An Interview about Galileo Galilei: He Avoided Being Burned at the Stake!

Apr 3, 2021 by

Galileo Galilei

Some years ago, as Your Serene Highness well knows, I discovered in the heavens many things that had not been seen before our own age.”

—Galileo Galilei, 1615

Michael F. Shaughnessy

1) Galileo Galilei (often simply known as Galileo) is one of our most famous scientists. Where was he born, and where did he go to school? 

It is widely thought that Galileo Galilei was responsible for establishing modern science by seeking to understand the world around him and the heavens by directly observing them. Galileo Galilei was born into the home of Vincenzo Galilei and Giulia Ammannati on February 15, 1564, in the Tuscany region of Pisa, Italy. He died on January 8, 1642, in Florence, Italy. Galileo was a talented lutenist, as was his father. The two collaborated on a musical experiment in 1588 involving pitch and its relationship to string tension. Galileo was also a skillful painter and poet.

Galileo was a student at the monastery school at Vallombrosa in Florence until he enrolled at the University of Pisa. Initially, he was interested in studying medicine, but later became captivated by mathematics and pursued an occupation concentrating on mathematics and Aristotelian philosophy. Galileo did not have his father’s support or encouragement with this career path and ultimately did not complete or earn a baccalaureate degree. He departed from the university in 1585 and continued studying mathematics at the University of Pisa with Ostilio Ricci, a Tuscan Court mathematician. He then traveled to visit Christopher Clavius in Rome, a mathematician teaching at the Jesuit Collegio Romano. Later, Galileo corresponded with Guildobaldo del Monte, who shared his interest in mathematics and astronomy.

Galileo earned his living for the following twenty years by lecturing, tutoring mathematics, and designing mathematical instruments, such as the hydrostatic balance. He also wrote manuscripts on physics topics relating to experiments on the motions of objects.

His first attempt to acquire the chair position at the University of Pisa was not granted; however, his reputation and recognition among mathematicians were gaining momentum, which eventually led to obtaining the chair position one year later in 1589. His hypothesized falling bodies experiments produced his manuscript On Motion. Because his experiment speculated that a heavy falling object’s speed is not relative to its weight, he leaned toward an Archimedean approach to problems, abandoning his previous ties with Aristotelian philosophy. This change in ideology did not fare well with Galileo’s workfellows, thus resulting in an unrenewed contract. Not to worry; with the assistance of his supporters, among them Christopher Clavius and Guildobaldo del Monte, the University of Padua welcomed him as their next chair of mathematics from 1592 through 1610. It has been reported that Galileo wished to pursue another position at the Medici court because he “did not like the wine in the Venice area and he had to teach too many students.”

Another life-changing event for Galileo happened in Padua, namely meeting Marina Gamba, the mother of his three children, Virginia, Livia, and Vincenzo.

While a university professor’s salary would typically be sufficient to secure one’s future at that time in history, Galileo had the additional obligation of being the head of his family’s household due to his father’s death in 1591. Fortunately, Galileo became aware of a recently invented instrument in the Netherlands, called the spyglass, which allowed one to view distant objects as if they were near. After studying the instrument, Galileo improved upon the instrument’s design and even learned how to grind the lenses himself, making progressively more powerful spyglasses–known as telescopes today.

2) The telescope was one of his tools. How did he use it—and what improvements and observations did he make?

The telescoping device was invented independently sometime around 1608 by Johannes (Hans) Lippershey (also known as Lipperhey) and Zacharias Jansen. As soon as he learned of it, Galileo started working with the telescope in the summer of 1609 while living at his home in Tuscany. During this time, he had built his first telescope, which he refined to make its magnification greater. Galileo made his two lenses by hand. For one of his lenses, the objective lens, he shaped it in a convex structure. The other lens was called the eyepiece. Galileo shaped it in the form of a concave configuration. With the new modifications, it still provided a crude device, so Galileo built a second telescope. He placed two lenses, one convex and the other concave, both at the objective end of the tube-like device, made of lead. This scope was still somewhat crude even by the standards of his day, with only about an 8- or 9-fold increase in magnification. See Figure 5. Nevertheless, Galileo used the instrument to observe locations as far away as 21 miles, from San Marco to Santa Giustina, Padua.

Figure 5. Galileo telescope.

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Galileo produced another modification. Together the two new lenses increased the magnification view to about 20 times the standard viewing size. He set up his refined telescope in the backyard of his home, in the middle of his garden. He pointed the telescope, which he called the spyglass, to the night skies to observe the heavens. The term telescope was coined in 1611, and at that time, Galileo switched to the new term instead of using a spyglass.

Galileo was reported to be the first person in world history to aim the telescope to the sky. This newly modified viewing device changed the character of the planets, which now looked like small moon-like discs rather than prominent stars. His fifth telescope enhanced the magnification to about 33-times the standard viewing size. The new modifications permitted Galileo to use the device as a piece of scientific equipment for collecting data.

