Humanities 3 V. The Scientific Revolution
Lecture 21 Two World Systems
Outline The Significance of the Debate Two Systems: Ptolomeic vs. Copernican Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican (1632) The Tower Argument
The Significance of the Debate Science versus the Religion/Bible: Which has authority to decide the truth about nature? (evolution, climate change) Rejection of an anthropocentric view of the universe (again compare evolution) How the debate gets settled: The nature of scientific arguments
Ptolemaic World System Based on Aristotle s cosmology (geocentric, Earth immobile) Celestial and sublunary bodies have fundamentally different properties (including different principles of motion) Aristotle s picture is empirically false: planets are not observed to move with constant circular motions In his Almagest, the Egyptian geometer Ptolemy (c. 80-c. 170 AD) devised a system of mathematical constructions that saved the phenomena
Aristotle s Universe 55 concentric spheres Earth is at the center and immobile outermost sphere is the primum mobile, whose motion moves the other spheres for Christians, beyond this is heaven where the angels and blessed reside
Ptolemy, Epicycle Model of Retrograde Planetary Motion
Ptolemaic motion showing eccentric (C), epicycle (P) and equant (Q)
Copernican System 1543 Publication of Copernicus On the Revolutions of the Celestial Orbs Heliocentric; Earth revolves around the sun and rotates on its axis No difference in the physical properties of terrestrial and celestial bodies (same principles of motion) