Galileo Galilei was a pioneer of modern physics and telescopic astronomy who was born
on February 15, 1564 near Pisa, Italy. As an Italian physicist and astronomer, he was the
first to use the telescope to discover many undiscovered realms of space. His discoveries
of sunspots, lunar mountains and valleys, and the satellites of Jupiter formed the basis
of modern astronomy. His discoveries opened a gateway into the unexplored areas of the
Galileo's education began in Vallambrosa where he was taught by monks, but the takeoff
point of his career was when he registered at the University of Pisa at the age of
seventeen. Here the power of mathematical reasoning interested him so much that he became
interested in applying it to the events that occur in nature. He learned of the studies of
Aristotle and studied his writings on logic, motion, and the structure of the universe.
Over the years Galileo criticized and refuted many of Aristotle's views. He became one of
the most relentless advocates agains of Aristotle's doctrines.
Galileo's studies began in the Cathedral of Pisa one day as Galileo was watching a lamp
which was swinging from the ceiling. He observed a rhythm in the swings of the lamp and
noticed that the lamp always took the same time to go from one end of its swing to the
other. His goal was to find out whether or not all of the swings took the same amount of
time. He and a friend both made pendulums and decided to count the number of oscillations
that the pendulums made in a given amount of time. They found that both pendulums made the
same number of oscillations at the same time. Thus he discovered the law of isochronism,
or equality of time, of oscillations. By 1586, Galileo left the University of Pisa and
went back to his family in Florence.
In Florence, Galileo applied himself to geometry and from the study of Euclid he soon
passed to that of other ancient mathematicians, especially Archimedes. Galileo found in
Archimedes a teacher for whom he learned the power and the wide intellectual range of
mathematical reasoning. Galileo studies produced two books that he wrote during this
period. One, in Latin contains the theorems to determine the center of gravity of solid
bodies, made him known among mathematicians of his time. The other book, called the Little
Balance, was written in Italian. In this book he told how he read the story of Archimedes
and the studies of bodies immersed in water.
In the fall of that year Galileo moved to Pisa where he studied while he continued to
teach at the university,In this period he resumed his study of motion and wrote a short
book in Latin know as De Motu (On Motion). In De Motu , Galileo tried to disprove some of
Aristotle's main views about motion. One of his objections was the assertion that motion,
in the absence of the direct action of a force, is maintained by the medium in which it
takes place. De Motu represents Galileo's first step in his systematic study of motion and
is a key point of reference in Galileo's intellectual development. It shows that Galileo
had already started the deep process of original thinking and critical revision of
Aristotle's principles which lasted throughout his life.
At about this time Galileo tried to disprove one particular statement of Aristotle's.
Aristotle said that when bodies of the same material but of different weights fall freely,
they fall with speeds proportional to their weights. To prove that lighter and heavier
objects fall at the same speed, Galileo climbed to the top of the Leaning Tower of Pisa
while the entire body of students gathered in the square below to watch the demonstration.
From the top of the tower, Galileo let go two objects of the same substance but of
different weights at exactly the same time. They reached the ground at exactly the same
time. This story may or may not be true, but was also proved by the Dutch mathematician
At the age of 28, Galileo moved to Padua. These years were said to be the best eighteen
years of his life. Galileo made friends among the nobles and the rich Venetians. One of
his most important friends, Giovanni Francesco Sagredo, used his position to improve
Galileo's status as a scientist at the University. Following Sagredo's death, Galileo's
friend lived on as a character in two of his best books, Dialogue of the Two Greatest
Systems of the World and Two New Sciences. Galileo became a professor at the University of
Padua and began to teach large numbers of students. During these years Galileo wrote many
of his greatest books, including Fortifications, Military Constructions, and Mechanics.
The last and most important was a true engineering textbook which described the action of
simple machines such as the lever, pulley, screw, and inclined plane. He also dealt with
problems of falling bodies which he took up again in later years.
