GALILEO GALILEI (1564-1642), Italian astronomer and experimental philosopher, was born at Pisa on Feb. 15, 1564. He came of a noble but impoverished Florentine family; his father Vincenzio Galilei was a competent mathematician and an able musician. Galileo received his early education at the monastery of Vallombrosa, near Florence. Here he studied Greek, Latin and logic but showed a distaste for the science he was taught. He acquired a reputation for intellectual aptitude as well as for me chanical invention. In 1581 his father sent him to the University of Pisa to study medicine.
Galileo was endowed with many natural gifts ; he was a skilful musician and showed a taste for painting and it seemed possible that he might develop in any direction determined by choice or by chance.
The Pendulum and Velocities.—In 1581, while watching a lamp set swinging in the cathedral of Pisa, he observed that, what ever the range of its oscillations, they were invariably executed in equal times. The experimental verification of this fact led him to the important discovery of the isochronism of the pendulum. He applied the new principle to the timing of the human pulse. Up to this time he was entirely ignorant of mathematics, his father having carefully held him aloof from a study which he rightly apprehended would lead to his total alienation from that of medicine. Accident, however, frustrated this purpose. A lesson in geometry which he overheard by chance awakened his interest, and with his father's reluctant permission he began to study mathematics and science. In he was withdrawn from the university, through lack of means, before he had taken a degree, and returned to Florence. He lectured to the Florentine Academy and in 1586 he published an essay descriptive of his invention of the hydrostatic balance, which rapidly made his name known throughout Italy. At the request of his first patron, the Marchese Guidubaldo del Monte of Pesaro, a man equally eminent in science and influential through family connections, he wrote, in 1588, a treatise on the centre of gravity in solids, which obtained for him the honourable though not lucrative post of mathematical lecturer at the Pisan university. During the ensuing two years (1589-91) he carried on that remarkable series of experiments by which he established the first principles of dynamics. From the Leaning Tower of Pisa he afforded to all the professors and stu dents of the university ocular demonstration that bodies of differ ent weights fall with the same velocities. He also showed that the path of a projectile is a parabola. His sarcasm roused the anger of men holding different views. He became unpopular, resigned his professorship and withdrew to Florence in 1591.
In the following year he was appointed to the chair of mathe matics at Padua. He remained here until 161o; his appointment was three times renewed and his salary continually increased.
Researches with the Telescope.—Galileo seems, at an early period of his life, to have adopted the Copernican theory of the solar system, but was deterred from avowing his opinions—as is proved by his letter to Kepler of Aug. 4, the fear of ridicule rather than of persecution. A rumour of the invention of the telescope (q.v.) which reached Venice in June 1609, suf ficed to set Galileo on the track; and he succeeded in producing a telescope of threefold magnifying power. Upon this first attempt he rapidly improved, until he attained to a power of 32, and his instruments, of which he manufactured hundreds with his own hands, were soon in request in every part of Europe. This form of telescope, which is known by his name, is used for opera-glasses. Galileo began to make observations with his telescope and pub lished the Sidereus Nuncius in 161o. He observed the mountain ous configuration of the moon and he showed that the Milky Way was a collection of lesser stars. An important discovery was that of Jupiter's satellites, first seen by Galileo on Jan. 7, 161o, and by him named Sidera Medicea, in honour of the grand duke of Tus cany, Cosimo II., who had been his pupil and was about to become his employer. This was a strong confirmation of the Corpernican theory which was triumphantly received by its advocates. Later he observed what appeared to him as the triple form of Saturn, the phases of Venus and sun spots.
In Sept. 1610 Galileo finally abandoned Padua for Florence. His researches with the telescope had been rewarded by the Venetian senate with the appointment for life to his professor ship, at a high salary. He was also appointed as philosopher and mathematician extraordinary to the grand duke of Tuscany. This sinecure presented him with an increased opportunity of con tinuing his scientific labours.
In 161I Galileo visited Rome, and exhibited the telescopic wonders of the heavens to the most eminent personages at the pontifical court. Encouraged by the flattering reception accorded to him, he ventured, in his Letters on the Solar Spots, printed at Rome in 1613, to take up a more decided position with regard to the Copernican theory. Galileo's brilliant researches, enhanced by his formidable dialectic and enthusiastic zeal, drew the atten tion of the authorities to the discrepancies between the new view of the solar system and certain passages in the Scriptures.
