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In the scientific firmament, Galileo’s star shines as brightly as that of Newton or Einstein. Yet how many of us know much about his life beyond his interest in the heavens and his troubles with the Church (which Pope John Paul II officially ended in 1992, 350 years after Galileo’s death)? In this timeline, turn back the clock to the late Italian Renaissance and relive the dramatic life of one of history’s foremost scientific geniuses. Lexi Krock


Galileo Galilei is born in Pisa on February 15. He is the first child of Vincenzo Galilei of Florence, a music teacher, and Giulia degli Ammannati of Pescia.


The city of Pisa

1579 Galileo studies Greek, Latin, and logic at the Benedictine monastery of Santa Maria di Vallombrosa and considers becoming a monk until his father expresses displeasure at the idea.
1581 Galileo begins his studies in September at the University of Pisa, where he studies medicine and mathematics. Though he is a diligent medical student, mostly to satisfy his father’s wish that he become a doctor, Galileo prefers mathematics.

Galileo, now 21, leaves the University of Pisa without a degree after four years of study. He spends the next four years giving private lessons in mathematics in Florence and Siena.


Sixteenth-Century Florence

1589 Galileo takes a teaching position at the University of Pisa. He refuses to wear the standard academic regalia, a black robe, dismissing the sartorial tradition as pretentious and cumbersome. University officials repeatedly impose fines on him for this transgression.
1591 Galileo’s father dies at 70, and Galileo becomes the primary financial provider for his family, which includes his mother, his married sister Virginia (whose dowry requires regular payments), his 16-year-old brother Michelangelo, and his unmarried sister Livia. Three other siblings died during childhood.

In December, Galileo becomes chair of the mathematics department at the University of Padua in the Republic of Venice. He gives lectures on geometry and astronomy in addition to private lessons on Euclid, Cosmography, and other subjects.


Euclid, Greek geometer, circa 300 B.C.

1595 Galileo develops his theory of the tides, asserting that they ebb and flowin relation to the Earth’s diurnal and annual movements. His theory, though elegantly conceived, is incorrect.

Galileo’s Big Mistake

For most of his adult life, from about the age of 30 until his death at 78 in the year 1642, Galileo propounded his theory of how the tides work. Yet a lengthy summary of our understanding of the tides that the British physicist Lord Kelvin published in 1882 mentions Galileo only in passing.

Why the cursory treatment? Because Galileo’s theory of the tides, while extremely well thought out and eminently reasonable considering the state of scientific knowledge at the time, was wrong. Indeed, it was the most significant stumble in an otherwise brilliant scientific career. A look back at Galileo’s big mistake offers clues to how and why such a great thinker could have made it, and what lessons it can offer us today.


Galileo appears before the Inquisition, circa 1632, in a 19th-century painting by Joseph-Nicolas Robert-Fleury. Photo credit: Corbis Images

A planet like a barge

It was in 1595 when Galileo, just shy of his 30th birthday, first came up with his explanation for the tides. The idea occurred to him while traveling on a barge that was ferrying freshwater to Venice. He noticed that whenever the barge’s speed or direction altered, the freshwater inside sloshed around accordingly. If the vessel suddenly ground to a halt on a sandbar, for instance, the water pushed up towards the bow then bounced back toward the stern, doing this several times with ever decreasing agitation until it returned to a level state.

Galileo realized that the Earth’s dual motion—its daily one around its axis and its annual one around the sun—might have the same effect on oceans and other great bodies of water as the barge had on its freshwater cargo. The key, as Galileo saw it, was that even though we don’t sense it, different parts of our planet move at different speeds depending on the time of day. It’s as if the Earth were a barge, which sped up, slowed down, and periodically changed direction.

Galileo fervently believed Copernicus was right, and he would tell the world.

In his “Treatise on the Tides,” a paper he wrote in 1616, Galileo used a diagram to explain why he thought this was true (see animated version below). In the animation, the Earth revolves in a counterclockwise direction both on its axis and around the sun.

Galileo1Galileo demonstrated that the combination of the two motions—the so-called “absolute motion”—is always fastest on the part of the circle near A, because that part is moving in the same direction for both the daily and annual movements. And it is always slowest on the part of the circle near B, because that part is moving in opposite directions for the daily and annual movements. He determined that because the two absolute motions are additive on one side of the globe and subtractive on the other, the extreme ends of any sufficiently large basin of water, such as the Mediterranean Sea, would be moving at different speeds. The daily acceleration and deceleration of that body of water, he reasoned, effects a sloshing within it—the tides.

