Galileo (born February 15, 1564, Pisa [Italy]—died January 8, 1642, Arcetri,
near Florence) Italian natural philosopher, astronomer, and mathematician
who made fundamental contributions to the sciences of motion, astronomy,
and strength of materials and to the development of the scientific method.
His formulation of (circular) inertia, the law of falling bodies,
and parabolic trajectories marked the beginning of a fundamental change in
the study of motion. His insistence that the book of nature was written in
the language of mathematics changed natural philosophy from a verbal,
qualitative account to a mathematical one in which experimentation became
a recognized method for discovering the facts of nature. Finally, his
discoveries with the telescope revolutionized astronomy and paved the way
for the acceptance of the Copernican heliocentric system, but
his advocacy of that system eventually resulted in an Inquisition process
against him.
Early life and career
Galileo was born in Pisa, Tuscany, on February 15, 1564, the oldest son
of Vincenzo Galilei, a musician who made important contributions to the
theory and practice of music and who may have performed some
experiments with Galileo in 1588–89 on the relationship between pitch and
the tension of strings. The family moved to Florence in the early 1570s,
where the Galilei family had lived for generations. In his middle teens
Galileo attended the monastery school at Vallombrosa, near Florence, and
then in 1581 matriculated at the University of Pisa, where he was to
study medicine. However, he became enamoured with mathematics and
decided to make the mathematical subjects and philosophy his profession,
against the protests of his father. Galileo then began to prepare himself to
teach Aristotelian philosophy and mathematics, and several of his lectures
have survived. In 1585 Galileo left the university without having obtained a
degree, and for several years he gave private lessons in the mathematical
subjects in Florence and Siena. During this period he designed a new form
of hydrostatic balance for weighing small quantities and wrote a
short treatise, La bilancetta (“The Little Balance”), that circulated in
manuscript form. He also began his studies on motion, which he pursued
steadily for the next two decades.
In 1588 Galileo applied for the chair of mathematics at the University of
Bologna but was unsuccessful. His reputation was, however, increasing, and
later that year he was asked to deliver two lectures to the Florentine
Academy, a prestigious literary group, on the arrangement of the world
in Dante’s Inferno. He also found some ingenious theorems on centres of
gravity (again, circulated in manuscript) that brought him recognition
among mathematicians and the patronage of Guidobaldo del Monte (1545–
1607), a nobleman and author of several important works on mechanics. As
a result, he obtained the chair of mathematics at the University of Pisa in
1589. There, according to his first biographer, Vincenzo Viviani (1622–
1703), Galileo demonstrated, by dropping bodies of different weights from
the top of the famous Leaning Tower, that the speed of fall of a heavy object
is not proportional to its weight, as Aristotle had claimed. The manuscript
tract De motu (On Motion), finished during this period, shows that Galileo
was abandoning Aristotelian notions about motion and was instead taking
an Archimedean approach to the problem. But his attacks on Aristotle made
him unpopular with his colleagues, and in 1592 his contract was not
renewed. His patrons, however, secured him the chair of mathematics at
the University of Padua, where he taught from 1592 until 1610.
Although Galileo’s salary was considerably higher there, his responsibilities
as the head of the family (his father had died in 1591) meant that he was
chronically pressed for money. His university salary could not cover all his
expenses, and he therefore took in well-to-do boarding students whom he
tutored privately in such subjects as fortification. He also sold a
proportional compass, or sector, of his own devising, made by an artisan
whom he employed in his house. Perhaps because of these financial
problems, he did not marry, but he did have an arrangement with a
Venetian woman, Marina Gamba, who bore him two daughters and a son. In
the midst of his busy life he continued his research on motion, and by 1609
he had determined that the distance fallen by a body is proportional to the
square of the elapsed time (the law of falling bodies) and that the trajectory
of a projectile is a parabola, both conclusions that contradicted
Aristotelian physics.
