galileo biography english

(1564-1642)

Who Was Galileo?

Galileo was an Italian astronomer, mathematician, physicist, philosopher and professor who made pioneering observations of nature with long-lasting implications for the study of physics.

Galileo Galilei was born in Pisa in the Duchy of Florence, Italy, on February 15, 1564.

Galileo was the first of six children born to Vincenzo Galilei, a well-known musician and music theorist, and Giulia Ammannati. In 1574, the family moved to Florence, where Galileo started his formal education at the Camaldolese monastery in Vallombrosa.

In 1583, Galileo entered the University of Pisa to study medicine. Armed with prodigious intelligence and drive, he soon became fascinated with many subjects, particularly mathematics and physics.

While at Pisa, Galileo was exposed to the Aristotelian view of the world, then the leading scientific authority and the only one sanctioned by the Roman Catholic Church.

At first, Galileo supported this view, like any other intellectual of his time, and was on track to be a university professor. However, due to financial difficulties, Galileo left the university in 1585 before earning his degree.

Career as a Professor

Galileo continued to study mathematics after leaving the university, supporting himself with minor teaching positions.

During this time he began his two-decade study on objects in motion and published The Little Balance , describing the hydrostatic principles of weighing small quantities, which brought him some fame. This gained him a teaching post at the University of Pisa, in 1589.

While there, Galileo conducted his fabled experiments with falling objects and produced his manuscript Du Motu (On Motion) , a departure from Aristotelian views about motion and falling objects. Galileo developed an arrogance about his work, and his strident criticisms of Aristotle left him isolated among his colleagues. In 1592, his contract with the University of Pisa was not renewed.

Galileo quickly found a new position at the University of Padua , teaching geometry, mechanics and astronomy. The appointment was fortunate, for his father had died in 1591, leaving Galileo entrusted with the care of his younger brother.

During his 18-year tenure at Padua, he gave entertaining lectures and attracted large crowds of followers, further increasing his fame and his sense of mission.

Daughters and Son

In 1600, Galileo met Marina Gamba, a Venetian woman, who bore him three children out of wedlock: daughters Virginia and Livia, and son Vincenzo. He never married Marina, possibly due to financial worries and possibly fearing his illegitimate children would threaten his social standing.

Galileo worried his daughters would never marry well, and when they were older, had them enter a convent. In 1616, at the San Mateo Convent, Virginia changed her name to Maria Celeste and Livia became Sister Arcangela, when they became nuns. Maria Celeste remained in contact and supported her father through letters until her death.

No letters from Arcangela survive. His son’s birth was eventually legitimized and he became a successful musician.

In July 1609, Galileo learned about a simple telescope built by Dutch eyeglass makers and soon developed one of his own. In August, he demonstrated it to some Venetian merchants, who saw its value for navigation and spotting ships. The merchants gave Galileo a salary to manufacture several of them.

Galileo’s ambition pushed him to go further, and in the fall of 1609 he made the fateful decision to turn his telescope toward the heavens. Using his telescope to explore the universe, Galileo observed the moon and found Venus had phases like the moon, proving it rotated around the sun, which refuted the Aristotelian doctrine that the Earth was the center of the universe.

He also discovered Jupiter had revolving moons that didn’t revolve around planet Earth. In 1613, he published his observations of sunspots, which also refuted Aristotelian doctrine that the sun was perfect.

Galileo published a number of books throughout his career, including:

The Operations of the Geometrical and Military Compass (1604), which revealed Galileo’s skills with experiments and practical technological applications.

The Starry Messenger (1610), a small booklet revealing Galileo’s discoveries that the moon was not flat and smooth but a sphere with mountains and craters.

Discourse on Bodies in Water (1612), which refuted the Aristotelian explanation of why objects float in water, saying that it wasn’t because of their flat shape, but instead the weight of the object in relation to the water it displaced.

Dialogue Concerning the Two Chief World Systems (1632), a discussion among three people: one who supports Copernicus' heliocentric theory of the universe, one who argues against it, and one who is impartial. Though Galileo claimed Dialogues was neutral, it was clearly not. The advocate of Aristotelian belief comes across as the simpleton, getting caught in his own arguments.

Two New Sciences (1638), a summary of Galileo’s life’s work on the science of motion and strength of materials.

What Did Galileo Discover?

In addition to the telescope and his numerous mathematical and scientific discoveries, in 1604 Galileo constructed a hydrostatic balance for measuring small objects.

That same year, he also refined his theories on motion and falling objects, and developed the universal law of acceleration, which all objects in the universe obeyed. He also devised a type of simple thermometer.

Thermometer

A simple glass-bulb thermometer known as a Galileo thermometer wasn't invented by Galileo, but was based on his understanding that the density of liquids changes based on its temperature.

A thermoscope that Galileo designed (or helped to design) is similar to modern-day thermometers. Inside the thermoscope, a liquid rises and falls in a glass tube as the temperature of the liquid rises or falls.

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Galileo and the Church

After Galileo built his telescope in 1609, he began mounting a body of evidence and openly supporting the Copernican theory that the earth and planets revolve around the sun. The Copernican theory, however, challenged the doctrine of Aristotle and the established order set by the Catholic Church.

In 1613, Galileo wrote a letter to a student to explain how Copernican theory did not contradict Biblical passages, stating that scripture was written from an earthly perspective and implied that science provided a different, more accurate perspective.

The letter was made public and Church Inquisition consultants pronounced Copernican theory heretical. In 1616, Galileo was ordered not to “hold, teach, or defend in any manner” the Copernican theory. Galileo obeyed the order for seven years, partly to make life easier and partly because he was a devoted Catholic.

In 1623, a friend of Galileo, Cardinal Maffeo Barberini, was elected as Pope Urban VIII. He allowed Galileo to pursue his work on astronomy and even encouraged him to publish it, on condition it be objective and not advocate Copernican theory. This led Galileo to publish Dialogue Concerning the Two Chief World Systems in 1632, which advocated the theory.

Church reaction was swift, and Galileo was summoned to Rome. Galileo’s Inquisition proceedings lasted from September 1632 to July 1633. During most of this time, Galileo was treated with respect and never imprisoned.

However, in a final attempt to break him, Galileo was threatened with torture, and he finally admitted he had supported Copernican theory, but privately held that his statements were correct. He was convicted of heresy and spent his remaining years under house arrest.

Though ordered not to have any visitors nor have any of his works printed outside of Italy, he ignored both. In 1634, a French translation of his study of forces and their effects on matter was published, and a year later, copies of the Dialogue were published in Holland.

While under house arrest, Galileo wrote Two New Sciences , published in Holland in 1638. By this time, Galileo had become blind and was in poor health.

In time, however, the Church couldn’t deny the truth in science. In 1758, it lifted the ban on most works supporting Copernican theory. It wasn't until 1835 that the Vatican dropped its opposition to heliocentrism altogether.

In the 20th century, several popes acknowledged the great work of Galileo, and in 1992, Pope John Paul II expressed regret about how the Galileo affair was handled.

Galileo died after suffering from a fever and heart palpitations on January 8, 1642, in Arcetri, near Florence, Italy.

Galileo's contribution to our understanding of the universe was significant not only for his discoveries, but for the methods he developed and the use of mathematics to prove them. He played a major role in the Scientific Revolution and earned the title "The Father of Modern Science."

QUICK FACTS

• Name: Galileo Galelei
• Birth Year: 1564
• Birth date: February 15, 1564
• Birth City: Pisa
• Birth Country: Italy
• Gender: Male
• Best Known For: Galileo was an Italian scientist and scholar whose inventions included the telescope. His discoveries laid the foundation for modern physics and astronomy.
• Science and Medicine
• Astrological Sign: Aquarius
• Monastery school at Vallombrosa, near Florence, University of Pisa
• Nacionalities
• Interesting Facts
• Galileo supported the Copernican theory, which supports a sun-centered solar system.
• Galileo was accused twice of heresy by the church for his beliefs. He remained under house arrest the remaining years of his life.
• Galileo devised his own telescope, in which he observed the moon and found Venus had phases like the moon, proving it rotated around the sun.
• Galileo played a major role in the scientific revolution and earned the moniker "The Father of Modern Science."
• Death Year: 1642
• Death date: January 8, 1642
• Death City: Arcetri
• Death Country: Italy

We strive for accuracy and fairness.If you see something that doesn't look right, contact us !