The chief objection to Galileo’s argument was that his model should have called for only one high tide a day, whereas there are roughly two. Galileo explained this away by stating that many other factors play a role in creating a specific tidal situation. These include the length of a basin, its orientation, its depth, the shape of its coasts, the effect of winds, and so forth.

animated gif of earthGalileo thought the tides occurred because of the Earth’s daily rotation around its axis and its yearly circuit around the sun. For his reasoning, and an explanation of this animation, see article. Enlarge Photo

A sensible theory

Galileo thought he had discovered the correct explanation for the tides, one that would sink the other leading hypothesis of the day—namely, that the moon triggered the tides. In 1609, the German mathematician Johannes Kepler had written, “The sphere of influence of the attraction which is in the moon extends as far as the Earth, and incites the waters up from the torrid zone….” But because Kepler and other proponents of the moon theory could offer no scientific explanation for such influence, Galileo, who worked strictly on the basis of what he could see with his own eyes, found it impossible to believe; the theory smacked of the occult, he declared. (It wasn’t until Sir Isaac Newton published his law of universal gravitation in 1687 that Kepler’s “attraction” gained a firm scientific footing. Today, of course, most schoolchildren know that Kepler was right: The moon’s gravitational tug gives rise to the tides.)

Nicolaus CopernicusGalileo also thought his tidal theory provided powerful support for the notion that the Earth revolved around the sun (and not the other way around, as virtually everyone in the world then believed). This notion was first advanced in the mid-16th century by the Polish astronomer Nicolaus Copernicus, who maintained that our planet’s rotation and revolution around the sun accounted for the apparent movement of the heavenly bodies. Galileo believed in the Copernican rather than the Ptolemaic system, which had held sway ever since Aristotle and his successor Ptolemy. The Ptolemaic view, that the Earth was the center of the universe and all heavenly bodies revolved around it, was insinuated by Catholic teachings to be supported by the Bible and was therefore upheld by the Vatican.

Even as he published his tidal theory in 1616, however, Galileo may have had some lingering doubts about it. At the end of the “Treatise,” he says he hopes his idea “does not turn out to be delusive, like a dream which gives a brief image of truth followed by an immediate certainty of falsity. This I submit to the judgment of intelligent investigators.” Such scientific investigators would not appear for some time, and then, alas, they would judge him wrong.


Galileo knew in his heart that Copernicus (seen here) was right about about the planets revolving around the sun, and he eventually let fly with his Dialogue. Enlarge Photo credit: © Heritage Images/Corbis



In deep water

In the meantime, Galileo had to face investigators of a different stripe: inquisitors. Within two months, the Inquisition placed Copernicus’s book on the ‘Index of Prohibited Books.’ Called before the Pope’s theological adviser, Roberto Cardinal Bellarmino, Galileo was forced to agree not to defend or hold Copernicus’s doctrine.

For years, Galileo kept his thoughts on the matter close to his chest, but eventually he could contain himself no longer. He fervently believed Copernicus was right, and he would tell the world. In 1632, Galileo published his Dialogue Concerning the Two Chief World Systems. During the six years he had labored on it, Galileo’s working title for the book had been On the Flux and Reflux of the Sea, and indeed, he reprised his 1616 argument in the last of the book’s four sections.

Even Galileo’s greatest blunder reveals his greatest strength as a scientist.

All the old arguments were there. At one point, for example, one of the book’s three main characters, Salviati, who is a thinly disguised stand-in for Galileo, says, “Among all the famous men who have philosophized [about the tides], I wonder more at Kepler than any of the rest. Though he is a free and acute genius, he has lent his assent to the moon’s dominance over the oceans and to other occult happenings and other such trifles.”

In a dramatic way, Galileo also broke his promise to obey the Church’s mandate not to espouse the Copernican system. The Dialogue represents, as another superior thinker, Albert Einstein, put it in a foreword to a 1953 edition of the book, “a downright roguish attempt to comply with this order in appearance and yet in fact to disregard it. Unfortunately, it turned out that the Holy Inquisition was unable to appreciate adequately such subtle humor.” Galileo was placed under house arrest and the Dialogue banned.

Lord KelvinIn the only mention of Galileo in his classic 1882 treatise on the tides, Lord Kelvin wrote that the Italian astronomer found the idea that the moon caused the tides “a lamentable piece of mysticism….” Enlarge

A strenuous quest

Einstein had keen insights on why Galileo acted so rashly. First of all, it was Galileo’s longing to find a mechanical proof of the Earth’s motion, Einstein felt, that misled him into not only formulating but clinging so tenaciously to his flawed theory on the tides. The planet’s motion accounts for the tides, Galileo posited, and the tides account for the planet’s motion—it was all so neat. Moreover, all Galileo’s astronomical observations convinced him that Copernicus was correct, and he wanted desperately to prove that fact scientifically. As Einstein put it, “His aim was to substitute for a petrified and barren system of ideas the unbiased and strenuous quest for a deeper and more consistent comprehension of the physical and astronomical facts.”

Even Galileo’s greatest blunder reveals his greatest strength as a scientist. Referring to the tidal theory, Stillman Drake, a leading Galileo scholar, once wrote, “The prime source of Galileo’s effectiveness was his bringing together of mathematics, astronomy, and physics in an inseparable relationship. Hence even a questionable example of such a relationship given by him was still capable of revealing to others what sort of thing should be sought after in constructing a scientific explanation.”

That is, even though Galileo was wrong in this instance, he was right about the direction of science (and showed it by example): away from a more speculative, deductive approach and towards a more empirical, experimental method. Galileo didn’t make it into Lord Kelvin’s seminal paper on the tides. But for all his work, including his tidal theory, he secured a place in history as the first modern scientist.


