Early Concepts of Motion: Galileo

GALILEO'S CONCEPTS OF MOTION 
Galileo Galilei (1564-1642) was a contemporary of Jo­hannes Kepler. Yet for some reason he seems not to have been significantly influenced in his work by Kep­ler or Kepler's three laws of planetary motion. Galileo approached an understanding of motions in the cos­mic world by studying terrestrial motion, especially falling bodies.
The dominant concepts of motion during the Re­naissance were still those of Aristotle, who had defined motion as either natural or forced. A rock falling toward the ground was an instance of natural motion, or the tendency of earthly materials to return to their natural place-the ground. And no cause was needed to assist the motion; it was a natu ral tendency. On the other hand a thrown rock required a force or cause both to set it into motion and to continue it in motion, and thus this was an unnatural tendency­referred to by Aristotle as forced motion.
Galileo did not formulate the principle of gravity that we recognize today; that was for Newton to do later. However, he did conceptualize a force as some­thing that brings about a change in the motion of bodies, with the earth exerting an attractive force (that is, gravity) that influences falling bodies. He also rec­ognized the tendency of bodies either at rest or in motion to resist a change in their non motion or mo­tion; we now call the resistance the inertia of the body. Thus to Galileo and to his scientific contem­porary in France, Rene Descartes (1596-1650), rest and uniform motion were natural states of affairs. To change such states, it was necessary to have a force act on the body regardless of whether it was falling straight down or moving across the surface of the earth. The departure from uniformity in motion is now referred to as accelerated motion.
THE TElESCOPE: OBSERVING MOTION IN THE HEAVENS 
Although mechanics was possibly his most significant accomplishment, Galileo also revolutionized astron­omy in 1609 by designing and building a telescope. As the first telescopic explorer of the heavens, he estab­lished his place in history through such discoveries as Jupiter's four large satellites, craters and mountains on the moon, the phases of Venus, and individual stars in the Milky Way. (The frontispiece to this chap­ter reproduces Galileo's drawings of the moon.) Kep­ler had been the first to demonstrate that the helio­centric system was valid, and Galileo gave the Copernican theory observational support: For exam­ple, he observed that on a smaller scale Jupiter's sat­ellites moving around the planet were analogous to the planets orbiting the sun. They were obviously heavenly bodies not in orbit about the earth, and here also was evidence disputing Aristotle's contention that a moving earth would leave the moon behind. Jupiter retains its satellites; logically, then, the earth can move around the sun without losing its satellite.
Theological hostility loomed over Galileo for supporting Copernican cosmology. Pope Paul V instructed his emissary, Cardinal Bellarmine, to warn eo against teaching or upholding Copernican e, and from the Holy Office in February 1616 is stern decree:

• The following propositions are to be censured: (1) that the sun is at the center of the world and the universe .... Unanimonsly, this proposition has been declared stupid and absurd as a philosophy, and formally heretic because it contradicts in express manner sentences in the Holy scripture...(2) that the Earth is not the center of the world motionless, but changes its place entirely according to its diurnal movement. Unanimously, this proposition declared false as a philosophy .... 

But more liberal Pope Urban VIII took office, and obtained permission to teach both the Ptol­and Copernican systems; he was, though, to present the latter as an unproved alternative. Encourged Galileo began work on a masterly astronomical commentary, which passed censorship and was published 1632 as The Dialogues of Galileo Galilei on Principal Systems of the World: The Ptolemaic Copernican. Powerful enemies soon convinced the Pope that Galileo had cast the Ptolemaic system in an unfavorable light, the book was officially banned, and in the year 1633 the great scientist was publicly humiliated before a papal tribunal in which he re­canted his Copernican views.
The last 9 years before his death in 1642 Galileo spent in his villa in Arcetri, some distance from Flo­rence, under strict house arrest. He was forbidden to publish, to discuss the forbidden philosophy, and even to speak to Protestants although he was able to finish Two New Sciences and have it published in Ley­den in the Netherlands in 1638. By then he was 74 and totally blind-about which he writes, " ... this uni­verse, which by my remarkable observations and clear demonstrations I have enlarged a hundred, nay a thousand fold beyond the limits universally accepted by the learned men of all previous ages, are now shriv­elled up for me into such a narrow compass as is filled by my own bodily sensations." The silencing of Gali­leo acted to silence Catholic scientists in the south of Europe, and from there the scientific revolution moved to northern Europe. Galileo died in the same year, 1642, that Isaac Newton was born in England.