Although he had no desire to raise the theological issue, it must be admitted that, the discussion once set on foot, he threw himself into it with characteristic impetuosity, and thus helped to pre cipitate a decision which it was in his interest to avert. Not only did Galileo explain adverse texts but he tried to produce scrip tural confirmation of the Copernican system. The agitation against him increased and in 1615 he received a semi-official warning to avoid theology and limit himself to physical reasoning. In Feb. 1616 the consulting theologians of the Holy Office characterized the proposition that the sun is immovable in the centre of the world and that the earth has a diurnal motion of rotation as heretical. Shortly afterwards Galileo was admonished by the pope, Paul V., not to "hold, teach or defend" the condemned doctrine. This injunction he promised to obey.
Galileo returned to Florence three months later, not ill-pleased, as his letters testify, with the result of his visit to Rome. For seven years, during which he led a life of studious retirement at Bellosguardo, near Florence, he maintained an almost unbroken silence. At the end of that time he appeared in public with his Saggiatore, in which he dealt with the nature of comets. His views were erroneous, since he held comets to be mere atmospheric ema nations reflecting sunlight after the fashion of a halo or a rain bow. The Saggiatore was printed at Rome in Oct. 1623 by the Academy of the Lincei, of which Galileo was a member, with a dedication to the new pope, Urban VIII., and notwithstanding some passages containing a covert defence of Copernican opinions, was received with acclamation by ecclesiastical no less than by scientific authorities.
It was at once evident that the whole tenor of this remarkable work was in flagrant contradiction with the edict passed 16 years before its publication, as well as with the author's personal pledge of conformity to it. The theological censures which the book did not fail to incur were not slow in making themselves felt. Towards the end of August the sale was prohibited ; on Oct. 1 the author was cited to Rome by the Inquisition. He pleaded his age and infirm health, but no excuse was admitted. At length, on Feb. 13, 1633, he arrived at Rome where he was detained but treated with indulgence. He was finally examined by the Inquisi tion on June 21 under menace of torture which was not carried out and which it was never intended to execute. The following day Galileo recanted and was sentenced to incarceration at the pleasure of the tribunal, and by way of penance was enjoined to recite once a week for three years the seven penitential psalms. This sentence was signed by seven cardinals, but did not receive the customary papal ratification. The legend according to which Galileo, rising from his knees after repeating the formula of abjuration, stamped on the ground, and exclaimed, "Eppur si muove!" is, as may readily be supposed, entirely apocryphal. Its earliest ascertained appearance is in the Abbe Irailli's Querelles litteraires (vol. iii., p. 49, 1761) .
Galileo remained in the custody of the Inquisition from the 21st to the 24th of June, on which day he was relegated to the Villa Medici on the Trinita de Monti. Thence, on July 6, he was permitted to depart for Siena, where he spent several months in the house of the archbishop, Ascanio Piccolomini, one of his numerous and trusty friends.
It was not until December that his earnest desire of returning to Florence was realized, and the remaining eight years of his life were spent in his villa at Arcetri, in the strict seclusion which was the prescribed condition of his comparative freedom.
His prodigious mental activity continued undiminished to the last. In 1636 he completed his Dialoghi delle nuove scienze, in which he recapitulated the results of his early experiments and mature meditations on the principles of mechanics. This in many respects his most valuable work was printed by the Elzevirs at Leyden in 1638, and excited admiration equally universal and more lasting than that accorded to his astronomical treatises. His last telescopic discovery—that of the moon's diurnal and monthly librations—was made in 1637, only a few months before he became blind. But the fire of his genius was not even yet extinct. He continued his scientific correspondence with unbroken interest and undiminished acumen ; he thought out the application of the pendulum to the regulation of clockwork, which Huygens successfully realized 15 years later; and he was engaged in dic tating to his disciples, Viviani and Torricelli, his latest ideas on the theory of impact when he was seized with the slow fever which resulted in his death on Jan. 8, 1642.
The direct services of permanent value which Galileo ren dered to astronomy are virtually summed up in his telescopic discoveries. To the theoretical perfection of the science he con tributed little or nothing. The circumstance, however, which most seriously detracts from his scientific reputation is his neg lect of the discoveries made during his lifetime by Kepler, the greatest of his contemporaries. His name is nevertheless justly associated with a vast extension of the bounds of the visible uni verse, and his telescopic observations are a standing monument to his ability. Within two years of their first discovery, he had constructed approximately accurate tables of the revolutions of Jupiter's satellites, and he proposed their frequent eclipses as a means of determining longitudes, not only on land, but at sea. The idea, though ingenious, has been found of little practical utility at sea. His observations on sun spots are noteworthy for their accuracy and the deductions he drew from them with regard to the rotation of the sun and the revolution of the earth.
