GALILEO GALILEI (1564–1642)

GALILEO GALILEI (1564–1642), Italian scientist. Born in Pisa, Galileo was the eldest of the six or seven children of Vincenzio Galilei, a merchant and music theorist, and Giulia Ammannati. He spent his childhood in Pisa and Florence; in the fall of 1581, upon his father's advice, he enrolled at the University of Pisa as a student of medicine. Not enthusiastic about this discipline, within two years he had begun to study Euclidean and Archimedean works privately and left the university in 1585 without a degree. He offered both public and private lessons in mathematics for the next three years and sought, unsuccessfully, to obtain a professorial chair at Bologna in 1588. His various meditations on and experiments with mechanics, metrology, and musical consonance, and his participation in a Florentine academy in this period, helped him secure the chair in mathematics at the University of Pisa in the fall of 1589.

By late 1592 Galileo had won a more prestigious post in mathematics at the University of Padua, and it was here that he undertook significant work in optics and catoptrics, magnetism, tidal theory, mechanics, and instrumentation. This last area was crucial to his financial well-being: in order to meet the demands incumbent upon him as the eldest son, and to supplement his professorial salary, Galileo offered private lessons to students in Padua, many of whom were eager to learn the various uses of a calculating instrument of his design. Galileo's extant writings in mechanics in these same years likewise reflect a strong interest in combining classical problems with actual devices for lifting, lowering, and guiding solid bodies and fluids.

Galileo may have become an adherent of the heliocentric world system posited by Nicolaus Copernicus (1473–1543) in the mid-1590s: so he asserted in 1597 in a letter to the German astronomer Johannes Kepler (1571–1630), discoverer of the laws of planetary motion. Certain conjectures regarding tidal theory reflect a cautious interest in the hypothesis of a mobile Earth, for tides were explained as a product of the globe's annual and diurnal motions, with variations in periodicity deriving from the particular shape of any large body of water. One might also infer Galileo's discreet support of the Copernican system through the attention he devoted in this period to speculative arguments derived from mechanics. The arena in which cosmogony and mechanics intersected was in a quantified approach to a myth mentioned in Plato's Timaeus involving the "creation point," or the place or places from which the Divine Architect originally dropped the various planets. These bodies, after falling toward the sun, would each reach and remain in the orbits to which they had been assigned. Scholars have suggested that around 1602–1604 Galileo did attempt to combine his still evolving understanding of the law of falling bodies and of the way such bodies behave when diverted into uniform orbital motion, with Kepler's estimated periods of revolution for Saturn, Mars, and Jupiter.

By the fall of November 1604 Galileo's attention was on the heavens, for the appearance of a new star seemed to offer strong evidence against Aristotelian conventions regarding an immutable world beyond the Moon. But his most explicitly Copernican conjectures concern the Moon; between 1605 and 1607 he and several of his closest associates had observed the ashen light reflected onto that body by Earth at the beginning and end of each lunar cycle. The rough and opaque body of Earth was, in other words, like other planets, tolerably bright; the corollary was, for some, that Earth likewise participated in "the dance of the stars." In this period Galileo was also engaged in more studies of motion and hydrostatics, and involved with additional work in magnetism.

By spring or summer 1609, Galileo was making celestial observations with the aid of a telescope at least three times more powerful than a prototype from The Hague. By November of that year, he had developed a telescope that magnified twenty times, and it was with this instrument that he undertook his observations of the lunar body. His Starry Messenger of 1610 shows that the telescope confirmed his earlier naked-eye impressions of both a rough lunar surface and of the ashen light, and that it allowed him to present certain of the Moon's features, most notably its peaks, valleys, and craters, in terms of their terrestrial counterparts. He used the shadows cast by a particular mountain on the Moon to calculate the average height of such formations. On the basis of these observations of the Moon's similarity to Earth, Galileo proposed a thoroughgoing revision of the Ptolemaic conception of the cosmos, and he promised to deliver such arguments in his System of the World, the forerunner to the eventual Dialogue concerning the Two Chief World Systems of 1632.

The greatest discoveries in the Starry Messenger lay in its final section, a description of the positions of the satellites of Jupiter from 7 January until 2 March 1610, when the treatise went to press. In these brief observations and in the spare diagrams that accompanied them, Galileo presented the orbital movements of four satellites, or Medici stars, whose very existence was new to virtually all of his audience. The fact that Jupiter had moons strongly suggested to him that Earth was neither unique nor central nor motionless: satellites revolving about a celestial body clearly did not prevent its movement.

By the end of 1610, Galileo, newly appointed as mathematician and philosopher at the court of the grand duke of Tuscany, had interpreted the phases of Venus as a confirmation of Copernican claims, and perhaps more importantly, evidence against the models of both Ptolemy and and the Danish astronomer Tycho Brahe (1546–1601), who posited that the five planets revolved around the Sun, which in turn revolved around Earth; Kepler obligingly published his letters on the matter in his Dioptrice of 1611. Galileo had some notion of sunspots by spring 1611, but his systematic study of the phenomena appears to date only to early 1612, when he had learned of the observations of several friends, and of the treatise of an eventual enemy, the Swabian Jesuit Christoph Scheiner (1573–1650). Galileo took immediate issue with Scheiner's initial conjecture that the spots were actually small stars orbiting and partially eclipsing the solar body, and he did not hesitate to expose both the Jesuit astronomer's ignorance of Galileo's recent findings concerning Venus, and the weakness of Scheiner's geometrical proofs. Because he saw no reason to subscribe to the Aristotelian fiction of the changeless heavens, Galileo's three letters on the subject offered the more consistent (though inaccurate) explanation of the sunspots as enormous masses of dark clouds constantly produced on the solar surface and moving uniformly over it before vanishing forever.