In 1609, Galileo was undoubtedly the first human being to point a telescope at Venus. To him, the image of Venus took the appearance of a disc without any distinguishing features to it. He did, however, notice that the planet had phases like those of the moon. See Figure 6. Galileo interpreted this observation to mean that the Earth orbited the sun. Galileo reasoned that Venus orbited the sun, too, rather than Venus or the sun orbiting the Earth, as was commonly thought in his day.

Figure 6. Moon phases as drawn by Galileo Galilei.

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In early 1610, Galileo pointed his refined device at the moon and discovered that it had mountains. He described the Milky Way as a structure filled with many stars. He even discovered the moons of Jupiter, having found four of them moving about the giant planet as independent bodies. Galileo’s findings were published in a book called “Starry Messenger,” also called The Sidereal Messenger,or Sidereus Nuncius, in March of 1610, dedicating and sending the new publication to his benefactor Cosimo II, the Grand Duke of Tuscany. See Figure 7. In July of 1610, the latter would appoint Galileo as the head mathematician, philosopher, and professor at the University of Pisa.

Figure 7. Cosimo II de’ Medici (1590–1621), Grand Duke of Tuscany

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By March of 1610, Galileo had fashioned over 100 telescopes, and only about a dozen of them could be used to see the moons of Jupiter effectively. He shared his homemade telescopes with heads of political states as gifts but made sure that his competitors did not receive them. Therefore, it would take about another 20 years before anyone else could verify the existence of Jupiter’s four moons discovered by Galileo.

Meanwhile, in July of 1610, Galileo focused his backyard telescope on Saturn. He saw it expand! He thought the planet had two other rather large planets sitting side-by-side along with Saturn. It looked like a triple-planet to him. Of course, we know in modern times that Galileo was observing the rings of Saturn.

3) He apparently “invented” the military compass—why is this important?

Figure 8. Galileo Galilei designed a geometrical and military compass.

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In 1597, Galileo began designing and constructing a new device, his first, called a “geometric and military compass.” See Figure 8. The new instrument was meant for conducting mathematical calculations quickly to solve problems in engineering and military situations. Interestingly, Galileo’s geometric and military compass was not a magnetic compass. In 1606, he published a description of his new gadget in a booklet titled Operations of the Geometric and Military Compass. The new device looked like a pair of metal rulers connected by a hinge, with an arched swivel, sort of like a fulcrum, that permitted Galileo to open it and adjust it to any desired angle, as shown in Figure 9.

Figure 9. Close-up detail from a geometrical and military compass designed by Galileo Galilei.

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By 1599, Galileo had made significant modifications to his military compass. Since his invention had numbers on his rulers, Galileo could perform calculations. The military compass could calculate the square roots precisely to place an army to an appropriate location on the battlefield. Another military application that his device could calculate was to assess the amount of charge to use for a given cannon on the battlefield.

Others who knew of the device soon adopted it for other purposes. For instance, shipbuilders found that they could use the instrument to calculate ship hulls’ shapes and build smaller-scale models of new ship designs for testing before being built to full sizes.

With demand for the invention ever-increasing, Galileo decided to hire a full-time military compass maker, whose family lived in the house with the Galilei family. The live-in device craftsman was paid with a salary, free housing, and a share of the profits from sales of the brass military compasses. Galileo did not make a significant profit under these terms since he also provided all the production materials for crafting the device. He did, however, make a more significant profit by teaching students how to use the device. Galileo also provided an instruction manual, written by his hand, to accompany the military compass. Three years later, he would have the instruction manual printed en masse and asked for a small fee to accompany the newly sold instruments.

4) His book Discourse on Floating Bodies helped establish hydrostatics—what exactly is hydrostatics—and why is this important?

During a royal dinner at the Florentine court, a chance incident embroiled Galileo in a public debate regarding ice and water. It would later have a dramatic effect on his life and career. The debate centered on what was known about hydrostatics. In Galileo’s time, the field of hydrostatics represented a phenomenon originally proposed inaccurately by Aristotle and later accurately refuted by Archimedes. Aristotle thought that ice was condensed water, which floated on top of a liquid water body because of the ice’s shape. He thought the ice shapes on the water prevented its ability to overcome the water resistance, and so the ice could not sink to the bottom. In contrast, Archimedes had purported that ice floated on top of liquid water because solid ice was lighter than liquid water. Rightly, Galileo agreed with Archimedes on this issue.

