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 an expert in many areas of science and was born in the city of Pisa, which was part of the region known as Florence. He was called the father of observational astronomy, modern physics, the scientific method, and modern science.

Galileo studied speed, gravity, the movement of objects, and the idea that motion continues unless stopped by force. He also worked in practical science and technology, describing how pendulums swing and how objects float or sink in water. He helped develop the thermoscope and created tools used in warfare, such as military compasses. Using a telescope he improved, he observed stars in the Milky Way, the changing appearance of Venus, the four largest moons of Jupiter, Saturn’s rings, craters on the Moon, and dark spots on the Sun. He also built an early microscope.

Galileo supported the idea that the Earth orbits the Sun, which was not accepted by the Catholic Church or some scientists. The Roman Inquisition looked into his ideas in 1615 and said they went against religious beliefs.

Later, Galileo wrote a book called Dialogue Concerning the Two Chief World Systems (1632), which seemed to criticize Pope Urban VIII and caused problems with the Pope and the Jesuits, who had previously supported him. He was tried by the Inquisition, found guilty of heresy, and forced to say he was wrong. He spent the rest of his life under house arrest. During this time, he wrote Two New Sciences (1638), which focused on movement and the strength of materials.

Early life and family

Galileo was born in Pisa (then part of the Duchy of Florence) on February 15, 1564. He was the first of six children of Vincenzo Galilei, a skilled lutenist, composer, and music theorist, 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 skilled 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 the dowries promised to his brothers-in-law, leading to legal disputes. He also borrowed money from Galileo to support his music and travels. These financial issues may have encouraged Galileo to create inventions to earn more money.

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 name shared the same origin as his family name, Galilei. Both his name and family name came from an ancestor, Galileo Bonaiuti, a respected physician, professor, and politician in Florence during the 15th century. When Galileo used his full name, it was sometimes “Galileo Galilei Linceo,” referencing his membership in the Accademia dei Lincei, an elite science group in the Papal States. It was common in 16th-century Tuscany for the eldest son to take the family name, so Galileo Galilei was not necessarily named after his ancestor Galileo Bonaiuti.

The Italian name “Galileo” (and the surname “Galilei”) comes from the Latin “Galilaeus,” meaning “of Galilee.” This biblical name was used in a joke. In 1614, during the Galileo affair, a Dominican priest named Tommaso Caccini criticized Galileo in a sermon, quoting the Book of Acts: “Men of Galilee, why do you stand looking into heaven?”

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

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 often ill. Vincenzo later became Galileo’s legal heir and married Sestilia Bocchineri.

Career and first scientific contributions

Galileo seriously considered becoming a priest when he was young. However, his father encouraged him to attend the University of Pisa in 1580 to study medicine. He was influenced by lectures from teachers such as Girolamo Borro, Domingo de Soto, and Francesco Buonamici in Florence. In 1581, while studying medicine, he noticed a swinging chandelier. He observed that the chandelier’s swings took the same amount of time, no matter how far it moved. When he returned home, he tested this by creating two pendulums of equal length. One was swung with a large arc, and the other with a small arc. Both kept time together. It was not until nearly 100 years later, with the work of Christiaan Huygens, that the pendulum’s tautochrone property was used to build accurate timepieces.

At first, Galileo avoided mathematics because physicians earned more money than mathematicians. However, after accidentally attending a geometry lecture, he convinced his father to let him study mathematics and natural philosophy instead of medicine. He created a thermoscope, a precursor to the thermometer, and published a book in 1586 about a hydrostatic balance he invented. This invention brought him recognition in academic circles. He also studied disegno, which included fine art, and in 1588 became an instructor at the Accademia delle Arti del Disegno in Florence. There, he taught perspective and chiaroscuro. That same year, he gave two lectures to the Florentine Academy about Dante’s Inferno, proposing a scientific model of its structure. Inspired by Renaissance art, 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 care of 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 linked to mathematics, astronomy, and medicine at the time. He also worked on hydraulic engineering, earning a patent for a horse-powered water pump in 1594.

