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Galile′o (găl′ĭ-lē̇′ō̇) (usual designation of Galileo Galilei), the founder of modern physics, was born at Pisa, Feb. 18, 1564, and died at Arcetri, near Florence, Jan. 8, 1642. In 1581, at the age of seventeen, he entered the University of Pisa where a brilliant career awaited him, not only in mathematics and mechanics but in literature, eloquence, music and art. From 1592 to 1610 he held a chair of mathematics in the University of Padua. The remainder of his life was mostly spent at Florence. It is easy to recall Galileo’s position in time, if we remember that he was born on the day of Michelangelo’s death and that he died in the year of Newton’s birth.

In popular estimation Galileo has ranked as a great astronomical discoverer, and rightly so; but it is now clear that his chief service to science is the establishment of modern dynamics.

To him we owe a clear statement of the fundamental assumption of physical science, namely, that nature always behaves in the same way in the same circumstances.

To him we owe also the ideas which underlie the first two of Newton’s laws of motion. See Dynamics.

He first enunciated the correct laws of falling bodies, showing that, if we neglect the resistance of the air, the path of a projectile must be an ellipse. In astronomy he did not invent the telescope, as is so often asserted; but he used this instrument with great skill to discover spots on the sun, measure its rotation-period, discover four of the satellites of Jupiter, observe the phases of Mercury and Venus and find the elongated form of Saturn which Huygens later showed to be due to the planet’s rings.

But no astronomical service of Galileo can outrank that which he did in establishing the Copernican system by showing the mechanical principles upon which the solar system is constructed. In 1616 he was warned by the Inquisition not to “hold, teach or defend” the Copernican system, and agreed to act upon their advice. Nevertheless, in 1632, he published his Dialogue Concerning the Two Great Systems of the Universe, which proved to be a somewhat disguised yet powerful argument for the Copernican view. As a result he was summoned to Rome in Oct., 1632, where he disavowed his adherence to the heliocentric system. The story which is often told of his recanting and then remarking concerning the earth, as he rose from his knees: “It does move, however!” is a pure fiction of later times.

An English translation of the more important of Galileo’s writings is sadly needed.