On the second evening of October in 1611, the Grand Duke Cosimo II of the Medici family held a royal dinner and had a special guest Cardinal Maffeo Barberini, who would in later years become Pope Urban VIII. As an after-dinner entertainment venue, the Grand Duke had arranged for Galileo to provide a scientific-based skit for his dinner guests. Galileo staged a debate in which the behavior of ice atop liquid water behaved according to the hydrostatic principles described by Archimedes (ice floated because it was lighter than liquid water) and which was sharply in contrast to the incorrect notions of Aristotle (ice was heavier than liquid water but its shape prevented sinking). Cardinal Barberini was invited to participate in the discussion that night and became an admirer of Galileo for many years afterward.

Shortly after the historic royal dinner, Galileo realized a rift between the philosophers regarding the Aristotelian versus the Archimedean views of ice and water. To settle the matter, he wrote his thoughts on the phenomenon in a book called Discourse on Floating Bodies to describe floating ice bodies’ actions on the surfaces of liquid water. While writing this Discourse, Galileo became ill and worked in seclusion while convalescing in bed. During this time, he received word from an artist friend of a disturbing nature. According to the rumor, specific envious individuals, described as evildoers by his artist friend, were adamantly opposed to his friendships with the duke and the Florentine archbishop. Furthermore, they opposed his views on floating ice bodies on water, and they had severe difficulties with his views on the motion of the Earth.

Galileo’s book was published during the spring season of 1612. Rather than quelling the rift, the book served to enhance the divide. In response to his Discourse on Floating Bodies, several books were published strongly opposing Galileo’s views on ice and water hydrostatics. While they were at it, the Aristotelians and the “evildoers” questioned Galileo’s notions regarding the Earth’s movement around the sun and his views regarding sunspots. Therefore, the debate regarding the nature of floating ice metamorphosed into a much more considerable controversy concerning the more severe transgression of heresy.

5) As I recall, Carl Sagan wrote a book on Comets—but was Galileo the very first to write on comets?

It is believed that the ancient Greek philosopher Aristotle was one of the first observers to write about comets in the 4th century B.C. He had inaccurately believed that comets were a part of the Earth’s high atmosphere, remnants of the atmosphere’s undefined disturbances. For numerous centuries after that, many agreed with this Aristotelian interpretation, including Galileo. Many in his day also believed that comets were evil omens and harbingers of disaster. The first genuinely accurate description of the comets arose out of the observations of a Danish astronomer named Tycho Brahe. In 1577 and 1585, Brahe observed two bright comets but without a telescope.

Brahe reasoned that these cometary visons had somehow crashed through the crystalline, celestial spheres that were postulated to make up the sky by Aristotle or that these glass ceilings did not exist. Brahe had chosen to believe that because of the famous nova of 1572, that the glass sphere was not immutable (i.e., not unchangeable) as Aristotle had proclaimed. Thus, Brahe reasoned that the comets were located beyond the planet Venus. Therefore, the crystalline spheres were nonexistent.

Regrettably, Galileo was chronically ill during the year 1618, when three bright comets made a glowing appearance in the skies of Italy. He was unable to point his telescopes at any of these historical objects because he was bed-ridden. However, other observers managed to use Galileo’s telescope designs to build and use them to make observations of these comets. While their observations appeared out of focus, Galileo’s contemporaries felt that the comets were located beyond the atmosphere of the Earth but closer than the moon. These interpretations were in line with Aristotle’s but were in stark disagreement with Tycho Brahe.

Another of Galileo’s contemporaries, however, a Jesuit-trained astronomer by the name of Orazio Grassi, made a convincing argument that because of the movement of a bright comet in November of 1618, its location was beyond the moon, closer to the sun.

Soon, a rift formed between the Aristotelian school of thought and those of Brahe and Grassi. Galileo was called upon to voice his opinion on the matter and weigh-in in writing. He wrote a book incognito called Discourse on the Comets under the name of his student Mario Guiducci instead of Galileo’s name. In the book, Galileo took the wrong side and voiced the opinion that comets were merely the manifestation of the sun’s reflections off the Earth’s vapors of the high atmosphere, consistent with the inaccurate hypotheses of Aristotle. Galileo specifically criticized Brahe and Grassi. By the time Galileo’s book was published, Brahe was already dead, but a much alive Father Grassi was incensed. He immediately replied with a book of own, though with the Latin nom de plume Lothario Sarsi. Grassi’s book was titled Libra Astronomica, and in it, he took Galileo to the task. Galileo replied with another book, called The Assayer, which was a direct attack on Grassi’s argument that Galileo’s version of the comet would be weightless on a balance used by assayers of gold ore. Grassi got personal and replied to Galileo’s The Assayer by publishing a retort called Winetaster, implying that Galileo was drunk when he wrote his book!

The much public debate between Grassi and Galileo grew even more personal. Both authors took to personal insults and mockery. The comet debate also grew dangerous.