CITATION INFORMATION

• Article Title: Galileo Biography
• Author: Biography.com Editors
• Website Name: The Biography.com website
• Url: https://www.biography.com/scientists/galileo
• Access Date:
• Publisher: A&E; Television Networks
• Last Updated: March 30, 2021
• Original Published Date: April 3, 2014
• And yet it moves.
• All truths are easy to understand once they are discovered; the point is to discover them.
• The Bible shows the way to go to heaven, not the way the heavens go.

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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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Biography Online

Galileo Galilei Biography

Short biography of Galileo

Galileo was born in Pisa, Duchy of Florence, Italy in 1564 to a poor but noble family.

His parents recognised their child’s innate intelligence and talents and made sacrifices to have him educated. At his father’s insistence, Galileo studied the profitable career of medicine. But, at the University of Pisa, Galileo became fascinated by a wide range of subjects. He also became critical of many of Aristotle ‘s teaching which had dominated education for the past 2,000 years.

Galileo was appointed to be a mathematics professor at the University of Pisa, but his strident criticisms of Aristotle left him isolated amongst his contemporaries. After three years of persecution, he resigned and went to the University of Padua, where he taught maths. His entertaining lectures attracted a large following, and he was able to spend the next 18 years pursuing his interests in astronomy and mechanics.

During this time, Galileo made important discoveries about gravity, inertia and also developed the forerunner of the thermometer. He also worked on the pendulum clock Galileo also worked tirelessly on the science of gnomonics (telling time by shadows) and the laws of motion.

It was in astronomy that Galileo became most famous. In particular, his support for heliocentrism garnered the opposition of the Holy Roman Catholic Church.

Galileo came to the same conclusions of Copernicus – that the sun was the centre of the universe and not the earth. Galileo was also a great admirer of Johannes Kepler for his work on planetary motions.

By inventing the world’s first powerful telescope, Galileo was able to make many ground-breaking explorations of the universe. Galileo’s His telescopes increased magnification from around just 2x to around 30x magnification. Using this new telescope he found that:

• Saturn had a beautiful ring of clouds.
• The moon was not flat but had mountains and craters.
• Using his own telescope, he discovered four moons of Jupiter – Io, Ganymede, Callisto, and Europa. He also noted these moons revolved around Jupiter rather than the sun.

To support the theory of heliocentrism, Galileo had the mathematical proofs of Copernicus but also new proofs from the science of astronomy. However, Galileo knew that publishing these studies would bring the disapproval of the church authorities. Yet, he also felt a willingness to risk the church’s displeasure.

“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.”

—Galileo Galilei, Letter to the Grand Duchess Christina

Galileo was a devout Catholic. He had considered the priesthood as a young man. However, he felt the church was mistaken to take the Bible as a literal source for all scientific studies. As Galileo stated: “The Bible shows the way to go to heaven, not the way the heavens go.” The church’s opposition to heliocentrism centred on Biblical sentences, such as “the world is firmly established, it cannot be moved.” 1 Chronicles 16:30. Galileo contended this was a mistaken view of faith and the Bible.

“Copernicus never discusses matters of religion or faith, nor does he use argument that depend in any way upon the authority of sacred writings which he might have interpreted erroneously. … He did not ignore the Bible, but he knew very well that if his doctrine were proved, then it could not contradict the Scriptures when they were rightly understood.”

Letter to the Grand Duchess Christina (1615)

The Church had already started to forbid Galileo’s teachings, especially anything that supported Copernicus. However, in 1623, a new pope, Pope Urban VIII seemed to be more liberally minded, and he allowed Galileo to publish his great works on astronomy – supporting the ideas of Copernicus.

However, after the publication of Dialogue Concerning the Two Chief World Systems , conservative elements within the Church sought to attack Galileo’s beliefs and writings. In this pamphlet, Galileo appeared to ridicule the words of Pope Urban VIII – making the Pope less sympathetic to Galileo. As a consequence, Galileo was arrested and imprisoned for several months. He was convicted of heresy and was forced to recant his beliefs.  One apocryphal story relates to how Galileo, after recanting his scientific beliefs, muttered under his breath – the rebellious phrase:

“ And yet it moves .”

He spent the remaining years of his life under house arrest at Arceti.

Galileo had three children, born out of wedlock to Marina Gamba. He was especially close to one of his daughters, Polissena; she took the name of Sister Maria Celeste and entered a convent near Arceti.

Despite being censured by the church, Galileo continued to make discoveries until death overtook him in 1642. Under house arrest, he was able to write Two New Sciences ; this summarised his earlier work on the new sciences now called kinematics and the strength of materials. One of Galileo’s significant contributions to the Scientific Revolution was to depict the laws of nature in mathematical terms but also to make an effective use of experiment and observation to develop theories.

“Philosophy is written in that great book which ever lies before our eyes — I mean the universe — but we cannot understand it if we do not first learn the language and grasp the symbols, in which it is written. This book is written in the mathematical language.”

– Galileo, Il Saggiatore (1623)

His law “A body moving on a level surface will continue in the same direction at constant speed unless disturbed.” was incorporated into Sir Isaac Newton’s laws of motion His influential work led many to call him the father of ‘Modern Physics.’ Albert Einstein would later pay tribute to the contributions of Galileo.

“ In advocating and fighting for the Copernican theory Galileo was not only motivated by a striving to simplify the representation of the celestial motions. His aim was to substitute for a petrified and barren system of ideas the unbiased and strenuous quest for a deeper and more consistent comprehension of the physical and astronomical facts. ”

Foreword, written by Einstein, to a 2001 edition of Galileo’s famous book Dialogue Concerning the Two Chief World Systems.

Galileo was blind by the time he passed away on 8 January 1642, aged 77.

Citation: Pettinger, Tejvan . “ Biography of Galileo Galilei” , Oxford, UK  www.biographyonline.net 23rd July 2011. Last updated 17th March 2018.

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Galileo Galilei

Galileo Galilei (1564–1642) has always played a key role in any history of science and, in many histories of philosophy, he is a, if not the, central figure of the scientific revolution of the 17 th Century. His work in physics or natural philosophy, astronomy, and the methodology of science still evoke debate after over 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, other internet resources, related entries.

Galileo was born on February 15, 1564 in Pisa. By the time he died on January 8, 1642 (but see problems with the date, Machamer 1998, pp. 24–5) 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 whole philosophical system.

Galileo and his family moved to Florence in 1572. He started to study for the priesthood, but left and enrolled for a medical degree at the University of Pisa. He never completed this degree, but instead studied mathematics notably with Ostilio Ricci, the mathematician of the Tuscan court. Later he visited the mathematician Christopher Clavius in Rome and started a correspondence with Guildobaldo del Monte. He applied and was turned down for a position in Bologna, but a few years later in 1589, with the help of Clavius and del Monte, he was appointed to the chair of mathematics in Pisa.

In 1592 he was appointed, at a much higher salary, to the position of mathematician at the University of Padua. While in Padua he met Marina Gamba, and in 1600 their daughter Virginia was born. In 1601 they had another daughter Livia, and in 1606 a son Vincenzo.

It was during his Paduan period that Galileo worked out much of his mechanics and began his work with the telescope. In 1610 he published The Starry Messenger , and soon after accepted a position as Mathematician,a non-teaching post at University of Pisa and Philosopher to the Grand Duke of Tuscany. A facsimile copy of The Library of Congress’ manuscript of The Starry Messenger and a symposium discussing details about the manuscript, may be found in Hessler and DeSimone 2013. Galileo had lobbied hard for this position at the Medici court and even named the moons of Jupiter, which he discovered, after the Medici. There were many reasons hewanted move, but he says he did not like the wine in the Venice area and he had to teach too many students. Late in 1610, the Collegio Romano in Rome, where Clavius taught, certified the results of Galileo’s telescopic observations. In 1611 he became a member of what is perhaps the first scientific society, the Academia dei Lincei.

In 1612 Galileo published a Discourse on Floating Bodies , and in 1613, Letters on the Sunspots . In this latter work he first expressed his position in favor of Copernicus. In 1614 both his daughters entered the Franciscan convent of Saint Mathew, near Florence. Virginia became Sister Maria Celeste and Livia, Sister Arcangela. Marina Gamba, their mother, had been left behind in Padua when Galileo moved to Florence.

In 1613–4 Galileo entered into discussions of Copernicanism through his student Benedetto Castelli, and wrote a Letter to Castelli . In 1616 he transformed this into the Letter to the Grand Duchess Christina . In February 1616, the Sacred Congregation of the Index condemned Copernicus’ book On the Revolution of the Heavenly Orbs , pending correction. Galileo then was called to an audience with Cardinal Robert Bellarmine and advised not to teach or defend Copernican theory.

