HPS 322: Michael J. White

Galileo Galilei (1564-1642)

I. An Advocate of ‘Two Books’ Conception of the Expression of God’s Intellect and Will: Revelation (The Bible and magisterium of Church) is one book; Nature, the language of which is mathematics, is the other book. A. Revelation speaks of matters relevant to our eternal welfare; it is not concerned with other (e.g., scientific) matters. B. Nature does not speak of matters directly relevant to our eternal welfare; hence, its language is often difficult to understand. “[Natural] is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it” (from The Assayer.) Question: What, then, is Nature’s message and what is its importance to us?

II. ‘Mitigated skepticism’: “For in our speculating we either seek to penetrate the true and internal essence of natural substances, or content ourselves with a knowledge of some of their properties. The former I hold to be as impossible an undertaking with regard to the closest elemental substances as with more remote celestial things. . . . “Hence I should infer that although it may be vain to seek to determine the true substance of sunspots, still it does not follow that we cannot know some properties of then, such as their location, motion, shape size opacity, mutability, generation, and dissolution” (from Letters on Sunspots). B. Distinction between primary qualities and secondary qualities. 1. Secondary qualities an effect of interaction between subject and object of sensation. 2. Primary qualities evidently ‘objective’ features of object of sensation a. Qualities that an object must retain in order to remain that (kind of) object b. Qualities that we cannot conceive an object as lacking while remaining that (kind of) object c. ‘Quantitative’ qualities of an object: i. e., those amenable to mathematical analysis III. A ‘PR Man’ for Heliocentric (Copernican–not Keplerian) Cosmology: In 1609-1610 Galileo ‘turns the telescope on the heavens’. A. Features of (our) moon’s surface B. Moons of Jupiter (“Medicean stars” or, later, “Galilean Moons”) C. Phases of Venus: important evidence against Ptolemaic cosmology but not against Tychonic cosmology D. Little change in apparent size of fixed stars (as opposed to apparent size of planets). Meaning?? E. What he didn’t see: stellar parallax

IV. Galileo begins the job of developing a ‘new physics’ in which to place the new cosmology A. Galileo’s use of ‘frames of reference’ and concepts of absolute vs. relative motion: his mastery of the Hellenistic tradition of geometrical analysis and his use of geometrical methods of representing motion at a constant speed (“uniform” motion) and motion of constant acceleration /deceleration (“uniformly difform” motion) that were developed by the ‘Oxford calculators’ and and in the 14th century; his use of the technique of simplification/idealization of physical phenomena. 1. Analysis of reversed rectilinear motion. 2. Phenomenon of ‘natural acceleration’, e.g., the behavior of motion of ‘dropped’ objects in the vicinity of earth’s surface. (See also selections from Dialogues concerning the Two Chief World Systems.) 3. ‘Time-square Law’ and techniques of ‘reduction’ of (constantly) accelerated motion to motion of constant velocity 4. Behavior of motion on inclined planes 5. Argument that the apparent gravitational acceleration of objects isn’t real (!!). (from Dialogues concerning the Two Chief World Systems). B. Several Central Points: 1. Uniform motion requires no cause in order to sustain it. There is a sense in which any body is ‘indifferent’ to motion, once that motion is impressed on it. 2. But what is ‘uniform motion’? It includes circular motion, which is the most important type of uniform motion. 3. “Since a body is indifferent to motion, it can participate freely in more than one motion at the same time” (Richard S. Westfall, Force in Newton’s Physics: The Science of Dynamics in the Seventeenth Century, p. 4).