Galilei (February 15, 1564 – January 8, 1642) was an Italian
physicist, astronomer, and philosopher who is closely associated
with the scientific revolution. His achievements include improvements
to the telescope, a variety of astronomical observations, the first
and second laws of motion, and effective support for Copernicanism.
has been referred to as the "father of modern astronomy",
as the "father of modern physics", and as "father
of science". His experimental work is widely considered complementary
to the writings of Francis Bacon in establishing the modern scientific
method. Galileo's career coincided with that of Johannes Kepler.
work of Galileo is considered to be a significant break from that
of Aristotle. In addition, his conflict with the Roman Catholic
Church is taken as a major early example of the conflict of authority
and freedom of thought, particularly with science, in Western
Galileo was born in Pisa, in the Tuscan region of Italy, the son
of Vincenzo Galilei, a mathematician and musician born in Florence
in 1520, and Giulia Ammannati, born in Pescia and married in 1563.
Galileo was their first child.
was tutored from a very young age. After that he attended the
University of Pisa, but was forced to cease his study there for
financial reasons. However, he was 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.
a devout Catholic, Galileo fathered three children out of wedlock.
All were the children of Galileo and Marina Gamba. Because of
their illegitimate birth, both girls were sent to the convent
of San Matteo in Arcetri at early ages.
Virginia (1600–1634) who took the name Maria Celeste upon
entering a convent. Galileo's eldest child, 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.
2. Livia (b. 1601) took the name Suor Arcangela, she was sick
for most of her life at the convent.
3. Vincenzio (b. 1606) was later legitimized and married Sestilia
1611, he went to Rome, where he joined the Accademia dei Lincei
and observed sunspots. In 1612, opposition arose to the Copernican
theories, which Galileo supported. In 1614, from the pulpit of
Santa Maria Novella, Father Tommaso Caccini (1574-1648) denounced
Galileo's opinions on the motion of the Earth, judging them dangerous
and close to heresy. Galileo went to Rome to defend himself against
in 1616, Cardinal Roberto Bellarmino (1542-1621) personally handed
Galileo an admonition enjoining him to neither advocate nor teach
Copernican astronomy, because it was contrary to the accepted
understanding of the Holy Scriptures. In 1622, Galileo wrote the
Saggiatore [The Assayer], which was approved and published in
1623. In 1624, he developed the first known example of the microscope.
1630, he returned to Rome to apply for a license to print the
Dialogo dei Massimi Sistemi [Dialogue on the Great World Systems],
published in Florence in 1632. But in October of that year, he
was ordered to appear before the Holy Office in Rome. The court
issued a sentence of condemnation and forced Galileo to abjure.
He was confined in Siena and eventually, in December 1633, he
was allowed to retire to his villa in Arcetri.
1634, he was deprived of the support of his beloved daughter,
Sister Maria Celeste (1600-1634), who died prematurely. In 1638,
when he was almost totally blind, the Discorsi e dimostrazioni
intorno a due nuove Scienze [Discourses and demonstrations on
two new Sciences] was published in Leiden. Galileo died alone
in Arcetri on January 8, 1642.
In the pantheon of the scientific revolution, Galileo takes a
high position because of his pioneering use of quantitative experiments
with results analyzed mathematically. There was no tradition of
such methods in European thought at that time; the great experimentalist
who immediately preceded Galileo, William Gilbert, did not use
a quantitative approach. However, Galileo's father, Vincenzo Galilei,
had performed experiments in which he discovered what may be the
oldest known non-linear relation in physics, between the tension
and the pitch of a stretched string.
also contributed to the rejection of blind allegiance to authority
(like the Church) or other thinkers (such as Aristotle) in matters
of science and to the separation of science from philosophy or
religion. These are the primary justifications for his description
as the "father of science".
the 20th century some authorities challenged the reality of Galileo's
experiments, in particular the distinguished French historian
of science Alexandre Koyré. 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.
