : Io, Europa, and Callisto. Four nights later, he discovered Ganymede. He determined that these moons were orbiting the planet since they would appear and disappear—a phenomenon he attributed to their movement behind Jupiter. He observed them further in 1620. Later astronomers overruled Galileo's names for them as Medicean stars and called them Galilean satellites . The demonstration that Jupiter had smaller bodies orbiting it was problematic for the Ptolemaic geocentric model of the universe, in which everything circled around the Earth .

Galileo also noted that Venus exhibited a full set of phases like the Moon . The heliocentric model developed by Copernicus predicted that all phases of Venus would be visible because its orbit 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. By contrast, Ptolemy 's geocentric model predicted that only the crescent and new phases of Venus would be seen, because Venus was thought to remain between the Sun and Earth during its orbit around the Earth. Galileo's observation of the phases of Venus proved that Venus orbited the Sun and supported (but did not prove) the heliocentric model.

Galileo was one of the first Europeans to observe sunspots, although there is evidence that Chinese astronomers had done so earlier. He also reinterpreted a sunspot observation from the time of Charlemagne , which formerly had been attributed (impossibly) to a transit of Mercury . The very existence of sunspots showed another difficulty with the notion of unchanging "perfection" of the heavens as assumed in the older philosophy. In addition, the annual variations in their motions, first noticed by Francesco Sizzi, presented great difficulties for either the geocentric system or that of Tycho Brahe . A dispute over priority in the discovery of sunspots led to a long and bitter feud with Christoph Scheiner. There is, however, little doubt that both of them were beaten by David Fabricius and his son Johannes.

Upon observing the patterns of light and shadow on the Moon's surface, Galileo deduced the existence of lunar mountains and craters. He even estimated the mountains' heights from these observations. This led him to the conclusion that the Moon was “rough and uneven, and just like the surface of the Earth itself,” and not a perfect sphere as Aristotle had claimed.

When Galileo examined the Milky Way, he realized that it was a multitude of densely packed stars , not nebulous (or cloud-like) as previously thought. He also located many other stars too distant to be visible with the naked eye.

In 1612, he observed the planet Neptune but did not realize that it was a planet and took no particular notice of it. It appears in his notebooks as one of many unremarkable dim stars.

Galileo's theoretical and experimental work on the motions of bodies, along with the largely independent work of Kepler and René Descartes , was a precursor of the classical mechanics developed by Sir Isaac Newton . He was a pioneer, at least in the European tradition, in performing rigorous experiments and insisting on a mathematical description of the laws of nature.

One of the most famous stories about Galileo is that he dropped balls of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass (excluding the limited effect of air resistance). This was contrary to what Aristotle had taught: that heavy objects fall faster than lighter ones, in direct proportion to weight. Though the story of the tower first appeared in a biography by Galileo's pupil, Vincenzo Viviani, it is no longer generally accepted as true. Moreover, Giambattista Benedetti had reached the same scientific conclusion years before, in 1553. Galileo, however, did perform experiments involving rolling balls down inclined planes , which proved the same thing: falling or rolling objects are accelerated independently of their mass. [Rolling is a slower version of falling, as long as the distribution of mass in the objects is the same.] Although Galileo was the first person to demonstrate this experimentally, he was not, contrary to popular belief, the first to argue that it was true. John Philoponus had argued for this view centuries earlier.

Galileo determined the correct mathematical law for acceleration: the total distance covered, starting from rest, is proportional to the square of the time. This law is regarded as a predecessor to the many scientific laws expressed later in mathematical form. He also concluded that objects retain their velocity unless a force —often friction —acts upon them, refuting the accepted Aristotelian hypothesis that objects "naturally" slow down and stop unless a force acts upon them. Here again, John Philoponus had proposed a similar (though erroneous) theory. Galileo's Principle of Inertia stated: "A body moving on a level surface will continue in the same direction at constant speed unless disturbed." This principle was incorporated into Newton's laws of motion (as the first law).