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Telescopic discoveries of Galileo
Galileo's telescopes
Two of Galileo's first telescopes; in the Museo Galileo, Florence.
Galileo's illustrations of the Moon
Galileo's sepia wash studies of the Moon, 1609; in the Biblioteca Nazionale, Florence.
(more)
Galileo Galilei: Copernican system
Illustration of the Copernican system of the universe from Galileo's Dialogo sopra i due
massimi sistemi del mondo, tolemaico e copernicano (Dialogue Concerning the Two
Chief World Systems, Ptolemaic & Copernican), 1632.(more)
At this point, however, Galileo’s career took a dramatic turn. In the spring
of 1609 he heard that in the Netherlands an instrument had been invented
that showed distant things as though they were nearby. By trial and error,
he quickly figured out the secret of the invention and made his own three-
powered spyglass from lenses for sale in spectacle makers’ shops. Others
had done the same; what set Galileo apart was that he quickly figured out
how to improve the instrument, taught himself the art of lens grinding, and
produced increasingly powerful telescopes. In August of that year he
presented an eight-powered instrument to the Venetian Senate (Padua was
in the Venetian Republic). He was rewarded with life tenure and a doubling
of his salary. Galileo was now one of the highest-paid professors at the
university. In the fall of 1609 Galileo began observing the heavens with
instruments that magnified up to 20 times. In December he drew
the Moon’s phases as seen through the telescope, showing that
the Moon’s surface is not smooth, as had been thought, but is rough and
uneven. In January 1610 he discovered four moons revolving around Jupiter.
He also found that the telescope showed many more stars than are visible
with the naked eye. These discoveries were earthshaking, and Galileo
quickly produced a little book, Sidereus Nuncius (The Sidereal Messenger),
in which he described them. He dedicated the book to Cosimo II de
Medici (1590–1621), the grand duke of his native Tuscany, whom he had
tutored in mathematics for several summers, and he named the moons of
Jupiter after the Medici family: the Sidera Medicea, or “Medicean Stars.”
Galileo was rewarded with an appointment as mathematician and
philosopher of the grand duke of Tuscany, and in the fall of 1610 he
returned in triumph to his native land.
Examine Aristotle's model of the solar system and note its failure to explain
phenomena like retrograde motion
Aristotle's theory of the solar system.
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Galileo was now a courtier and lived the life of a gentleman. Before he left
Padua he had discovered the puzzling appearance of Saturn, later to be
shown as caused by a ring surrounding it, and in Florence he discovered
that Venus goes through phases just as the Moon does. Although these
discoveries did not prove that Earth is a planet orbiting the Sun, they
undermined Aristotelian cosmology: the absolute difference between the
corrupt earthly region and the perfect and unchanging heavens was proved
wrong by the mountainous surface of the Moon, the moons of Jupiter
showed that there had to be more than one centre of motion in the universe,
and the phases of Venus showed that it (and, by implication, Mercury)
revolves around the Sun. As a result, Galileo was confirmed in his belief,
which he had probably held for decades but which had not been central to
his studies, that the Sun is the centre of the universe and that Earth is a
planet, as Copernicus had argued. Galileo’s conversion to Copernicanism
would be a key turning point in the Scientific Revolution.
Galileo Galilei: sunspots
Illustration from Galileo's Istoria e dimostrazioni intorno alle macchie solari e loro
accidenti (“History and Demonstrations Concerning Sunspots and Their Properties,” or
“Letters on Sunspots”), 1613.(more)
German mathematician Christoph Scheiner.
Christoph Scheiner observing sunspots, c. 1620.
After a brief controversy about floating bodies, Galileo again turned his
attention to the heavens and entered a debate with Christoph
Scheiner (1573–1650), a German Jesuit and professor of mathematics
at Ingolstadt, about the nature of sunspots (of which Galileo was an
independent discoverer). This controversy resulted in Galileo’s Istoria e
dimostrazioni intorno alle macchie solari e loro accidenti (“History and
Demonstrations Concerning Sunspots and Their Properties,” or “Letters on
Sunspots”), which appeared in 1613. Against Scheiner, who, in an effort to
save the perfection of the Sun, argued that sunspots are satellites of the
Sun, Galileo argued that the spots are on or near the Sun’s surface, and
he bolstered his argument with a series of detailed engravings of his
observations.
