Galileo Galilei

← Encyclopedia

Galileo Galilei was an Italian mathematician, natural philosopher, and practicing astrologer whose telescopic observations after 1609 helped end the Earth-centered cosmos that European astronomy and astrology had shared for fifteen centuries. The same instrument that made his name in the new science turned, in time, against the old art he had earned a living by.

That second fact is easy to lose, and worth keeping. He was born at Pisa in February 1564, the son of Vincenzo Galilei, a lutenist and music theorist whose own quarrels with received authority left a mark on the boy. After an abandoned medical course and a stretch teaching at Pisa, Galileo took up the chair of mathematics at the University of Padua on 25 September 1592, where he remained for eighteen years — the most productive and settled period of his life. That chair was not an abstraction. In the Italian universities the mathematicus was a recognized profession that combined geometry, the construction of instruments, the casting of nativities, and the teaching of astrology, and Padua in particular carried one of Europe’s strongest traditions of medical astrology. Most of Galileo’s students were medical students, and he lectured to them on how to draw and judge a birth chart. This was not ornamental. The doctrine they were learning held that the seven planets governed the body’s four humours and the temperaments built from them, that the zodiacal signs ruled the parts of the body in sequence, and that the timing of a disease and the choice of a moment to bleed or purge could be read off the positions of the heavens — the discipline the ancients called iatromathematics. A physician who could not cast a figure was, by the standard of the age, half trained.

A competent professional, not a private scorner

The image of Galileo as a clear-eyed empiricist who tolerated astrology only as a paymaster’s nuisance does not survive contact with the documents. Surviving papers in his own hand show him doing the work in earnest. He cast horoscopes for himself — two distinct nativities for his own birth — and for the two daughters he had by Marina Gamba, Virginia and Livia, sketching the figures and working through the interpretive apparatus rather than merely tabulating positions. He drew charts for friends, for paying clients among the Venetian and Paduan nobility, and, with an eye to advancement, for the patrons whose favor he was courting, the Medici among them. The manuscript material — some fifty folios of notes, calculations, and figures — was long left out of the public record; the antiquarian Antonio Favaro, who edited the great national edition of Galileo’s Opere, addressed the subject in an essay on “Galileo astrologo” against considerable embarrassment, but reduced the working papers to a couple of pages. The full corpus, under the Galilean title Astrologica nonnulla, was not printed in the national edition until a 2017 appendix edited by Germana Ernst. Historians who have worked through the manuscripts — Darrel Rutkin and Nicholas Kollerstrom among them — find a man who knew the technicalities well and treated the practice as genuine, not as a sham he was quietly contemptuous of. By the lights of his own century he was an ordinary practitioner of judicial astrology, the branch of divination that reads character and event from the chart of a moment, working inside a learned tradition whose technical canon ran from Ptolemy’s Tetrabiblos through the Arabic masters to the Renaissance handbooks.

The chart that mattered most to his career was not his own. When he turned a spyglass on the heavens in late 1609 and found the bodies that would make him famous, the patron he set out to win was Cosimo II de’ Medici, Grand Duke of Tuscany — Galileo’s former pupil. He had already cast figures touching the Medici house, and the dedication that follows reads as the move of a man fluent in the symbolic economy of the courts: the planet whose new attendants he had discovered was the very planet to which the prince’s fortunes were astrologically tied.

The winter that did not fit

The discoveries came fast. In Padua in the winter of 1609–1610, working with a spyglass he had improved into an instrument of roughly twentyfold magnification, Galileo turned it on the night sky and found things no eye had reported. The Moon was not a polished sphere of the heavens but a body of mountains and valleys, its terminator broken and ragged, casting shadows that lengthened and shrank like the shadows of earthly peaks. The Milky Way dissolved under the lens into a dust of separate stars. And on successive nights in January 1610 he found three, then four, points of light strung in a line beside Jupiter — points that shifted their places from night to night and never strayed from the planet, which could only mean they were circling it. Across forty-four observations between 7 January and 2 March 1610 he established their orbits. Here, plainly visible, was a center of motion that was not the Earth: four bodies wheeling about a wandering planet, a small system within the larger one.