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 were consistent with Galileo's report. Later research
into Galileo's unpublished working papers from as early as 1604
clearly showed the reality of the experiments and even indicated
the particular results that led to the time-squared law (Drake,
Although the popular idea of Galileo's inventing the telescope
is inaccurate, he improved the device and was one of the first
to use it to observe the sky, and for a time was one of very few
people able to construct one good enough for the purpose. Based
only on sketchy descriptions of the telescope, invented in the
Netherlands in 1608, Galileo made one with about 8x magnification,
and then made improved models up to about 20x.
August 25, 1609, he demonstrated his first telescope to Venetian
lawmakers. His work on the device also 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
January 7, 1610 Galileo discovered three of Jupiter's four largest
satellites (moons): Io, Europa, and Callisto. He discovered Ganymede
four nights later. He determined that these moons were orbiting
the planet since they would appear and disappear, which he attributed
to their movement behind Jupiter.
made additional observations of them in 1620. Later astronomers
overruled Galileo's naming of these objects, changing his Medicean
stars to Galilean satellites. The demonstration that a planet
had smaller planets orbiting it was problematic for the orderly,
comprehensive picture of the geocentric model of the universe,
in which everything circled around the Earth.
noted that Venus exhibited a full set of phases like the Moon.
The heliocentric model of the solar system developed by Copernicus
predicted that all phases would be visible since the orbit of
Venus 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
contrast, the geocentric model of Ptolemy predicted that only
crescent and new phases would be seen, since Venus was thought
to remain between the Sun and Earth during its orbit around the
Earth. Galileo's observations of the phases of Venus proved that
it orbited the Sun and lent support to (but did not prove) the
was one of the first Europeans to observe sunspots, although there
is evidence that Chinese astronomers had done so before. 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 unchanging perfection of the heavens as assumed in the
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; in fact, there can be little doubt that both of them
were beaten by David Fabricius and his son Johannes.
was the first to report lunar mountains and craters, whose existence
he deduced from the patterns of light and shadow on the Moon's
surface. 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.
observed the Milky Way, previously believed to be nebulous, and
found it to be a multitude of stars, packed so densely that they
appeared to be clouds from Earth. He also located many other stars
too distant to be visible with the naked eye. Galileo observed
the planet Neptune in 1612, 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.
claims of scientific errors and misconduct
Although Galileo is generally considered one of the first modern
scientists, as evidenced by his position in the sunspot controversy,
he is often said to have arrogantly considered himself to be the
sole proprietor of the discoveries in astronomy.
he never accepted Kepler's elliptical orbits for the planets,
holding to the circular orbits of Copernicus, which still employed
epicycles to account for irregularities in planetary motions.
(The circle was considered the "perfect" shape.)
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.)
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, of course, is true in the Sun's frame of
reference; but it is by no means adequate to explain the tides.
commentators consider that Galileo developed this position simply
to justify his own opinion because the theory was not based on
any real scientific observations. If his theory was correct, there
would be only one high tide per day and it would happen at noon.
The fact that there are two daily high tides at Venice instead
of one, and that they travel around the clock, Galileo and his
contemporaries knew, but he dismissed as due to several secondary
causes, such as the shape of the sea, its depth, and other things.
the imputation that Galileo 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).
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.
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 not now generally accepted as true. Moreover, Giambattista
Benedetti had reached the same scientific conclusion years before,
Galileo did perform experiments involving rolling balls down inclined
planes, which proved the same thing: falling or rolling objects
(rolling is a slower version of falling, as long as the distribution
of mass in the objects is the same) are accelerated independently
of their mass. (Although Galileo was the first person to demonstrate
this via experiment, he was not (contrary to popular belief) the
first to argue that it was true. John Philoponus had argued this
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 later scientific laws expressed 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 (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 (1st
also noted that a pendulum's swings always take the same amount
of time, independently 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)
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. While he could reach no conclusion
on whether light propagated instantaneously, he recognized that
the distance between the hilltops was perhaps too small for a
is lesser known for, yet still 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).
his 1632 Dialogue Galileo presented a physical theory to account
for tides, based on the motion of the Earth. If correct, this
would have been a strong argument for the reality of the Earth's
motion. (The original title for the book, in fact, described it
as a dialogue on the tides; the reference to tides was removed
by order of the Inquisition.)
