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Galileo Galilei - Father of Modern Science

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Actor Leo McKern as Galileo in the 1964 BBC production of Bertolt Brecht's play 'The Life of Galileo'.
I believe that good philosophers fly alone, like eagles, and not in flocks like starlings. It is true that because eagles are rare birds they are little seen and less heard, while birds that fly like starlings fill the sky with shrieks and cries, and wherever they settle befoul the Earth beneath them.
– extract from Il Saggiatore (The Assayer), by Galileo Galilei (published October 1623)

Galileo Galilei has been called 'the Father of Modern Science' by modern-day scientists. Galileo might not have been the first man to use a telescope to gaze into outer space, but he was certainly the first to accurately record his sightings. Among his discoveries were the main moons of Jupiter, which are now collectively called 'the Galilean moons' in his honour. Galileo accepted the Copernican theory, which placed the Sun, not the Earth, at the centre of the known Universe. When he tried to persuade others on this point, he was arrested and put on trial by the Catholic Church, who promoted the geocentric view as taught by the ancient Greek philosophers. This became one of history's most infamous clashes between faith and science. While under house arrest he continued to write, and study the heavens.


Galileo's father Vincenzo Galilei was a lute player who earned a living teaching music theory in Florence, Italy. In 1562 he married Giulia Ammannati and they moved to Pisa, where Galileo was born in 1564. He was the oldest of six children, two of whom died in infancy. When he was 17 Galileo enrolled at the University of Pisa to study medicine, but he switched to mathematics after working out that a pendulum could be used to regulate a clock. Then Galileo's father became ill and could no longer pay for his son's further education, so at the age of 21 Galileo left university and returned to the family home to become the breadwinner. He was engaged as a private tutor and was able to financially support the family. In 1591 Vincenzo Galilei died and Galileo assumed responsibility for his youngest brother Michelangelo.

Discoveries and Telescopes

Galileo was one of the first scientists in modern times to arrive at conclusions from observation and practical experimentation. When he was 22 he wrote a paper called La Bilancetta (The Little Balance) on hydrostatic balance; that is weighing things in air and water with the intention of discovering the components of objects. He also performed extensive experiments with pendulums. By 1589, around the time he turned 25, Galileo was appointed head of the department of mathematics at the University of Pisa. The post there was neither well-paid nor prestigious, but during the next three years he built himself something of a reputation.

Galileo taught his students about the 'Equivalence Principle' (universality of free fall) by dropping objects of varying weights, like a cannonball and perhaps a small wooden ball, to show that things accelerate at the same rate, regardless of their mass. Galileo, being Earthbound, had to choose objects for which the effects of air resistance are negligible. Over four centuries later, on the airless surface of the Moon, Commander Dave Scott, one of the Apollo 15 astronauts, carried out a demonstration of the Equivalence Principle using a hammer and a falcon feather, and proved Galileo was right.

In 1592 he was offered the post of professor of mathematics at the University of Padua, with a salary three times that he had earned at Pisa. He lectured there for the next 18 years. During the year 1593 he invented a pump for raising water and was granted a patent the following year. Another invention in 1593 was the termometro lento (slow thermometer). It depends upon the fact that a liquid becomes less dense as its temperature rises. Galileo's thermometer had a glass cylinder in which glass globes of slightly differing weights sank one by one as the temperature of the liquid in which they were immersed rose. Conversely, when the temperature fell, the glass globes rose again. Each globe was marked with an individual temperature. The lowest of the group floating towards the top of the liquid indicated the current temperature1.

In 1597 Galileo devised 'a mathematical instrument consisting of two rulers connected at one end by a joint and marked with several scales' which he called a Sector. He initially built it himself and sold the 'geometric and military compass' to his wealthy students. Later he instructed a craftsman to build it, and published an instruction manual for its use, which was far more profitable.


On the night of 9/10 October, 1604, a 'new star' appeared in the heavens above Italy. The German astronomer Johannes Kepler (1571-1630) studied it from 17 October, and it was subsequently named after him. Galileo viewed the phenomenon himself on Christmas Eve. By the new year 1605, he prepared to lecture on the event at the University of Padua. He proposed that the new star proved that the heavens weren't fixed, and that it was further away than the Moon. This was a radical belief and also a dangerous point of view, the official view was that the heavens were eternal and unchanging. The 'new star' was in fact a supernova, an exploding star in the constellation Ophiuchus, now catalogued SN 1604 and dubbed 'Kepler's Supernova'. Supernovae are so rare that none have been witnessed in our own galaxy since the 1604 event, although some have been recorded in other galaxies.

Galileo's Children

Galileo never married, but he did have a relationship with Marina di Andrea Gamba of Venice who gave him two daughters, Virginia (born 1600) and Livia (born 1601), and a son Vincenzio (born 1606). They lived in Galileo's house in Padua, but the girls went with their father when he moved to Florence in 1610 to take up a position at the Medici court. Vincenzio joined them in 1613 when his mother married Giovanni Bartoluzzi. The girls were placed in a convent and went on to become nuns named Sister Maria Celeste (Virginia) and Sister Arcangela (Livia). Galileo succeeded in his application to have his son made legitimate by the Grand Duke of Tuscany. Sister Maria Celeste is the subject of a book, Galileo's Daughter by Dava Sobel.

