Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), often called Galileo Galilei, was an Italian astronomer, physicist, and engineer. He was born in Pisa, which was part of the Duchy of Florence at the time. Galileo is known as the father of observational astronomy, modern classical physics, the scientific method, and modern science.

Galileo studied speed, velocity, gravity, free fall, the principle of relativity, inertia, and projectile motion. He also worked in applied science and technology, describing the properties of pendulums and tools for measuring weight in water. He helped develop the thermoscope and invented several military compasses. Using an improved telescope, he observed stars in the Milky Way, the phases of Venus, Jupiter’s four largest moons, Saturn’s rings, lunar craters, and sunspots. He also built an early microscope.

Galileo supported the idea that Earth orbits the Sun, which was called Copernican heliocentrism. This caused conflict with the Catholic Church and some astronomers. In 1615, the Roman Inquisition investigated his ideas and concluded they contradicted accepted interpretations of the Bible.

Later, Galileo defended his views in a book called Dialogue Concerning the Two Chief World Systems (1632). This book seemed to argue against and mock Pope Urban VIII, causing the Pope and the Jesuits to turn against Galileo. He was tried by the Inquisition, found "vehemently suspect of heresy," and forced to recant. He spent the rest of his life under house arrest. During this time, he wrote Two New Sciences (1638), which focused on the study of motion and the strength of materials.

Early life and family

Galileo was born in Pisa (then part of Florence) on February 15, 1564. He was the first of six children of Vincenzo Galilei, a skilled lutenist, composer, and music teacher, and Giulia Ammannati, the daughter of a wealthy merchant. Vincenzo and Giulia married in 1562, when he was 42 and she was 24. Galileo became a talented lutenist himself.

Three of Galileo’s five siblings lived past infancy. The youngest, Michelangelo (or Michelagnolo), also became a lutenist and composer. He caused financial problems for Galileo for the rest of his life. Michelangelo could not pay his share of his father’s promised dowries to his brothers-in-law, who later tried to take legal action to collect the money. Michelangelo also sometimes borrowed money from Galileo to support his music and travels. These financial challenges may have influenced Galileo’s early interest in creating inventions to earn more income.

When Galileo was eight, his family moved to Florence, but he stayed with Muzio Tedaldi for two years. At age ten, he joined his family in Florence and studied under Jacopo Borghini. From 1575 to 1578, he studied subjects like logic at the Vallombrosa Abbey, located about 30 kilometers (19 miles) southeast of Florence.

Galileo often used only his first name. At the time, surnames were not required in Italy, and his first name, Galileo, shared the same origin as his family name, Galilei. Both names came from an ancestor named Galileo Bonaiuti, a respected physician, professor, and politician in Florence during the 15th century. When Galileo used both names, he sometimes wrote "Galileo Galilei Linceo," referring to his membership in the Accademia dei Lincei, an elite science group in the Papal States. It was common for families in Tuscany during the mid-16th century to name their eldest son after the family surname. Therefore, Galileo Galilei was not necessarily named after his ancestor, Galileo Bonaiuti.

The Italian name "Galileo" (and the surname "Galilei") comes from the Latin word "Galilaeus," meaning "from Galilee." This biblical name was the subject of a (supposed) pun. In 1614, during the Galileo affair, a Dominican priest named Tommaso Caccini criticized Galileo in a sermon, quoting the Bible: "Men of Galilee, why do you stand looking up into heaven?"

Though a devout Catholic, Galileo had three children out of wedlock with Marina Gamba: two daughters, Virginia (born 1600) and Livia (born 1601), and a son, Vincenzo (born 1606). Because the children were born outside of marriage, Galileo believed the daughters could not marry and would need costly support or very expensive dowries, similar to the financial issues he faced with two of his sisters. Their only acceptable option was to become nuns. Both daughters joined the convent of San Matteo in Arcetri.

Virginia took the name Maria Celeste when she entered the convent. She died on April 2, 1634, and is buried with Galileo at the Basilica of Santa Croce in Florence. Livia took the name Sister Arcangela and was sick most of her life. Vincenzo later became Galileo’s legal heir and married Sestilia Bocchineri.

