68 Fun Facts About Galileo Galilei

Which physicist was born on 8 January 1942, exactly 300 years after Galileo died?

Stephen Hawking counted Galileo among his inspirations, and his official biography calls the dates a fitting symmetry. In 1981 he first set out his idea of a universe with no boundary at a conference held in the Vatican.

Galileo's father Vincenzo was a leading figure in which field?

Music: Vincenzo Galilei was a court musician and theorist who put the old rules of harmony to the test, experimenting on how a string's tension changes its pitch. His son may well have helped him.

At his father's urging, Galileo enrolled at the University of Pisa to study what?

Medicine was his father's choice, made after calling the boy home from a monastery where he was thinking of becoming a monk. It never held him: after four years he left the university without a degree.

In the famous story of the swinging cathedral lamp, the student Galileo timed its swings against what?

His own heartbeat, as the story goes: wide swings and narrow ones seemed to take the same time. Doubters point out that the lamp shown to visitors today was hung only in 1587, after he had left the university.

In the traditional story, what chance event drew the young Galileo away from his university course?

Attending a geometry lecture by Ostilio Ricci, a court tutor in Florence, is the usual account. Galileo then brought Ricci home to help talk his reluctant father into letting him change studies.

Galileo's first scientific work, written in 1586, described his design for what?

A hydrostatic balance: at 22 he refined the practice of weighing metals first dry and then under water, so that the share of gold in an alloy could be read from the scale. The tract, La Bilancetta, circulated in manuscript.

The gal, a unit named after Galileo and used by geophysicists, measures what?

Acceleration: one gal is a change of one centimetre per second every second. Gravity surveyors work in thousandths of it, because a mountain or a pocket of light rock shifts the local pull by only that much.

In 1588 the Florentine Academy invited Galileo to lecture on the shape, location and size of what?

Dante's Inferno: he measured the poem's hell like a surveyor, even working out how thick its roof must be. The physicist Mark Peterson traces his later discovery of scaling laws back to these two lectures.

From 1592 to 1610 Galileo taught geometry, mechanics and astronomy at which university?

Padua paid him far better than his first post, yet as head of a family he was always short of money: he ran a workshop at home, sold instruments of his own design and took in private pupils.

In 1594 Galileo was granted a patent for what device?

A horse-powered water pump, invented the year before. He needed the money: his father's death had left him head of the family, with a sister's large dowry still to be paid.

What happened to Galileo's two illegitimate daughters, Virginia and Livia?

They became nuns. Galileo apparently judged daughters born out of wedlock unmarriageable, and he placed both girls in the convent of San Matteo in Arcetri for life while they were still children.

At his final interrogation in 1633, the Inquisition formally threatened Galileo with what?

Torture. It stayed a threat: he stuck to his denial that he had held Copernican views since 1616, was never racked or thrown in a dungeon, and was sentenced the next day.

To prove Galileo right about falling bodies, an Apollo 15 astronaut dropped a hammer and what on the lunar surface?

A feather. In the lunar vacuum the falcon feather fell exactly as fast as the geological hammer, and the two hit the dust together, live on television.

In August 1609 Galileo demonstrated one of his first telescopes to whom?

Venetian lawmakers. Galileo presented an eight-power spyglass to the Senate, which rewarded him by doubling his salary and making his university post a job for life.

Which short book set out Galileo's first discoveries with the telescope?

The Starry Messenger. Galileo rushed it into print in March 1610, about two months after his January sightings, and by the next spring the Jesuits' own astronomers had confirmed what he saw and feted him.

Why is Thomas Harriot's telescope drawing of the Moon, made before Galileo's, so little known?

Never published. Harriot kept his Moon sketches among his private papers all his life, while Galileo had his own drawings in print within four months of making them.

On 7 January 1610 Galileo noticed three 'fixed stars' in a line beside which planet?

Jupiter. About a week later he realised the 'stars' were moons circling the planet, which showed that not everything in the sky revolves around the Earth.

