Galileo Galilei

• 1. Introduction

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  Galileo Galilei was born in Pisa on the 15th of February 1564, the first of six children of a leading lutenist. He died on the 8th of January 1642, aged 77, after a fever and heart palpitations. Between those two dates lived a man who pointed an improved telescope at the night sky and saw things no one had described before. He watched four points of light shift their positions beside Jupiter night after night. He saw Venus pass through a full set of phases like the Moon. He found the Milky Way to be a multitude of stars packed so densely they looked like clouds. Albert Einstein called him the father of modern physics. Stephen Hawking judged that he bears more responsibility for the birth of modern science than anybody else. So why did the Roman Inquisition declare him vehemently suspect of heresy, ban his book, and confine him to his villa for the rest of his life? How did a pious Catholic, once supported by a pope and the Jesuits, end up reading penitential psalms under house arrest? And what did he discover about falling bodies, pendulums, and the language of nature that still anchors physics today?

• 2. The Improved Telescope

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  On the 25th of August 1609, Galileo demonstrated one of his early telescopes to Venetian lawmakers, an instrument that magnified roughly eight or nine times. He had built it perhaps from descriptions alone of the device Hans Lippershey tried to patent in the Netherlands the year before. His first version managed about three times magnification. He later pushed improved versions to around thirty times. With a Galilean telescope, an observer could see magnified, upright images of things on Earth, what became known as a spyglass. Galileo sold these as a profitable sideline to merchants who found them useful at sea and as items of trade. On the 30th of November 1609, he aimed his telescope at the Moon. Thomas Harriot, an English mathematician, had looked four months earlier but saw only a strange spottedness. Galileo deduced that the uneven waning came from light blocked by lunar mountains and craters. He drew topographical charts and estimated the heights of those mountains. The Moon was not the translucent, perfect sphere Aristotle had claimed. In March 1610 he published his first telescopic observations in a brief treatise, Sidereus Nuncius, the Starry Messenger, the first scientific treatise published from observations made through a telescope.

• 3. The Medicean Stars

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  On the 7th of January 1610, Galileo saw what he described as three fixed stars, totally invisible by their smallness, lying in a straight line near Jupiter. On following nights their positions shifted in ways fixed stars never could. On the 10th of January one had vanished, hidden, he reasoned, behind Jupiter. On the 13th he counted four distinct objects, and on the 15th he concluded they were orbiting the planet. He had found Jupiter's four largest moons. Galileo named the group the Medicean stars, honoring his future patron Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers renamed them the Galilean satellites for their discoverer. Simon Marius independently discovered them on the 8th of January 1610 and gave the names still used: Io, Europa, Ganymede, and Callisto, published in his Mundus Iovialis in 1614. A planet with smaller bodies circling it broke the rules of Aristotelian cosmology, which held that all heavenly bodies must circle the Earth. Many astronomers refused to believe it. When Galileo demonstrated the telescope in Bologna, attendees struggled to see the moons. Martin Horky noticed that some fixed stars, such as Spica Virginis, appeared double, and took this as proof the instrument deceived. Christopher Clavius's observatory in Rome confirmed the sightings and gave Galileo a hero's welcome when he visited the next year. By mid-1611 he had measured the satellites' periods with an accuracy Johannes Kepler had believed impossible.

• 4. A Universe That Changes

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  From September 1610, Galileo observed that Venus exhibits a full set of phases, crescent through gibbous to full, like the Moon. In Ptolemy's geocentric model, the orbit of Venus could not produce that full range. Traditionally Venus was placed entirely on the near side of the Sun, where it could show only crescent and new phases. After Galileo's observations of all its phases, the Ptolemaic model became untenable. The great majority of astronomers converted to geo-heliocentric models such as the Tychonic system. Sunspots posed another problem for the unchanging perfection of the heavens taught in orthodox Aristotelian physics. Francesco Sizzi and others observed an apparent annual variation in their trajectories in 1612 to 1613. A dispute over who first discovered sunspots drew Galileo into a long and bitter feud with the Jesuit Christoph Scheiner. Mark Welser, to whom Scheiner had announced his finding, asked Galileo's opinion, and neither man knew of Johannes Fabricius' earlier work. Galileo had already disproved the immutability of the heavens once before. Ottavio Brenzoni's letter of the 15th of January 1605 brought the supernova of 1572 and the nova of 1601 to his notice. Observing Kepler's supernova in 1604, he found no detectable diurnal parallax in these new stars and concluded they lay far away, among the distant stars.

• 5. The Mathematics Of Motion

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  In The Assayer, published in 1623, Galileo wrote that philosophy is written in the grand book of the universe, in the language of mathematics, its characters triangles, circles, and other geometric figures. He was among the first modern thinkers to state plainly that the laws of nature are mathematical. To compare measurements made on different days and in different laboratories, he set up standards of length and time, building a reproducible foundation for confirming laws by inductive reasoning. His father Vincenzo Galilei, a lutenist and music theorist, had already shown perhaps the oldest known non-linear relation in physics: for a stretched string, the pitch varies as the square root of the tension. Young Galileo could watch his own father expand the Pythagorean tradition of music. Galileo proposed that a falling body accelerates uniformly when the medium's resistance stays negligible, or in a vacuum. He derived the law that the distance fallen from rest is proportional to the square of the elapsed time. Most of his experiments with falling bodies used inclined planes, where problems of timing and air resistance were much reduced. The famous tale of dropping balls from the Leaning Tower of Pisa comes from his pupil Vincenzo Viviani. There is no account by Galileo himself, and historians generally accept it was at most a thought experiment that did not take place. In Two New Sciences, Salviati, regarded as Galileo's spokesman, held that in a medium totally devoid of all resistance all bodies would fall with the same speed.