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.
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.
While Galileo's application of mathematics to experimental physics
was innovative, his mathematical methods were the standard ones
of the day. 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 before,
thanks to accurate translations by Tartaglia and others; but by
the end of Galileo's life it was being superseded by the algebraic
methods of Descartes, which a modern mind finds incomparably easier
produced one piece of original and even prophetic work in mathematics:
Galileo's paradox, which 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 to what we now call technology
as distinct from pure physics, and suggested others. 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
1595–1598, Galileo devised and improved a "Geometric
and Military Compass" suitable for use by gunners and surveyors.
This expanded on earlier instruments designed by Niccolo Tartaglia
and Guidobaldo del Monte. For gunners, it offered, in addition
to a new and safer way of elevating cannons accurately, 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.
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.
1609, Galileo was among the first to use a refracting telescope
as an instrument to observe stars, planets or moons.
1610, he used a telescope as a compound microscope, and he made
improved microscopes in 1623 and after. This appears to be the
first clearly documented use of the compound microscope.
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 would make possible the determination of longitude. 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.
his last year, when totally blind, he designed an escapement mechanism
for a pendulum clock. The first fully operational pendulum clock
was made by Christiaan Huygens in the 1650s.
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.
Psalms 93 and 104, and Ecclesiastes 1:5 speak of the motion of
celestial bodies and the suspended position of the earth. Galileo
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, particularly when the
scripture in question 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. In fact, it is the earth's rotation
which gives the impression of the sun in motion across the sky.
1616 the attacks on Galileo had reached a head, and he went to
Rome to try to persuade the 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. For the next several years Galileo stayed well
away from the controversy.
revived his project of writing a book on the subject, encouraged
by the election of Cardinal Barberini as Pope Urban VIII in 1623.
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.
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. Only the latter of those
requests was fulfilled by Galileo. Whether unknowingly or deliberate,
Simplicius, the defender of the Aristotelian Geocentric view in
Dialogue Concerning the Two Chief World Systems, was often caught
in his own errors and sometimes came across as a fool.
fact made Dialogue Concerning the Two Chief World Systems 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 agree Galileo did not act out of malice and felt
blindsided by the reaction to his book. However, the pope did
not take the public ridicule lightly, nor the blatant bias. Galileo
had alienated one of his biggest and most powerful supporters,
the pope, and was called to Rome to explain himself.
the loss of many of his defenders in Rome because of Dialogue
Concerning the Two Chief World Systems, Galileo was ordered to
stand trial on suspicion of heresy in 1633. The sentence of the
Inquisition was in three essential parts:
was required to recant his heliocentric ideas, which were condemned
as "formally heretical"; he was ordered imprisoned;
the sentence was later commuted to house arrest. His offending
Dialogue was banned; and in an action not announced at the trial,
publication of any of his works was forbidden, including any he
might write in the future.
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 while Galileo was under house arrest when
he dedicated his time to one of his finest works, Two New Sciences.
This book has received high praise from both Sir Isaac Newton
and Albert Einstein. As a result of this work, Galileo is often
called, the "father of modern physics".
was reburied on sacred ground at Santa Croce in 1737. He was formally
rehabilitated in 1741, when Pope Benedict XIV authorized the publication
of Galileo's complete scientific works (a censored edition had
been published in 1718), and in 1758 the general prohibition against
heliocentrism was removed from the Index. On 31 October 1992,
Pope John Paul II expressed regret for how the Galileo affair
was handled, as the result of a study conducted by the Pontifical
Council for Culture.