Italian physicist and astronomer whose work founded the modern scientific method of deducing laws to
explain the results of observation and experiment. In physics, Galileo discovered the properties of the
pendulum, invented the thermometer, and formulated the laws that govern the motion of falling bodies. In
astronomy, Galileo was the first to use the telescope to make observations of the moon, sun, planets, and
stars.
Galileo was born in Pisa on February 15, 1564. His full name was Galileo Galilei, his father being Vincenzio
Galilei (c.1520–1591), a musician and mathematician. Galileo received his early education from a private tutor
at Pisa until 1575, when his family moved to Florence. He then studied at a monastery until 1581, when he
returned to Pisa to study medicine at the university. Galileo was attracted to mathematics rather than
medicine, however, and also began to take an interest in physics. In about 1583 he discovered that a
pendulum always swings to and fro in the same period of time regardless of the amplitude (length) of the
swing. He is said to have made this discovery by using his pulse to time the swing of a lamp in Pisa Cathedral.
Galileo remained at Pisa until 1585, when he left without receiving a degree and returned home to Florence.
There he studied the works of Euclid and Archimedes, and in 1586 extended Archimedes' work in hydrostatics
by inventing an improved version of the hydrostatic balance for measuring specific gravity. At this time,
Galileo's father was investigating the ratios of the tensions and lengths of vibrating strings that produce
consonant intervals in music, and this work may well have demonstrated to Galileo that the validity of a
mathematical formula could be tested by practical experiment.
In 1589 Galileo became professor of mathematics at the University of Pisa. He attacked the theories of
Aristotle that then prevailed in physics, allegedly demonstrating that unequal weights fall at the same speed
by dropping two cannon balls of different weights from the Leaning Tower of Pisa to show that they hit the
ground together. Galileo also published his first ideas on motion in De motu/On Motion (1590).
Galileo remained in Pisa until 1592, when he became professor of mathematics at Padua. He flourished at
Padua, and refined his ideas on motion over the next 17 years. He deduced the law of falling bodies in 1604
and came to an understanding of the nature of acceleration. In 1609 he was diverted from his work in physics
by reports of the telescope, which had been invented in Holland the year before. Galileo immediately
constructed his own telescopes and set about observing the heavens, publishing his findings in Sidereus
nuncius/The Starry Messenger (1610). The book was a sensation throughout Europe and brought Galileo
immediate fame. It resulted in a lifetime appointment to the University of Padua, but Galileo rejected the post
and later in 1610 became mathematician and philosopher to the Grand Duke of Tuscany, under whose
patronage he continued his scientific work at Florence.
In 1612 Galileo returned to hydrostatics and published a study of the behavior of bodies in water, in which he
championed Archimedes against Aristotle. In the following year he performed the same service for Nicolaus
Copernicus, publicly espousing the heliocentric system. This viewpoint aroused the opposition of the church,
which declared it to be heretical and in 1616 Galileo was instructed to abandon the Copernican theory. Galileo
continued his studies in astronomy and mechanics without publicly supporting Copernicus although he was
personally convinced. In 1624 Urban VIII became pope and Galileo obtained permission to present the
arguments for the rival heliocentric and geocentric systems in an impartial way. The result, Dialogue
Concerning The Two Chief World Systems, was published in 1632, but it was scarcely impartial. Galileo used
the evidence of his telescope observations and experiments on motion to favor Copernicus, and immediately
fell foul of the church again. The book was banned and Galileo was taken to Rome to face trial on a charge of
heresy in April 1633. Forced to abjure his belief that the earth moves around the sun, Galileo is reputed to
have muttered "Eppur si muove" ("Yet it does move").
Galileo was sentenced to life imprisonment, but the sentence was commuted to house arrest and he was
confined to his villa at Arcetri near Florence for the rest of his life. He continued to work on physics, and
summed up his life's work in Discourses and Mathematical Discoveries Concerning Two New Sciences. The
manuscript of this book was smuggled out of Italy and published in Holland in 1638. By this time Galileo was
blind, but he still continued his scientific studies. In these last years, he designed a pendulum clock. Galileo
died in Arcetri on January 8, 1642.