• 6. Ship, Cabin, And Pendulum

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  Shut yourself up with some friend in the main cabin below the decks of a ship, Galileo wrote, with flies, butterflies, a bowl of water with fish, and a bottle dripping into a vessel below. With the ship standing still, the animals fly equally to all sides, the fish swim in all directions, the drops fall straight down. Then have the ship move at any steady speed in a straight line. You will discover not the least change in any of these effects, nor could you tell whether the ship was moving or standing still. This thought experiment, voiced by Salviati in the Dialogue, states the principle of relativity: the laws of physics are the same in any system moving at constant speed in a straight line. The principle later framed Newton's laws of motion and sits at the center of Einstein's special theory of relativity. In 1581, while studying medicine, Galileo watched a swinging chandelier shift between larger and smaller arcs. Timing it against his pulse, it seemed to take the same time to swing no matter the sweep. At home he set up two pendulums of equal length, swung one wide and one small, and found they kept time together. He claimed a simple pendulum is isochronous, though Christiaan Huygens later found this only approximately true. It was Huygens, almost a hundred years on, who turned the pendulum into an accurate timepiece.

• 7. Instruments And Engineering

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  Between 1595 and 1598, Galileo devised and improved a geometric and military compass for gunners and surveyors, building on instruments by Niccolò Tartaglia and Guidobaldo del Monte. For gunners it offered a safer way to elevate cannons and to compute the gunpowder charge for cannonballs of different sizes and materials. As a geometric instrument it could construct any regular polygon and compute the area of any polygon or circular sector. The instrument maker Marc'Antonio Mazzoleni produced more than a hundred of these compasses, which Galileo sold for 50 lire, offering a course of instruction in their use for 120 lire. In 1593 he built a thermometer that moved water in a tube using the expansion and contraction of air in a bulb. By 1624 Galileo had used a compound microscope, giving one to Cardinal Zollern in May of that year for presentation to the Duke of Bavaria. The fellow Lincean Giovanni Faber coined the word microscope from the Greek for small and to look at, meant as a companion to telescope. The name telescope itself was coined for Galileo's instrument by the Greek mathematician Giovanni Demisiani at a banquet in 1611. Having found the orbital periods of Jupiter's satellites, Galileo proposed in 1612 using their positions as a universal clock to fix longitude. Spain and later Holland had set large prizes for solving the longitude problem, vital to safe navigation. Observing the moons from a rolling ship proved too difficult. The method was first successfully applied by Giovanni Domenico Cassini in 1681 and used for large land surveys, including the remapping of France.

• 8. Heresy And House Arrest

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  Cardinal Bellarmine wrote in 1615 that the Copernican system could not be defended without a true physical demonstration that the Sun does not circle the Earth but the Earth circles the Sun. Galileo thought his theory of the tides supplied that proof, attributing the seas' motion to the Earth's daily rotation and yearly revolution. The theory in fact fails, implying only one high tide per day, and he dismissed the idea, known to Kepler, that the Moon causes the tides. In 1615 his writings on heliocentrism were submitted to the Roman Inquisition by Father Niccolò Lorini, who cited Galileo's letter to Benedetto Castelli. In February 1616 an Inquisitorial commission declared heliocentrism foolish and absurd in philosophy and formally heretical. On the 26th of February, Bellarmine ordered Galileo to abandon completely the opinion that the Sun stands still at the center and the Earth moves, and not to hold, teach, or defend it in any way. Encouraged by the election of his admirer Cardinal Maffeo Barberini as Pope Urban VIII in 1623, Galileo wrote the Dialogue Concerning the Two Chief World Systems, published in 1632 with the Inquisition's authorization. Urban had asked him to give arguments for and against heliocentrism and not to advocate it. But Simplicio, the defender of the Aristotelian view, was repeatedly caught in his own errors, and the name carried the connotation of simpleton in Italian. The Pope took the perceived disrespect badly. Galileo was called to Rome, arrived in February 1633, and faced inquisitor Vincenzo Maculani. On the 22nd of June the sentence came: vehemently suspect of heresy, required to abjure, curse, and detest his opinions. Imprisonment was commuted to house arrest. Allowed to return to his villa at Arcetri in 1634, he was ordered to read the Seven Penitential Psalms once a week for three years, a burden his daughter Maria Celeste took upon herself. There he wrote Two New Sciences, published in Holland in 1638 to avoid Catholic censorship. He went completely blind that same year. According to legend, after recanting he muttered that the Earth still moves, but the earliest written account of those words dates to a century after his death.