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(Galileo Galilei) (găl'ĭlē`ō; gälēlĕ`ō gälēlĕ`ē), 1564–1642, great Italian astronomer, mathematician, and physicist. By his persistent investigation of natural laws he laid foundations for modern experimental science, and by the construction of astronomical telescopes he greatly enlarged humanity's vision and conception of the universe. He gave a mathematical formulation to many physical laws.

Contributions to Physics

His early studies, at the Univ. of Pisa, were in medicine, but he was soon drawn to mathematics and physics. It is said that at the age of 19, in the cathedral of Pisa, he timed the oscillations of a swinging lamp by means of his pulse beats and found the time for each swing to be the same, no matter what the amplitude of the oscillation, thus discovering the isochronal nature of the pendulum, which he verified by experiment. Galileo soon became known through his invention of a hydrostatic balance and his treatise on the center of gravity of solid bodies. While professor (1589–92) at the Univ. of Pisa, he initiated his experiments concerning the laws of bodies in motion, which brought results so contradictory to the accepted teachings of Aristotle that strong antagonism was aroused. He found that bodies do not fall with velocities proportional to their weights, but he did not arrive at the correct conclusion (that the velocity is proportional to time and independent of both weight and density) until perhaps 20 years later. The famous story in which Galileo is said to have dropped weights from the Leaning Tower of Pisa is apocryphal. The actual experiment was performed by Simon Stevin several years before Galileo's work. However, Galileo did find that the path of a projectile is a parabola, and he is credited with conclusions foreshadowing Newton's laws of motion.

Contributions to Astronomy

In 1592 he began lecturing on mathematics at the Univ. of Padua, where he remained for 18 years. There, in 1609, having heard reports of a simple magnifying instrument put together by a lens-grinder in Holland, he constructed the first known complete astronomical telescope. Exploring the heavens with his new aid, Galileo discovered that the moon, shining with reflected light, had an uneven, mountainous surface and that the Milky Way was made up of numerous separate stars. In 1610 he discovered the four largest satellites of Jupiter, the first satellites of a planet other than Earth to be detected. He observed and studied the oval shape of Saturn (the limitations of his telescope prevented the resolving of Saturn's rings), the phases of Venus, and the spots on the sun. His investigations confirmed his acceptance of the Copernican theory of the solar system; but he did not openly declare a doctrine so opposed to accepted beliefs until 1613, when he issued a work on sunspots. Meanwhile, in 1610, he had gone to Florence as philosopher and mathematician to Cosimo II de' Medici, grand duke of Tuscany, and as mathematician at the Univ. of Pisa.

Conflict with the Church

In 1611 he visited Rome to display the telescope to the papal court. In 1616 the system of Copernicus was denounced as dangerous to faith, and Galileo, summoned to Rome, was warned not to uphold it or teach it. But in 1632 he published a work written for the nonspecialist, Dialogo … sopra i due massimi sistemi del mondo [dialogue on the two chief systems of the world] (tr. 1661; rev. and ed. by Giorgio de Santillana, 1953; new tr. by Stillman Drake, 1953, rev. 1967); that work, which supported the Copernican systemCopernican system,
first modern European theory of planetary motion that was heliocentric, i.e., that placed the sun motionless at the center of the solar system with all the planets, including the earth, revolving around it.
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 as opposed to the Ptolemaic, marked a turning point in scientific and philosophical thought. Again summoned to Rome, he was tried (1633) by the Inquisition and brought to the point of making an abjuration of all beliefs and writings that held the sun to be the central body and the earth a moving body revolving with the other planets about it. Since 1761, accounts of the trial have concluded with the statement that Galileo, as he arose from his knees, exclaimed sotto voce, "E pur si muove" [nevertheless it does move]. That statement was long considered legendary, but it was discovered written on a portrait of Galileo completed c.1640.

After the Inquisition trial Galileo was sentenced to an enforced residence in Siena. He was later allowed to live in seclusion at Arcetri near Florence, and it is likely that Galileo's statement of defiance was made as he left Siena for Arcetri. In spite of infirmities and, at the last, blindness, Galileo continued the pursuit of scientific truth until his death. His last book, Dialogues Concerning Two New Sciences (tr., 3d ed. 1939, repr. 1952), which contains most of his contributions to physics, appeared in 1638. In 1979 Pope John Paul II asked that the 1633 conviction be annulled. However, since teaching the Copernican theory had been banned in 1616, it was technically possible that a new trial could find Galileo guilty; thus it was suggested that the 1616 prohibition be reversed, and this happened in 1992. The pope concluded that while 17th-century theologians based their decision on the knowledge available to them at the time, they had wronged Galileo by not recognizing the difference between a question relating to scientific investigation and one falling into the realm of doctrine of the faith.