Galileo made several fundamental contributions to mechanics. He rejected the impetus theory that a force or
push is required to sustain motion, and identified motion in a horizontal plane and rotation as being “neutral”
motions that are neither natural nor forced. Galileo realized that gravity not only causes a body to fall, but
that it also determines the motion of rising bodies and furthermore that gravity extends to the center of the
earth. He found that the distance traveled by a falling body is proportional to the square of the time of
descent. Galileo then showed that the motion of a projectile is made up of two components: one component
consists of uniform motion in a horizontal direction and the other component is vertical motion under
acceleration or deceleration due to gravity. This explanation was used by Galileo to refute objections to
Copernicus based on the argument that birds and clouds would not be carried along with a turning or moving
earth. He explained that a horizontal component of motion provided by the earth always exists to keep such
objects in position even though they are not attached to the ground.
Galileo came to an understanding of uniform velocity and uniform acceleration by measuring the time it takes
for bodies to move various distances. In an era without clocks, it was difficult to measure time accurately and
Galileo may have used his pulse or a water clock—but not, ironically, the pendulum—to do so. He had the
brilliant idea of slowing vertical motion by measuring the movement of balls rolling down inclined planes,
realizing that the vertical component of this motion is a uniform acceleration due to gravity while the
horizontal component is a uniform velocity. Even so, this work was arduous and it took Galileo many years to
arrive at the correct expression of the law of falling bodies. This was presented in the Two New Sciences,
together with his derivation that the square of the period of a pendulum varies with its length (and is
independent of the mass of the pendulum bob). Galileo also deduced by combining horizontal and vertical
motion that the trajectory of a projectile is a parabola. He furthermore realized that the law of falling bodies is
perfectly obeyed only in a vacuum, and that air resistance always causes a uniform terminal velocity to be
reached.
The other new science of Galileo's masterwork was engineering, particularly the science of structures. His
main contribution was to point out that the dimensions of a structure are important to its stability: a small
structure will stand whereas a larger structure of the same relative dimensions may collapse. Using the laws
of levers, Galileo went on to examine the strengths of the materials necessary to support structures.
Galileo's other achievements include the invention of the thermometer in 1593. This device consisted of a bulb
of air that expanded or contracted as the temperature changed, causing the level of a column of water to rise
or fall. Galileo's thermometer was very inaccurate because it neglected the effect of atmospheric pressure, but
it is historically important as one of the first measuring instruments in science.
Galileo's astronomical discoveries were a tribute to both his scientific curiosity and his ability to devise new
techniques with instruments. Within two years of first building a telescope he had compiled fairly accurate
tables of the orbits of four of Jupiter's satellites and proposed that their frequent eclipses could serve as a
means of determining longitude on land and at sea. His observations on sunspots are noteworthy for their
accuracy and for the deductions he drew from them regarding the rotation of the sun and the orbit of the
earth. He believed, however—following both Greek and medieval tradition—that orbits must be circular, not
elliptical, in order to maintain the fabric of the cosmos in a state of perfection. This preconception prevented
him from deriving a full formulation of the law of inertia, which he himself discovered although it is usually
attributed to the contemporary French mathematician René Descartes. Lacking the theory of Newtonian
gravity, he hoped to explain the paths of the planets in terms of circular inertial orbits around the sun.
Galileo was a pugnacious and sarcastic man, especially when confronted with those who could not or would
not admit the validity of his observations and arguments. This characteristic symbolizes his position in the
history of science, for with Galileo the idea that experiment and observation can be used to prove the validity
of a proposed mathematical description of a phenomenon really became a working method in science.
Although he built on the views and work of Archimedes and Copernicus, in his achievements Galileo can be
considered to have founded not only classical mechanics and observational astronomy, but the method of
modern science overall. In mechanics Isaac Newton developed Galileo's work into a full understanding of
inertia, which Galileo had not quite grasped, and arrived at the basic laws of motion underlying Galileo's
discoveries.
Italian physicist and astronomer whose work founded the modern scientific method of deducing laws to
explain the results of observation and experiment. In physics, Galileo discovered the properties of the
pendulum, invented the thermometer, and formulated the laws that govern the motion of falling bodies. In
astronomy, Galileo was the first to use the telescope to make observations of the moon, sun, planets, and
stars.