In 1623 Galileo published The Assayer dealing with the comets and arguing they were sublunary phenomena. In this book, he made some of his most famous methodological pronouncements including the claim the book of nature is written in the language of mathematics.

The same year Maffeo Barberini, Galileo’s supporter and friend, was elected Pope Urban VIII. Galileo felt empowered to begin work on his Dialogues concerning the Two Great World Systems . It was published with an imprimatur from Florence (and not Rome) in 1632. Shortly afterwards the Inquisition banned its sale, and Galileo was ordered to Rome for trial. In 1633 he was condemned. 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 under house arrest, his daughter, Maria Celeste died (cf. Sobel 1999). At this time he began work on his final book, Discourses and Mathematical Demonstrations concerning Two New Sciences . This book was smuggled out of Italy and published in Holland. Galileo died early in 1642. Due to his conviction, he was buried obscurely until 1737.

For detailed biographical material, the best and classic work dealing with Galileo’s life and scientific achievements is Stillman Drake’s Galileo at Work (1978). More recently, J.L. Heilbron has written a magnificent biography, Galileo , that touches on all the multiple facets of Galileo’s life (2010). A strange popularization based somewhat on Heilbron’s book, by Adam Gopik, appeared in The New Yorker in 2013.

For many people, in the Seventeenth Century as well as today, Galileo was and is seen as the ‘hero’ of modern science. Galileo discovered many things: with his telescope, he first saw the moons of Jupiter and the mountains on the Moon; he determined the parabolic path of projectiles and calculated the law of free fall on the basis of experiment. He is known for defending and making popular the Copernican system, using the telescope to examine the heavens, inventing the microscope, dropping stones from towers and masts, playing with pendula and clocks, being the first ‘real’ experimental scientist, advocating the relativity of motion, and creating a mathematical physics. His major claim to fame probably comes from his trial by the Catholic Inquisition and his purported role as heroic rational, modern man in the subsequent history of the ‘warfare’ between science and religion. This is no small set of accomplishments for one 17 th -century Italian, who was the son of a court musician and who left the University of Pisa without a degree.

One of the good things about dealing with such momentous times and people is that they are full of interpretive fecundity. Galileo and his work provide one such occasion. Since his death in 1642, 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), but this is not our topic here.

Philosophically, Galileo has been used to exemplify many different themes, usually as a side bar to what the particular writer wished to make the hallmark of the scientific revolution or the nature of good science. Whatever was good about the new science or science in general, it was Galileo who started it. One early 20th Century tradition of Galileo scholarship used to divvy up Galileo’s work into three or four parts: (1) his physics, (2) his astronomy, and (3) his methodology, which could include his method of Biblical interpretation and his thoughts about the nature of proof or demonstration. In this tradition, typical treatments dealt with his physical and astronomical discoveries and their background and/or who were Galileo’s predecessors. More philosophically, many would ask how his mathematics relates to his natural philosophy? How did he produce a telescope and use his telescopic observations to provide evidence in favor of Copernicanism (Reeves 2008)? Was he an experimentalist (Settle 1961, 196, 1983, 1992; Palmieri 2008), a mathematical Platonist (Koyré 1939), an Aristotelian emphasizing experience (Geymonat 1954), precursor of modern positivist science (Drake 1978), or maybe an Archimedean (Machamer 1998), who might have used a revised Scholastic method of proof (Wallace 1992)? Or did he have no method and just fly like an eagle in the way that geniuses do (Feyerabend 1975)? Behind each of these claims there was some attempt to place Galileo in an intellectual context that brought out the background to his achievements. Some emphasize his debt to the artisan/engineer practical tradition (Rossi 1962), others his mathematics (Giusti1993, Peterson 2011,, Feldhay 1998, Palmieri 2001, 2003, Renn 2002, Palmerino 2015,), some his mixed (or subalternate) mathematics (Machamer 1978, 1998, Lennox 1986, Wallace 1992), others his debt to atomism (Shea 1972, Redondi 1983), and some his use of Hellenistic and Medieval impetus theory (Duhem 1954, Claggett 1966, Shapere 1974) or the idea that discoveries bring new data into science (Wootton (2015).

Yet most everyone in this tradition seemed to think the three areas—physics, astronomy and methodology—were somewhat distinct and represented 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 (Moss 1993, Feldhay 1998, Spranzi 2004), the power structures of his social milieu (Biagioli 1993, 2006), his personal quest for acknowledgment (Shea and Artigas 2003) and more generally has emphasized the larger social and cultural history, specifically the court and papal culture, in which Galileo functioned (Redondi 1983, Biagioli 1993, 2006, 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 this path out I shall show why, at the end of his life, Galileo felt compelled (in some sense of necessity) to write the Discourses Concerning 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 blind and under house arrest. 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 by Biener 2004 and 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 have 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 signals this goal clearly when he leaves Padua in 1611 to return to Florence and the court of the Medici and asks for the title Philosopher as well as Mathematician . This was not just a status-affirming request, but also a reflection of his large-scale goal. What Galileo accomplished by the end of his life in 1642 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 concepts 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, cf. selections in Lindberg 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 1998, Adams et al . 2017). However phrased, Galileo’s main move was to de-throne the Aristotelian physical categories of the one celestial (the aether or fifth element) and four terrestrial elements (fire, air, water and earth) and their differential directional natures of motion (circular,  and up and down). In their place he left only one element, corporeal matter, and a different way of describing the properties and motions of matter in terms of the mathematics of the equilibria of proportional relations (Palmieri 2001) that were typified by the Archimedian simple machines—the balance, the inclined plane, the lever, and, he includes, the pendulum (Machamer 1998, 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. But this would take more explaining (Dijksterhuis 1950, Machamer et al. 2000, Gaukroger 2009).

As a main focus underlying 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 didn’t realize that this was his grand goal until the time he actually wrote the Discourses on the Two New Sciences in 1638. 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 actually not before the Dialogues on the Two Chief World Systems of 1632. Before 1632, he did not have the theory and evidence he needed to support his claim about unified, singular matter. He had thought deeply about the nature of matter before 1610 and had tried to work out how best to describe matter, but the idea of unified matter theory had to wait on the establishment of principles of matter’s motion on a moving earth. And this he did not do until the Dialogues .

Galileo began his critique of Aristotle in the 1590 manuscript, De Motu . The first part of this manuscript deals with terrestrial matter and argues that Aristotle’s theory has it wrong. For Aristotle, sublunary or terrestrial matter is of four kinds [earth, air, water, and fire] and has two forms, heavy and light, which by nature are different principles of (natural) motion, down and up. Galileo, using an Archimedian model of floating bodies and later the balance, argues that there is only one principle of motion, the heavy ( gravitas ), and that lightness (or levitas ) is to be explained by the heavy bodies moving so as to displace or extrude other bits of matter in such a direction that explains why the other bits rise. So on his view heaviness (or gravity) is the cause of all natural terrestrial motion. But this left him with a problem as to the nature of the heavy, the nature of gravitas ? In De Motu , he argued that the moving arms of a balance could be used as a model for treating all problems of motion. In this model heaviness is the proportionality of weight of one object on one arm of a balance to that of the weight of another body on the other arm of the balance. In the context of floating bodies, weight is the ‘weight’ of one body minus weight of the medium.

Galileo realized quickly these characterizations were insufficient, and so began to explore how heaviness was relative to the different specific gravities of bodies having the same volume. He was trying to figure out what is 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 standard measures of heaviness that would work across different substances. So at this point he did not have useful replacement categories.

A while later, in his 1600 manuscript, Le Mecaniche (Galileo 1600/1960) he introduces the concept of momento , a quasi force concept that applies to a body at a moment and which is somehow proportional to weight or specific gravity (Galluzzi 1979). Still, he has no good way to measure or compare specific gravities of bodies of different kinds and his notebooks during this early 17 th -century period reflect his trying again and again to find a way to bring all matter under a single proportional measuring scale. He tries to study acceleration along an inclined plane and to find a way to think of what changes acceleration brings. In this regard and during this period he attempts to examine the properties of percussive effect of bodies of different specific gravities, or how they have differential impacts. Yet the details and categories of how to properly treat weight and movement elude him.

