GALILEI, Galileo, Italian physicist and astronomer: b. Pisa, 14 Feb. 1564; d. Arcetri, 8 Jan. 1642. His father, Vincenzo Galilei, an impoverished nobleman of Florence, caused him to be instructed in Latin and Greek, drawing and music, and he very early showed a strong inclination to mechanical labors. In 1581 Galileo entered the University of Pisa, to attend lectures on medicine and the Aristotelian philosophy. Here he became con spicuous in refusing to accept without question the dogmatic statements of his teachers. That spirit of observation for which he was dis tmguished was early developed. When he was only 19 the swinging of a lamp suspended from the ceiling of the cathedral in Pisa led him to discover the isochronism of • the pendu lum, which he was the first to apply as a measure of time. He studied mathematics under Ostilio Ricci, soon exhausted Euclid and Archimedes, and was led, by the works of the latter, in 1586, to the invention of the hydro static balance, by which the specific gravity of solids might be ascertained with much accuracy.
He now devoted his attention exclusively to mathematics and natural science, and in 1589 was made professor of mathematics in the Uni versity of Pisa. In the presence of numerous spectators he went through with his experi ments, which he performed on the leaning tower of Pisa, to show that weight has no in fluence on the velocity of falling bodies. By this means he excited the opposition of the adherents of Aristotle to such a degree, that after two years he was forced to resign his professorship. Through the influence of the Marchese Guidobaldo, the Senate of Venice, in 1592, appointed him professor of mathe matics in Padua. Here he remained till 1610, He lectured with unparalleled success. Scholars from the most distant regions of Europe crowded about him. In 1597 he in vented the sector.
One of the most important mathematical dis coveries which he made at a period subsequent to this is that the spaces through which a body falls, in equal times, increase as the numbers 1, 3, 5, 7• that is, if a body falls 16 feet in the first second, it will fall 48 in the next second, 80 in the third, and so on. Whether the thermom eter was his invention it is difficult to determine; perhaps he only improved it. By means of a telescope, constructed by himself, he made a series of the most important discoveries. He found that the moon, like the earth, has an un even surface; and he taught his scholars to measure the height of its mountains by their shadow. A particular nebula he resolved into individual stars, and even conjectured that the whole Milky Way, with good instruments, might be resolved in the same manner. His
most remarkable discovery was that of Jupiter's satellites, 7 Jan. 1610. He likewise observed Saturn's ring, though he had not a just idea with regard to it. He saw the sun's spots some what later, and inferred, from their regular ad vance from east to west, the rotation of the sun, and the inclination of its axis to the plane of the ecliptic.
Galileo's name, meantime, had grown so celebrated that Cosmo II, grand duke of Tus cany, appointed him grand-ducal mathematician and philosopher, and invited him to become first instructor in mathematics at Pisa. Here he gained a decisive victory for the Copernican system by the discovery of the varying phases of Mercury, Venus and Mars; as the motion of these planets about the sun, and their de pendence on it for light, were thus established beyond the possibility of doubt. He wrote a work afterward on the floating and sinking of solid bodies in water, and in this, as well as in all his other writings, scattered the seeds of many new doctrines.
While thus employed in enlarging the field of natural philosophy, a tremendous storm was gathering about his own head. He had de clared himself in favor of the Copernican sys tem, in his work on the sun's spots, and was therefore denounced as a heretic by his enemies. In 1611 he visited Rome for the first time, where he was honorably received, and where a favorable report was made on his writings by the mathematicians of the Collegio Romano at the instance of Cardinal Bellarmin. On his re turn to Florence, however, he became more and more involved in controversy, which gradually' took a theological turn, and in the course of which he declared the literal understanding of the utterances of Scripture with regard to physical phenomena to lead to absurdities. From Rome he received, in the name of the Cardinal Barberini (afterward Pope Urban VIII), the warning not to overstep the limits of mathematics and physics, but he paid no heed to the well-meant advice. The monks preached against him, and in 1616 he found himself again obliged to proceed to Rome, where he is said to have pledged himself to abstain for the future from promulgating his system either orally or otherwise. The genuineness of the document on the basis of which this is asserted, has, however, been questioned in modern times, and the controversy regarding this matter is not yet finally settled.