Galileo also noted that a pendulum's swings always take the same amount of time, independent of the amplitude. The story goes that he came to this conclusion by watching the swings of the bronze chandelier in the cathedral of Pisa, using his pulse to time it. While Galileo believed this equality of period to be exact, it is only an approximation appropriate to small amplitudes. It is good enough to regulate a clock , however, as Galileo may have been the first to realize. (See Technology below.)

In the early 1600s, Galileo and an assistant tried to measure the speed of light. They stood on different hilltops, each holding a shuttered lantern. Galileo would open his shutter, and, as soon as his assistant saw the flash, he would open his shutter. At a distance of less than a mile, Galileo could detect no delay in the round-trip time greater than when he and the assistant were only a few yards apart. Although he could reach no conclusion on whether light propagated instantaneously, he recognized that the distance between the hilltops was perhaps too short for a good measurement.

Galileo is lesser known for but nevertheless credited with being one of the first to understand sound frequency. After scraping a chisel at different speeds, he linked the pitch of sound to the spacing of the chisel's skips (frequency).

In his 1632 Dialogue Concerning the Two Chief World Systems , Galileo presented a physical theory to account for tides , based on the Earth's motion. Had it been correct, it would have been a strong argument in support of the idea that the Earth moves. (The original title for the book described it as a dialogue on the tides; the reference to tides was removed by order of the Inquisition.) His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. Kepler and others correctly associated the Moon with an influence over the tides, based on empirical data. A proper physical theory of the tides, however, was not available until Newton.

Galileo also put forward the basic principle of relativity, that the laws of physics are the same in any system that is moving at a constant speed in a straight line, regardless of its particular speed or direction. Hence, there is no absolute motion or absolute rest. This principle provided the basic framework for Newton's laws of motion and is the "infinite speed of light" approximation to Einstein 's special theory of relativity.

Mathematics

Although Galileo's application of mathematics to experimental physics was innovative, his mathematical methods were the standard ones of the time. The analyses and proofs relied heavily on the Eudoxian theory of proportion, as set forth in the fifth book of Euclid's Elements. This theory had become available only a century earlier, thanks to accurate translations by Niccolo Fontana Tartaglia and others. By the end of Galileo's life, however, it was being superseded by the algebraic methods of Descartes , which a modern finds incomparably easier to follow.

Galileo produced one piece of original and even prophetic work in mathematics, known as Galileo's paradox. It shows that there are as many perfect squares as there are whole numbers, even though most numbers are not perfect squares. Such seeming contradictions were brought under control 250 years later, in the work of Georg Cantor.

Galileo made a few contributions and suggested others to what we now call technology , as distinct from pure physics. This is not the same distinction as made by Aristotle, who would have considered all Galileo's physics as techne or useful knowledge, as opposed to episteme , or philosophical investigation into the causes of things.

Between 1595 and 1598, Galileo devised and improved a "Geometric and Military Compass" suitable for use by artillery gunners and surveyors. It expanded on earlier instruments designed by Niccolo Tartaglia and Guidobaldo del Monte. Besides providing a new and safer way of elevating cannons accurately, it offered gunners a way of quickly computing the charge of gunpowder for cannonballs of different sizes and materials. As a geometric instrument, it enabled the construction of any regular polygon , computation of the area of any polygon or circular sector, and a variety of other calculations.

About 1606–1607 (or possibly earlier), Galileo made a thermometer , using the expansion and contraction of air in a bulb to move water in an attached tube.

In 1609, Galileo was among the first to use a refracting telescope as an instrument to observe stars, planets or moons. Then, in 1610 he used a telescope as a compound microscope and made improved microscopes in 1623 and after. This appears to be the first clearly documented use of the compound microscope.

In 1612, having determined the orbital periods of Jupiter's satellites, Galileo proposed that with sufficiently accurate knowledge of their orbits, one could use their positions as a universal clock, and this knowledge would also make it possible to determine longitudes . He worked on this problem from time to time during the remainder of his life, but the practical problems were severe. The method was first successfully applied by Giovanni Domenico Cassini in 1681 and was later used extensively for land surveys; for navigation, the first practical method was the chronometer of John Harrison.

In his last year of life, when totally blind , Galileo designed an escapement mechanism for a pendulum clock. The first fully operational pendulum clock was made by Christiaan Huygens in the 1650s.