One of Galileo’s problems was that the Archimedian simple machines that he was using as his model of intelligibility, especially the balance, are not easily conceived of in a dynamic way (but see Machamer and Woody 1994). Except for the inclined plane, time is not a property of the action of simple machines that one would normally attend to. In discussing a balance, 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 in his Postils to Rocco ca. 1634–45 does; see Palmieri 2005). The converse is also true. It is difficult to model ‘dynamic’ phenomena that deal with the rate of change of different bodies as problems of balance arms moving upwards or downwards because of differential weights. So it was that Galileo’s classic dynamic puzzle about how to describe time and the force of percussion, or the force of body’s impact, would remain unsolved, He could not, throughout his life find systematic relations among specific gravities, height of fall and percussive forces. In the Fifth Day of the Discouses, he presciently explores the concept of the force of percussion . This concept will become, after his death, one of the most fecund ways to think about matter.

In 1603–9, Galileo worked long at doing experiments on inclined planes and most importantly with pendula. The pendulum again exhibited to Galileo that acceleration and, therefore, time is a crucial variable. Moreover, isochrony—equal times for equal lengths of string, despite different weights—goes someway towards showing that time is a possible form for describing the equilibrium (or ratio) that needs to be made explicit in representing motion. It also shows that in at least one case time can displace weight as a crucial variable. Work on the force of percussion and inclined planes also emphasized acceleration and time, and during this time (ca. 1608) he wrote a little treatise on acceleration that remained unpublished.

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 accepts, probably as early as the 1594 draft of Le Mecaniche , that natural motions might be accelerated. But that accelerated motion is properly measured against time is an idea enabled only later, chiefly through his failure to find any satisfactory dependence on place and specific gravity. Galileo must have observed that the speeds of bodies increase as they move downwards and, perhaps, do so naturally, particularly in the cases of the pendulum, the inclined plane, in free fall, and during projectile motion. Also at this time he begins to think about percussive force, the force that a body acquires during its motion that shows upon impact. For many years he thinks that the correct science of these changes should describe how bodies change according to where they are on their paths. Specifically, it seems that height is crucial. Percussive force is directly related to height and the motion of the pendulum seems to involve essentially 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.)

The law of free fall, expressed as time squared, was discovered by Galileo through the inclined plane experiments (Drake 1999, v. 2), but he attempted to find an explanation of this relation, and the equivalent mean proportional relation, through a velocity-distance relation. His later and correct definition of natural acceleration as dependent on time is an insight gained through recognizing the physical significance of the mean proportional relation (Machamer and Hepburn 2004; for a different analysis of Galileo’s discovery of free fall see Renn et al. 2004.) Yet Galileo would not publish anything making time central to motion until 1638, in Discourses on the Two New Sciences (Galileo 1638/1954.) But let us return to the main matter.

In 1609 Galileo begins his work with the telescope. Many interpreters have taken this to be an interlude irrelevant to his physics. The Starry Messenger , which describes his early telescopic discoveries, was published in 1610. There are many ways to describe Galileo’s findings but for present purposes they are remarkable as his start at dismantling of the celestial/terrestrial distinction (Feyerabend 1975). Perhaps the most unequivocal case of this is when he analogizes the mountains on the moon to mountains in Bohemia. The abandonment of the heaven/earth dichotomy implied that all matter is of the same kind, whether celestial or terrestrial. 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 for earth, fire and the heavens. This is a far stronger claim than he had made back in 1590. In addition, he described of his discovery of the four moons circling Jupiter, which he called politically the Medicean stars (after the ruling family in Florence, his patrons). In the Copernican system, the earth having a moon revolve around it was unique and so seemingly problematic. Jupiter’s having planets made the earth-moon system non-unique and so again the earth became like the other planets.  Some fascinating background and treatments of this period of Galileo’s life and motivations have recently appeared (Biagoli 2006, Reeves 2008, and the essays in Hessler and De Simone 2013).

In 1611, at the request of Cardinal Robert Bellarmine, the professors at the Collegio Romano confirmed Galileo’s telescopic observations, with a slight dissent from Father Clavius, who felt that the moon’s surface was probably not uneven. Later that year Clavius changed his mind.

A few years later in his Letters on the Sunspots (1612), Galileo enumerated more reasons for the breakdown of the celestial/terrestrial distinction. Basically the ideas here were that the sun has spots ( maculae ) and rotated in circular motion, and, most importantly Venus had phases just like the moon, which was the spatial key to physically locating Venus as being between the Sun and the earth, and as revolving around the Sun. In these letters he claimed that the new telescopic evidence supported the Copernican theory. 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 were 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 super-lunary nature of comets, which the great Danish astronomer Tycho Brahe had demonstrated earlier.

Yet even with all these changes, two things were missing. First, he needed to work out some general principles concerning the nature of motion for this new unified matter. Specifically, given his Copernicanism, he needed to work out, at least qualitatively, a way of thinking about the motions of matter on a moving earth. The change here was not just the shift from a Ptolemaic, Earth-centered planetary system to a Sun-centered Copernican model. For Galileo, this shift was also from a mathematical planetary model to a physically realizable cosmography. It was necessary for him to describe the planets and the earth as real material bodies. In this respect Galileo differed dramatically from Ptolemy, Copernicus, or even Tycho Brahe, who had demolished the crystalline spheres by his comets-as-celestial argument and flirted with physical models (Westman 1976). So on the new Galilean scheme there is only one kind of matter, and it may have only one kind of motion natural to it. Therefore, he had to devise (or shall we say, discover) principles of local motion that will fit a central sun, planets moving around that sun, and a daily whirling earth.

This he did by introducing two new principles. In Day One of his Dialogues on the Two Chief World Systems (1632), Galileo argued that all natural motion is circular. Then, in Day Two, he introduced his version of the famous principle of the relativity of observed motion. This latter held that motions in common among bodies could not be observed. Only those motions differing from a shared common motion could be seen as moving. The joint effect of these two principles was to say that all matter shares a common motion, circular, and so only motions different from the common, say up and down motion, could be directly observed. Of course, neither of the principles originated 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.

In Day Three, Galileo dramatically argues for the Copernican system. Salviati, the persona of Galileo, 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. Earlier in Day One, he had repeated his claims from The Starry Messenger , noting that the earth must be like the moon in being spherical, dense and solid, and having rugged mountains. Clearly the moon could not be a crystalline sphere as held by some Aristotelians.

In the Dialogues , things are more complicated than we have just sketched. Galileo, as noted, argues for a circular natural motion, so that all things on the earth and in the atmosphere revolve in a common motion with the earth so that the principle of the relativity of observed motion will apply to phenomena such as balls dropped from the masts of moving ships. Yet he also introduces at places a straight-line natural motion. For example, in Day Three, he gives a quasi account for a Coriolis-type effect for the winds circulating about the earth by means of this straight-line motion (Hooper 1998). Further, in Day Four, when he is giving his proof of the Copernican theory by sketching out how the three-way moving earth mechanically moves the tides, he nuances his matter theory by attributing to the element water the power of retaining an impetus for motion such that it can provide 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 in De Motu in 1590, with submerged bodies, but more importantly he learned much more while working through his dispute over floating bodies ( Discourse on Floating Bodies , 1612). In fact a large part of this 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 (cf. Palmieri, 1998, 2004a).

The final chapter of Galileo’s scientific story comes in 1638 with the publication of Discourses of the Two New Sciences . The second science, discussed (so to speak) in the last two days, dealt with the principles of local motion. These have been much commented upon in the Galilean literature. Here is where he enunciates the law of free fall, the parabolic path for projectiles and his physical “discoveries” (Drake 1999, v. 2). But the first two days, the first science, has been much misunderstood and little discussed. This first science, misleadingly, has 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 first 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 1998, Machamer and Hepburn 2004, and the detailed work spelling this out by Biener 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. Remember that specific gravities did not work.

So it is in Day One that he begins to discuss how to describe, mathematically (or geometrically), the causes of how beams break. He is searching for the mathematical description of the essential nature of matter. He rules out certain questions that might use infinite atoms as basis for this discussion, and continues on giving reasons for various properties that matter has. Among these are questions of the constitution of matter, properties of matter due to its heaviness, the properties of the media within which bodies move and what is the cause of a body’s coherence as a single material body. The most famous of these discussions is his account of acceleration of falling bodies, that whatever their weight would fall equally fast in a vacuum. The Second Day lays out the mathematical principles concerning how bodies break. He does this all by reducing the problems of matter to problems of how a lever and a balance function. Something he had begun back in 1590, though this time he believes he is getting it right, showing mathematically how bits of matter solidify and stick together, and do so by showing how they break into bits. The ultimate explanation of the “sticking” eluded him since he felt he would have to deal with infinitesimals to really solve this problem.