Career and first scientific contributions

Although Galileo seriously considered becoming a priest as a young man, his father encouraged him to attend the University of Pisa in 1580 to study medicine. He was influenced by teachers such as Girolamo Borro, Domingo de Soto, and Francesco Buonamici from Florence. In 1581, while studying medicine, he noticed a swinging chandelier whose motion was affected by air currents. He observed that the chandelier took the same amount of time to swing back and forth, regardless of how far it moved. When he tested this idea at home using two pendulums of equal length, he found they kept time together. It was not until nearly 100 years later, with the work of Christiaan Huygens, that the pendulum's consistent timing was used to create accurate clocks. At the time, Galileo had avoided studying mathematics because physicians earned more money than mathematicians. However, after attending a geometry lecture, he convinced his father to let him switch to studying mathematics and natural philosophy instead of medicine. He created a thermoscope, an early version of a thermometer, and in 1586 published a book about a hydrostatic balance he invented, which earned him recognition from scholars. Galileo also studied disegno, a term for fine art, and in 1588 became an instructor at the Accademia delle Arti del Disegno in Florence, teaching perspective and chiaroscuro. That same year, he gave two lectures on Dante’s Inferno to the Florentine Academy, proposing a scientific model of its structure. Inspired by the city’s artistic traditions, Galileo developed an appreciation for aesthetics. While teaching at the Accademia, he formed a lifelong friendship with the painter Cigoli.

In 1589, Galileo was appointed to the mathematics chair at the University of Pisa. In 1591, his father died, and Galileo took responsibility for caring for his younger brother, Michelagnolo. In 1592, he moved to the University of Padua, where he taught geometry, mechanics, and astronomy until 1610. During this time, he made important discoveries in both basic science and practical applications. He studied astrology, which was then linked to mathematics, astronomy, and medicine. He also worked on hydraulic engineering, earning a patent in 1594 for a horse-powered water pump.

Tycho Brahe and others had observed a supernova in 1572. In 1605, Galileo learned about this event and another in 1601 through a letter from Ottavio Brenzoni. He later studied the 1604 supernova, noting that these stars showed no detectable movement due to Earth’s rotation, which led him to conclude they were distant stars. This finding challenged the idea that the heavens were unchanging, as Aristotle had claimed.

Inspired by descriptions of the first telescope, which Hans Lippershey tried to patent in 1608, Galileo built his own telescope in 1609 with about 3× magnification. He later improved it to 30× magnification. His telescope produced upright, magnified images of Earth and was used for observing the sky. On 25 August 1609, he demonstrated one of his telescopes to Venetian lawmakers. He sold his telescopes to merchants who found them useful for navigation and trade. In March 1610, he published his early telescope observations in a book called Sidereus Nuncius (Starry Messenger).

On 30 November 1609, Galileo used his telescope to observe the Moon. Though others had seen the Moon through a telescope before, Galileo was the first to explain that its uneven surface was caused by mountains and craters blocking sunlight. He made detailed maps of the Moon and estimated the heights of its mountains. His findings showed the Moon was not a perfect, translucent sphere, as Aristotle had believed, nor was it the "eternal pearl" described by Dante. Galileo is sometimes credited with discovering lunar libration in 1632, though others may have done so earlier.

Cigoli, a friend of Galileo, painted a realistic image of the Moon, likely using his own telescope for observation.

On 7 January 1610, Galileo noticed three faint stars near Jupiter. Over the next few days, he observed that these stars moved relative to Jupiter, suggesting they were not fixed stars but instead orbiting it. He concluded they were four moons orbiting Jupiter, a discovery that supported Copernicus’ idea that Earth and other planets orbit the Sun. He named the moons the Medicean stars in honor of Cosimo II de’ Medici and his brothers. Later, they were renamed Galilean satellites after Galileo. Simon Marius independently discovered the moons in 1610 and gave them the names Io, Europa, Ganymede, and Callisto, which are still used today.

Galileo’s discovery of Jupiter’s moons caused debate among scientists. The idea that a planet could have smaller planets orbiting it contradicted the belief that all celestial bodies revolved around Earth. Some astronomers struggled to confirm his findings, while others, like Christopher Clavius, confirmed them and welcomed Galileo as a hero. Over the next 18 months, Galileo made precise measurements of the moons’ orbits, a task Johannes Kepler had thought impossible.

Galileo saw practical value in his discovery. Accurate timekeeping was essential for determining a ship’s position at sea, which was critical for safe navigation. Solving the longitude problem was important for maritime travel, and large rewards were offered by Spain and later by Holland for solutions.

Scientific contributions

Galileo Galilei made important discoveries about how objects move by combining experiments with math. At the same time, other scientists like William Gilbert studied magnetism and electricity in a more descriptive way. Galileo’s father, Vincenzo Galilei, was a musician and theorist who discovered that the pitch of a stretched string changes based on the square root of the tension. This idea connected music and physics, as instrument makers had long known that dividing a string into whole-number parts creates harmonious sounds. These early links between math, music, and science influenced young Galileo.

Galileo was one of the first scientists to clearly state that the laws of nature can be described using math. In The Assayer, he wrote that the universe is written in the language of mathematics, using shapes like triangles and circles. His work built on ideas from earlier philosophers and helped separate science from philosophy and religion. He believed that observations and experiments could change scientific ideas.