He created sketches of various inventions, such as a candle-and-mirror combination to reflect light throughout a building; an automatic tomato picker; a pocket comb that doubled as an eating utensil; and what appears to be a ballpoint pen.

Accusations against Galileo of scientific errors and misconduct

Although Galileo is generally considered to be one of the first modern scientists, he is often said to have arrogantly considered himself to be the "sole proprietor" of discoveries in astronomy, as exemplified by his position in the sunspot controversy. Furthermore, he never accepted Kepler's elliptical orbits for the planets, holding to the Copernican circular orbits that employed epicycles to account for irregularities in planetary motions. Before Kepler, people held to the notion that orbits of heavenly bodies were circular because the circle was considered the "perfect" shape.

Concerning his theory on tides, Galileo attributed them to momentum, despite his great knowledge of the ideas of relative motion and Kepler's better theories using the Moon as the cause. (Neither of these great scientists, however, had a workable physical theory of tides. This had to wait for the work of Newton.) Galileo stated in his Dialogue that if the Earth spins on its axis and is traveling at a certain speed around the Sun, parts of the Earth must travel "faster" at night and "slower" during the day. This view is by no means adequate to explain the tides.

Many commentators consider that Galileo developed this position merely to justify his own opinion because the theory was not based on any real scientific observations. If his theory were correct, there would be only one high tide per day and it would happen at noon. Galileo and his contemporaries knew that there are two daily high tides at Venice instead of one, and that they travel around the clock. He, however, attributed that observation to several secondary causes, such as the shape of the sea and its depth. Against the imputation that he was guilty of some kind of deceit in making these arguments, one may take the position of Albert Einstein , as one who had done original work in physics, that Galileo developed his “fascinating arguments” and accepted them too uncritically out of a desire for a physical proof of the motion of the Earth (Einstein 1952).

In the twentieth century, some authorities—in particular, the distinguished French historian of science Alexandre Koyré—challenged some of Galileo's alleged experiments. The experiments reported in Two New Sciences to determine the law of acceleration of falling bodies, for instance, required accurate measurements of time, which appeared to be impossible with the technology of the 1600s. According to Koyré, the law was arrived at deductively, and the experiments were merely illustrative thought experiments.

Later research, however, has validated the experiments. The experiments on falling bodies (actually rolling balls) were replicated using the methods described by Galileo (Settle 1961), and the precision of the results was consistent with Galileo's report. Research into Galileo's unpublished working papers from as early as 1604 clearly showed the validity of the experiments and even indicated the particular results that led to the time-squared law (Drake 1973).

Controversy between Galileo and the Church

Partly because of such scriptures as Psalms 93 and 104 and Ecclesiastes 1:5, which speak of the motion of celestial bodies and the suspended position of the Earth, and partly because of philosophical views derived from Ptolemy and others, the Catholic Church and religious authorities of the day held to a geocentric, Ptolemaic cosmology. Galileo, on the other hand, defended heliocentrism and claimed it was not contrary to those Scripture passages. He took Augustine's position on Scripture: not to take every passage too literally. This particularly applies when it is a book of poetry and songs, not a book of instructions or history. The writers of the Scripture wrote from the perspective of the terrestrial world, and from that vantage point the Sun does rise and set. As we know now, it is the Earth's rotation that gives the impression of the Sun's motion across the sky.

By 1616, the attacks on Galileo had reached a head, and he went to Rome to try to persuade Church authorities not to ban his ideas. In the end, Cardinal Bellarmine, acting on directives from the Inquisition, delivered him an order not to "hold or defend" the idea that the Earth moves and the Sun stands still at the center. The decree did not prevent Galileo from hypothesizing heliocentrism, but for the next several years, he stayed away from the controversy.

In 1623, he revived his project of writing a book on the subject, encouraged by the election of Cardinal Barberini as Pope Urban VIII. Barberini was a friend and admirer of Galileo and had opposed the condemnation of Galileo in 1616. The book Dialogue Concerning the Two Chief World Systems was published in 1632, with formal authorization from the Inquisition and papal permission.

