Galileo Affair - 1600s Revolution in Cosmology

1600s Revolution in Cosmology

Galileo began his telescopic observations in the later part of 1609, and by March 1610 was able to publish a small book, The Starry Messenger (Sidereus Nuncius), relating some discoveries that had not been dreamed of in the philosophy of the time: mountains on the Moon, lesser moons in orbit around Jupiter, and the resolution of what had been thought to be very cloudy masses in the sky (nebulae) into collections of stars too faint to see individually without a telescope. Other observations followed, including the phases of Venus and the existence of sunspots.

None of these findings, which were difficult at first for other astronomers to verify, proved that the Earth moved, or directly contradicted either Aristotle's model or Christian doctrine. However, they caused difficulties for natural philosophers of the time, as they contradicted scientific and philosophical ideas based on those of Aristotle and Ptolemy and closely associated with the Catholic Church then (despite their being pagan authors). In particular, the phases of Venus, which showed it to circle the sun, and the observation of moons orbiting Jupiter contradicted the geocentric model of Ptolemy, in which the motion of all celestial bodies was centered on the Earth.

Jesuit astronomers, experts both in Church teachings, science, and in natural philosophy, were at first skeptical and hostile to the new ideas; however, within a year or two the availability of good telescopes enabled them to repeat the observations. In 1611, Galileo visited the Collegium Romanum in Rome, where the Jesuit astronomers by that time had repeated his observations. Christoph Grienberger, one of the Jesuit scholars on the faculty, sympathized with Galileo’s theories, but was asked to defend the Aristotelian viewpoint by Claudio Acquaviva, the Father General of the Jesuits. Not all of Galileo's claims were completely accepted: Christopher Clavius, the most distinguished astronomer of his age, never was reconciled to the idea of mountains on the Moon, and outside the collegium many still disputed the reality of the observations. In a letter to Kepler of August 1610, Galileo complained that some of the philosophers who opposed his discoveries had refused even to look through a telescope:

My dear Kepler, I wish that we might laugh at the remarkable stupidity of the common herd. What do you have to say about the principal philosophers of this academy who are filled with the stubbornness of an asp and do not want to look at either the planets, the moon or the telescope, even though I have freely and deliberately offered them the opportunity a thousand times? Truly, just as the asp stops its ears, so do these philosophers shut their eyes to the light of truth."

Geocentrists who did verify and accept Galileo's findings had an alternative to Ptolemy's model in an alternative geocentric (or "geo-heliocentric") model proposed some decades earlier by Tycho Brahe—a model, in which, for example, Venus circled the sun.

Galileo became involved in a dispute over priority in the discovery of sunspots with Christoph Scheiner, a prominent Jesuit. This became a bitter lifelong feud. Neither of them, however, was the first to recognise sunspots—the Chinese had already been familiar with them for centuries.

At this time, Galileo also engaged in a dispute over the reasons that objects float or sink in water, siding with Archimedes against Aristotle. The debate was unfriendly, and Galileo's blunt and sometimes sarcastic style, though not extraordinary in academic debates of the time, made him enemies. During this controversy one of Galileo's friends, the painter, Lodovico Cardi da Cigoli, informed him that a group of malicious opponents, which Cigoli subsequently referred to derisively as "the Pigeon league," was plotting to cause him trouble over the motion of the earth, or anything else that would serve the purpose. According to Cigoli, one of the plotters had asked a priest to denounce Galileo's views from the pulpit, but the latter had refused. Nevertheless, three years later another priest, Tommaso Caccini, did in fact do precisely that, as described below.