Tycho Brahe and others had observed a supernova in 1572. In 1605, Ottavio Brenzoni wrote to Galileo about this event and another nova from 1601. Galileo later observed Kepler’s Supernova in 1604. Since these stars showed no detectable diurnal parallax, he concluded they were far away, disproving the idea that the heavens were unchanging, as Aristotle had claimed.

In 1609, Galileo built a telescope with about 3× magnification, inspired by descriptions of Hans Lippershey’s invention in the Netherlands. 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 early telescopes to Venetian lawmakers. He sold telescopes to merchants, who found them useful for navigation and trade. In March 1610, he published his first telescopic 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 telescopes before, Galileo was the first to explain that its uneven surface was caused by mountains and craters blocking sunlight. He also created detailed maps of the Moon’s surface, estimating the heights of its mountains. His observations showed the Moon was not a perfect sphere, as Aristotle had believed, nor was it a "planet" as Dante had described. Galileo is sometimes credited with discovering lunar libration in latitude in 1632, though others may have discovered it earlier.

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

On 7 January 1610, Galileo observed three small objects near Jupiter. Over the next few nights, he noticed these objects changed position relative to Jupiter, suggesting they were not fixed stars. On 10 January, one of the objects disappeared, which he believed was because it was hidden behind Jupiter. By 15 January, he concluded they were orbiting Jupiter. This discovery supported Copernicus’ idea that Earth and other planets orbit the Sun. Galileo named the four moons the Medicean stars, honoring Cosimo II de’ Medici and his brothers. Later, they were renamed Galilean satellites in his honor. Simon Marius independently discovered them in 1610 and gave them the names Io, Europa, Ganymede, and Callisto.

Galileo’s discovery of Jupiter’s moons caused controversy. The idea that a planet had smaller planets orbiting it contradicted the belief that all heavenly bodies revolved around Earth. Some astronomers struggled to confirm his findings. When he demonstrated his telescope in Bologna, some attendees saw double stars, like Spica Virginis, and questioned the telescope’s accuracy. However, Christopher Clavius’s observatory in Rome confirmed the moons’ existence, and Galileo received praise when he visited Rome. Over the next 18 months, Galileo made precise measurements of the moons’ orbits, a task Johannes Kepler had thought impossible.

Galileo saw a practical use for his discovery. Accurate navigation at sea required knowing a ship’s east-west position, which depended on synchronizing ship clocks with clocks at the prime meridian. Solving the longitude problem was vital for safe travel and offered large rewards from Spain and later Holland.

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 tested how the tension in a stretched string affects its pitch. His findings showed that pitch changes in a way that is not a straight line, which was an early example of a non-linear relationship in physics. This idea connected to the Pythagorean tradition of music, where dividing a string into whole numbers creates musical scales. This tradition showed that math had long been linked to music and science, and young Galileo saw his father’s work as an extension of that knowledge.

Galileo was one of the first scientists to clearly say that natural laws can be described with math. In his book The Assayer, he wrote that the universe is written in the language of mathematics, using shapes like triangles and circles. His math work built on ideas from earlier scholars, which he studied during his time learning philosophy. His research helped separate science from philosophy and religion, a major step in human thinking. He was open to changing his ideas if experiments showed something different.

To test his theories, Galileo needed ways to measure length and time consistently so results could be compared across different experiments. This allowed scientists to use math and observations to confirm laws. Galileo understood how math, theory, and experiments work together. He studied parabolas, which are U-shaped curves, and showed that a projectile, like a cannonball, follows a parabolic path when only gravity acts on it. He noted that for very large objects, like Earth-sized ones, this might not be true, but for smaller distances, like those used in his time, the path is close to a parabola.

Using a refracting telescope, Galileo discovered the Moon’s surface was not smooth. In 1610, he saw Jupiter’s four largest moons and observed Venus’s phases. He also thought Saturn’s rings were two separate planets. In 1612, he noticed Neptune moving but did not recognize it as a planet.