Pope Urban VIII personally asked Galileo to give arguments for and against heliocentrism in the book, and to be careful not to advocate heliocentrism. He made another request—that his own views on the matter be included in Galileo's book. Galileo fulfilled only the latter of those requests, using a character named Simplicius to defend the geocentric view. Whether intentionally or not, Galileo portrayed Simplicius as someone who got caught in his own errors and sometimes came across as a fool. This fact made Dialogue appear as an advocacy book, an attack on Aristotelian geocentrism and defense of the Copernican theory. To add insult to injury, Galileo put the words of Pope Urban VIII into the mouth of Simplicius. Most historians take the view that Galileo did not act out of malice and felt blindsided by the reaction to his book. The pope, however, did not take the public ridicule lightly, nor the blatant bias. Galileo had alienated the pope, one of his biggest and most powerful supporters, and was called to Rome to explain himself.

With the loss of many of his defenders in Rome, Galileo was ordered to stand trial on suspicion of heresy in 1633. The sentence of the Inquisition had three essential parts:

• Galileo was required to recant his heliocentric ideas, which were condemned as “formally heretical.”
• He was ordered imprisoned. This sentence was later commuted to house arrest.
• His offending Dialogue was banned. In an action not announced at the trial, publication of any of his works was forbidden, including any he might write in the future.

After a period with the friendly Ascanio Piccolomini (the Archbishop of Siena), Galileo was allowed to return to his villa at Arcetri near Florence, where he spent the remainder of his life under house arrest. It was then that Galileo dedicated his time to one of his finest works, Two New Sciences . Based on this book, which received high praise from both Sir Isaac Newton and Albert Einstein , Galileo is often called the "father of modern physics."

On October 31, 1992, Pope John Paul II officially announced that the Catholic Church had mishandled the case.

Named after Galileo

• The Galileo mission to Jupiter
• The Galilean moons of Jupiter
• Galileo Regio on Ganymede
• Galilaei crater on the Moon
• Galilaei crater on Mars
• Asteroid 697 Galilea (named on the occasion of the 300th anniversary of the discovery of the Galilean moons)
• Galileo (unit of acceleration)
• Galileo Positioning System
• Galileo Stadium in Miami, Florida

Galileo's writings

• Dialogue Concerning Two New Sciences , 1638, Lowys Elzevir (Louis Elsevier) Leiden (in Italian, Discorsi e Dimostrazioni Matematiche, intorno á due nuoue scienze Leida, Appresso gli Elsevirii, 1638)
• Letters on Sunspots
• The Assayer (in Italian, Il Saggiatore )
• Dialogue Concerning the Two Chief World Systems , 1632 (in Italian, Dialogo dei due massimi sistemi del mondo )
• The Starry Messenger , 1610, Venice (in Latin , Sidereus Nuncius )
• Letter to Grand Duchess Christina

Writings on Galileo

• Galileo Galilei , an opera by Philip Glass
• Galileo , a play by Bertolt Brecht
• Lamp at Midnight , a play by Barrie Stavis
• Galileo's Daughter , a Memoir by Dava Sobel

References ISBN links support NWE through referral fees

• Drake, Stillman. 1953. Dialogue Concerning the Two Chief World Systems . Berkeley, CA: University of California Press. ISBN 978-0375757662
• Drake, Stillman. 1957. Discoveries and Opinions of Galileo . New York: Doubleday & Company. ISBN 978-0385092395
• Drake, Stillman. 1973. "Galileo's Discovery of the Law of Free Fall." Scientific American v. 228, #5, pp. 84-92.
• Drake, Stillman. 1978. Galileo At Work . Chicago: University of Chicago Press. ISBN 978-0226162263
• Einstein, Albert. 1952. Foreword to (Drake, 1953).
• Fantoli, Annibale. 2003. Galileo — For Copernicanism and the Church , third English edition. Vatican Observatory Publications. ISBN 978-8820974275
• Fillmore, Charles. [1931] 2004. Metaphysical Bible Dictionary . Unity Village, Missouri: Unity House. ISBN 978-0871590671
• Hellman, Hal. 1999. Great Feuds in Science. Ten of the Liveliest Disputes Ever . New York: Wiley. ISBN 978-0471350668
• Lessl, Thomas. 2000. " The Galileo Legend ." New Oxford Review , 27-33. Retrieved December 13, 2012.
• Newall, Paul. 2005. "The Galileo Affair." Retrieved December 13, 2012.
• Settle, Thomas B. 1961. "An Experiment in the History of Science." Science , 133:19-23.
• Sobel, Dava. 1999. Galileo's Daughter . Penguin Books. ISBN 978-0140280555
• White, Andrew Dickson. 1898. A History of the Warfare of Science with Theology in Christendom . Retrieved December 13, 2012.