The Telescope

Galileo made his first telescope in 1609, having heard about the miraculous contraption called a spyglass that could magnify things three times being patented in the Netherlands. His own prototype managed the same, but changes eventually improved later ones to ×20 magnification. While others were using spyglasses to keep watch on ships at sea, Galileo turned his telescope to the night sky. He was able to study the surface of the Moon in intimate detail. He managed to catalogue the phases of Venus, which added weight to the heliocentric theory proposed by his predecessors Nicolaus Copernicus (1473-1543), Kepler and Fr Giordano Bruno (1548-16002). But it was when he turned his telescope on the planet Jupiter that he witnessed something which secured his place in history.

The Moons of Jupiter

The intention of the Holy Spirit is to teach how to go to Heaven, and not how go the heavens.

While Galileo had complete faith that the Bible was a sure map to Heaven, he did not believe that it was ever intended to be an accurate map of the heavens. For that only careful observations of the natural world would do - and he saw no conflict in this, even if the church did. Galileo's discovery of Jupiter's four main moons in January 1610, which he called the the 'Medicean Planets' after the important Medici family who were his patrons, provided evidence against geocentricity, which went against what was taught in the Scriptures.

At the time only six planets were known, and only one moon3, our own. When moons were discovered encircling Jupiter, clearly these moons were orbiting another planet, not the Earth. This was strong evidence that the Earth was just one of many planets instead of being at the centre of things like the Bible said. In other words, the Earth was not necessarily the centre of the Universe. Changing the order of things in the heavens was considered blasphemous (showing disrespect or scorn for God) because interpretations of the Bible (Old Testament) imply that God made the heavens with the Earth at the centre.

The Longitude Problem

Galileo was not slow to realise that his discovery could be put to a practical use, and attempted to turn it to his financial advantage. King Philip of Spain had offered up a substantial prize to anyone who could come up with a reliable and accurate method to establish the longitude position of a ship at sea. In order to do this, it was necessary to know the exact time4. Galileo worked out that the occultation of Jupiter's moons happened at wholly predictable times and could therefore be used to calculate the time on Earth. He produced a reference table of the approximately 1,000 occurrences that took place each year, and applied for the prize.

Although the observation was relatively straightforward on land, the practicalities of taking an accurate observation from a rolling ship, and the inconvenient fact that the object was not visible during daylight, or for half of the year, were insoluble. Galileo's idea was turned down by the Spanish, so he tried his luck with the Dutch, another seafaring nation. Again he was turned down, but he was presented with a gold chain for a good attempt5. Galileo continued to experiment, producing the 'Celatone', a helmet incorporating a telescopic eyepiece to assist the observer, but the problems were insurmountable and it was eventually abandoned.

The Earth Moves and the Sun Rotates

Nature does not follow precise rules, nor will every question about the Universe be answered. Some people find the answer to a question; Galileo scored some hits but he also made mistakes. Galileo described the tidal motions of the seas as being due to the rotation of the Earth. He knew nothing of the universal force of gravity6 and would have put the Moon's influence, had it been suggested at the time, down to astrological thinking. From studying sunspots he deduced that because they moved across the face of the Sun, that the Sun must rotate. This was a radical view and one that was not accepted at the time, but it was indeed true, and he safely7 demonstrated the movement of sunspots to assembled clergy to prove his point.

Missing the Boat with Neptune

Galileo observed the outermost planet Neptune in December 1612, and again in January 1613, but he never classified it as anything special - although it's quite possible Galileo wrote about what he thought he was witnessing in code or in the form of a puzzle. Therefore, because he didn't announce what he suspected was a new planet, Galileo is not acknowledged as the discoverer8 of Neptune. However, University of Melbourne physicist Professor David N Jamieson has studied Galileo's notebooks and he declared:

Galileo may indeed have formed the hypothesis that he had seen a new planet which had moved right across the field of view during his observations of Jupiter over the month of January 1613.

Prof Jamieson gave a series of lectures during 2009 as part of the International Year of Astronomy and his promotion of Galileo as the original discoverer of Neptune formed part of these.

The Inquisition

The head of the Catholic Church from 1605-21, Pope Paul V, issued an order that 'heliocentric ideas could not be defended'. The Church believed that science had shown that the Earth was in the centre. The ancient Greek astronomer Ptolemy had perfected the geocentric theory, based on careful observations, and it was very accurate at predicting the positions of the planets. A letter was sent to Galileo from Cardinal Bellarmine warning him against persisting in his belief of heliocentrism, where the Sun is the central body and everything else in the Solar System orbits it. It was Cardinal Bellarmine who had presided over the trial of Fr Giordano Bruno who was burned at the stake in 1600, and Galileo was well aware of this. Galileo took no notice and this stubbornness resulted in his being summoned before the Pope in 1616. The Church said that Galileo could say that the Sun was the centre of the Solar System if he could prove it. He couldn't, and Galileo was subsequently forbidden from teaching or promoting his theories about heliocentrism.