Galileo’s Copernicanism
Galileo’s increasingly overt Copernicanism began to cause trouble for him.
In 1613 he wrote a letter to his student Benedetto Castelli (1577–1644)
in Pisa about the problem of squaring the Copernican theory with certain
biblical passages. Inaccurate copies of this letter were sent by Galileo’s
enemies to the Inquisition in Rome, and he had to retrieve the letter and
send an accurate copy. Several Dominican fathers in Florence lodged
complaints against Galileo in Rome, and Galileo went to Rome to defend the
Copernican cause and his good name. Before leaving, he finished an
expanded version of the letter to Castelli, now addressed to the grand
duke’s mother and good friend of Galileo, the dowager Christina. In
his Letter to the Grand Duchess Christina, Galileo discussed the problem of
interpreting biblical passages with regard to scientific discoveries but,
except for one example, did not actually interpret the Bible. That task had
been reserved for approved theologians in the wake of the Council of
Trent (1545–63) and the beginning of the Catholic Counter-Reformation.
But the tide in Rome was turning against the Copernican theory, and in
1615, when the cleric Paolo Antonio Foscarini (c. 1565–1616) published a
book arguing that the Copernican theory did not conflict with scripture,
Inquisition consultants examined the question and pronounced the
Copernican theory heretical. Foscarini’s book was banned, as were some
more technical and nontheological works, such as Johannes
Kepler’s Epitome of Copernican Astronomy. Copernicus’s own 1543
book, De revolutionibus orbium coelestium libri vi (“Six Books Concerning
the Revolutions of the Heavenly Orbs”), was suspended until corrected.
Galileo was not mentioned directly in the decree, but he
was admonished by Robert Cardinal Bellarmine (1542–1621) not to “hold or
defend” the Copernican theory. An improperly prepared document placed in
the Inquisition files at this time states that Galileo was admonished “not to
hold, teach, or defend” the Copernican theory “in any way whatever, either
orally or in writing.”
Why was Galileo persecuted for his discoveries?
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Galileo was thus effectively muzzled on the Copernican issue. Only slowly
did he recover from this setback. Through a student, he entered a
controversy about the nature of comets occasioned by the appearance of
three comets in 1618. After several exchanges, mainly with Orazio Grassi
(1583–1654), a professor of mathematics at the Collegio Romano, he finally
entered the argument under his own name. Il saggiatore (The Assayer),
published in 1623, was a brilliant polemic on physical reality and an
exposition of the new scientific method. Galileo here discussed the method
of the newly emerging science, arguing:
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 letters in which it is composed. 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.
frontispiece to Galileo's Dialogue Concerning the Two Chief World Systems,
Ptolemaic & Copernican
Frontispiece to Galileo's Dialogue Concerning the Two Chief World Systems, Ptolemaic &
Copernican (1632). From left are Aristotle, Ptolemy, and Copernicus. Ptolemy holds an
astrolabe, Copernicus a model of a planet orbiting the Sun.(more)
He also drew a distinction between the properties of external objects and
the sensations they cause in us—i.e., the distinction between primary and
secondary qualities. Publication of Il saggiatore came at
an auspicious moment, for Maffeo Cardinal Barberini (1568–1644), a friend,
admirer, and patron of Galileo for a decade, was named Pope Urban VIII as
the book was going to press. Galileo’s friends quickly arranged to have it
dedicated to the new pope. In 1624 Galileo went to Rome and had six
interviews with Urban VIII. Galileo told the pope about his theory of
the tides (developed earlier), which he put forward as proof of the annual
and diurnal motions of Earth. The pope gave Galileo permission to write a
book about theories of the universe but warned him to treat the Copernican
theory only hypothetically. The book, Dialogo sopra i due massimi sistemi
del mondo, tolemaico e copernicano (Dialogue Concerning the Two Chief
World Systems, Ptolemaic & Copernican), was finished in 1630, and Galileo
sent it to the Roman censor. Because of an outbreak of the plague,
communications between Florence and Rome were interrupted, and Galileo
asked for the censoring to be done instead in Florence. The Roman censor
had a number of serious criticisms of the book and forwarded these to his
colleagues in Florence. After writing a preface in which he professed that
what followed was written hypothetically, Galileo had little trouble getting
the book through the Florentine censors, and it appeared in Florence in
1632.
