THE ESSENTIAL GALILEO...excerpt

"[.03] A main reason for delay was that beginning in 1609 Galileo became actively involved in astronomy. To be sure, he had been previously acquainted with the new theory of a moving earth published by Nicolaus Copernicus in 1543. He had been appreciative of the fact that Copernicus had advanced a novel argument supporting that ancient idea, namely, a detailed mathematical demonstration that the known facts about the motion of the heavenly bodies could be explained more systematically and coherently (not just more simply) if we attribute to the earth a daily axial rotation and an annual heliocentric revolution. Galileo had acquired the general impression that this geokinetic theory was more consistent with the new physics he was researching than was the geostatic theory. In particular, he had also been attracted to Copernicanism because he thought that the earth's motion could best explain why the tides occur. But he had not articulated, let alone published, this general impression and this particular feeling. "On the other hand, Galileo had been acutely aware of the considerable evidence against Copernicanism. The earth's motion seemed epistemologically absurd because it contradicted direct sense experience. It seemed astronomically false because it had consequences that could not be observed, such as the similarity between terrestrial and the heavenly bodies, Venus' phases, and annual stellar parallax. It seemed mechanically impossible because the available laws of motion implied that bodies on a rotating earth would, for example, follow a slanted rather than vertical path in free fall, and would be thrown off by centrifugal force. And it seemed theologically heretical because it contradicted the literal meaning and the traditional interpretation of some passages in the Bible. Until 1609 Galileo apparently judged that the anti-Copernican arguments far outweighed the pro-Copernican ones. Thus we find him teaching geostatic astronomy in his courses and reacting in a lukewarm and evasive manner when an enthusiastic Copernican like Johannes Kepler tried to engage him. "[.4] However, the telescopic discoveries that began in 1609 led Galileo to a major reassessment of Copernicanism, and so for the next seven years he was seriously and explicitly involved in astronomical research and discussions. In 1609 he perfected the telescope to such an extent as to make it an astronomically useful instrument that could be duplicated by others for sometime. By its means he made several startling discoveries, which he immediately published in The Sidereal Messenger (Venice, 1610): that the moon's surface is full of mountains and valleys; that innumerable other stars exist beside those visible with the naked eye; that the Milky Way and nebulas are dense collections of large numbers of stars; and that the planet Jupiter has four moons revolving around it at different distances and with different periods. As a result, Galileo became a celebrity, resigned his professorship at Padua, was appointed Philosopher and Chief Mathematician to the grand duke of Tuscany, and moved to Florence the same year. Soon thereafter, he discovered sunspots on the phases of Venus." p.4-5, "Introduction," THE ESSENTIAL GALILEO, edited and translated by Maurice A. Finocchiaro (Hackett Pubs, Indianapolis: 2008)