In 1597, Galileo built an instrument which illustrates both his competence in
mathematical calculation and his great craftsmanship. This device was known as the
geometric and military compass. It was a combination of a divider and slide rule. It
helped to solve a large number of mathematical and geometric problems, including the
extraction of square and cubic roots. Because the end of the sixteenth century was a
period of conflict, Galileo stressed military uses of this instrument. For instance, the
compass was used to determine relations of weight and size of cannon balls, to regulate
the front and side formations of armies, and to measure the inclination of a wall. Galileo
sold so many that he could not produce enough to fit the demand. The compass later became
one of the most useful and widely used inventions.
Galileo's most important scientific achievements in the period at Padua were in two
fields: the study of motion and astronomy. In the field of motion he resumed the study
started in Pisa with observations on pendulums and falling bodies. Galileo published these
observations in 1638, in the book Two New Sciences.
Later, Galileo began to admire the work of a man named William Gilbert, who had
published a book called De Magnete (Concerning the Magnet). Magnetism had been a well
known property of lodestone and iron at that time and was used by navigators to make
magnetic needles. Galileo, who did not accept the authority of written word, performed
Gilbert's experiments himself. He wrote a new book called The Dialogue, in which he told
of how he became convinced that the power of the stone is not increased by the weight of
Galileo's research on magnetism was yet a minor episode in his scientific activity.
On October 9, 1604, a new star appeared in the sky. This new star emerging aroused
Galileo's scientific curiosity and initiated his career as a stargazer. Shortly after the
star appeared, Galileo began to make systematic observations He began to measure its
height in the sky and began to check whether its position in respect to other stars varied
from night to night. After having observed it for several weeks, he gave three public
lectures to describe his findings and drew large crowds. A very important invention
emerged in 1609. In the early summer he was in Venice when he heard that a certain Fleming
had constructed an eye-glass by which distant objects were seen nearby. Galileo went back
to Padua at once to think of how he could build a similar instrument. He concluded that
the effect could be achieved by the combination of a convex and concave glass. Galileo
mounted the two lenses at the ends of a lead tube, and his first telescope came into
At once he realized the great importance of his instrument, both in science and in
practical life. He realized it might modify the art of war because it would permit
sighting of the enemy at much greater distance that usual. Galileo must have shown the
instrument to his friends or talked about it, for soon the rumor spread that he had
invented it. In a letter to his brother in law Galileo stated: "And news having
reached Venice...I was called by the Most Serene Signory six days ago, to whom it was my
duty to show it as well as to the whole Senate, to the intense astonishment of all."
Galileo indeed showed his telescope and it indeed astonished all.
Senators and noblemen gathered around in the Palazzo Ducale. The professor from Padua
stood with his telescope on the marble floor. His telescope was a sheet of metal, covered
by crimson sateen, of about the length of 24 inches and the diameter of about one and
three quarter inches in diameter, with two glasses, one at each end. Galileo, needing more
room climbed the steep stairs of the bell tower of San Marco across the Piazzetta and
demonstrated its ability to represent an object fifty miles off as appearing five miles
away. Galileo's telescope opened a new door to the universe.
Once more Galileo showed his craftsmanship. The first telescope he built had a
magnification of three diameters, the second eight diameters and finally he built one
magnifying 33 diameters. He increased the size of his lenses, stopping at the point where
a further increase would result in the distortion of images. No instrument but the
telescope could give Galileo the feeling of how much the power of the senses could be
enlarged. For the first time he could see distant things as well as if they were close by
and discover the existence of objects that up till then had been too far away to be
visible. Many new things in space appeared to him.
The moon appeared very different to him, and among its new strange features were
prominences and cavities in its surface. Many of these were comparable to mountains and
crevices in the Earth's surface. Galileo calculated the height of lunar mountains and
concluded correctly that some are as high as four miles. he calculated that these
mountains were higher than any mountain on the Earth.Mount Everest is actually five and a
half miles high.
The telescope still had more marvels marvels in store. Galileo soon discovered four
small planets revolving around the planet Jupiter, as the moon revolves around the Earth.
He named Jupiter's satellites the Medicean Satellites and this remained his favorite
astronomical discovery. He observed the satellites of Jupiter and studied their motion. He
used this to predict the satellite's positions so accurately that navigators would be able
to rely on them to guide their ships. He spent year after year observing the Medicean
planets with his telescope.