Galileo named his four newly found moons the 'Medicean stars' after the Grand Duke of where?

Tuscany. The flattery worked: within months Cosimo II de' Medici gave Galileo a court post for life, and he left his university job and moved back to Florence.

Which astronomer suggested the names Io, Europa, Ganymede and Callisto for the four moons Galileo discovered?

Johannes Kepler. Simon Marius, who claimed to have seen the moons himself, printed Kepler's suggestion, but astronomers ignored the names and used plain numbers for more than two centuries.

Galileo proposed eclipses of his newly found moons as a clock to solve which navigation problem?

Longitude. The method failed on a rolling deck but worked on land, where French astronomers used it to redraw the map of France and found the country smaller than everyone had thought.

Galileo's 1610 discovery that Venus shows a full set of phases made which model of the cosmos untenable?

The Ptolemaic. Yet the phases fitted Tycho Brahe's rival scheme too, with planets circling the Sun and the Sun circling a fixed Earth, so many astronomers switched to that compromise, not to Copernicus.

When Galileo first observed Saturn in 1610, what did he take its rings to be?

Two companion planets. He drew Saturn as a large globe with a smaller one on each side, then in 1612 found both had vanished and wondered whether Saturn had swallowed his children.

Which planet did Galileo record in his notebooks in 1612 as just another dim star?

Neptune. It went unrecognised as a planet until 1846, when astronomers found it by calculating where an unseen world must lie to be pulling Uranus off its predicted path.

Galileo's long feud with the Jesuit Christoph Scheiner was over who first discovered what?

Sunspots. While the two men quarrelled over who came first, neither knew that a German, Johannes Fabricius, had already beaten both of them into print with a book in 1611.

What did Galileo's telescope reveal the Milky Way to be?

A dense mass of stars. Until then its glow had been taken for a kind of cloud; his spyglass broke it into countless stars too faint and too crowded for the naked eye to tell apart.

Ganymede, the largest moon Galileo discovered, is wider than Mercury but far lighter because it is rich in what?

Water ice. Ganymede is so icy that it has under half of Mercury's mass, and in 2015 Hubble found signs of a salty ocean beneath the crust by watching how the moon's auroras rock.

The Dialogue Concerning the Two Chief World Systems was originally titled after which natural phenomenon?

The tides. Galileo believed the seas' ebb and flow proved that the Earth circles the Sun, so Church censors made him strike the tides from the title rather than seem to endorse his proof.

Which 1975 Queen hit calls out the name Galileo again and again?

Bohemian Rhapsody. Queen's producer recalled that its middle section kept swelling because the singer kept arriving at the studio with a few more 'Galileos' to overdub.

In Galileo's Dialogue, which of the three speakers stands in for Galileo himself?

Salviati. He and the neutral listener, Sagredo, bear the names of two real friends of Galileo's who had died long before the book came out, one in 1614 and one in 1620.

Galileo's The Assayer grew out of a quarrel with a Jesuit about which objects, three of which appeared in 1618?

Comets. Galileo was on the wrong side: he suggested they might be mere tricks of light near the Earth, while the Jesuit Orazio Grassi rightly placed them far out in space.

Which pope was so charmed by The Assayer that he had it read aloud to him at table?

Urban VIII. Nine years later the same pope felt mocked by Galileo's Dialogue, set up the commission that sent him before the Inquisition and refused every plea to pardon him.

In The Assayer, Galileo wrote that the book of the universe 'is written in the language of' what?

Mathematics. Its letters, he wrote, are triangles, circles and other figures, and the idea helped turn the study of nature from wordy argument into measurement and calculation.

In 1616, which cardinal delivered the Inquisition's order that Galileo abandon heliocentrism?

Bellarmine. Galileo later got the cardinal to certify in writing that he had not been tried or punished, and 17 years on he produced that letter in his own defence before the Inquisition.

Galileo's Letter to the Grand Duchess Christina said which book is no guide to astronomy?