Soon after their debate commenced, an edict was declared that the writings in the books of the time had better be careful with the treatment of Nicolaus Copernicus, who had postulated that the Earth revolved around the sun, a notion referred to as the heliocentric universe. The Copernican notion of a sun-centered universe went directly against the widely accepted idea of an Earth-centered universe in which all the heavenly bodies revolved around the Earth—the Earth was supposed to be the center of the universe, with the night sky consisting of the crystalline sphere and its pinpoints of starlight, comets included.

In 1616, Galileo had already been warned, if not reprimanded, by a cardinal and the Pope to steer clear of studying the motions of the heavenly bodies, and his comet debate had become dangerously close to this touchy issue. However, in his new friendship with Pope Urban VIII, Galileo felt at liberty to address the comet debate, the civil nature of which seemed to get away from him, his colleagues, and his detractors.

Today we know that comets are beyond the sun and the moon and consist of enormous masses of ice water, with traces of methane and ammonia, and they are known to orbit the sun in parabolic or elliptical shapes.

6) His Dialogue Concerning the Two Chief World Systems was a book that got Galileo summoned to Rome—and got him imprisoned—but at least he was not burned at the stake. What was going on in the Church at that time?

Galileo’s purported path to the burning stake was thwarted, but its course had its origins in antiquity. Galileo started on his perilous pathway to the stake when he wrote in his 1632 book Dialogue Concerning the Two Chief World Systems that the Earth and the heavenly bodies, like the planets and the stars, followed a so-called Copernicus model as they moved about in the sky. According to Nicolaus Copernicus, who was a priest, the planets moved in orbits around the sun. In 1543, Copernicus, who was fearful of being burned at the stake for such heresy, had anonymously published the shocking idea.

Beginning with Aristotle, it was widely believed that the Earth was stationary and all the objects in the sky, like the moon, the sun, and the five known planets, orbited the fixed Earth as the middle of the known universe. During the second century A.D., the Greek astronomer Claudius Ptolemy built upon Aristotle’s view and wrote that the stationary Earth was surrounded by a series of perfectly rounded spheres that moved the heavenly bodies around the center of the universe—the Earth. This ancient notion of an Earth-centered universe continued into the era of Copernicus and Galileo. The ancient Aristotelian and Ptolemaic views of the Earth, the known planets, and the moon were accepted by the Church as official doctrine.

Using his telescope in 1609, Galileo discovered Jupiter’s moons. He further observed that its moons orbited Jupiter, implying that not everything heavenly revolved around the Earth. Galileo discovered that Jupiter’s moons did not orbit Earth—they orbited Jupiter!

At about this time, a German cleric turned astronomer, Johannes Kepler, built upon Copernicus and published two principles of planetary motion, one of which stated that the planets moved about in elliptical orbits (sort of like ovals), rather than in the Ptolemaic scheme of the perfect circular orbits.

In 1619, Kepler continued with his ideas and published an elaborate mathematically driven third law of planetary orbits. The main implication from Kepler’s Laws was that the planets mathematically behaved as if they orbited the sun rather than the Earth.

Galileo finished writing his book towards the end of 1629, a manuscript that his daughter Suor Maria Celeste had helped him complete. He began the arduous process of having it accepted by Church officials for licensing and printing. In the spring of 1630, Galileo, with his family in tow, journeyed to Rome, where he spent two months meeting with officials to have his book approved for publication. In June, Galileo received authorization for publication with minor revisions, including a change in its title. Soon after, an outbreak of the deadly plague occurred, delaying publication. In the interim, powerful Church officials, like the Dominican Father Niccolo Riccardi, who oversaw the final book printing, requested the manuscript to read it one more time! After some negotiations, Father Riccardi agreed to let an underling of his choosing read it instead of going through the trouble of mailing the manuscript during plague times. Thus, Father Giacinto Stefani read it and quickly approved it. Yet, Father Riccardi delayed final approval for printing the book, even though the Pope’s officials, the Pope himself, and Stefani had already given their consent. Father Riccardi read through it and required that the manuscript undergo revisions, and Galileo dutifully, though grudgingly, made the necessary alterations. With final approval for printing authorized, the book was released in February of 1632.

In Galileo’s Dialogue Concerning the Two Chief World Systems, the two systems of the world he had dealt with concerned the views of Copernicus and Ptolemy. Copernicus had stated that the planets revolved around the sun, and Ptolemy postulated that the planets did so in an oval-shaped pattern. Soon after the publication of Galileo’s book, it received a great deal of attention from two camps—one in favor of the Earth-centered universe in which the heavenly bodies had orbited and which was the view the Church had adopted, and the other view favoring the sun as the center of the planetary orbits. A variety of responses to Galileo’s book had emerged, ranging from praise to grave concerns, all of which got the attention of the Pope, who then prohibited the further sales of Galileo’s book. Meanwhile, the Pope appointed a special papal commission to investigate the issue. Somehow, Pope Urban had been convinced that Galileo had deceived him, having learned that Galileo in 1615 had been officially warned and had agreed not to discuss Copernicus. However, in 1624, Galileo had specifically told Pope Urban about new work concerning Copernican theory. In 1630, Pope Urban was soundly upset about the perceived deceit on Galileo’s part. Galileo was, thus, summoned to Rome and report to the Inquisition to stand trial for heresy!