Less than a month later Galileo announced another detail noticed by him. He described
the phases of Venus. He said that he began to observe Venus with his instrument and saw it
was growing in size daily, keeping its roundness until it eventually arrived a great
distance from the sun and started to lose its roundness on its eastern side. A few days
later, it diminished to a half circle. Then it transformed into a horned shape and it
became thinner until it vanished. Copernicus had been disappointed not to see the phases
of Venus. The fact is that the phases of Venus are undetectable with the naked eye.
Soon afterward he made another important observation, that of sunspots. He described
them as being dark spots seen in the solar disk. Some were always produced and others are
being dissolved. They varied in duration from one or two days to thirty or forty. Their
shapes continually changed, some quickly and violently, and some more slowly and
moderately. In addition to changing shape, some of them divided into three or four, and
some of them united into one. Besides all of these disordered movements they have in
common a general uniform motion across the face of the sun in parallel lines. From this
motion was learned that the sun was absolutely spherical, that it rotates west to east
around its center, and completes its entire revolution in about a lunar month. Thus the
sun had blemishes on its surface, and it rotates around its axis as the Earth and moon do.
During all of these years, the Inquisition had been observing Galileo. The Inquisition
was the high tribunal of the Roman Church. During the Counter Reformation, the Inquisition
played a very important role. Working in secrecy, it spotted, investigated, and prosecuted
those suspected of heresy, trying to suppress it before it could spread. It exerted
censorship to avoid the distortion of the Catholic doctrine, and issued the Index of
Prohibited Books. To be tried by the Inquisition was something no one could take lightly.
Galileo kept talking in favor of the Copernican doctrine, and while he busied himself
with this, the Inquisition went on examining his case. On February 23 a group of eleven
theologians was convened to examine his beliefs in science, or at least what the
Inquisition said were his beliefs. Two propositions were submitted by the theologians: 1)
The Sun is the center of the world and hence immovable of local motion. 2) The Earth is
not the center of the world, or immovable, but moves according to the whole itself and
also with diurnal motion. Neither of these were quoted from his writings. on the Pope's
instruction, Cardinal Bellarmine called Galileo to his palace and told Galileo that he was
not to hold, teach, or defend the condemned opinion of Copernicus. A few days later De
Revolutionibus, which the Pope had accepted, was condemned and prohibited until it be
Later, with the publish of The Dialogue, there was a clash of personalities between
Pope Urban VIII and Galileo. Behind this clash were the long standing traditions and
authority of the Church and the need for freedom of thought in science. The Church would
not easily accept views that went against tradition. As it happened, the conflict between
science and theology gathered so early that the Church took strong action against the
founder of modern science. Galileo was summoned to Rome. He pleaded for time and clemency,
but the Pope was inflexible. The trial of Galileo as a heretic had begun.
Galileo arrived in Rome in February 1633 and stayed there five months. These months
were filled wit questionings by the Inquisitors. The purpose was to see whether Galileo
was still holding the prohibited doctrine and whether in writing The Dialogue he had
disobeyed orders. The trial turned into gradual spiritual pressure in which they tried to
make Galileo believe a sin that he had not committed. On June 22, 1633, Galileo was taken
to the large hall of a monastery in the center of Rome. He knelt as the sentence was read
to him: "We say, pronounce, sentence and declare that you, Galileo, for the things
found in the trial and confessed by you have made yourself suspected of heresy. We order
that the book The Dialogue by Galileo Galilei be prohibited by public edict. We condemn
you to formal prison...and we impose on you that for the next three years you say the
seven penitential psalms once a week."
In the final years Galileo completed his book Two New Sciences. Galileo's eyes had
bothered him for many years. He had been complaining of eye pain because of long hours of
studying. He lost sight in his right eye first, then the left. During the last four years
of his life he was completely blind. In November 1641 a slow fever seized Galileo and his
arthritic pains had become more. This was his last illness. He died on January 8, 1642 at
almost 78 years of age.