Galileo studied sunspots, the Milky Way, and stars, including how to measure their size without a telescope. He created the term "Aurora Borealis" in 1619 to describe lights in the sky caused by solar wind interacting with Earth’s magnetosphere.

Galileo contributed to engineering, not just physics. He designed a geometric and military compass for gunners and surveyors, improving earlier tools. This device helped calculate cannonpowder amounts and construct shapes. He sold over 100 of these tools with instructions.

In 1593, he built a thermometer using air expansion to move water. In 1609, he used a refracting telescope to observe celestial objects, and the word "telescope" was coined for his invention. He later used a microscope, and the term "microscope" was created from Greek words meaning "to look at small things." His microscope images were among the first clear records of compound microscopes.

In 1612, Galileo proposed using Jupiter’s moons as a clock to measure longitude, but practical challenges made this difficult. This method was later used for land surveys and by explorers like Zebulon Pike. For sea travel, a portable marine chronometer, like John Harrison’s, was eventually needed. Late in life, he designed a pendulum clock escapement, though it was not built until after Christiaan Huygens made the first pendulum clock.

Galileo advised on engineering projects to control river flooding, such as a plan for the Bisenzio River in 1630. He noted that ball bearings can cause friction when they rub together, and suggested enclosing them in a cage to reduce this. This idea was first described by Galileo in the 17th century.

Galileo’s work on motion, along with Kepler and Descartes, helped lead to classical mechanics developed by Isaac Newton. He tested pendulums, and it is said he began by observing a chandelier in Pisa’s cathedral, using his pulse as a timer. His first recorded interest in pendulums was in a book published after his death.

Death

Galileo continued to meet with visitors until his death on January 8, 1642, when he was 77 years old. He died after suffering from a fever and a fast heartbeat. 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 ancestors. He also planned to build a stone monument in Galileo’s honor.

These plans were not carried out, however, because Pope Urban VIII and his nephew, Cardinal Francesco Barberini, objected. They did so because Galileo had been judged by the Catholic Church for "strong suspicion of heresy." Instead, Galileo was buried in a small room next to the novices' chapel, located at the end of a corridor near the southern transept of the basilica.

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

Legacy

The Galileo affair was mostly forgotten after Galileo's death, and the conflict over his ideas became less important. In 1718, the Inquisition allowed the printing of Galileo's works again, but the controversial Dialogue was not included. In 1741, Pope Benedict XIV approved the publication of Galileo's full scientific writings, which included a version of the Dialogue that had some parts removed. In 1758, the Church stopped banning books that supported the idea that the Earth orbits the Sun, but it still banned uncensored versions of the Dialogue and Copernicus's book, De Revolutionibus. In 1835, these works were finally removed from the Church's list of banned books, ending official opposition to the heliocentric theory.

Interest in the Galileo affair grew again in the early 1800s when Protestant writers used it to criticize Roman Catholicism. People have continued to discuss the event over time. In 1939, Pope Pius XII praised Galileo in a speech to the Pontifical Academy of Sciences, calling him a brave scientist who faced challenges without fear. His advisor, Robert Leiber, noted that Pius XII supported science and felt sorry that Galileo had been treated unfairly.

In 1990, Cardinal Ratzinger (later Pope Benedict XVI) spoke about the Galileo affair, saying it showed how modern society and science struggle with uncertainty. He mentioned the philosopher Paul Feyerabend, who claimed the Church acted reasonably by considering the effects of Galileo's ideas. However, the Cardinal did not say if he agreed with Feyerabend. He warned against using these views to quickly defend the Church.

In 1992, Pope John Paul II admitted the Inquisition had made a mistake by condemning Galileo for proving the Earth moves around the Sun. He said the Church's leaders at the time did not understand the difference between the Bible and how it was interpreted.

