Galileo Galilei

views updated

Galileo Galilei

Excerpt from The Starry Messenger (1610)

Reprinted in The Achievement of Galileo

Edited by James Brophy and Henry Paolucci
Published in 1962

Ascientific revolution occurred during the Renaissance through the influence of humanists who took a renewed interest in the work of ancient philosophers. Humanism was a movement initiated in Florence, Italy, in the mid-1300s by scholars who set out to revive the culture of ancient Greece and Rome (called the classical period). They hoped to start a cultural rebirth, or renaissance, that would end what they believed was the "barbarism" of the Middle Ages, the thousand-year period that began with the fall of the Roman Empire in the fourth and fifth centuries. Scientists were particularly interested in giving Greek texts updated translations and interpretations. They developed new theories that eventually replaced the Greek concepts that had dominated science for almost two thousand years. By the sixteenth century science had become a separate field from philosophy and technology, which had been the major areas of thought in ancient times. (Philosophy is devoted to a search for a general understanding of values and reality through speculative thinking. Technology is the application of practical knowledge, such as engineering.) An even more important development was that science now had a practical function. For instance, scientists were asking how things happened in nature, whereas the ancients were mainly concerned with why things happened. This shift in thinking had a profound impact on all aspects of life, and by the end of the 1600s science had replaced Christianity (a religion founded by Jesus of Nazareth, called the Christ) as the center of European civilization.

The most significant advances took place in the field of astronomy, the study of the number, size, and motions of heavenly bodies. At the beginning of the Renaissance astronomy was still linked with cosmology and astrology as it had been since ancient times. Cosmology is the study of the nature of the universe as an ordered structure, and it is closely allied with philosophy and theology (the study of religious faith, practice, and experience). Astrology is the "science" of the influences of heavenly bodies on earthly matters, including the lives and fortunes of humans. Some of the most famous Renaissance views of the universe, such as infinity (un-limited time and space), were developed not by astronomers but by philosophers and theologians (those who study religion). These ideas were then incorporated into astronomy. In fact, the scientific revolution began in astronomy, with the work of Polish astronomer Nicolaus Copernicus (1473–1543).

In 1543 Copernicus published De revolutionibus orbium coelestium (On the Revolution of the Heavenly Spheres), in which he gave proof that the Sun is the center of the universe. This idea was extremely controversial. At the time all astronomers accepted the theory of the ancient Egyptian scholar Ptolemy (Claudius Ptolemaeus; c. C. E. 100–c. 170), who stated that the Sun revolved around the Earth. Moreover, this view was enforced by the Roman Catholic Church, which found evidence for it in the Bible (the Christian holy book). Copernicus died only a few hours after De revolutionibus orbium coelestium was printed, however, and his work went virtually unnoticed for decades. Then in 1604 the Italian astronomer Galileo Galilei (called Galileo; 1564–1642) publicly declared that he supported Copernicus's theory of a Sun-centered universe.

A professor of physical sciences (study of the Earth and the universe) at the University of Padua in the Republic of Venice, Galileo had been attracting attention since he joined the faculty in 1592. At that time he gave three public lectures before overflow audiences in Venice. He argued that a new star, which had appeared earlier that year, was major evidence in support of Copernicus's views. More important was a letter in which he stated his theory of natural motion. By natural motion Galileo meant that a body will fall freely in space, and he proposed the law of free fall to account for this phenomenon. This concept contradicted the accepted view that the universe was a perfectly ordered and fixed system in which no body can freely move on its own.

In 1609 Galileo learned about the success of some Dutch spectacle, or eyeglass, makers in combining lenses (pieces of glass that reflect images) into what later came to be called the telescope. He feverishly set to work, and on August 25 he presented to the Venetian Senate a telescope as his own invention. The success was tremendous. He obtained a lifelong contract at the University of Padua, but he also stirred up resentment when it was learned that he was not the original inventor. Within a few months, however, Galileo had made sensational discoveries, which he published on March 12, 1610, under the title Sidereus nuncius (The Starry Messenger). The booklet took the world of science by storm.