At first, Galileo tried to convince the authorities that he was too frail of health to travel and report to them. A panel of physicians examined a bed-ridden Galileo and sent a report describing a lengthy list of ailments to the Inquisition authorities, who soundly rejected it. They replied that Galileo must nevertheless report to them in Rome of his own free will, or he would be dragged to Rome in chains!

In February of 1633, Galileo arrived in the Holy City of Rome for trial by the Holy Office of the Inquisition, which officially began in April. It is said that Pope Urban could have had Galileo arrested while awaiting tribunal, the pontiff instead permitted Galileo to be a house guest, though in seclusion, at the Tuscan embassy, a place Galileo had stayed on prior visits with the Vatican.

Official transcripts of Galileo’s Inquisition have survived. In these historical documents concerning the trial, Galileo was asked whether he recalled having been warned by Cardinal Bellarmine not to write about the motions of the Earth. Galileo replied that he indeed had been so warned but that he had interpreted the Cardinal’s warnings to be only that he (Galileo) speak of Earth’s motion as purely hypothetical, not factually. Galileo went on to argue that in his Dialogue Concerning the Two Chief World Systems, he never specifically stated that the Earth moved, only that the arguments in favor of its motion were inconclusive, as advised by Cardinal Bellarmine. See Figure 10.

Figure 10. Galileo Galilei was facing the Roman Inquisition.

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At an impasse, out-of-court plea bargaining negotiations were undertaken. Instead of the most or the least serious of charges, Galileo could opt for pleading guilty to an intermediate charge. Meanwhile, Galileo reviewed his book and realized that the evidence was not entirely in his favor. In the book, Galileo had most certainly given a convincing and quite logical impression that the Earth was doing the moving about around the sun and, thus, not as the Church was given to believe. He testified, thus, that he was now sorry about having given this offending impression, and the prosecutors took it as a confession, at which point they proceed to deliver a conviction. The only remaining issue was whether Galileo had had malicious intent when he provided strong evidence in his book about the motion of the Earth.

The authorities, as was the standard practice and as the court records indicate, had decided to interrogate Galileo under a so-called verbal threat of being tortured. According to the transcript documents, Galileo was given the verbal threat of torture. He replied that he would be willing to undergo the torture rather than admit that he had malicious intent. In the end, Galileo was not tortured, but he was convicted of “vehement suspicion of heresy” for writing about the offending belief that the Earth moved around the sun. He was forced to recant his transgressions verbally, and his book was banned. He was also sentenced to life in prison, a sentence that was immediately reduced to house arrest for the remainder of his life. Thus, Galileo was spared from being burned alive at the stake, unlike the Dominican priest, Giordano Bruno, who was burned alive for having committed essentially the same offense in 1600.

7) The Discourse on Two New Sciences—was a book that discussed projectiles and falling bodies, and I guess, indirectly gravity. How was this book received?

Galileo wrote his final and most famous book of all, his Discourse on Two New Sciences, soon after having undergone trial for his life before the Holy Office of the Inquisition by Church authorities in 1633. During the tribunal, he agreed to the lesser confession of intermediate heresy, and he began to serve out his life-sentence of house arrest for his crime.

At first, he spent a week in seclusion at the spacious Villa of the Medici, owned by his friend the Grand Duke of Tuscany, Ferdinando II. Then Galileo spent five months as a guest of his friend, the Archbishop of Siena. He had been spared being burned at the stake, and he consoled himself with a new project: another book! It would be his last one. At the archbishop’s home, he wrote the first part of the book. In December of 1633, Galileo would finally arrive at his villa in Arcetri, Italy, to finish out his life sentence, to complete his book, and to die.

Some historians argue that had Galileo not undergone the life-threatening Inquisition at the hands of the Church authorities regarding the movement theories of the heavenly bodies, he likely never would have written his most famous book, yet, the so-called Discourse on Two New Sciences. There are several translations of his original 1638 book, each with differences in their titles. One title closest to his original is Discourses and Mathematical Demonstrations Relating to Two New Sciences, and another version is called Dialogues Concerning Two New Sciences. Many historians of science denote Galileo’s historically famous book simply as Two New Sciences.

In Galileo’s last and most critically acclaimed book, the Two New Sciences, he adhered to the same format as he had with the previous book, his Dialogue, the one that had gotten him into and out of trouble with the Inquisition. Both of Galileo’s books took the form of spirited conversations between three thinly disguised fictional characters named Simplicio, Sagredo, and Salviati. Each of the characters has particular importance, as well as an origin.

