Galileo Galilei

Chapter 1 · The Musician's Son Who Measured Things

Pisa, Padua, and the physics of motion

1564 – 1608 · Pisa · Padua · Florence

Galileo was born in Pisa on 15 February 1564. His father Vincenzo was a musician and music theorist who ran experiments on strings under tension to test received doctrine about musical intervals — probably the first person Galileo watched testing an authority against a measurement.

He entered the University of Pisa to study medicine at his father's insistence, discovered mathematics, and left in 1585 without a degree. He taught privately, wrote on centres of gravity, and secured the chair of mathematics at Pisa in 1589 and then at Padua in 1592, where he spent eighteen years he later called the happiest of his life.

His physics from this period reshaped the subject. He studied pendulums and established that the period depends on length rather than the size of the swing. He rolled balls down inclined planes, slowing gravity enough to time it with a water clock and his own pulse, and showed that distance travelled grows with the square of elapsed time — uniform acceleration. He argued that in the absence of air resistance all bodies fall at the same rate regardless of weight, contradicting Aristotle. The story of dropping weights from the Leaning Tower comes from a biography written by his assistant Viviani decades later and is probably not literally true; the inclined plane experiments are documented in his own working papers.

He supplemented a modest salary by making and selling a military compass and by taking in student lodgers. He had three children with Marina Gamba, to whom he was not married; his daughters entered a convent, and the elder, Sister Maria Celeste, wrote him letters that survive and are among the warmest documents of his life.

Why this matters

Galileo's inclined-plane work established that motion can be described by mathematical law — the foundational move that Newton later completed.

You have the physicist before the telescope. What would you ask him?

Ask Galileo

• “How did an inclined plane let you measure falling?”
• “Did you really drop weights from the Leaning Tower?”
• “What did your father's experiments teach you?”
• “Why was Aristotle's account of falling bodies wrong?”
• “How did you make a living as a mathematics professor?”

Chapter 2 · The Starry Messenger

Twenty nights that changed the sky

1609 – 1613 · Padua · Venice · Florence

In 1609 Galileo heard that Dutch spectacle-makers had produced a device that made distant objects appear closer. Without seeing one he worked out the optics and built his own, grinding the lenses himself, and quickly reached magnifications of twenty and then about thirty times — far better than anything else available.

He demonstrated it to Venetian senators on the campanile of St Mark's, showing them ships hours before they were visible to the naked eye, and had his salary doubled. Then he pointed it upward.

*Sidereus Nuncius*, published in March 1610 after only a few months of observation, reported that the Moon has mountains and craters and is not a perfect sphere; that the Milky Way resolves into countless individual stars; and that Jupiter has four moons orbiting it, which he named the Medicean stars in a successful bid for Medici patronage.

That last point mattered enormously. A central objection to Copernicus was that the Earth could not move because it would leave its Moon behind. Jupiter demonstrably carried four moons with it while it moved.

He went on to observe the full cycle of phases of Venus, which the Ptolemaic system could not produce, and sunspots, which showed the Sun changing and imperfect.

None of this proved the Earth moves — the Tychonic system, in which the planets orbit the Sun and the Sun orbits a stationary Earth, accounted for the phases equally well, and the absence of observable stellar parallax was a real argument against Copernicus. Galileo's actual proof, based on the tides, was wrong.

Why this matters

The telescope turned astronomy from a mathematical exercise into an observational science, and Galileo's rapid publication is a model of how new instruments transform a field.

You have the new sky. What would you ask him?

Ask Galileo

• “Why did Jupiter's moons matter so much for Copernicus?”
• “What did the phases of Venus rule out?”
• “Why were mountains on the Moon controversial?”
• “Why did you name Jupiter's moons after the Medici?”
• “Did you actually have proof that the Earth moves?”

Chapter 3 · How the Heavens Go

Arguing about scripture, and the warning of 1616

1613 – 1623 · Florence · Rome

The conflict was not a simple clash of science and religion. Many churchmen were interested in Galileo's observations, and Jesuit astronomers at the Collegio Romano confirmed them. The problem was that Galileo, a mathematician and layman, began telling theologians how to interpret scripture.

In his 1615 *Letter to the Grand Duchess Christina* he argued that scripture is written in ordinary language to be understood by ordinary people, that it teaches salvation rather than astronomy, and — quoting Cardinal Baronius — that the Bible tells us how to go to heaven, not how the heavens go. Where a demonstrated physical truth conflicts with a literal reading, he wrote, the reading must be revised.

This was reasonable, and it was also exactly the sort of argument the Council of Trent had forbidden laypeople to make in the middle of the Reformation, when Protestant claims to interpret scripture individually were the central threat.

In 1616 a Church commission declared the Sun-centred model formally heretical. Cardinal Bellarmine met Galileo and warned him. Precisely what he was told became the crux of his later trial: Bellarmine's certificate said he must not hold or defend the doctrine, while a disputed unsigned minute in the file said he had been ordered not to teach or discuss it in any way.

The distinction mattered, because Copernicanism could still be discussed as a mathematical hypothesis. Galileo largely kept quiet for seven years.

In 1623 his friend and admirer Maffeo Barberini became Pope Urban VIII, and granted him permission to write about both systems, provided he treated them as hypotheses and included the argument that God could produce the same appearances by other means.