Things to Remember While Reading an Excerpt from The Starry Messenger:

The telescope was a revolutionary scientific advancement. Prior to the development of telescopic lenses, astronomers had to rely on observations with the naked eye and on mathematical calculations when they studied stars and planets. Although some of their data were accurate, they could usually make only speculations because there was no way to have a close view of distant objects.
In The Starry Messenger Galileo gave astonishing evidence about mountains on the Earth's Moon and about moons circling Jupiter. He also identified a large number of stars, especially in the belt of the Milky Way, a galaxy (very large group of stars), of which the Earth's solar system is a part.
In the following excerpt from The Starry Messenger Galileo described how he made his telescope. He then detailed his observations of the surface of the Moon, which he found to be rough and uneven. This was an important discovery because astronomers at the time accepted the theory of the ancient Greek philosopher Aristotle (384–322 b.c.) that the surface of the Moon is perfectly smooth.

What happened next…

The view of the heavens changed drastically, and so did Galileo's life. In 1610 he accepted the position of mathematician in Florence, Italy, at the court of Duke Cosimo II de' Medici (1590–1621). Galileo's move to Florence turned out to be highly unwise. In the beginning everything went well. He made a triumphal visit to Rome in 1611, and the next year his Discourse on Bodies in Water was published. In this work he disclosed his discovery of the phases of the planet Venus, which proved the truth of the Copernican theory that celestial bodies travel around the Sun. Galileo's aim was to make a detailed description of the universe according to the theories of Copernicus and to develop a new form of physics (a science that deals with energy and matter and their interaction). A major obstacle was the traditional belief, stated in the Bible, that the Earth is the center of the universe. To deal with the difficulties raised by the Scripture (text of the Bible), Galileo addressed religious issues. He was assisted by church leaders, such as Monsignor Piero Dini and Father Benedetto Castelli, his best scientific pupil. In letters to Dini and Castelli, Galileo produced essays that now rank among the best writings of biblical analysis of those times. His longest letter was addressed to Grand Duchess Christina of Tuscany. In all of the letters he discarded the idea of an Earth-centered universe in favor of the theory that the Earth revolves around the Sun. As the letters were more widely circulated, a confrontation with church authorities became inevitable. In 1616 Cardinal Robert Bellarmine issued an order that forbade Galileo from teaching, printing, or defending the Copernican doctrine of the motion of the Earth.

"A Grand Revolution"

At the conclusion of The Starry Messenger Galileo summarized evidence that supported the Copernican theory of a universe with the Sun, not the Earth, at its center. In an amused tone he noted that some people were quite happy to accept the idea that planets revolve around the Sun. Yet they seemed to be offended by any notion that the Earth might move around the Sun along with the planets.

Galileo wrote:

Here we have a fine and elegant argument for quieting the doubts of those who, while accepting with tranquil mind the revolutions of the planets about the sun in the Copernican system, are mightily disturbed to have the moon alone revolve about the earth and accompany it in an annual rotation about the sun. Some have believed that this structure of the universe should be rejected as impossible. But now we have not just one planet rotating about another while both run through a great orbit around the sun; our own eyes show us four stars which wander around Jupiter as does the moon around the earth, while all together trace out a grand revolution about the sun in the space of twelve years.

Brophy, James, and Henry Paolucci, eds. The Achievement of Galileo, p. 28.

Galileo agreed not to promote Copernicus's views. Nevertheless, he was determined to have the order overturned. The next year Galileo had six audiences, or formal meetings, with Pope Urban VIII (1568–1644; reigned 1623–44). Urban promised a pension for Galileo's son, Vincenzio, but he did not grant Galileo permission to resume his work on a new description of the universe. Before departing for Florence, Galileo was informed that the pope had remarked that he did not believe the Roman Catholic Church would ever declare the Copernican theory to be heretical, but he was also certain the theory could never be proven. This news gave Galileo encouragement to go ahead with the great undertaking of his life, the Dialogue concerning the Two Chief World Systems.

Galileo's Dialogue was published in 1632. The book features four main topics discussed by three speakers in dialogue, or conversation, form on four consecutive days. The speakers are Simplicius, Salvati, and Sagredo. Simplicius represents Aristotle, Salviati is a spokesman for Galileo, and Sagredo plays the role of an arbiter (one who makes the final judgment on an issue), who leans heavily toward Galileo. The first day is devoted to the criticism of the alleged perfection of the universe, as claimed by Aristotle. Here Galileo made use of his discovery of the "imperfections" of the Moon, namely, its rugged surface as revealed by the telescope. The second day is a discussion of the rotation of the Earth on its axis (an imaginary line extending through the center of the Earth from north to south) as an explanation of various aspects of the universe. During the third day the orbital motion of the Earth around the Sun is debated. A main issue is the undisturbed nature of the surface of the Earth in spite of its double motion—that is, its revolving on an axis while at the same time orbiting around the Sun. The discussion on the fourth day shows that the tides (the rhythmical rising and falling of oceans and other bodies of water) are proof of the Earth's twofold motion. In this section Galileo seems to contradict the contention of the third day discussion, that the surface of the Earth remains undisturbed by its double motion. The tides, which cause the regular movement of oceans, show that the Earth's surface is in fact affected by the twofold motion.