The idea of a universal force of gravitation seems to have hovered on the borders of this great man's mind, without ever fully entering it. More valid instances of the anticipation of modern discoveries may be found in his prevision that a small annual parallax would eventually be found for some of the fixed stars, and that extra-Saturnian planets would at some future time be ascertained to exist, and in his conviction that light travels with a measurable although, in relation to terrestrial distances, infinite velocity. Although Galileo discovered, in 161o, a means of adapting his telescope to the examination of minute objects, he did not become acquainted with the compound microscope until 1624 when he saw one of Drebbel's instruments in Rome, and, with characteristic ingenuity, immediately introduced some ma terial improvements into its construction.
The most substantial part of his work consisted undoubtedly in his contributions towards the establishment of mechanics as a science. Some valuable but isolated facts and theorems had been previously discovered and proved, but it was he who first clearly grasped the idea of force as a mechanical agent. The interdependence of motion and force was not indeed formulated into definite laws by Galileo, but his writings on dynamics are everywhere suggestive of those laws, and his solutions of dynam ical problems involve their recognition. In this branch of science he paved the way for Newton. The extraordinary advances made by him were owing to his happy method of applying mathematical analysis to physical problems.
The method which was peculiarly his consisted in the combi nation of experiment with calculation—in the transformation of the concrete into the abstract, and the assiduous comparison of results. This was applied to the investigation of the laws of falling bodies, of equilibrium and motion on an inclined plane and of the motion of a projectile. The latter, together with his definition of momentum and other parts of his work, implied a knowledge of the laws of motion as later stated by Newton, but Galileo did not enunciate these laws in a definite form. In his Discorso intorno alle cose the stanno su l'acqua, published in 1612, he used the principle of virtual velocities to demonstrate the more elementary theorems of hydrostatics, deducing from it the equilibrium of fluid in a siphon, and worked out the conditions for the flotation of solid bodies in a liquid. He also constructed an elementary form of thermometer.
The first complete edition of Galileo's writings was published at Florence (1842-56) , in 16 vols., under the supervision of Eugenio Alberi. Besides the works already enumerated, it contained the Sermones de mote gravium composed at Pisa between 1589 and 1591 ; his letters to his friends, with many of their replies, as well as several of the essays of his scientific opponents ; his laudatory comments on the Orlando Furioso, and depreciatory notes on the Gerusalemme Liberata, some stanzas and sonnets of no great merit, together with the sketch of a comedy ; finally, a reprint of Viviani's Life, with valuable notes and corrections. The original documents from the archives of the Inquisition, relating to the events of 1616 and recovered from Paris in 1846 by the efforts of Count Rossi, and now in the Vatican library, were to a limited extent made public by Monsignor Marino-Marini in 185o, and more unreservedly by M. Henri de l'Epinois, in an essay entitled "Galilee, son proces, sa con demnation," published in 1867 in the Revue des questions historiques. He was followed by Karl von Gebler, who, in an able and exhaustive but somewhat prejudiced work, Galileo Galilei and die romische Curie (Stuttgart, 1876), sought to impeach the authenticity of a document of prime importance in the trial of 1633. He was victoriously answered by Domenico Berti, in Il Processo originale di Galileo Galilei (1876), and by M. de l'Epinois, with Les pieces du proces de Galilee (1877) . The touching letters of Galileo's eldest daughter, Sister Maria Celeste, to her father were printed in 1864 by Carlo Arduini, in a publication entitled La Primogenita di Galileo Galilei.
The issue of a "national edition" of the Works of Galileo, in 20 large volumes, was begun at Florence in 189o. It includes a mass of previously inedited correspondence and other documents, collected by the indefatigable director, Antonio Favaro, among whose numerous publications on Galilean subjects may be mentioned: Galileo e lo studio di Padova (2 vols., 1883) ; Scampoli Galileani (12 series, 1886-97) ; Nuovi Studii Galileani (1891) ; Galileo Galilei e Suor Maria Celeste (1891) . See also Th. Henri Martin, Galilee, les droits de la science et la methode des sciences physiques (1868) ; Private Life of Galileo (by Mrs. Olney, 1870) ; J. J. Fahie, Galileo; his Life and Work (1903) ; J. A. C. Oudemans and J. Bosscha, Galilee et Marius (19o3) . The relations of Galileo to the church are temperately and ably dis cussed by F. R. Wegg-Prosser in Galileo and his Judges (1889) , and in two articles published in the American Catholic Quarterly for April and July 1901. "La questione Copernicana dal primo al secondo processo di Galileo," Civilta cattolica, anno 78 Vol. 2, p. 229-242, 345, Vol. 3, p. 228-242 (Roma, 1927). (A. M. C.; X.)