The Bible. Galileo borrowed a cardinal's quip to make the point: Scripture teaches the way to salvation, not the way the skies turn.

In the Dialogue, the character Simplicio defends which view of the cosmos?

Aristotelian geocentrism. Galileo said the name honoured an ancient commentator on Aristotle, but it also suggests 'simpleton', and he gave Simplicio the pope's own favourite argument to defend.

Galileo's 1633 sentence found him 'vehemently suspect of' what?

Heresy. The judges offered to absolve him provided he would 'abjure, curse and detest' his Copernican views, which he did at a formal ceremony, and they banned his Dialogue by public edict.

Galileo's sentence of formal imprisonment was soon commuted to what?

House arrest. He spent his last eight years confined to his villa outside Florence, yet he finished his last great book on physics there and had it spirited abroad to be printed.

As part of his sentence, Galileo was ordered to recite what once a week for three years?

The Seven Penitential Psalms. His elder daughter took the weekly recital on herself in his place, but she died less than four months after he came home.

According to legend, what did Galileo mutter after recanting heliocentrism?

'And yet it moves'. Neither the trial records nor his first biographer mention the remark, and it is not found in print until 1757, more than a century after his death.

To avoid Catholic censorship, Galileo's Two New Sciences (1638) was published where?

Holland. The Inquisition had banned him from publishing anything ever again, so the manuscript was smuggled out of Italy to the Dutch printer Louis Elzevir, who had visited him at home.

Galileo's last book, Two New Sciences, dealt with the strength of materials and the science of what?

Motion. It finally printed the laws of falling bodies and projectiles that Galileo had worked out decades earlier and then set aside when the telescope took over his life.

In which city did the Inquisition put Galileo on trial in 1633?

Rome. Ailing and nearly 69, he asked for the hearing to be moved to Florence but was told to come or be brought in chains; once there he lodged with an ambassador, never in a dungeon.

Galileo argued that a ball rolling on a perfectly smooth, endless level surface would do what?

Carry on for ever. The textbooks of his day said things move only while something pushes them; his reversal became the law of inertia, though he pictured the endless path as a circle round the Earth.

Galileo's telescope design, which shows things the right way up, is the one used in which device?

Opera glasses. A concave eyepiece keeps the picture upright and the tubes short, which suits the theatre; the price is a field of view so narrow that Galileo could fit only a quarter of the full Moon in his.

Galileo's early thermometer tracked the swelling and shrinking of what inside a glass bulb?

Air. He warmed a small glass flask in his hands and upended its long neck in water; as the air inside cooled and shrank, the water climbed up the neck.

The word 'telescope', coined for Galileo's instrument at a banquet in 1611, was built from words in which ancient language?

Greek. It joins the words for 'far' and 'watcher', and the banquet marked Galileo's election to the Lincean Academy, an honour he valued enough to sign himself 'Linceo' ever after.

Late in life and totally blind, Galileo talked his son through the design of what?

A pendulum clock. Unable to draw, he had his son sketch the mechanism, but it was not built in his lifetime, and the first working pendulum clock was Christiaan Huygens's in the 1650s.

Galileo's paradox, that there seem to be as many square numbers as whole numbers, is about which idea?

Infinity. Every number has a square, yet most numbers are not squares, so Galileo decided that 'more' and 'fewer' cannot apply to infinite collections; set theory now says the two match.

Galileo's thought experiment about flies and fish in a ship's cabin illustrates which principle?

Relativity. Below deck on a smoothly sailing ship, he argued, nothing the flies, the fish or dripping water do can tell you whether you are moving, a point Einstein later built on.

Galileo showed that the distance a body falls from rest is proportional to what?

The square of the time. So a stone falls four times as far in two seconds as in one, a rule Galileo had worked out by about 1604 but kept out of print for more than thirty years.

Why do many historians doubt that Galileo really dropped weights from the Leaning Tower?

He left no record of it. The tale first appears in a biography that his pupil Vincenzo Viviani wrote twelve years after Galileo's death, and which was not printed until 1717.