The name Simplicio was purported to relate to a well-known philosopher, Simplicius of Cilicia (born in 480 B.C. and died 560 B.C.), who was an ardent follower of the Aristotelian theory of planetary motions. However, Galileo loosely based the character Simplicio on two of his contemporaries, both of whom were his opponents on the issue of Copernicus theory.

One of these adversaries was an outspoken detractor named Lodovico Delle Colombe, a well-respected member of the Florentine Academy and self-proclaimed “anti-Galileo” critic. Galileo’s proponents insulted Colombe and his so-called flawed reasoning by referring to him as a member of the “pigeon league,” which was a take on Colombe’s meaning “dove” in Italian. Another apparent adversary of Galileo was thought to be Cesare Cremonini, of the University of Padua, who had taken to debate Galileo concerning his ideas publicly. It did not escape anyone that the term Simplicio used by Galileo was dangerously close to the Italian term “sempliciotto,” meaning simpleton!

The character Sagredo was based on a student colleague and close friend of Galileo named Giovanfrancesco Sagredo, who was born in 1571 and died in Venice in 1620. In both books, the character Sagredo was portrayed as an intelligent and wealthy colleague who frequently sided with the third personality, Salviati, named after a real-life and close friend Filippo Salviati, who was born in 1582 and died in 1614. However, the books’ character of Salviati was meant to denote Galileo himself. The real Filippo Salviati had been a gracious host of Galileo numerous times and had provided the opportunity for him to convalesce from his illnesses and to write. Since both of Galileo’s real-life friends were already dead in 1633 and were terribly missed, he resolved to bring them back to life in his books, and in doing so, he immortalized them famously.

In Galileo’s Dialogue, the three characters spent four days in lively debates regarding the organization of the moving bodies in the heavens and the flow of water in the sea. In his Two New Sciences, the same protagonists underwent another series of conversations, which lasted another four days, discussing physics, mechanics, the strength of materials, and the motions of particular objects. The trio, Simplicio, Sagredo, and Salviati, would convene at Sagredo’s palace but this time, however, their discourse was somewhat less lively, less boisterous, and more muted and polite in their tone, considering what had just happened to their author, Galileo, who nearly lost his life, the reader can understand his caution this time around. Nevertheless, the trio delved carefully into conversations about the weight of air or the speed of light. They specifically conversed on the strength of materials, the nature of falling objects, the motions of objects as they fell to the ground, and on the motions of projectiles, such as cannonballs. In so doing, the speakers Simplicio, Sagredo, and Salviati consider at length, especially in mathematical terms, the motion of bodies and their constant acceleration as they fell along an incline. Here, Galileo made the insightful discovery that various falling bodies had a constant acceleration because of the Earth’s gravitational pull rather than because of the relative weight or mass of the falling objects. Galileo’s idea contrasted with that of Aristotle, who proclaimed that objects fell at different rates due to their relative mass or weight, failing to consider the role of air friction.

In the Two New Sciences, the discussants paid attention to bodies in motion. They reconsidered the problem, a dangerous discourse, of the Earth and its movement. Aristotle had argued the Earth did not move as people could not detect its motion. Galileo refuted this notion by arguing that a boat on the water in which its sailors might drop an object like a rock from a specific point, such as the top of its mainmast. He reasoned that the rock would fall on the same spot on the ship, the mainmast base, whether the ship was stationary or moving.

Galileo’s Two New Sciences was published in July of 1638 by arranging to have the finished manuscript printed in Leiden, Holland, where it was considered safe from the Catholic Inquisition. Galileo feigned no knowledge of its printing in a Protestant country to circumvent another call to Rome and a second Inquisition. The book quickly became a best-seller.

In modern times, David R. Scott, astronaut and commander of the Apollo 15 mission to the moon, conducted a famous experiment on the lunar surface. With much of the world watching live in 1970, Scott simultaneously dropped a hammer and a feather from his gloved hands, letting them fall to the moon’s surface. The two objects dropped at the same rate and landed at the same time. Hence, commander Scott proclaimed that Galileo had been correct regarding the nature of the constant rate of acceleration of falling objects, as measured in the vacuum of space.

8) Ironically, in October 1989, Galileo, a spacecraft—left the space shuttle Atlantis, and this probe reached Jupiter—and its four moons—Galileo had seen these four moons 385 years before using his telescope!