In 2008, the head of the Pontifical Academy of Sciences, Nicola Cabibbo, planned to build a statue of Galileo inside the Vatican. Later that year, Pope Benedict XVI praised Galileo's work in astronomy. However, the statue plan was later stopped.

Stephen Hawking said Galileo played a major role in starting modern science, and Albert Einstein called him the father of modern science. Einstein wrote that Galileo fought against ideas based on authority, believing only evidence and careful thinking could prove truth.

John G. Simmons noted that Galileo helped change how science was viewed. His discoveries, such as the four moons of Jupiter (Io, Europa, Ganymede, and Callisto), are now called the Galilean moons. Other scientific terms and projects, like the Galileo spacecraft, are named after him.

In 2009, the United Nations chose the year as the International Year of Astronomy to celebrate the 400th anniversary of Galileo's first telescope observations.

Writings

Galileo wrote about scientific tools in his early works. One example is a 1586 book called The Little Balance that explains how to measure objects in air or water. Another is a 1606 book titled Le Operazioni del Compasso Geometrico et Militare, which describes how to use a tool for geometry and military purposes.

His early studies on motion and mechanics include a work from around 1590 called On Motion and another from around 1600 called Mechanics. These writings used older ideas about how objects move in fluids and how their weight affects falling speed. They also discussed a theory that movement slows down over time unless something keeps it going.

In 1610, Galileo published The Starry Messenger, the first scientific book based on telescope observations. It described:
– moons orbiting Jupiter
– the rough surface of the Moon
– many stars not visible to the naked eye, especially those in the Milky Way
– differences between planets and stars, with planets appearing as small circles and stars as points of light

In 1613, Galileo wrote Letters on Sunspots, which reported his telescope observations of sunspots and the full cycle of Venus’s phases. He also noted mysterious features around Saturn that later disappeared. In 1615, he wrote a letter to the Grand Duchess Christina, revising an earlier letter that was criticized by the Inquisition. In 1616, after being told not to support the Copernican view, Galileo wrote a private letter to a cardinal about the tides, based on the idea that Earth moves. In 1619, one of his students published a work arguing against a Jesuit theory about comets.

In 1623, Galileo wrote The Assayer, which criticized ideas based on Aristotle’s authority and promoted experiments and math in science. Pope Urban liked the book so much he had it read to him. In 1632, Galileo published Dialogue Concerning the Two Chief World Systems, which supported the Copernican theory. This led to his trial and a ban on publishing. In 1638, he published Discourses and Mathematical Demonstrations Relating to Two New Sciences in Holland, outside the Inquisition’s control.

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 later years, Galileo had a library of at least 598 books at Villa Il Gioiello near Florence. While under house arrest, he could not write or publish but still met visitors who brought him new scientific books.

Galileo’s will did not mention his books, but after his death, most of his possessions, including his library, went to his son, Vincenzo Galilei Jr. His wife, Sestilia Bocchineri, inherited the collection after his son’s death in 1649.

Vincenzo Viviani, Galileo’s former assistant, collected Galileo’s books and papers to publish them. This plan never happened, and Viviani left many items to the Hospital of Santa Maria Nuova in Florence. Some books were sent to other libraries, like the Biblioteca Comunale degli Intronati in Siena and the Biblioteca Magliabechiana, which later became the Biblioteca Nazionale Centrale di Firenze.

A small part of Viviani’s collection, including Galileo’s manuscripts and works by others, went to his nephew, Abbot Jacopo Panzanini. After Panzanini’s death, the collection passed to his great-nephews, Carlo and Angelo Panzanini. Over time, the books were sold or lost. In the 1700s, a Florentine senator named Giovanni Battista Clemente de’Nelli bought the remaining books and manuscripts, realizing their value.

Nelli cared for the collection until his death in 1793. His sons tried to sell it to the French government, but the Grand Duke of Tuscany, Ferdinand III, bought it instead. The archive was then placed in the Biblioteca Palatina.