“The Bible shows the way to go to heaven, not the way the heavens go.”

— Quoted by Galileo, attributing it to Cardinal Cesare Baronio, in his Letter to the Grand Duchess Christina, 1615

Why this matters

The dispute was about who had authority to interpret scripture during the Counter-Reformation — which is why a mathematical argument turned into a heresy proceeding.

You are at the point where the argument became dangerous. What would you ask him?

Ask Galileo

• “Why did you write to the Grand Duchess about scripture?”
• “What exactly were you told in 1616?”
• “Why was a layman interpreting the Bible so provocative?”
• “Did Jesuit astronomers accept your observations?”
• “What did Urban VIII actually permit you to write?”

Chapter 4 · The Dialogue and the Trial

A character called Simplicio

1632 – 1633 · Florence · Rome

The *Dialogue Concerning the Two Chief World Systems*, published in 1632 with official approval, is a conversation between three characters: Salviati, who argues for Copernicus, Sagredo, an intelligent neutral, and Simplicio, who defends Aristotle and Ptolemy.

It was written in Italian rather than Latin, which meant educated laypeople could read it. It is brilliant, funny and not remotely balanced: Simplicio is repeatedly made to look foolish. And Galileo gave Simplicio — whose name suggests simpleton — the pope's own argument that God could produce any appearance by unknown means, placing it at the end as a conclusion nobody in the dialogue takes seriously.

Urban VIII, under severe political pressure during the Thirty Years' War and vulnerable to charges of leniency, took it as a personal humiliation. The book was banned and Galileo was summoned to Rome.

The trial in 1633 turned on the disputed 1616 minute. Threatened with torture — shown the instruments, in the formal procedure of the time — the sixty-nine-year-old Galileo agreed to a plea, and was then convicted anyway of being vehemently suspect of heresy. He was required to kneel and abjure, curse and detest his errors.

The story that he muttered *eppur si muove* — and yet it moves — first appears more than a century later and has no contemporary support.

He was placed under house arrest at Arcetri for the remaining nine years of his life. His daughter Maria Celeste, whose letters had sustained him, died months later.

Why this matters

Galileo's trial became the defining symbol of institutional power suppressing inquiry — and the details show that tact, politics and timing mattered as much as the astronomy.

You have the trial. What would you ask him?

Ask Galileo

• “Why did you put the pope's argument in Simplicio's mouth?”
• “Why write in Italian rather than Latin?”
• “What was it like to be shown the instruments of torture?”
• “Did you ever say 'and yet it moves'?”
• “Should you have recanted?”

Chapter 5 · Blind at Arcetri

The best book, written under house arrest

1634 – 1642 and after · Arcetri · Leiden

Confined and forbidden to publish, Galileo went back to the physics he had worked on forty years earlier and wrote the book most physicists consider his finest: *Discourses and Mathematical Demonstrations Relating to Two New Sciences*.

It covers the strength of materials — why a structure cannot simply be scaled up, since volume grows faster than cross-sectional area, an argument that founded engineering mechanics — and the mathematics of motion. In it he sets out uniform acceleration, shows that a projectile's path is a parabola by treating horizontal and vertical motion independently, and states the principle that a body in motion continues unless something stops it, which becomes Newton's first law.

The manuscript was smuggled out of Italy and published in Leiden in 1638 by a Protestant printer beyond the reach of the Inquisition.

By then he was blind, probably from glaucoma and cataracts, possibly worsened by observing the Sun. He continued to work through his students Viviani and Torricelli, who visited him at Arcetri. He died there on 8 January 1642, aged seventy-seven. The Church refused a monument; his remains were moved to a proper tomb in Santa Croce in 1737.

The Church's position moved slowly. Works advocating heliocentrism were removed from the Index in 1758, and in 1992 a papal commission concluded that Galileo's judges had erred.

His real legacy is not the trial. It is the argument that nature is written in the language of mathematics, and that a claim about the physical world is settled by measurement rather than by authority.

Why this matters

Two New Sciences is the direct bridge from Galileo to Newton: the mathematics of motion in it is what the Principia builds upon.

You have the last book and the long aftermath. What would you ask him?

Ask Galileo

• “How did you write your best book under house arrest?”
• “Why can't a structure simply be scaled up?”
• “How did you work out that projectiles follow parabolas?”
• “What did it cost you to keep working while blind?”
• “What does it mean that nature is written in mathematics?”

What Galileo changed

Galileo established the telescope as an astronomical instrument, provided the observations that made the Sun-centred system credible, and — in Two New Sciences — created the mathematics of motion that Newton built on. His insistence that physical claims are decided by measurement rather than authority is the foundation of experimental science.

A debate that continues

Historians dispute how far the trial was about astronomy at all rather than about scriptural authority, papal politics and Galileo's own tactlessness, and continue to argue over the status of the disputed 1616 injunction.

Keep exploring — ask Galileo

• “How should a scientist behave when the evidence is not yet decisive?”
• “What did you most want your Italian readers to understand?”
• “What would have persuaded your opponents?”

Related lives

• Johannes Kepler — Discoverer of Planetary Motion
• Isaac Newton — Lucasian Professor · President of the Royal Society
• Michelangelo — Il Divino · Sculptor of the Sistine

Related themes

The Scientific Revolution · Forces and motion · Science and religion