Indeed, Galileo discovered these four moons of Jupiter. On the 7th of January in 1610, Galileo used his telescope in his backyard garden. He saw three “bright stars” adjacent in a line around Jupiter, thinking he had found “Medicean” planets. One of these “planet-star” types of objects was located on one side of Jupiter, while the other two were found on the other side of the giant planet. The next night, however, Galileo saw that the three planet-stars were on one side, on the west side, of Jupiter. On the 10th night of January, Galileo observed that one of the planet-stars disappeared altogether, and the remaining two were now on the other side (eastside!) of the planet Jupiter. The next night Galileo saw that the two moons were still on the east side of Jupiter, but larger! He continued this frenzy of observations of Jupiter’s “attending bodies,” making over 60 separate observatory experiments in all. He concluded that these four objects which moved about Jupiter were not stars, nor even planets, but rather moons that orbited the planet Jupiter.

When Galileo announced these findings, he immediately became famous worldwide. He had skeptics, however, who questioned Galileo’s veracity. They argued that, according to Aristotle, the heavens were supposed to be unchangeable, and Galileo was proclaiming that elements of the heavens were moving about!

Interestingly, he refused to provide a telescope to another famous scientist and astronomer named Johannes Kepler, whose mathematical studies accurately describe the moons of Jupiter that Galileo had discovered with the telescope. Kepler’s descriptions were known then as the First Law or the Law of Orbits. In 1609 Kepler devised a set of mathematical equations to illustrate the orbits of the known planets, which he described as elliptical, like ovals, rather than circular. Galileo chose to answer Kepler’s questions by writing letters to him only in anagrams so that Kepler would be unable to decipher the answers. After that, Galileo refused to correspond with Kepler. Galileo even ignored all references to Kepler’s books when writing up his works for publication.

Galileo called the four moons of Jupiter, I, II, III, and IV, collectively called the Jovian moons. Today we know that the four moons that Galileo first discovered, which are the largest, are individually called Io, Europa, Ganymede, and Callisto.

In 1995, the Galileo spacecraft arrived at Jupiter, having been launched from Earth by NASA onboard the Space Shuttle Atlantis in 1989. The Galileo spaceship was an orbiting vehicle and entry probe. After conducting Venus and Earth’s flybys to arrive at Jupiter, Galileo became the first human-made device to orbit the giant planet.

The Galileo contained an impressive array of instruments, including a high-resolution imager device, an infrared spectrophotometer used for mapping, a wide-range ultra-violet spectrophotometer, and a radiation detector called a photopolarimeter-radiometer capable of measuring solar and thermal radiation. Galileo also harbored a particle detector to measure the speed, mass, and electrical charges of dust particles. The spacecraft had an energy measuring particle detector capable of analyzing ion amounts and energy levels of ionic charges in space. The heavy-ion detector could measure emissions from carbon or even nickel. Galileo had a magnetometer designed to detect incoming components of electromagnetic fields around Jupiter and its large moons. The spaceship had two plasma wave detection instruments for measuring levels of energy, mass and a unit for plasma electric field detection.

The Galileo entry probe device had another array of instruments for measuring the atmospheric components of Jupiter itself. On December 7 of 1995, the probe entered Jupiter’s atmosphere. It had been the first human-made object ever to do so. It took measurements for about an hour before being destroyed by the immense gravity of Jupiter.

Galileo conducted close-up studies of Jupiter’s moons, and many of these projects were published in modern scientific literature. Though Galileo Galilei discovered four moons of Jupiter, we know that as of this writing, the giant planet has 79 moons in orbit around it.

Of the many findings by the modern spacecraft, fundamental discoveries include the fact that the moon Europa has a surface of solid ice and an underground ocean composed of water! This discovery is striking as it supports the notion that living microbes might be found in Europa’s global under-ice sea of liquid water! Just as strikingly, moons Ganymede and Callisto are believed to harbor a layer of liquid saltwater! Thus, the Jovian (Galilean) moons may be harbingers of alien life within our solar system. The Galileo spaceship discovered that the moon Io has geologically active volcanoes! Ganymede has, like Earth, a protective electromagnetic field. Thus, should any life be found there, such as microbial life, these aliens would be theoretically protected from the sun’s damaging rays and other potentially dangerous outer space emissions.

Figure 11. Galileo launch on Soyuz on October 21, 2011.

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On the 21st day of October in 2011, the Arianespace launcher lifted off into space aboard the Soyuz rocket carrying a payload called the Galileo In-Orbit Validation Elements 1 and 2 (Giove-1 and -2). See Figure 11. These spacecraft are meant to provide satellite positioning and navigation systems for European nations. A total of 18 such space-faring craft are slated to be launched and deployed to make the entire Galileo constellation come online.

9) Galileo was also vindicated by the Vatican 350 years after his death. What happened, and how did he die?

Attempting to rehabilitate Galileo in 1979, Pope John Paul II requested a reexamination of Galileo’s Inquisition trial, asking that a panel of astronomers, theologians, scholarly experts, and historians consider all the available evidence accumulated since 1633 when Galileo was convicted of heresy and sentenced to life in prison. Galileo was even made to deliver a humiliating denouncement of his scientific findings going against the official teachings of the Church. He served his sentence as a heretic under house arrest and died on the evening of January 8, 1642. He was quietly buried on the Santa Croce parish grounds in Florence in an unmarked grave behind the sacristy under its bell tower.