To compare measurements across different experiments, Galileo created standards for measuring length and time. This allowed scientists to test mathematical rules using observations. He understood that the path of a moving object, like a projectile, follows a parabola. He noted that this shape is ideal for objects moving under gravity without air resistance. However, he acknowledged that for very large objects, like those the size of Earth, the path might not be a perfect parabola.

Galileo used a refracting telescope to observe the Moon’s rough surface and the four largest moons of Jupiter. He also saw the phases of Venus and studied Saturn, though he thought its rings were two separate planets. In 1612, he noticed Neptune moving in the sky but did not recognize it as a planet. He studied sunspots, the Milky Way, and stars, including how to measure their size without a telescope.

He coined the term Aurora Borealis in 1619 to describe lights in the sky caused by solar wind interacting with Earth’s magnetic field.

Galileo contributed to engineering by improving a geometric and military compass used by soldiers and surveyors. This tool helped calculate gunpowder amounts and construct shapes. He also built a thermometer that used air expansion to measure temperature.

In 1609, Galileo was among the first to use a refracting telescope for astronomy. The word telescope was later created for his instrument. He also used a microscope to study tiny objects, and the word microscope was coined to describe his invention.

Galileo proposed using the orbits of Jupiter’s moons as a way to measure longitude, but this method was difficult to use at sea. Later, a portable clock called a marine chronometer solved this problem. Near the end of his life, he designed a pendulum clock escapement, though it was not built until after Christiaan Huygens created the first working pendulum clock.

Galileo advised on engineering projects to control flooding, such as a plan to redirect a river near Florence.

He studied ball bearings and suggested enclosing them in cages to reduce friction between the balls.

Galileo’s work on motion, along with studies by Johannes Kepler and René Descartes, helped lead to the development of classical mechanics by Isaac Newton.

Galileo experimented with pendulums, possibly inspired by watching a chandelier swing in a cathedral. His observations contributed to understanding how pendulums move.

Death

Galileo continued to meet visitors until his death on January 8, 1642, when he was 77 years old. He died from a fever and heart problems. The Grand Duke of Tuscany, Ferdinando II, wanted to bury Galileo in the main part of the Basilica of Santa Croce, near the tombs of his father and other family members. He also planned to build a marble mausoleum to honor Galileo.

However, these plans were stopped because Pope Urban VIII and his nephew, Cardinal Francesco Barberini, objected. They did this because Galileo had been declared guilty by the Catholic Church of "vehement suspicion of heresy." Instead, Galileo was buried in a small room near the novices' chapel, which is located at the end of a corridor from the southern transept of the basilica to the sacristy.

In 1737, Galileo was reburied in the main part of the basilica after a monument was built there in his honor. During this move, three fingers and a tooth were taken from his remains. One of the fingers is now displayed at the Museo Galileo in Florence, Italy.

Legacy

After Galileo's death, the Galileo affair was mostly forgotten, and the controversy decreased. The Inquisition lifted the ban on reprinting Galileo's works in 1718, allowing publication of his writings (excluding the condemned Dialogue) in Florence. In 1741, Pope Benedict XIV allowed the publication of Galileo's complete scientific works, including a slightly censored version of the Dialogue. In 1758, the general ban on books supporting heliocentrism was removed from the Index of prohibited books. However, the specific ban on uncensored versions of the Dialogue and Copernicus's De Revolutionibus remained. In 1835, the church no longer opposed heliocentrism after removing these works from the Index.

Interest in the Galileo affair grew again in the early 1800s when Protestants used it to criticize the Catholic Church. Interest in the affair has increased and decreased over time. In 1939, Pope Pius XII, in his first speech to the Pontifical Academy of Sciences, called Galileo one of the "most daring heroes of research… not afraid of the challenges or risks on the way." His advisor, Robert Leiber, wrote that Pius XII was careful not to block scientific progress too soon and regretted the treatment of Galileo.

In 1990, Cardinal Ratzinger (later Pope Benedict XVI) mentioned current views on the Galileo affair, calling it an example that shows how modern times and science question themselves. He cited philosopher Paul Feyerabend, who said the Church was more reasonable than Galileo and considered the consequences of his ideas. Cardinal Ratzinger did not clearly agree or disagree with Feyerabend's statements. He said it would be unwise to use such views to create a quick apology.

In 1992, Pope John Paul II admitted the Inquisition made a mistake in condemning Galileo for saying Earth revolves around the Sun. He noted that the theologians who judged Galileo did not understand the difference between the Bible and its interpretation.