See biographies by L. Geymonat (tr. 1965), J. L. Heilbron (2010), and D. Wooton (2010); studies by G. de Santillana (1955), S. Drake (1970, 1978, and 1980), and W. R. Shea (1973); G. de Santillana, The Crime of Galileo (1955, repr. 1976); M. A. Finocchiaro, Galileo and the Art of Reasoning (1980).

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(gal-ă-lee -oh, -lay -) A NASA mission to Jupiter and its moons, launched Oct. 18 1989 from the space shuttle Atlantis. With insufficient power to fly directly to Jupiter, Galileo first followed a circuitous 3-year flight path through the inner Solar System using the gravity of the celestial bodies it passed to catapult it on its way. Its trajectory involved the following events: Venus flyby (Feb. 1990), Earth flyby (Dec. 1990), flyby of asteroid (951) Gaspra in the inner asteroid belt (Oct. 1991), second Earth flyby (Dec. 1992). Its instruments sent back excellent images and data on Venus, the Moon, Earth, and Gaspra. The mission received a potentially disastrous blow when Galileo's communications system was crippled after its main transmission antenna failed to open properly in Apr. 1991. As a result, data could be relayed to Earth only using a slow weak backup link, greatly reducing the number of transmitted images. Despite this problem, the spacecraft was to prove a major success.

Following the second Earth flyby, Galileo had gained sufficient momentum to reach Jupiter (see gravity assist). On its flight there, it flew past the asteroid (243) Ida (Aug. 1993), where it discovered the first asteroidal moon, Dactyl, about 1.5 km in diameter, orbiting its primary at a distance of about 100 km. Galileo reached Jupiter in Dec. 1995, entering a 198-day, highly elliptical orbit around the gas giant. Among its first tasks was to relay data back to Earth from a probe it had released in 1995. The probe plunged into Jupiter just as Galileo entered orbit and sent back information on the physical and chemical nature of the upper reaches of the planet's deep atmosphere before being destroyed.

Galileo's mission to the Jovian system was at first set to run for 23 months. During this first period – its primary mission – the spacecraft made observations of Jupiter and flew by the Galilean satellites Ganymede, Callisto, and Europa. It also returned long-range images of the volcanically active Io. Its endurance in the hostile radiation-filled environment around Jupiter and the excellence of the scientific data it sent back to Earth despite its communication problems persuaded NASA to prolong its stay after the end of its primary mission in Dec. 1997. It embarked on extended observations of Europa and Io. In 1999, it made its first close encounter with Io, returning during this and later visits valuable data on the satellite itself and on the Io torus, the doughnut-shaped cloud of highly energetic charged particles that surround the satellite's orbit. In late 2000, Galileo teamed up with the Saturn-bound Cassini–Huygens spacecraft to investigate the interaction between Jupiter's magnetosphere and the solar wind. The Galileo spacecraft remained operational in Jupiter's vicinity for nearly 8 years. In all, it made 35 circuits of Jupiter and completed 34 flybys of its major satellites. It returned a rich stream of information that transformed our understanding of the giant planet itself as well as its retinue of moons and its dusty rings. Altogether, it spent almost 14 years in space, traveling a total distance of 4 631 778 000 km. On Sept. 21 2003, Galileo was deliberately guided into Jupiter's atmosphere to be crushed out of existence.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006


McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


(1564–1642) Italian mathematician, astronomer, and physicist. [Ital. Hist.: EB, IV: 388]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.


full name Galileo Galilei. 1564--1642, Italian mathematician, astronomer, and physicist. He discovered the isochronism of the pendulum and demonstrated that falling bodies of different weights descend at the same rate. He perfected the refracting telescope, which led to his discovery of Jupiter's satellites, sunspots, and craters on the moon. He was forced by the Inquisition to recant his support of the Copernican system
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


["Galileo: A Strongly Typed Interactive Conceptual Language", A. Albano et al, ACM Trans Database Sys 10(2):230-260 (June 1985)].
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A satellite-based radio navigation system initiated by the European Union and European Space Administration (ESA). Using 24 satellites and six spares, Galileo was designed to interoperate with the U.S. GPS and Russian GLONASS systems. The first Galileo test satellite (GIOVE-A) was launched in late 2005, and the initial system went live in 2016. With 22 active satellites as of 2020, positioning accuracy is one meter (GPS is 20m; GLONASS is 60m). For more information, visit See GNSS and GPS.
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