Most of Galileo's real experiments on falling bodies used what to slow the fall?

Inclined planes. A ball rolling down a gentle slope takes seconds, not a blink, to cover the same drop, which let Galileo mark where it was at equal intervals and made drag matter far less.

To test the speed of light, Galileo had two distant observers signal each other with what?

Shuttered lanterns. Standing less than a mile apart, the pair could detect no delay at all, and Galileo concluded only that light, if not instantaneous, must be extraordinarily fast.

Which follower of Galileo invented the barometer to settle a puzzle his master had got wrong?

Evangelista Torricelli. Galileo had blamed the limited pull of a vacuum for the way a tall column of liquid breaks; Torricelli showed it is the weight of the atmosphere that holds the column up.

For sailors on a rolling ship, Galileo designed a small telescope fixed to what?

A helmet. His 'celatone' carried the telescope on a visor so the wearer could keep it aimed as the deck pitched, and he later proposed seating the observer in a bowl floating on oil instead.

Galileo is buried near Michelangelo in which Florence basilica?

Santa Croce. The pope would not allow a monument to a condemned man, so his body lay in a side room for 95 years, until a grand tomb funded by a bequest from his pupil Viviani was ready.

What was removed from Galileo's remains when he was reburied in 1737?

Three fingers and a tooth. Admirers treated the condemned man's bones like a saint's relics, and his middle finger ended up on show under glass in the Florence museum that bears his name.

Galileo's Dialogue stayed on the Church's Index of Prohibited Books for roughly how long?

200 years. The blanket ban on books teaching a Sun-centred cosmos was dropped in the 1750s, but the Dialogue itself, condemned at his trial in 1633, stayed listed into the nineteenth century.

Which pope admitted, more than 350 years after the trial, that the Church had erred in condemning Galileo?

John Paul II. He had called for the case to be reopened in 1979, at a ceremony marking Einstein's centenary, and the inquiry that followed took 13 years to report.

Which playwright wrote Life of Galileo?

Bertolt Brecht. He wrote it in exile from Hitler's Germany, then reworked it in English with the actor Charles Laughton, who played Galileo when that version opened in Los Angeles in 1947.

Albert Einstein called Galileo the father of what?

Modern science. Einstein's reason was that Galileo saw, and drummed into everyone else, that pure reasoning without observation tells us nothing about the real world.

Which scientist was born months after Galileo died?

Isaac Newton. He arrived on Christmas Day 1642 by England's old calendar, but that was already 4 January 1643 in Galileo's Italy, so the two share a year only if the calendars are mixed.

A Galileo critic reputedly preached, 'Ye men of Galilee, why stand ye' doing what?

Gazing up into heaven. The line, a pun on Galileo's name, is credited to the friar Tommaso Caccini, whose attack led a senior Dominican to write Galileo an apology on behalf of the order.

To protect his claim, Galileo first announced his strange 1610 sighting of Saturn in what form?

An anagram. He sent friends 37 scrambled letters hiding one short sentence, so that he could prove later that he had seen it first while keeping rivals in the dark about what 'it' was.

By 1624 Galileo had used and given away which instrument besides the telescope?

A compound microscope. One friend in the Lincean Academy used it to publish close-up drawings of a bee, and another member coined the word 'microscope' for the new instrument.

To outdo the weak spyglasses on sale in 1609, Galileo taught himself which craft?

Lens grinding. Spectacle-shop lenses gave only about three-fold magnification; by shaping his own he reached eight-fold within two months and twenty-fold before the year was out.

Galileo's friend Cigoli painted the Virgin Mary standing on a Moon that, unusually for its day, showed what?

Craters. In a chapel dome he swapped the smooth, perfect orb of tradition for the pockmarked Moon his friend had just drawn, one of the first times the telescope's discoveries appear in a painting.

Galileo showed that a cannonball, once fired, follows which curve?

A parabola. He got it by treating the flight as two movements at once: a steady drift forwards and a fall that keeps speeding up.