The second science, Days Three and Four of Discorsi , dealt with proper principles of local motion, but this was now motion for all matter (not just sublunary stuff) and it took the categories of time and acceleration as basic. Interestingly Galileo, here again, revisited or felt the need to include some anti-Aristotelian points about motion as he had done back in 1590. The most famous example of his doing this, is his “beautiful thought experiment”, whereby he compares two bodies of the same material of different sizes and points out that according to Aristotle they fall at different speeds, the heavier one faster. Then, he says, join the bodies together. In this case the lightness of the small one ought to slow down the faster larger one, and so they together fall as a speed less than the heavy fell in the first instance. Then his punch line: but one might also conceive of the two bodies joined as being one larger body, in which case it would fall even more quickly. So there is a contradiction in the Aristotelian position (Palmieri 2005). His projected Fifth Day would have treated the grand principle of the power of matter in motion due to impact. He calls it the force of percussion, which deals with two bodies interacting. This problem he does not solve, and it won’t be solved until René Descartes, probably following Isaac Beeckman, turns the problem into finding the equilibrium points for colliding bodies.

The sketch above provides the basis for understanding Galileo’s changes. He has a new science of matter, a new physical cosmography, and a new science of local motion. In all these he is using a mathematical mode of description based upon, though somewhat changed from, the proportional geometry of Euclid, Book VI and Archimedes (for details on the change see Palmieri 2002).

It is in this way that Galileo developed the new 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 treated in a way that allowed for coherent discussion of the principles of motion. That a unified matter became accepted and its nature became one of the problems for the ‘new science’ that followed was due to Galileo. Thereafter, matter really mattered.

No account of Galileo’s importance to philosophy can be complete if it does not discuss Galileo’s condemnation and the Galileo affair (Finocchiaro 1989). The end of the episode is simply stated. In late 1632, after publishing Dialogues on the Two Chief World Systems , Galileo was ordered to go to Rome to be examined by the Holy Office of the Inquisition. In January 1633, a very ill Galileo made an arduous journey to Rome. Finally, in April 1633 Galileo was called before the Holy Office. This was tantamount to a charge of heresy, and he was urged to repent (Shea and Artigas, 183f). Specifically, he had been charged with teaching and defending the Copernican doctrine that holds that the Sun is at the center of the universe and that the earth moves. This doctrine had been deemed heretical in 1616, and Copernicus’ book had been placed on the Index of Prohibited Books, pending correction.

Galileo was called four times for a hearing; the last was on June 21, 1633. The next day, 22 June, Galileo was taken to the church of Santa Maria sopra Minerva, and ordered to kneel while his sentence was read. It was declared that he was “vehemently suspect of heresy”. Galileo was 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 centre 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 194)

Galileo was not imprisoned but had his sentence commuted to house arrest. In December 1633 he was allowed to retire to his villa in Arcetri, outside of Florence. During this time he finished his last book, Discourses on the Two New Sciences , which was published in 1638, in Holland, by Louis Elzivier. The book does not mention Copernicanism at all, and Galileo professed amazement at how it could have been published. He died on January 8, 1642.

There has been much controversy over the events leading up to Galileo’s trial, and it seems that each year we learn more about what actually happened. There is also controversy over the legitimacy of the charges against Galileo, both in terms of their content and judicial procedure. The summary judgment about this latter point is that the Church most probably acted within its authority and on ‘good’ grounds given the condemnation of Copernicus, and, as we shall see, the fact that Galileo had been warned by Cardinal Bellarmine earlier in 1616 not to defend or teach Copernicanism. There were also a number of political factors given the Counter Reformation, the 30 Years War (Miller 2008), and the problems with the papacy of Urban VIII that served as further impetus to Galileo’s condemnation (McMullin, ed. 2005). It has even been argued (Redondi 1983) that the charge of Copernicanism was a compromise plea bargain to avoid the truly heretical charge of atomism. Though this latter hypothesis has not found many willing supporters.

Legitimacy of the content, that is, of the condemnation of Copernicus, is much more problematic. Galileo had addressed this problem in 1615, when he wrote his Letter to Castelli (which was transformed into the Letter to the Grand Duchess Christina ). In this letter he had argued that, of course, the Bible was an inspired text, yet two truths could not contradict one another. So in cases where it was known that science had achieved a true result, the Bible ought to be interpreted in such a way that makes it compatible with this truth. The Bible, he argued, was an historical document written for common people at an historical time, and it had to be written in language that would make sense to them and lead them towards the true religion.

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 right 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) were only hypotheses and due to their mathematical, purely calculatory character were not susceptible to physical proof. This is a sort of instrumentalist, anti-realist position (Duhem 1985, Machamer 1976). 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 the phenomena. Subtler versions, without an Aquinian metaphysical bias, of this position have been argued subsequently and more fully by van Fraassen (1996) and others. Less sweepingly, it could reasonably be argued that both Ptolemy and Copernicus’ theories were primarily mathematical, and that what Galileo was defending was not Copernicus’ theory per se, but a physical realization of it. In fact, it might be better to say the Copernican theory that Galileo was constructing was a physical realization of parts of Copernicus’ theory, which, by the way, dispensed with all the mathematical trappings (eccentrics, epicycles, Tusi couples and the like). Galileo would be led to such a view by his concern with matter theory. Of course, put this way we are faced with the question of what constitutes identity conditions for a theory, or being the same theory. There is clearly a way in which Galileo’s Copernicus is not Copernicus and most certainly not Kepler.

The other aspect of all this which has been hotly debated is: what constitutes proof or demonstration of a scientific claim? In 1616, the same year that Copernicus’ book was placed on the Index of Prohibited Books, Galileo was called before Cardinal Robert Bellarmine, head of the Holy Office of the Inquisition and warned not to defend or teach Copernicanism. During this year Galileo also completed a manuscript, On the Ebb and Flow of the Tides . The argument of this manuscript will turn up 17 years later as day Four of Galileo’s Dialogues concerning the Two Chief World Systems . This argument, about the tides, Galileo believed provided proof of the truth of the Copernican theory. But insofar as it possibly does, it provides an argument for the physical plausibility of Galileo’s Copernican theory. Let’s look more closely at his argument.

Galileo argues that the motion of the earth (diurnal and axial) is the only conceivable (or maybe plausible) physical cause for the reciprocal regular motion of the tides. He restricts the possible class of causes to mechanical motions, and so rules out Kepler’s attribution of the moon as a cause. How could the moon without any connection to the seas cause the tides to ebb and flow? Such an explanation would be the invocation of magic or occult powers. So the motion of the earth causes the waters in the basins of the seas to slosh back and forth, and since the earth’s diurnal and axial rotation is regular, so are the periods of the tides; the backward movement is due to the residual impetus built up in the water during its slosh. Differences in tidal flows are due to the differences in the physical conformations of the basins in which they flow (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 be impressed. First, they do not accept Galileo’s restriction of possible causes to mechanically intelligible causes. Second, the tidal argument does not directly deal with the annual motion of the earth about the sun. And third, the argument does not touch anything about the central position of the sun or about the periods of the planets as calculated by Copernicus. So at its best, Galileo’s argument is an inference to the best partial explanation of one point in Copernicus’ theory. Yet when this argument is added to the earlier telescopic observations that show the improbabilities of the older celestial picture, to the fact that Venus has phases like the moon and so must revolve around the sun, to the principle of the relativity of perceived motion which neutralizes the physical motion arguments against a moving earth, it was enough for Galileo to believe that he had the necessary proof to convince the Copernican 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 Discourses on 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, and Mayer 2012. To understand the long, tortuous, and fascinating aftermath of the Galileo affair see Finocchiaro 2005, and for John Paul II’s attempt see George Coyne’s article in McMullin 2005.

Primary Sources: Galileo’s Works

The main body of Galileo’s work is collected in Le Opere di Galileo Galilei , Edizione Nazionale, 20 vols., edited by Antonio Favaro, Florence: Barbera, 1890-1909; reprinted 1929-1939 and 1964–1966.