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In the Dialogue’s witty conversation between Salviati (representing Galileo),
Sagredo (the intelligent layman), and Simplicio (the dyed-in-the-wool
Aristotelian), Galileo gathered together all the arguments (mostly based on
his own telescopic discoveries) for the Copernican theory and against the
traditional geocentric cosmology. As opposed to Aristotle’s, Galileo’s
approach to cosmology is fundamentally spatial and geometric: Earth’s axis
retains its orientation in space as Earth circles the Sun, and bodies not
under a force retain their velocity (although this inertia is ultimately
circular). But in giving Simplicio the final word, that God could have made
the universe any way he wanted to and still made it appear to us the way it
does, he put Pope Urban VIII’s favourite argument in the mouth of the
person who had been ridiculed throughout the dialogue. The reaction
against the book was swift. The pope convened a special commission to
examine the book and make recommendations; the commission found that
Galileo had not really treated the Copernican theory hypothetically and
recommended that a case be brought against him by the Inquisition. Galileo
was summoned to Rome in 1633. During his first appearance before the
Inquisition, he was confronted with the 1616 edict recording that he was
forbidden to discuss the Copernican theory. In his defense Galileo produced
a letter from Cardinal Bellarmine, by then dead, stating that he was
admonished only not to hold or defend the theory. The case was at
somewhat of an impasse, and, in what can only be called a plea bargain,
Galileo confessed to having overstated his case. He was pronounced to be
vehemently suspect of heresy and was condemned to life imprisonment and
was made to abjure formally. There is no evidence that at this time he
whispered, “Eppur si muove” (“And yet it moves”). It should be noted that
Galileo was never in a dungeon or tortured; during the Inquisition process
he stayed mostly at the house of the Tuscan ambassador to the Vatican and
for a short time in a comfortable apartment in the Inquisition building. (For
a note on actions taken by Galileo’s defenders and by the church in the
centuries since the trial, see BTW: Galileo’s condemnation.) After the
process he spent six months at the palace of Ascanio Piccolomini (c. 1590–
1671), the archbishop of Siena and a friend and patron, and then moved
into a villa near Arcetri, in the hills above Florence. He spent the rest of his
life there. Galileo’s daughter Sister Maria Celeste, who was in a nearby
nunnery, was a great comfort to her father until her untimely death in 1634.
Galileo was then 70 years old. Yet he kept working. In Siena he had begun a
new book on the sciences of motion and strength of materials. There he
wrote up his unpublished studies that had been interrupted by his interest
in the telescope in 1609 and pursued intermittently since. The book was
spirited out of Italy and published in Leiden, the Netherlands, in 1638 under
the title Discorsi e dimostrazioni matematiche intorno a due nuove scienze
attenenti alla meccanica (Dialogues Concerning Two New Sciences). Galileo
here treated for the first time the bending and breaking of beams and
summarized his mathematical and experimental investigations of motion,
including the law of falling bodies and the parabolic path of projectiles as a
result of the mixing of two motions, constant speed and
uniform acceleration. By then Galileo had become blind, and he spent his
time working with a young student, Vincenzo Viviani, who was with him
when he died on January 8, 1642.