In 1982, Pope John Paul II formally convened a special blue-ribbon panel called the Galileo Commission to form smaller study groups and reinvestigate the Galileo affair’s original case. Ten years later, the Commission concluded its charge, and Pope John Paul II admitted that not only had the 1633 Inquisition been a mistake, but that Galileo had been theologically accurate in his biblical interpretations. Galileo had been, after all, a devoted Catholic for the entirety of his life, even though he had dutifully undergone the rigors of the Inquisition. However, Pope John Paul II equivocated about the issue, reasoning that Galileo’s wish to simply convey novel ideas regarding the Earth’s motion relative to the heavenly bodies was just as reasonable as his adversaries’ inclination to disagree with him. In any case, Pope John Paul II declared that Galileo’s case and his ultimate vindication would serve as an excellent illustration of the harmony between theology and science.

10) Scientifically, what were his most significant contributions?

Galileo Galilei has been widely regarded for having established the foundations of modern science. He was known to have invoked the so-called scientific method of making conclusions about the natural worlds through direct observation. Most historians of science and biographers would agree that Galileo’s most famous scientific discovery deals with the Earth. He demonstrated through careful observation of the heavenly skies that our home planet moves around the sun, contrary to what was believed for more than a thousand years. Galileo’s evidence that the Earth orbits the sun shook the very foundations of academia and Church law.

In his time, he was, at a relatively young age, already famous. According to one legend, Galileo was a medical student attending mass one day in 1583 at the Cathedral of Pisa and noticed a swinging lamp or chandelier oscillated in synchrony with the cords’ lengths that held the lights. The longer the cords, the less frequent the oscillations, and the shorter the cords, the more frequent the oscillation. He studied ways to invent a prototype for pendulum-based clocks based on the kinetic energies of the pendulums. While he figured out some of the physics behind pendulums’ kinetic work, the applied efforts to building clocks remained unfinished. Others like Christiaan Huygens patented the pendulum system based on the foundation established by Galileo.

An important discovery attributed to Galileo has to do with the law of inertia. He is thought to have developed the physics-based concept that an object at rest or in motion will stay as such until some external force is applied to the said object. Isaac Newton would later build upon the inertia concept to formulate his laws dealing with the movements of objects. These concepts are taught in physics courses all over the world in modern times.

The military and geometric compass device invented by Galileo permitted one to make calculations rapidly. The device has two rulers connected by a curved ruler, which permitted the user to calculate the angle settings for firing a cannon or computing monetary exchange rates. See Figure 12.

Figure 12. Close-up detail from a geometrical and military compass designed by Galileo Galilei.

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As mentioned above, Galileo refined the magnification capabilities of the telescope with sufficient resolution to observe the night skies in his backyard. He observed the moon closely, discovering that it had mountainous regions. He studied the known planets, discovering that Jupiter had moons of its own.

He would deduce that the Earth and the planets of the solar system moved about the sun. With his advanced telescope, he characterized the phases of Venues and observed how they were like our own moon. Galileo studied the sunspots. Though he did not manage to figure out what they were, he concluded that the sun rotated because the sunspots moved. He discovered that the Milky Way was comprised of an untold number of stars. Modern images of the center of our galaxy by spacecraft-based telescopes, like the Hubble, provide definitive evidence that Galileo was correct about the stellar make-up of our galaxy’s center.

Interestingly, one farfetched legend has it that Galileo once climbed the Leaning Tower of Pisa to conduct a famous experiment. Historians of science are doubtful this incident ever happened with Galileo. The story has its apparent origins with Galileo’s biographical writings by a devoted student, Vincenzo Viviani, who famously wrote about Galileo’s gravitational acceleration concept. In the legend, Galileo dropped objects from the famous Leaning Tower and found that objects of different weights fell to the Earth at an equivalent rate. Though the famous incident involving Galileo is very likely untrue, it is known that a version of the same sort of experiment was performed on the surface of the moon in the early 1970s with astronaut Dave Scott. It is also known that scientists have “reenacted” Galileo’s thought experiment at the Leaning Tower of Pisa in more recent times.

Figure 13. A 2009 Commemorative stamp of Galileo Galilei.

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In his lifetime, Galileo was celebrated and famous. The accolades continue even to this day. Notable among the honors given to Galileo is, for example, his portrait on the cover of a postage stamp, commemorated in 2009, as seen in Figure 13. Undoubtedly, Galileo Galilei’s contributions will continue to permit new generations of scientists to build upon the foundation of science, astronomy, mathematics, physics, and engineering that he helped to establish.

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