• 1590, On Motion , translated I.E. Drabkin, Madison: University of Wisconsin Press, 1960.
• 1600, On Mechanics , S. Drake (trans.), Madison: University of Wisconsin Press, 1960.
• 1610, The Starry Messenger , A. van Helden (ed.), Chicago: University of Chicago Press, 1989.
• 1613, Letters on the Sunspots , selections in S. Drake, (ed.), The Discoveries and Opinions of Galileo , New York: Anchor, 1957.
• 1623, Il Saggiatore , The Assayer , translated by Stillman Drake, in The Controversy of the Comets of 1618 , Philadelphia: The University of Pennsylvania Press 1960.
• 1632, Dialogue Concerning the Two Chief World Systems , S. Drake (trans.), Berkeley: University of California Press, 1967.
• 1638, Dialogues Concerning Two New Sciences , H. Crew and A. de Salvio (trans.), Dover Publications, Inc., New York, 1954, 1974. A better translation is: Galilei, Galileo. [ Discourses on the ] Two New Sciences , S. Drake (trans.), Madison: University of Wisconsin Press, 1974; 2nd edition, 1989 & 2000 Toronto: Wall and Emerson.

Secondary Sources

• 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.
• Bedini, Silvio A., 1991, The Pulse of Time: Galileo Galilei, the Determination of Longitude, and the Pendulum Clock , Florence: Olschki.
• –––, 1967, Galileo and the Measure of Time , Florence: Olschki.
• Biagioli, Mario, 1993, Galileo Courtier , Chicago: University of Chicago Press.
• –––, 1990, “Galileo’s System of Patronage,” History of Science , 28: 1–61.
• –––, 2006, Galileo’s Instruments of Credit :Tekescopes, 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.
• Carugo, Adriano and Crombie, A. C., 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.
• Claggett, Marshall, 1966, The Science of Mechanics in the Middle Ages , Madison: University of Wisconsin Press.
• Crombie, A. C., 1975, “Sources of Galileo’s Early Natural Philosophy,” in Reason, Experiment, and Mysticism in the Scientific Revolution , Edited by Maria Luisa Righini Bonelli and William R. Shea, pp. 157–175. New York: Science History Publications.
• Dijksterhuis, E.J., 1961 [1950], The Mechanization of the World Picture , translated by C Dikshoorn, Oxford: Oxford University Press.
• Drake, Stillman, 1957, Discoveries and Opinions of Galileo , Garden City, NY: Doubleday.
• –––, 1978, Galileo at Work: His Scientific Biography , 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 volumes, Toronto: University of Toronto Press.
• Duhem, Pierre, 1954, Le Systeme du monde , 6 volumes, Paris: Hermann.
• –––, 1985, To Save the Phenomena: An Essay on the Idea of Physical Theory from Plato to Galileo , translated Roger Ariew, Chicago: University of Chicago Press.
• Feldhay, Rivka, 1995, Galileo and the Church: Political Inquisition or Critical Dialogue , New York, NY: Cambridge University Press.
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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up this entry topic 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 Biblioteca Nazionale Centrale, Florence Istituto e Museo di Storia della Scienza, Florence Max Planck Institute for the History of Science, Berlin.
• The Galileo Project , contains Dava Sobel’s translations of all 124 letters from Suor Maria Celeste to Galileo in the sequence in which they were written, maintained by Albert Van Helden.
• Galileo Galilei , The Institute and Museum of the History of Science of Florence, Italy.

Copernicus, Nicolaus | -->matter --> | natural philosophy: in the Renaissance | religion: and science

Acknowledgments

Thanks to Zvi Biener and Paolo Palmieri for commenting on earlier drafts of this entry.

Copyright © 2017 by Peter Machamer < machamerpeter @ gmail . com >

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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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Nola Taylor Tillman is a contributing writer for Space.com. She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children. Follow her on Twitter at @NolaTRedd

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Biography of Galileo Galilei, Renaissance Philosopher and Inventor

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The University of Pisa

Becoming a mathematician, the leaning tower of pisa, the university of padua, building a spyglass (telescope).

• Galileo's Observations of the Moon
• Discovery of Jupiter's Satellites
• Seeing Saturn's Rings

Charged With Heresy

The final trial, inquisition and death, the vatican pardons galileo in 1992.

Galileo Galilei (February 15, 1564–January 8, 1642) was a famous inventor , mathematician, astronomer, and philosopher whose inventive mind and stubborn nature ran him into trouble with the Inquisition.

Fast Facts: Galileo Galilei

• Known For : Italian Renaissance philosopher, inventor, and polymath who faced the wrath of the Inquisition for his astronomical studies
• Born : February 15, 1564 in Pisa, Italy
• Parents : Vincenzo and Giulia Ammannati Galilei (m. July 5, 1562)
• Died : January 8, 1642 in Arcetri, Italy
• Education : Privately tutored; Jesuit monastery, University of Pisa
• Published Works : "The Starry Messenger"
• Spouse : None; Marina Gamba, mistress (1600–1610)
• Children : Virginia (1600), Livia Antonia (1601), Vincenzo (1606)

Galileo was born in Pisa, Italy on February 15, 1564, the oldest of seven children of Giulia Ammannati and Vincenzo Galilei. His father (c. 1525–1591) was a gifted lute musician and wool trader and wanted his son to study medicine because there was more money in that field. Vincenzo was attached to the court and was often traveling. The family was originally named Bonaiuti, but they had an illustrious ancestor named Galileo Bonaiuti (1370–1450) who was a physician and public officer in Pisa. One branch of the family broke off and began calling itself Galilei ("of Galileo"), and so Galileo Galilei was doubly named after him.

As a child, Galileo made mechanical models of ships and watermills, learned to play the lute to a professional standard, and showed an aptitude for painting and drawing. Originally tutored by a man named Jacopo Borghini, Galileo was sent to the Camaldlese monastery at Vallambroso to study grammar, logic, and rhetoric. He found the contemplative life to his liking, and after four years he joined the community as a novice. This was not exactly what his father had in mind, so Galileo was hastily withdrawn from the monastery. In 1581 at the age of 17, he entered the University of Pisa to study medicine , as his father wished.

At age 20, Galileo noticed a lamp swinging overhead while he was in a cathedral. Curious to find out how long it took the lamp to swing back and forth, he used his pulse to time large and small swings. Galileo discovered something that no one else had ever realized: the period of each swing was exactly the same. The law of the pendulum, which would eventually be used to regulate clocks , made Galileo Galilei instantly famous.

Except for mathematics , Galileo was soon bored with the university and the study of medicine. Uninvited, he attended the lecture of court mathematician Ostilio Ricci—who had been assigned by the Duke of Tuscany to teach the court attendants in math, and Galileo was not one of those. Galileo followed up the lecture by reading Euclid on his own; he sent a set of questions to Ricci, the content of which greatly impressed the scholar.

Galileo's family considered his mathematical studies subsidiary to medicine, but when Vincenzo was informed that their son was in danger of flunking out, he worked out a compromise so that Galileo could be tutored in mathematics by Ricci full-time. Galileo's father was hardly overjoyed about this turn of events because a mathematician's earning power was roughly around that of a musician, but it seemed that this might yet allow Galileo to successfully complete his college education. The compromise didn't work out, for Galileo soon left the University of Pisa without a degree.

After he flunked out, Galileo started tutoring students in mathematics to earn a living. He did some experimenting with floating objects, developing a balance that could tell him that a piece of gold, for example, was 19.3 times heavier than the same volume of water. He also started campaigning for his life's ambition: a position on the mathematics faculty at a major university. Although Galileo was clearly brilliant, he had offended many people in the field and they would choose other candidates for vacancies.

Ironically, it was a lecture on literature that would turn Galileo's fortunes. The Academy of Florence had been arguing over a 100-year-old controversy: what were the location, shape, and dimensions of Dante's Inferno? Galileo wanted to seriously answer the question from the point of view of a scientist. Extrapolating from Dante's line that the giant Nimrod's "face was about as long/and just as wide as St. Peter's cone in Rome," Galileo deduced that Lucifer himself was 2,000 arm-lengths long. The audience was impressed, and within the year, Galileo had received a three-year appointment to the University of Pisa, the same university that never granted him a degree.

When Galileo arrived at the University, some debate had started up on one of Aristotle's "laws" of nature: that heavier objects fell faster than lighter objects. Aristotle's word had been accepted as gospel truth, and there had been few attempts to actually test Aristotle's conclusions by actually conducting an experiment.

According to legend, Galileo decided to try. He needed to be able to drop the objects from a great height. The perfect building was right at hand—the Tower of Pisa , which was 54 meters (177 feet) tall. Galileo climbed to the top of the building carrying a variety of balls of varying sizes and weights and dumped them off the top. They all landed at the base of the building at the same time (legend says that the demonstration was witnessed by a huge crowd of students and professors). Aristotle was wrong.