Put on trial by church

Church officials were outraged by the Dialogue, which proved that Galileo supported Copernicus's ideas. Galileo was summoned to Rome to appear before the Inquisition, a church court set up to punish heretics (those who violate the laws of God and the church). The proceedings dragged on from the fall of 1632 to the summer of 1633. During that time Galileo was allowed to stay at the home of the Florentine ambassador in Rome. He was never subjected to physical force, but he had to inflict torture upon himself by publicly rejecting the doctrine that the Earth moved around the Sun. In 1633 Dialogue was placed on the Index of Prohibited Books by the Holy Office. The Holy Office was a branch of the church established as part of the Inquisition to review the content of all publications. The Index of Prohibited Books was a list of works the church considered to be heretical.

After the trial Galileo was placed under house arrest (not permitted to leave the premises) in Siena, a province in western Italy. The following December he was given permission to live in his own villa (country house) at Arcetri, a village near Florence. He was not supposed to have any visitors, but this order was not obeyed. The church was also unable to prevent the printing of Galileo's works outside Italy. Over the next five years translations of his writings were published in France and Holland. But the most important event in this connection took place in 1638, when Galileo's Two New Sciences was printed in Leiden, Holland. Like the Dialogue, this work is in the form of discussions over a period of four days. Topics include the mechanical resistance of materials, the atomic composition of matter, the nature of vacuum, the vibrations of pendulums, uniform and accelerated motion, and the projectile motion of a cannonball. Galileo spent his last years partially blind. He died in 1642.

Did you know…

Although Galileo proposed a radically new concept of the universe, he remained a Christian to the end of his life. He believed that the world was made by a rational creator (God) who gave order to everything according to weight, measure, and number. Galileo stated this faith in the closing pages of the first day of the Dialogue. He described the human mind as the most excellent product of the creator, because it could recognize mathematical truths.
In 1820 the condemnation of Copernicanism was revoked, or withdrawn, by the Holy Office. This action was taken on the basis of discoveries made by two little-known Italian astronomers. The first, Giovanni Battista Guglielmini, offered proof of the Earth's rotation by conducting experiments with bodies falling from a high tower. In Bologna, between 1789 and 1792, he made measurements and observed that the bodies moved slightly to the east, thus proving that the Earth moves in a twofold motion. The second astronomer, Giuseppe Calendrelli, measured the parallax (an apparent change in an object's position caused by a change in the position of the observer) of star Alpha in the constellation Lyra, also confirming such motion. He presented his findings to Pope Pius VII in a work published in 1806. During a debate over whether to approve an astronomy text in 1820, Benedetto Olivieri, head of the Holy Office, determined that the experiments of Guglielmini and Calendrelli demonstrated the Earth's twofold movement. As a result of Guglielmini's and Calendrelli's findings, Galileo's Dialogue was finally removed from the Index of Prohibited Books in 1835.
On November 10, 1979, Pope John Paul II (1920– ; elected 1978) acknowledged that Galileo had suffered a great deal from the church. The pope called for a frank reexamination of the famous trial. On July 3, 1981, he appointed a special commission to study and publish all available documents relating to the trial. The pope presented the results of the commission's work to the Pontifical (pope's) Academy of Sciences on October 31, 1992. The report made clear that astronomy was in a state of transition when Galileo was silenced in 1616 and when he was forced to withdraw his theory in 1633. Furthermore, scripture scholars were confused about cosmology. The report stated that Galileo had not demonstrated the Earth's motion, and theologians had been mistaken in their assessment of his teachings. But in the 1990s, the pope continued, science was so complex it was almost impossible to certify scientific discoveries as being absolutely true. The best one can hope for, he said, is that discoveries be "seriously and solidly grounded." He went on to state that the function of the Pontifical Academy was to advise the church if the degree of probability of a discovery is such "that it would be imprudent [unwise] or unreasonable to reject it." In other words, a scientific discovery should not be rejected if there is any possibility that it can be proven.