Galileo Galilei

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Portrait of Galileo Galilei by Giusto Sustermans.

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

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

Family and early career

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

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

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

Experimental science

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


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

It was on this page that Galileo first noted an observation of the moons of Jupiter. This observation upset the notion that all celestial bodies revolve around the Earth. Galileo published a full description in Sidereus Nuncius in March 1610.

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

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

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

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

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

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


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

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

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

Dome of the cathedral of Pisa with the "lamp of Galileo"

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

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

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

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

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


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

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


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

Statue outside the Uffizi, Florence

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

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

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

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

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

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

Accusations against Galileo of scientific errors and misconduct

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

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

Galileo Galilei

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

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

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

Controversy between Galileo and the Church

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

Did you know?
Galileo was accused of heresy in 1633 for his support of Nicolaus Copernicus' heliocentrism and it was not until 1992 that Pope John Paul II announced that the Catholic Church's denunciation of Galileo's work had been a tragic error

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

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

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

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

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

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

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

Named after Galileo

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

Galileo's writings

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

Writings on Galileo

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

ISBN links support NWE through referral fees

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

External links

All links retrieved April 17, 2024.


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