It might have helped the junior member of the faculty if Galileo had not continued to behave rudely toward his colleagues. "Men are like wine flasks," he once said to a group of students, "Look at…bottles with the handsome labels. When you taste them, they are full of air or perfume or rouge. These are bottles fit only to pee into!" Perhaps not surprisingly, the University of Pisa chose not to renew Galileo's contract.

Galileo Galilei moved on to the University of Padua. By 1593, he was desperate and in need of additional cash. His father had died, so Galileo was now head of his family. Debts were pressing down on him, most notably the dowry for one of his sisters, which was to be paid in installments over decades. (A dowry could be thousands of crowns, and Galileo's annual salary was 180 crowns.) Debtor's prison was a real threat if Galileo returned to Florence.

What Galileo needed was to come up with some sort of device that could make him a tidy profit. A rudimentary thermometer (which, for the first time, allowed temperature variations to be measured) and an ingenious device to raise water from aquifers found no market. He found greater success in 1596 with a military compass that could be used to accurately aim cannonballs. A modified civilian version that could be used for land surveying came out in 1597 and ended up earning a fair amount of money for Galileo. It helped his profit margin that the instruments were sold for three times the cost of manufacture, he offered classes on how to use the instrument, and the actual toolmaker was paid dirt-poor wages.

Galileo needed the money to support his siblings, his mistress (21-year-old Marina Gamba), and his three children (two daughters and a boy). By 1602, Galileo's name was famous enough to help bring in students to the University, where Galileo was busily experimenting with magnets .

During a vacation to Venice in 1609, Galileo Galilei heard rumors that a Dutch spectacle-maker had invented a device that made distant objects seem near at hand (at first called the spyglass and later renamed the  telescope ). A patent had been requested, but not yet granted. The methods were being kept secret because it was obviously of tremendous military value for Holland.

Galileo Galilei was determined to attempt to construct his own spyglass. After a frantic 24 hours of experimentation, working only on instinct and bits of rumors—he had never actually seen the Dutch spyglass—he built a three-power telescope. After some refinement, he brought a 10-power telescope to Venice and demonstrated it to a highly impressed Senate. His salary was promptly raised, and he was honored with proclamations.

Galileo's Observations of the Moon

If he had stopped here and become a man of wealth and leisure, Galileo Galilei might be a mere footnote in history. Instead, a revolution started when, one fall evening, the scientist trained his telescope on an object in the sky that all people at that time believed must be a perfect, smooth, polished heavenly body—the moon.

To his astonishment, Galileo Galilei viewed a surface that was uneven, rough, and full of cavities and prominences. Many people insisted that Galileo Galilei was wrong, including a mathematician who insisted that even if Galileo was seeing a rough surface on the Moon, that only meant that the entire moon had to be covered in invisible, transparent, smooth crystal.

Discovery of Jupiter's Satellites

Months passed, and his telescopes improved. On January 7, 1610, he turned his 30-power telescope toward Jupiter and found three small, bright stars near the planet. One was off to the west, the other two were to the east, all three in a straight line. The following evening, Galileo once again took a look at Jupiter and found that all three of the "stars" were now west of the planet, still in a straight line.

Observations over the following weeks led Galileo to the inescapable conclusion that these small "stars" were actually small satellites that were rotating around Jupiter. If there were satellites that didn't move around the Earth, wasn't it possible that the Earth was not the center of the universe? Couldn't the  Copernican  idea of the sun resting at the center of the solar system be correct?

Galileo Galilei published his findings in a small book titled "The Starry Messenger." A total of 550 copies were published in March 1610, to tremendous public acclaim and excitement. It was the only one of Galileo's writings in Latin; most of his work was published in Tuscan.

Seeing Saturn's Rings

There continued to be more discoveries via the new telescope: the appearance of bumps next to the planet Saturn (Galileo thought they were companion stars; the "stars" were actually the edges of Saturn's rings), spots on the Sun's surface (though others had actually seen the spots before), and seeing Venus change from a full disk to a sliver of light.

For Galileo Galilei, saying that the Earth went around the Sun changed everything since he was contradicting the teachings of the Catholic Church. While some of the church's mathematicians wrote that his observations were clearly correct, many members of the church believed that he must be wrong.

In December 1613, one of the scientist's friends told him how a powerful member of the nobility said that she could not see how his observations could be true since they would contradict the Bible. The woman quoted a passage in Joshua in which God causes the sun to stand still and lengthen the day. How could this mean anything other than that the sun went around the Earth?

Galileo was a religious man and agreed that the Bible could never be wrong. However, he said, the interpreters of the Bible could make mistakes, and it was a mistake to assume that the Bible had to be taken literally. That was one of Galileo's major mistakes. At that time, only church priests were allowed to interpret the Bible or define God's intentions. It was absolutely unthinkable for a mere member of the public to do so.

Some of the church clergy started responding, accusing him of heresy. Some clerics went to the Inquisition, the Catholic Church court that investigated charges of heresy, and formally accused Galileo Galilei. This was a very serious matter. In 1600, a man named Giordano Bruno was convicted of being a heretic for believing that the Earth moved about the sun and that there were many planets throughout the universe where life—living creations of God—existed. Bruno was burned to death.

However, Galileo was found innocent of all charges and was cautioned not to teach the Copernican system. Sixteen years later, all that would change.

The following years saw Galileo work on other projects. With his telescope he watched the movements of Jupiter's moons , recorded them as a list, and then came up with a way to use these measurements as a navigation tool. He developed a contraption that would allow a ship captain to navigate with his hands on the wheel, but the contraption looked like a horned helmet.

As another amusement, Galileo started writing about ocean tides. Instead of writing his arguments as a scientific paper, he found that it was much more interesting to have an imaginary conversation, or dialogue, between three fictional characters. One character, who would support Galileo's side of the argument, was brilliant. Another character would be open to either side of the argument. The final character, named Simplicio, was dogmatic and foolish, representing all of Galileo's enemies who ignored any evidence that Galileo was right. Soon, he wrote up a similar dialogue called "Dialogue on the Two Great Systems of the World." This book talked about the Copernican system .

"Dialogue" was an immediate hit with the public, but not, of course, with the church. The pope suspected that he was the model for Simplicio. He ordered the book banned and also ordered the scientist to appear before the Inquisition in Rome for the crime of teaching the Copernican theory after being ordered not to do so.

Galileo Galilei was 68 years old and sick. Threatened with torture, he publicly confessed that he had been wrong to have said that the Earth moves around the Sun. Legend then has it that after his confession, Galileo quietly whispered, "and yet, it moves."

Unlike many less famous prisoners, he was allowed to live under house arrest in his house outside of Florence and near one of his daughters, a nun. Until his death in 1642, he continued to investigate other areas of science. Amazingly, he even published a book on force and motion although he had been blinded by an eye infection.

The Church eventually lifted the ban on Galileo's Dialogue in 1822—by that time, it was common knowledge that the Earth was not the center of the Universe. Still later, there were statements by the Vatican Council in the early 1960s and in 1979 that implied that Galileo was pardoned and that he had suffered at the hands of the church. Finally, in 1992, three years after Galileo Galilei's namesake had been launched on its way to Jupiter, the Vatican formally and publicly cleared Galileo of any wrongdoing.

• Drake, Stillman. "Galileo at Work: His Scientific Biography." Mineola, New York: Dover Publications Inc., 2003.
• Reston, Jr., James. "Galileo: A Life." Washington DC: BeardBooks, 2000. 
• Van Helden, Albert. "Galileo: Italian Philosopher, Astronomer and Mathematician." Encyclopedia Britannica , February 11, 2019.
• Wootton, David. Galileo: "Watcher of the Skies." New Haven, Connecticut: Yale University Press, 2010.
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 MacTutor

Galileo galilei.

About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons believed while other denied them. A few days later the report was confirmed by a letter I received from a Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to investigate means by which I might arrive at the invention of a similar instrument. This I did soon afterwards, my basis being the doctrine of refraction.

In about two months, December and January, he made more discoveries that changed the world than anyone has ever made before or since.

I hold that the Sun is located at the centre of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover ... I confirm this view not only by refuting Ptolemy 's and Aristotle 's arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.

Philosophy is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the characters in which it is written. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it; without these one is wandering in a dark labyrinth.

I assume that the speed acquired by the same movable object over different inclinations of the plane are equal whenever the heights of those planes are equal.