Galileo's next writing, the Letter to the Grand Duchess Christina, was of little scientific importance, for it neither offered new observations nor announced novel astronomical hypotheses, and was published only in 1636 in a Latin translation. In terms of the sort of interpretation it offered—a brilliant analysis of the Old Testament verse Joshua 10:12 as compatible with a heliocentric universe and incompatible with a geocentric one—the Letter was among the boldest and most ill-advised moves of Galileo's career. His confidence in his reading, for all of its economy, appears to have been misplaced, and by early 1615 a complaint had been lodged with the Inquisition. In a meeting whose general tenor and purpose are still the subject of debate, Galileo met with Robert Cardinal Bellarmine in February 1616, but was not asked to abjure his Copernican beliefs. The Edict of 1616 formally prohibited books attempting to reconcile Scripture and the hypothesis of a mobile Earth, and stipulated that Copernicus's On the Revolutions of the Heavenly Spheres was suspended until such passages could be struck through. While Galileo appears not to have seen the edict as of particular concern to him, rivals immediately recognized its impact on the astronomer's career.

The controversy between Galileo and the Jesuit astronomer Orazio Grassi ranged from the fall of 1618, when three comets emerged, to 1626, when Grassi published his third and final work on the phenomena. Galileo's principal discussion of the comets, the Assayer, appeared in 1623. Although Galileo could no longer openly defend Copernicanism, and did not have an accurate explanation of the comets, he recognized flaws in many of Grassi's arguments, particularly in the implicit support that Grassi gave to the Tychonic world system. The Assayer contains important discussions of the usefulness of parallax and of the causes of telescopic magnification of distant bodies, several of Galileo's clearest formulations of his own methodology, and some of the most caustic and amusing moments of any scientific controversy.

The synthesis of Galileo's decades of astronomical observations, speculation, and revision, the Dialogue concerning the Two Chief World Systems, Ptolemaic and Copernican, was published in Florence in 1632. Divided into four days of exchanges between the learned Salviati, the cultured Sagredo, and the tireless Aristotelian Simplicio, the Dialogue examines and discards traditional arguments distinguishing the motions, substance, and final purpose of celestial and terrestrial bodies, discusses the experimental and logical evidence for Earth's diurnal and annual movements, presents the particulars of the orbits and telescopic appearance of the other planets, draws on the emergent science of magnetism as well as upon observations of the new stars of 1572 and 1604, the fixed stars, Moon spots, and sunspots, and concludes with an ample discussion of Galileo's theory of tides. The tempo and variety of the Dialogue are surely part of its enormous appeal: the speakers move easily from minute calculations to the most abstruse philosophical speculations without losing sight of their goal of assessing the two chief world systems. But to suggest, as Galileo did, that the work involves equally qualified opponents, or recognizes the merits of aspects of both views, or presents Copernicanism as merely hypothetical, is to err: Simplicio is overmatched from the outset, a rather inept spokesman for the Ptolemaic position throughout, and effectively silenced by his companions in the last pages of the Dialogue.

Summoned to Rome to account for his publication, Galileo recanted on 22 June 1633. Although depressed and humiliated by this turn of events, he soon focused on the Two New Sciences Pertaining to Mechanics and Local Motions. Published in Leiden in 1638, his last great work is in dialogue form, and again involves Salviati, Sagredo, and Simplicio. The product of a warring age, it is set in Venice's arsenal, the site of the republic's shipbuilding and munitions production. It has as one focus the "supernatural violence" with which projectiles are fired, presents the legendary burning mirrors of antiquity as plausible weapons, discusses at length notions of impact and resistance, is dedicated to a member of the noblesse d'épée, and refers to the battlefield death of one of Galileo's former students and fellow experimenters. That said, the Two New Sciences also attend to nonmilitary matters such as the void, the speed of light, the principle of the balance, musical intervals, the role of scale in very large structures or animals, uniformly accelerated or natural motion, and the Platonic "creation point." The true fight, as Galileo's dedication and several asides suggest, is for the reestablishment of his scientific and ethical reputation, and despite the burden of illness and old age, the stricture of house arrest, and his renunciation of cosmological issues, the victory was his.

See also Astronomy ; Brahe, Tycho ; Copernicus, Nicolaus ; Kepler, Johannes ; Optics ; Scientific Instruments .

BIBLIOGRAPHY

Primary Sources

Galilei, Galileo. Dialogue concerning the Two Chief World Systems. Translated by Stillman Drake. 2nd rev. ed. Berkeley, 1967.

——. Discourse on the Comets. In The Controversy on the Comets of 1618. Translated by Stillman Drake and C. D. O'Malley. Philadelphia, 1960.

——. Sidereus Nuncius or the Sidereal Messenger. Translated and with an introduction, commentary, and notes by Albert Van Helden. Chicago, 1989.

——. Two New Sciences. Translated with an introduction and notes by Stillman Drake. Madison, Wis., 1974.

Secondary Sources

Biagioli, Mario. Galileo, Courtier: The Practice of Science in the Culture of Absolutism. Chicago, 1993.

Drake, Stillman. Essays on Galileo and the History and Philosophy of Science. Selected and introduced by N. M. Swerdlow and T. H. Levere. 3 vols. Toronto, 1999.

——. Galileo at Work: His Intellectual Biography. Chicago, 1978.

Redondi, Pietro. Galileo: Heretic. Translated by Raymond Rosenthal. Princeton, 1987.

Eileen A. Reeves