He published in haste. Sidereus Nuncius — the Starry Messenger — appeared at Venice in March 1610, the first scientific book grounded in telescopic observation, and in it he named Jupiter’s four attendants the Medicean Stars for the ruling house of Tuscany. The gift landed. Cosimo II appointed him court mathematician and philosopher, and Galileo left the security of Padua for Florence and the patronage he had angled for. The discovery and its dedication were, at once, a contribution to astronomy and a transaction in the older art: the Jovian moons were both a fact about the cosmos and a compliment paid in the grammar of the heavens.

The observations went on, and each one chipped at the inherited picture. Turning the lens on the Sun, he traced dark spots crossing its face and changing as they crossed — blemishes on a body that the physics of Aristotle, still the framework taught in every university, held to be perfect and incorruptible, and proof besides that the Sun turned on its axis. And in the last months of 1610 he watched Venus move through a full cycle of phases, from a small bright disc to a thin crescent and back, exactly as the Moon does. This was the hinge. In the strict geocentric scheme inherited from Ptolemy, Venus was carried on an epicycle that kept it always between the Earth and the Sun; from the Earth it could therefore never show more than a crescent, never a full or near-full disc. The full set of phases was impossible on that arrangement and entirely natural on one in which Venus circles the Sun — sometimes near, showing a crescent, sometimes far across on the Sun’s other side, showing a gibbous, nearly full face. Cautious of being forestalled, Galileo first announced the result in a sealed Latin anagram, Haec immatura a me iam frustra leguntur o y, and only later unscrambled it to read Cynthiae figuras aemulatur mater amorum — the mother of loves imitates the shapes of Cynthia, that is, Venus mimics the phases of the Moon. The phases did not by themselves prove the Earth moved; they were equally consistent with the compromise system of Tycho Brahe, in which the planets circle the Sun while the Sun circles a stationary Earth. But they killed the pure Ptolemaic ordering outright, and they made the Sun-centered arrangement of Copernicus, on which Johannes Kepler was at that moment building his laws of planetary motion, suddenly the more economical reading of the sky.

The claim that the arrangement was real

What turned discovery into danger was not observation but interpretation. Astronomers had long handled the Copernican model as a calculating device — a way to predict positions without any commitment to the Earth actually moving. Galileo refused that evasion. He held that the heliocentric arrangement was not a mathematical fiction but the way things physically stood, and he said so in print and in correspondence, drawing the cosmology into open conflict with the literal reading of certain passages of scripture. In 1616 the Holy Office, with Cardinal Robert Bellarmine conveying the decision, declared the doctrine that the Sun stands still and the Earth moves to be philosophically false and contrary to scripture; Galileo was admonished, and Copernicus’s De revolutionibus suspended pending correction. He had a younger, bolder contemporary as a cautionary precedent: Giordano Bruno, the Nolan philosopher of an infinite, centerless universe of innumerable worlds, had been burned at Rome in 1600 after an eight-year Inquisition trial — and it was the Padua chair Bruno had hoped for, and failed to get, that went instead to Galileo.

For sixteen years Galileo trod carefully. Then, believing he had latitude from a sympathetic pope, Urban VIII, he published in 1632 the Dialogo sopra i due massimi sistemi del mondo — the Dialogue Concerning the Two Chief World Systems — a dramatised debate between a Copernican and a Ptolemaic spokesman in which the defense of the old cosmos was put into the mouth of a character named Simplicio. The pope took the slight personally, and the political ground gave way. Summoned to Rome, tried before the Roman Inquisition through the spring of 1633, Galileo was found, on 22 June 1633, to be vehemently suspect of heresy — a grave charge that stopped short of declared heresy — and made to abjure, curse, and detest the opinion that the Earth moves. The Dialogo was placed on the Index. He passed the rest of his life under house arrest at his villa at Arcetri above Florence, going blind, supervised but not silenced; there he completed the Discorsi, his work on the science of motion and materials, which Protestant printers in the Dutch Republic carried into print in 1638. He died at Arcetri on 8 January 1642.