External links

All links retrieved April 17, 2024.

• Biography of Galileo Galilei with links to related objects conserved in the Institute and Museum of the History of Science in Florence, Italy
• Galileo's "Notes on Motion" - online digital edition with transcriptions
• The Galileo Project at Rice University
• Electronic representation of Galilei's notes on motion (MS. 72)
• PBS Nova Online: Galileo's Battle for the Heavens
• Stanford Encyclopedia of Philosophy entry
• Galileo Galilei, in the Catholic Encyclopedia found online on New Advent, an orthodox Catholic website

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At this point, however, Galileo’s career took a dramatic turn. In the spring of 1609 he heard that in the Netherlands an instrument had been invented that showed distant things as though they were nearby. By trial and error, he quickly figured out the secret of the invention and made his own three-powered spyglass from lenses for sale in spectacle makers’ shops. Others had done the same; what set Galileo apart was that he quickly figured out how to improve the instrument, taught himself the art of lens grinding, and produced increasingly powerful telescopes . In August of that year he presented an eight-powered instrument to the Venetian Senate (Padua was in the Venetian Republic). He was rewarded with life tenure and a doubling of his salary. Galileo was now one of the highest-paid professors at the university. In the fall of 1609 Galileo began observing the heavens with instruments that magnified up to 20 times. In December he drew the Moon ’s phases as seen through the telescope , showing that the Moon’s surface is not smooth, as had been thought, but is rough and uneven. In January 1610 he discovered four moons revolving around Jupiter . He also found that the telescope showed many more stars than are visible with the naked eye. These discoveries were earthshaking, and Galileo quickly produced a little book, Sidereus Nuncius ( The Sidereal Messenger ), in which he described them. He dedicated the book to Cosimo II de Medici (1590–1621), the grand duke of his native Tuscany , whom he had tutored in mathematics for several summers, and he named the moons of Jupiter after the Medici family : the Sidera Medicea, or “Medicean Stars.” Galileo was rewarded with an appointment as mathematician and philosopher of the grand duke of Tuscany, and in the fall of 1610 he returned in triumph to his native land.

Galileo was now a courtier and lived the life of a gentleman. Before he left Padua he had discovered the puzzling appearance of Saturn , later to be shown as caused by a ring surrounding it, and in Florence he discovered that Venus goes through phases just as the Moon does. Although these discoveries did not prove that Earth is a planet orbiting the Sun , they undermined Aristotelian cosmology: the absolute difference between the corrupt earthly region and the perfect and unchanging heavens was proved wrong by the mountainous surface of the Moon, the moons of Jupiter showed that there had to be more than one centre of motion in the universe , and the phases of Venus showed that it (and, by implication , Mercury ) revolves around the Sun. As a result, Galileo was confirmed in his belief, which he had probably held for decades but which had not been central to his studies, that the Sun is the centre of the universe and that Earth is a planet, as Copernicus had argued. Galileo’s conversion to Copernicanism would be a key turning point in the Scientific Revolution .

After a brief controversy about floating bodies, Galileo again turned his attention to the heavens and entered a debate with Christoph Scheiner (1573–1650), a German Jesuit and professor of mathematics at Ingolstadt , about the nature of sunspots (of which Galileo was an independent discoverer). This controversy resulted in Galileo’s Istoria e dimostrazioni intorno alle macchie solari e loro accidenti (“History and Demonstrations Concerning Sunspots and Their Properties,” or “Letters on Sunspots”), which appeared in 1613. Against Scheiner, who, in an effort to save the perfection of the Sun, argued that sunspots are satellites of the Sun, Galileo argued that the spots are on or near the Sun’s surface, and he bolstered his argument with a series of detailed engravings of his observations.