Galileo invents a geometric and military compass, which has a commercial use as a pocket calculator. He hires a full-time instrument maker to mass-produce the compass, publishes a companion manual to the instrument, and gives lessons on its use.


Galileo’s geometric and military compass

1599 Galileo, 36, begins a relationship with 22-year-old Marina Gamba of Venice.
1600 In August, Galileo and Marina Gamba’s first daughter, Virginia, is born out of wedlock.
1601 In August, Livia, Galileo and Marina Gamba’s second child, is born almost exactly one year after her sister.

Galileo conducts experiments with a pendulum on the measurement of time increments (see His Experiments: Pendulum). He explains his findings in a letter to Santorio Santorio, a doctor friend in Venice, who then successfully uses a pendulum to measure his patients’ pulses.


Galileo’s pendulum looked similar to this one.

1607 Marina Gamba gives birth to Vincenzio, Galileo’s only son, in August.
1609 In May, Galileo learns of the invention of telescopic lenses in the Netherlands, which can be used to see objects at a distance. Within a month, he creates his own three-powered telescope (see His Telescope). Throughout the summer and fall Galileo continues to work on his telescope and begins toobserve the night sky through it. He presents an eight-powered telescope to the Senate in Venice and is awarded tenure at the University of Padua.

From November 30 to December 19, Galileo observes the moon through his telescope.


Galileo demonstrates his compass to the Venetian senate.

1610 On January 7, Galileo sees three bright stars near Jupiter; six days later he spies a fourth. Within a week he determines these are Jupiter’s satellites.

In Rome, Jesuit mathematicians at the Collegio Romano certify Galileo’s celestial discoveries, which include Saturn, sunspots, and the satellites of Jupiter, among other things.


Galileo was the first to observe Jupiter’s moons through a telescope.

1612 Galileo publishes Bodies That Stay Atop Water or Move Within It in Florence.

Galileo publishes his Sunspot Letters. Virginia and Livia Galilei, Galileo’s daughters, enter the Convent of San Matteo in Arcetri. They both take the habit within a year.


Galileo introduces a student to his telescope.

1614 Tommaso Caccini, a Dominican friar, delivers a sermon in Florence in which he denounces as heretics Galileo and others who subscribe to the Copernican view of the heavens (that the Earth revolves around the sun and not the other way around). Shortly thereafter, one of Caccini’s superiors sends Galileo a written apology. Later this year, Caccini is deposed by the Roman Inquisition.

In January, Galileo writes about his theory of the tides, arguing that it proves the movement of the Earth and the central position of the sun. He addresses his writing to Cardinal Alessandro Orsini.

Pope Paul V orders Robert Cardinal Bellarmine, the so-called “hammer of the heretics,” to warn Galileo against defending Copernican theory.


The universe according to Galileo, with the sun at its center

1619 Marina Gamba dies in February. She and Galileo were never married and never lived under the same roof.
1620 Galileo’s mother dies in September at the age of 82.
1623 In February, Roman censors give permission for Galileo’s book The Assayer to be printed. The book serves as a retort to Orazio Grassi, a teacher of mathematics, on the subject of comets, including their weight and composition, and meditates on the primacy of experimental science over the opinions of the popular majority.

Galileo travels to Rome, where he has audiences with Pope Urban VIII and several cardinals. The Pope grants Galileo permission to address Copernican theory in his writing on the condition that he only lend it the weight of a hypothesis.


Pope Urban VIII


Galileo finishes his work in April for Dialogue Concerning the Two Chief World Systems, which includes his treatise on the tides. It is published two years later.


Pope Urban VIII suspends distribution of Galileo’s Dialogue and appoints a commission to examine the book. The case is referred to the Inquisition, and Galileo is summoned from Florence to Rome.


Galileo appears before the Inquisition.


Copernican theory

In April, the Inquisition formally interrogates Galileo, who has been detained in the building of the Inquisition for several weeks. Galileo agrees to plead guilty in order to receive a lenient sentence, and on April 30 he confesses that he advocated Copernican theory too vigorously in the Dialogue. He agrees to modify his opinions in his next work.In June, the Pope orders Galileo imprisoned indefinitely under house arrest. Galileo makes his way back to his villa in Arcetri, near Florence, where he spends the remainder of his life under house arrest.

Galileo begins work on his Discourse Concerning Two New Sciences.


A 1633 letter from within the Inquisition condemning Galileo

1634 Galileo’s daughter Virginia, known as Sister Maria Celeste, dies in the Convent of San Matteo in April.
1637 Galileo, in failing health for several years, loses his eyesight. He petitions the Inquisition to be freed for medical reasons. His request is denied but in March the Inquisition gives Galileo permission to attend religious services on holidays.
1638 Discourse Concerning Two New Sciences is published in Holland.
1641 Galileo conceives of a pendulum-controlled clock.

Galileo dies in Arcetri on January 8. Isaac Newton is born in England on December 25.


Galileo died at age 78.

His Telescope
(and Sir Isaac’s)

On Galileo’s refractor and Newton’s reflector.