The publication of Galileo's book Dialogue Concerning the Two Chief World Systems happened during the papacy of Pope Urban VIII, who was portrayed as an idiot in the book for not agreeing with Galileo. Pope Urban VIII was the former Cardinal Maffeo Barberini, Galileo's supporter and friend, who is probably best remembered for comparing sneezing with orgasm9. Galileo was summoned before him to recant his theory in 1633. During a trial which lasted three weeks he did eventually recant, but he was found guilty of heresy and sentenced to a term in prison and a penance of reciting the Seven Penitential Psalms once a week for the next three years. However, he was let off the prison term and allowed to return home while he could get his daughters (who were nuns) to say the prayers for him. He remained under house arrest for the rest of his life. The Inquisition also banned publication of any work by Galileo, including any he might write in the future.


Galileo spent his remaining years at his home in Arcetri, near Florence, writing, using his telescope and indulging in earthly pleasures such as wine, which he described as 'light held together by moisture'. Among his regular visitors was his great friend Archbishop Piccolomini, who shared his passion for wine. Using his remaining good eye, Galileo was able to create a method for measuring the diameters of stars and the distance between them. He also wrote another and perhaps his greatest treatise, Discorsi e dimostrazioni matematiche, intorno a due nuove scienze (Discourses and Mathematical Demonstrations Relating to Two New Sciences), covering motion and the principles of mechanics, which was published in the Netherlands (because his work was banned in Italy) in 1638. By then Galileo was totally blind. When he was dying, Pope Urban VIII sent his old friend a blessing10. Galileo died on 8 January, 1642, and his remains were interred in consecrated ground at the Basilica di Santa Croce di Firenze. Although the pope did not allow a monument to be erected over his tomb, according to the Catholic Encyclopedia Galileo was never excommunicated.

Working out the Speed of Light

Galileo failed in his own attempts to measure the speed of light. In 1675/6 the Danish astronomer Ole Christensen Rømer11 calculated a reasonable estimate of the speed of light (2/3rds the actual figure) after he adjusted Galileo's table of eclipses, making allowances for the distance between the Earth and Jupiter, which varies considerably dependant upon their relative positions in their orbits.


Following a 13-year investigation of the Inquisition's treatment of Galileo, Pope John Paul II said that the 17th Century theologians were working with the knowledge available to them at the time. However, he acknowledged that the Church was wrong to condemn him. To rectify the wrong done to Galileo, preparations began for his rehabilitation by the Pope, and this act was performed in 1992. Pope Benedict XVI praised Galileo's contribution to astronomy in 2008.

Tributes and Celebrations

Galileo Galilei has had many tributes made to him, and honours include having the lunar crater Galilaei, the International Airport in Pisa, Italy, a planetarium in Buenos Aires, Argentina, and the Galileo spacecraft named after him, and a play about his life by Bertolt Brecht. He's even included in the lyrics of the Queen hit 'Bohemian Rhapsody'. One of the greatest scientists of the 20th Century, Professor Stephen Hawking (who was born on the anniversary of Galileo's death), stated:

Galileo, perhaps more than any other single person, was responsible for the birth of modern science.

The 400th anniversary of Galileo's use of the telescope, 2009, was designated the International Year of Astronomy, and many events were organised in his honour. Other tributes included the issue of a €25 coin which features his image and that of his telescope.

1Temperatures today are taken using thermometers that contain alcohol or mercury, although a recent improvement has been the introduction of an electronic circuit device which fits in the patient's ear.2This was not a natural death. After a lengthy trial, Bruno was sentenced to be roasted alive at the stake by the Catholic Inquisition for his blasphemy and heresy.3Two moons orbit the planet Mars, which is much closer to the Earth than Jupiter, but Mars' moons are small, captured asteroids which cannot be seen without powerful optical aid. They weren't discovered until the 19th Century.4This problem was eventually solved thanks to the technical genius of John Harrison (1693-1776).5It would have been very ironic if the consolation prize had been a watch.6This is because Sir Isaac Newton had not yet been born. Newton was born in Lincolnshire, England, almost exactly a year after Galileo's death.7You can't look at the Sun through a telescope, you'd be instantly blinded. However, you can set up your equipment to face the Sun and have its image projected onto another surface which is safe to look at. This is how transits of Venus and solar eclipses are witnessed.8That honour went to French mathematician Urbain Le Verrier over two centuries later in 1846, who detected the planet based on discrepancies in the orbit of Uranus.9Pope Urban VIII threatened tobacco smokers with excommunication because their sneezing sounded like sexual ecstacy. Woe betide anyone with a cold then.10This was a special event as the Pope doesn't bless everybody personally.11Other spellings of his surname are Roemer, Römer, and Romer.

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