Galileo Galilei

1. Galileo Galilei Dialogue Concerning the Two Chief World Systems

2. Foreword (Einstein) • Galileo possesses a passionate will, the intelligence and the courage to face the ignorance of his time (and the authority of the church) • His work is an unprecedented model of rational thinking in the sciences • Overcomes the anthropocentric and mythical thinking of his time • Lead men to an objective and causal attitude toward the cosmos • Possesses an unusual literary gift, an impressively suggestive and precise language that can be understood by anyone

3. In the DIALOGUE : an extremely lively and persuasive exposition of the prevailing views on the structure of the cosmos: “There is space, and within it there is a preferred point, the center of the universe. Matter tends to approach this point as closely as possible. Consequently matter has assumed a spherical shape (earth). Sun, moon and stars are prevented from falling toward the center of the universe by being fastened onto rigid (transparent) spherical shells whose centers are identical with that of the universe. The spherical shells revolve around the immovable globe with slightly differing angular velocities. The lunar shell has the smallest radius. It encloses everything “terrestrial.” The outer shells represent the heavenly bodies, eternal, indestructible, and inalterable while the earth is perishable, transitory and “corruptible”

4. This naïve description is a deterioration of the Greeks, Aristotle’s and Ptolemy’s ideas concerning the celestial bodies and their motions • Lacking a theory of mechanics, the Greeks reduced all complicated motions to the simplest ones they could conceive (uniform circular motions). Circular motion is still discernible in Galileo (he did not fully recognize the law of inertia) • Galileo’s aim: to substitute the petrified system with a deeper and more consistent comprehension of the physical and astronomical facts • The form of the dialogue (Aristotle) • to apply his literary talent to the sharp and vivid confrontation of opinions • To avoid an open commitment to controversial issues (had been forbidden to advocate the Copernican theory)

5. The Dialogue is an attempt to comply with the order in appearance and yet to disregard it in fact • Theory of the center of the universe (earth): bodies fall on earth (centripetal force) and cause its spherical form (unsatisfactory) • The theory does not account for the spherical shape of all other heavenly bodies • There must exist a relation between the spherical form of earth, planets and stars (concept of center of the universe declines) • It is recognized that the effect of gravity on free falling bodies manifests itself in a vertical acceleration of constant value • Knowledge is achieved that the presence of matter (earth) causes an acceleration of free bodies (at the surface of the earth)

6. Once the conception of the center of the universe is removed, there falls also the idea of the earth’s immovability (and of the earth’s (and man’s) exceptional role in the system) • It is simpler to assume a rotation of the earth on its axis than the rotation of all planets and stars around the earth • Problem: the power of observation (in XVII) is limited and could not reveal the “true orbits” of the planets • Kepler solved the question (although his findings are not evident in Galileo’s analysis) • Empirical and speculative methods go hand in hand in Galileo • He often uses logical deduction (that is put aside when incompatible with empirical findings). He leans not toward “factual knowledge” but toward “comprehension.”

7. To the Discerning Reader • Dialogue used so digressions can be made (complexity of the arguments) • Interlocutors • Giovanni Sagredo (Venice- noble extraction and trenchant wit) • Filippo Salviati (Florence- eminence of blood, fortune and intellect) They are both dead • Simplicius (represents the Peripatetic school) No real name is given (creation of a character) • The gentlemen have led such discussions and debates in reality (fictional reality) • Casuality of the discussions is now formalized on the written page (hence the dialogue receives structure, purpose and meaning)

8. How is the relation among the characters of the dialogue • The topic of the conversation (from the ebbs to the mobility of the earth). What process is that? • The natural process and causes are observed (417 - 1) • The effects must be studies and understood in order to derive the causes of natural phenomena (2) • Experience is fundamental (observation) (3) • What others have seen or observed is unriliable (direct polemic against the Aristotelians) (4) • The path of discovery has just been opened (418 -1) • From the primary cause produces secondary effects that are similar in nature (418 - 2) • Ebb periods occur daily, monthly and yearly (418 - 3) • Reasons adduced by Simplicius, are based on? (419 - 1) • Salviati’s confutation of Simplicius (420 - 1)

9. Those who believe these stories have poetic minds (two kinds -the inventors of fables and their believers) (420-2) • Aristotle ascribes to miracles all things whose causes are hidden 412 -1) • If causes can be proven how can we doubt? (421 - 2) • Arguments adduced by Simplicius go against natural limitations. The experience of Venice (422/3 - 1) • Natural and true phenomena happen necessarily (424-1) • Explanation of the tides (irregular velocity of earth motion) (424 - 2) • The example of the barge (possible experiment) (425 -1) • Three conclusions can be reached: a) no need for added water b) level of water in the middle does not noticeably vary c) water runs more (to and from) at the extremities

10. Observation: from the known to the unknown: process of deduction (from the barge to the Mediterranean) • diurnal and annual rotation at once accelerate and retard the motion of the earth (427 -1) • Five phenomena occur in rotation and revolution (428/9) • Largeness of basin determines visibility of the event (431) • Timing of the tides, six hours in the Mediterranean (433-1) • Impulses can go in the same directions, effects are greater tides, opposing impulses, motionless state of waters (435) • Variety of incidental phenomena (wind-rivers) • Simplicio’s question on the winds and refutation by Salviati (436-40) • Sagredo’s experience confirms the validity of Salviati’s east wind theory (440) • Reasons why tides are of greater or lesser magnitude (monthly and annual motion)

11. Complexity of phenomena depends on the composite motions of the planet. Additions and subtractions of the diurnal whirling to the annual motion (446) • Salviati’s Digression: the experiment of the pendulum (450) • Earth’s and moon’s orbits (453) are irregular • Sagredo’s comments on Salviati’s explanation (454) • How come the irregularity has gone unnoticed? (454-1) • Irregularity was not noticed on account of the distance and greater orbits of planets • Many variables and incidental causes cannot be determined or fixed (complexity of phenomena) • Language used is very factual, denotative rather than connotative (458-9)

12. Conclusion • More than the diurnal motion of the earth is needed to explain the motion of the tides (monthly-yearly) • Tides are produced by the uneven motion of earth and by a variety of undeterminable variables (winds, rivers etc.) • The tides confirm the mobility of the earth • In favor of the Copernican system a) stoppings and retrograde motions of the planets b) revolution of the sun upon itself c) ebbing and flowing of tides