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Galileo Galilei's Invention & Contributions

List of Discoveries of Galileo Galilei

Few individuals have had as profound an impact on science as Italian physicist and astronomer Galileo Galilei, whose groundbreaking inventions and discoveries earned him the title "father of modern science.'' With contributions in math, physics, and astronomy, Galileo's innovative, experiment-driven approach to science made him a key figure of the Scientific Revolution of the 16th and 17th centuries. During this time, he all but disproved the Aristotelian physics and cosmology that had previously dominated the sciences in Europe.

TL;DR (Too Long; Didn't Read)

Italian scientist Galileo Galilei made major contributions to math, physics and astronomy during the Scientific Revolution of the 16th and 17th Century. The so-called "father of modern science," his work on proving the heliocentric model of the galaxy brought him into conflict with the Catholic church.

Experiments in Motion

The law of falling bodies is one of Galileo's key contributions to physics. It states that objects fall at the same speed regardless of weight or shape. Through his experiments, Galileo countered the pervasive Aristotelian view, which held that heavier objects fall faster than lighter objects. The distance an object travels, he calculated, is proportional to the square of the time it takes the object to reach the ground. Galileo also first developed the concept of inertia — the idea that an object remains in rest or in motion until acted on by another force — which became the basis for one of Isaac Newton's laws of motion.

Geometric and Military Compass

In 1598, Galileo began selling a geometric and military compass of his own design, though the profits were minimal. Consisting of two rulers attached at right angles with a third, curved ruler between them, Galileo's compass — known as a sector — had multiple functions. Soldiers in the military used it to measure the elevation of a cannon's barrel, while merchants employed it to calculate currency exchange rates.

An Improved Telescope

While he did not invent the telescope, the enhancements Galileo made to original Dutch versions of the instrument enabled him to make new empirical discoveries. While early telescopes magnified objects by three times, Galileo learned to grind lenses — an advancement that eventually created a telescope with a magnifying factor of 30x. With his unprecedentedly powerful telescopes, Galileo was the first to observe the uneven, cratered surface of the moon; Jupiter's four largest satellites, dubbed the Galilean moons; dark spots on the surface of the sun, known as sunspots; and the phases of Venus. The telescope also revealed that the universe contained many more stars not visible to the naked eye.

The Case for Heliocentrism

In the 16th century, Polish astronomer Nicolaus Copernicus became the first scientist to promote a model of the solar system in which the Earth orbited its sun rather than the other way around. Galileo's observations discredited the Aristotelian theory of an Earth-centered solar system in favor of the Copernican heliocentric model. The presence of moons in orbit around Jupiter suggested that the Earth was not the sole center of motion in the cosmos, as Aristotle had proposed. Furthermore, the realization that the surface of the moon is rough disproved the Aristotelian view of a perfect, immutable celestial realm. Galileo's discoveries — including the theory of solar rotation, as suggested by shifts in sunspots — incurred the wrath of the Catholic Church, which espoused the Aristotelian system. Upon finding him guilty of heresy in 1633, the Roman Inquisition forced Galileo to rescind his support of heliocentrism and sentenced him to house imprisonment — he would eventually die, still under arrest, in 1642.

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• Encyclopedia Britannica: Galileo
• History.com: Galileo Galilei - Facts and Summary
• The Galileo Project: Science - On Motion
• Smithsonian Magazine: Science - Galileo's Instruments
• The Galileo Project: Science - Copernican System
• SparkNotes: The Scientific Revolution (1550-1700) - The Re-Formation of the Heavens
• The Physics Classroom: Inertia and Mass
• The Galileo Project: Science - Sector
• Space.com: Galileo Galilei: Biography, Inventions and Other Facts
• Smithsonian Magazine: Galileo’s Revolutionary Vision Helped Usher In Modern Astronomy

About the Author

Since beginning her career as a professional journalist in 2007, Nathalie Alonso has covered a myriad of topics, including arts, culture and travel, for newspapers and magazines in New York City. She holds a B.A. in American Studies from Columbia University and lives in Queens with her two cats.

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