The time in which a certain distance is traversed by an object moving under uniform acceleration from rest is equal to the time in which the same distance would be traversed by the same movable object moving at a uniform speed of one half the maximum and final speed of the previous uniformly accelerated motion.

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Additional Resources ( show )

Other pages about Galileo Galilei:

• Preface to Letters from Galileo
• Galileo's Confession
• Galileo's Dialogue
• The title page from Discorsi (1638)
• ... and another page
• ... and yet another page
• Multiple entries in The Mathematical Gazetteer of the British Isles ,
• Astronomy: The Dynamics of the Solar System
• Astronomy: The Structure of the Solar System
• Anstruther Solar System model
• Sci Hi blog
• Miller's postage stamps
• Heinz Klaus Strick biography

Other websites about Galileo Galilei:

• Dictionary of Scientific Biography
• Encyclopaedia Britannica
• Science Museum Florence
• The Galileo Project
• Michael Fowler ( Dialogue concerning two new sciences )
• The Catholic Encyclopedia
• Sci Hi blog ( Jupiter's moons )
• Sci Hi blog ( Galileo's telescope )
• Internet Encyclopedia of Philosophy
• Openculture ( Galileo's so-called "moon drawings" )
• MathSciNet Author profile

Honours ( show )

Honours awarded to Galileo Galilei

• Lunar features Crater Galilaei and Rima Galilaei
• Paris street names Rue Galilée ( = Galileo ) (16 th Arrondissement )
• Popular biographies list Number 9

Cross-references ( show )

• History Topics: A brief history of cosmology
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Galileo Galilei

• Occupation: Scientist, mathematician, and Astronomer
• Born: February 15, 1564 in Pisa, Italy
• Best known for: Improving the telescope to be used to study the planets and stars

• Galileo published the first scientific paper based on observations made through a telescope in 1610. It was called The Starry Messenger .
• In later years, the Catholic Church changed their views on Galileo and stated that they regretted how he was treated.
• Galileo noticed that the planet Saturn wasn't round. It was later discovered that Saturn had rings.
• A year before his death he came up with a pendulum design used for keeping time.
• He once said that "The Sun, with all those planets revolving around it…can still ripen a bunch of grapes as if it had nothing else in the universe to do."
• Listen to a recorded reading of this page:

Galileo Galilei discovered that the Earth orbits the sun, an observation for which he was put under house arrest for the last eight years of his life.

Galileo Matters More Than Ever on His 450th Birthday

The scientist's insights continue to enrich our world.

Not every astronomer can claim to have enjoyed both the attention of the Inquisition and the Indigo Girls , but then again, Galileo was no ordinary genius.

Discoverer of moons, toppler of Aristotle's physics, and celebrated loser of history's most famous heresy trial, Galileo Galilei's greatest invention, in truth, was our own modern world.

On the 450th anniversary of his birth today, February 15, 2014, it's worth taking a telescopic look at the achievements of this unparalleled genius of the Renaissance. Born in 1564 in Pisa, Italy, Galileo lived to the age of 77 , a life span that saw the start of the scientific revolution in Europe. (See also: " Galileo's Telescope at 400 .")

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Galileo is still in the news. An optical illusion he discovered in the 1600s caused Venus to appear much larger and blurrier—a "radiant crown," as Galileo called it—when seen through a telescope than when viewed with the naked eye.

The puzzle was finally understood just this week. Neuroscientists from the State University of New York College of Optometry report that the answer lies in the wiring of our visual brain cells . The brain responds to light and dark objects differently, so the brightness of a planet distorts its apparent size when it is seen against the dark background of space.

Heaven and Hell

"Infinite thanks to God," Galileo wrote in 1610, "for being so kind as to make me alone the first observer of marvels kept hidden."

He was celebrating his discovery of Jupiter's four large moons : Io, Europa, Ganymede, and Callisto. Originally he wanted to name the moons after his noble patrons, four brothers of Florence's famed Medici family called Cosimo, Francesco, Carl, and Lorenzo. Other astronomers, perhaps thankfully, assigned more elevated names to the moons, ones taken from mythology for the consorts of Jupiter, king of the gods.

Those moons revealed that some objects revolved around something other than the Earth, which helped Galileo to discover heliocentrism, or the fact that the Earth circles the sun. This finding, in turn, would earn him the attention of the Inquisition, which investigated religious rebellion and heresy in the world of 16th-century Italy. The Vatican officially apologized in 2000 for Galileo's heresy trial, which resulted in the scientist being kept under house arrest for the last eight years of his life.

Using telescopes he built, Galileo was the first to identify the four large moons of Jupiter.

Cannonball Physics

Galileo's most famous experiment, which he likely never really performed, was the 1589 dropping of cannonballs with different masses off the Leaning Tower of Pisa . The goal of this experiment was to show that objects fall at a uniform rate, that gravity doesn't make heavier objects fall faster.

That notion was contrary to classic Greek physics, which held that heavier objects fall faster . In 1971 the experiment was repeated on the moon (to remove the effects of air resistance) by Apollo 15 astronauts , who dropped a hammer and a feather to confirm Galileo's observation.

What was notable about the experiment was precisely that it was an experiment. Earlier models of scientific inquiry were reasoned entirely in the mind or argued from theological principles. Galileo, by contrast, advanced the fundamental idea that science relied on experiments to prove its contentions. This simple idea—prove it—was radical at the time.

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Galileo went even further, pioneering the idea that mathematics are essential to scientific observations, and abjuring the literary hand-waving of ancient texts. He was the father of mathematical physics, reporting his observations in tables that inspired today's lab books. The diagrams he made depicting astronomical objects are clear forerunners to modern ones.

He put his knowledge to practical use, grinding the lenses for improved telescopes that allowed him to make astronomical discoveries ahead of other scholars—spotting moons, finding sunspots, and peering into the craters of Earth's moon.

As recounted in his volume Starry Messenger , Galileo crafted a telescope for the sailing masters of Venice that magnified views by at least eight times, helping them look out for pirates on trading voyages.

According to legend, Galileo dropped a cannonball and a wooden ball from the Leaning Tower of Pisa to see which fell faster.

Open Access Science

In his writings and books, some published in Holland to avoid the wrath of the Inquisition, Galileo gave shape to an ideal that exists in the scientific community to this very day: that scientists are united in their quest to understand the unknown and that their voyage of discovery transcends national borders. Galileo widely corresponded with other natural philosophers and the great innovative minds of his time, such as Johannes Kepler , who first wrote down the laws of orbital motion.

In writing down their discoveries, Galileo and his contemporaries created the beginnings of the system of scientific correspondence that we know today as scientific journals, where discoveries are openly described by their methods, results, and possible shortfalls.

This was quite a contrast to the gnomic writings of alchemists, who cloaked their recipes in mythological allusions and double-talk. The open discourse of the scientific enterprise is one of the abiding gifts of the Renaissance. (Although it is worth noting that Galileo resorted to scrambling news of his findings in code in letters to Kepler.)

Galileo not only wrote to fellow scholars, he also wrote for the public, notes historian Doug Linder in his account of the trial of Galileo . "He seemed compelled to act as a consultant in natural philosophy to all who would listen," Linder writes. "He wrote in tracts, pamphlets, letters, and dialogues—not in the turgid, polysyllabic manner of a university pedant, but simply and directly."

That talent for communication was quite likely what got him in hot water with religious authorities, ending with the heresy trial, one of history's crueler attacks on independent thinking. The trial haunted the Vatican for centuries; its treatment of Galileo added momentum to the Enlightenment's demand for intellectual freedom, which opened the way for such documents as the U.S. Constitution and the United Nations Universal Declaration of Human Rights .

The scientist's communication skills were said to be overbearing at times. "Everyone agrees that Galileo was an incorrigible egotist, so full of himself that he repeatedly misjudged his ability to persuade the authorities of his own opinions," astronomical historian Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics noted in a recent review of Galileo biographies .

Whatever his flaws, "Galileo was the most articulate spokesman for the new astronomy, the pioneer who set observational astronomy on its modern track," Gingerich said.

And beyond his scientific achievements, Galileo is remembered in the popular imagination as a courageous truthseeker, a view expressed in the song "Galileo" by the folk rock group Indigo Girls. He was, they sing, "king of night vision, king of insight."

Remembered, too, 450 years after his birth, is Galileo's (likely apocryphal) rejoinder to the Inquisitors, " Yet still, it moves ." He was talking about the Earth, which today everyone knows as one of many planets, a place happily shaped more by Galileo than by any of his persecutors.

Follow Dan Vergano on Twitter .

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