The deeper irony

The irony runs deeper than biography. Astrology as it was practiced rested on the geocentric cosmos: the planets as wandering lights moving against the fixed stars, their positions read off a sky arranged around the human observer, their influences descending through a graded order of spheres with the Earth at the sump of it all. That cosmology was the shared property of the age — the same framework that underwrote the medical astrology Galileo taught at Padua, the celestial magic of Marsilio Ficino, and the vast macrocosm-microcosm engravings of Robert Fludd, in which the chain of correspondences bound the human body to the turning heavens. To move the Earth among the planets and the Sun to the center was, eventually, to pull the floor out from under that arrangement — not by refuting astrology point by point, but by dissolving the cosmology it assumed. The fixed stars receded to unmeasured distance; the spheres dissolved; the privileged central Earth on which the whole scheme of descending influence had been hung became one planet among several. Galileo himself seems not to have drawn that conclusion; he kept casting charts. The unravelling was the slow work of the century after him — carried by Kepler’s ellipses, by the new philosophy of René Descartes and the experimental program of Francis Bacon, and brought to its mechanical settlement by Isaac Newton — and it was carried in part by the instrument he had pointed at the sky. This is the threshold the sibling account of Renaissance natural philosophy maps in full: the contested ground, neither magic nor science as later ages would draw the line, on which men like Nicholas of Cusa had already begun to imagine a universe without a fixed center.

The textual record and the scholarship

The documentary base for Galileo is unusually full, and most of it is freely available. The standard corpus is the Edizione Nazionale delle Opere di Galileo Galilei, the twenty-volume national edition assembled by Antonio Favaro between 1890 and 1909, against which all serious work is checked; the 2017 Astrologica nonnulla appendix edited by Germana Ernst finally brought the astrological manuscripts into that record. The pivotal early book, Sidereus Nuncius (Venice, 1610), survives in numerous digitized copies. The biographical and philosophical scholarship is anchored by the entry of Peter Machamer and David Marshall Miller in the Stanford Encyclopedia of Philosophy, which surveys the discoveries, the two trials, and the disputes over Galileo’s method, and by John Heilbron’s Galileo (Oxford, 2010), the leading modern life.

On the specifically astrological question, the case that Galileo practiced judicial astrology seriously and competently is made by H. Darrel Rutkin in his study of Galileo as astrologer and at length in Sapientia Astrologica (Springer, 2019), and by Nicholas Kollerstrom, whose “How Galileo Dedicated the Moons of Jupiter to Cosimo II de’ Medici” in Culture and Cosmos 8 (2004) reads the Medicean dedication against the Grand Duke’s nativity. The wider tradition into which this practice fits — the technical canon from the Hellenistic astrologers through Ptolemy and the Arabic masters to the Renaissance handbooks — is set out in Nicholas Campion’s two-volume History of Western Astrology and in Anthony Grafton’s Cardano’s Cosmos (Harvard, 1999), which reconstructs the working life of a sixteenth-century professional astrologer in granular detail. For the trial as a procedural event, Maurice Finocchiaro’s documentary collections remain the standard, and the contrast with Galileo’s executed contemporary is laid out across the Bruno scholarship from Frances Yates to Ingrid Rowland. What the manuscripts and the secondary literature together establish is a figure considerably stranger than the textbook allows — at once the founder of a physics built on measurement and a working judicial astrologer who never repudiated the art.

He is remembered now as a founder of modern physical science, and that remembrance is largely deserved. But the figure who fits the textbook — the clear-eyed empiricist breaking with superstition — flattens a more interesting man, one who stood with one foot in the old learning he served and one in the observations that would unmake it, apparently without feeling the ground shift.

→ Related: Ptolemy · Rene Descartes · Francis Bacon · Divination · Jupiter · Venus · Aristotle · Astrology · Renaissance Natural Philosophy · Johannes Kepler · Giordano Bruno · Isaac Newton · Marsilio Ficino · Robert Fludd · Nicholas Of Cusa