In 2008, Nicola Cab

Writings

Galileo's early writings about scientific tools include a 1586 book called The Little Balance (La Billancetta), which explains how to use a balance to measure the weight of objects in air or water. He also wrote a 1606 manual titled Le Operazioni del Compasso Geometrico et Militare, which describes how to use a geometrical and military compass.

His early writings about motion and mechanics include a work from around 1590 called De Motu (On Motion) and another from around 1600 called Le Mecaniche (Mechanics). De Motu was based on ideas from ancient scientists and suggested that the speed of an object falling in a fluid depends on how much heavier it is than the fluid. It also included a theory that moving objects lose energy over time, meaning they would eventually stop moving unless acted on by another force.

In 1610, Galileo published The Starry Messenger (Sidereus Nuncius), the first scientific book based on observations made with a telescope. This book described:
– the four moons of Jupiter
– the rough, uneven surface of the Moon
– many stars not visible to the naked eye, especially those that make up the Milky Way
– differences between the appearance of planets and stars, with planets looking like small discs and stars appearing as tiny points of light

In 1613, Galileo wrote Letters on Sunspots, which described his telescope observations of dark spots on the Sun and suggested that the Sun and heavens might not be perfect. This work also included his observations of Venus’s full set of phases and the strange "appendages" of Saturn, which later disappeared. In 1615, he wrote a letter to the Grand Duchess Christina, which was not published until 1636. This letter was a revised version of an earlier letter that had been criticized by the Inquisition. In 1616, after being told not to support the Copernican idea that Earth moves around the Sun, Galileo wrote a private letter to a cardinal called Discourse on the Tides, which used the Copernican model to explain ocean movements. In 1619, one of his students published a work called Discourse on the Comets, which argued against a religious group’s explanation of comets.

In 1623, Galileo published The Assayer (Il Saggiatore), which criticized ideas based on ancient authority and supported using experiments and math to study science. The book was very popular, and the Pope even had it read to him. In 1632, Galileo published Dialogue Concerning the Two Chief World Systems (Dialogo sopra i due massimi sistemi del mondo), which compared the Copernican model with the traditional Earth-centered model. Even though he tried to follow rules set by the Inquisition, the book’s support for the Copernican idea led to Galileo being punished and banned from publishing. In 1638, he published Discourses and Mathematical Demonstrations Relating to Two New Sciences (Discorsi e Dimostrazioni Matematiche, intorno a due nuove scienze) in Holland, where the Inquisition had no power.

Galileo’s main written works include:
– The Little Balance (1586; in Italian: La Bilancetta)
– On Motion (c. 1590; in Latin: De Motu Antiquiora)
– Mechanics (c. 1600; in Italian: Le Mecaniche)
– The Operations of Geometrical and Military Compass (1606; in Italian: Le operazioni del compasso geometrico et militare)
– The Starry Messenger (1610; in Latin: Sidereus Nuncius)
– Discourse on Floating Bodies (1612; in Italian: Discorso intorno alle cose che stanno in su l'acqua, o che in quella si muovono)
– History and Demonstration Concerning Sunspots (1613; in Italian: Istoria e dimostrazioni intorno alle macchie solari)
– Letter to the Grand Duchess Christina (1615; published in 1636)
– Discourse on the Tides (1616; in Italian: Discorso del flusso e reflusso del mare)
– Discourse on the Comets (1619; in Italian: Discorso delle Comete)
– The Assayer (1623; in Italian: Il Saggiatore)
– Dialogue Concerning the Two Chief World Systems (1632; in Italian: Dialogo sopra i due massimi sistemi del mondo)
– Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638; in Italian: Discorsi e Dimostrazioni Matematiche, intorno a due nuove scienze)

In his final years, Galileo kept a library of at least 598 books at Villa Il Gioiello, near Florence. While under house arrest, he was not allowed to write or publish, but he still met visitors who brought him new scientific books from Europe.

Galileo’s will did not mention his books or papers. After his death, his library was given to his son, Vincenzo Galilei Jr. When Vincenzo died in 1649, the collection went to his wife, Sestilia Bocchineri.

Vincenzo Viviani, a former student of Galileo, collected Galileo’s books, papers, and unedited manuscripts to preserve them. Though Viviani planned to publish them, he never completed the project. In his will, Viviani gave many of the items to a hospital in Florence. Some books were sent to other libraries, like the Biblioteca Comunale degli Intronati in Siena. Later, books not related to medicine were moved to the Biblioteca Magliabechiana, which became part of the Biblioteca Nazionale Centrale di Firenze.

A small group of Galileo’s manuscripts, along with works by his friends Evangelista Torricelli and Benedetto Castelli, was left to Viviani’s nephew, Abbot Jac