GALILEI, GALILEO (b. Pisa, Italy, 15 February 1564; d. Arcetri, Italy, 8 January 1642), physics, astronomy.

    assumed proportionality of speed to space traversed. Accordingly, he introduced in its place two propositions drawn from mechanics, which he submitted for criticism to Valerio. Galileo received Valerio's reply in July 1609, just after his attention had again been diverted from mechanics, this time by news of the invention of the telescope.

The Telescope.

A Dutch lens-grinder, Hans Lipperhey, had applied in October 1608 to Count Maurice of Nassau for a patent on a device to make distant objects appear closer. Sarpi, whose extensive correspondence (maintained for theological and political reasons) kept him currently informed, learned of this device within a month. Somewhat skeptical, he applied for further information to Jacques Badovere (Giacomo Badoer), a former pupil of Galileo's then at Paris. In due course the report was confirmed. Galileo heard discussions of the news during a visit to Venice in July 1609, learned from Sarpi that the device was real, and probably heard of the simultaneous arrival at Padua of a foreigner who had brought one to Italy. He hastened back to Padua, found that the foreigner had left for Venice, and at once attempted to construct such a device himself. In this he quickly succeeded, sent word of it to Sarpi, and applied himself to the improvement of the instrument. Sarpi, who had meanwhile been selected by the Venetian government to assess the value of the device offered for sale to them by the stranger, discouraged its purchase. Late in August, Galileo arrived at Venice with a nine-power telescope, three times as effective as the other. The practical value of this instrument to a maritime power obtained for him a lifetime appointment to the university, with an unprecedented salary for the chair of mathematics. The official document he received, however, did not conform to his understanding of the terms he had accepted. As a result, he pressed his application for a post at the Tuscan court, begun a year or two earlier.

Galileo's swift improvement of the telescope continued until, at the end of 1609, he had one of about thirty power. This was the practicable limit for a telescope of the Galilean type, with plano-convex objective and plano-concave eyepiece. He turned this new instrument to the skies early in January 1610, with startling results. Not only was the moon revealed to be mountainous and the Milky Way to be a congeries of separate stars, contrary to Aristotelian principles, but a host of new fixed stars and four satellites of Jupiter were promptly discovered. Working with great haste but impressive accuracy, Galileo recited these discoveries in the Sidereus nuncius, published at Venice early in March 1610.

His sudden fame assisted Galileo in his negotiations at Florence. Moreover, the new discoveries made him reluctant to continue teaching the old astronomy. In the summer of 1610, he resigned the chair at Padua and returned to Florence as mathematician and philosopher to the grand duke of Tuscany, and chief mathematician of the University of Pisa, without obligation to teach.

Galileo's book created excitement throughout Europe and a second edition was published in the same year at Frankfurt. Kepler endorsed it in two small books, the Dissertatio cum Nuncio Sidereo, published before he had personally observed the new phenomena, and the Narratio de observatis a se quatuor Jovis satellitibus, published a few months later. Other writers attacked the claimed discoveries as a fraud. Galileo did not enter the controversy but applied himself to further observations. He discovered, later in 1610, the oval appearance of Saturn and the phases of Venus. His telescope was inadequate to resolve Saturn's rings, which he took to be satellites very close to the planet. The phases of Venus removed a serious objection to the Copernican system, and he saw in the satellites of Jupiter a miniature planetary system in which, as in the Copernican astronomy, it could no longer be held that all moving heavenly bodies revolved exclusively about the earth.

Early in 1611 Galileo journeyed to Rome to exhibit his telescopic discoveries. The Jesuits of the Roman College, who had at first been dubious, confirmed them and honored Galileo. Federico Cesi feted Galileo and made him a member of the Lincean Academy, the first truly scientific academy, founded in 1603. The pope and several cardinals also showed their esteem for Galileo.

Controversies at Florence.

Shortly after his return to Florence, Galileo became involved in a controversy over floating bodies. In that controversy an important role was played by Colombe, who became the leader of a group of dissident professors and intriguing courtiers that resented Galileo's position at court. Maffeo Barberini—then a cardinal but later to become pope—took Galileo's side in the dispute. Turning again to physics, Galileo composed and published a book on the behavior of bodies placed in water (Discurso ... intorno alle cose che stanno in su l'acqua, o in quella si muovono), in support of Archimedes and against Aristotle, of which two editions appeared in 1612. Using the concept of moment and the principle of virtual velocities, Galileo extended the scope of the Archimedean work beyond purely hydrostatic considerations.

While this work was in progress, Galileo received from Marcus Welser of Augsburg a short treatise on sunspots that Welser had published pseudonymously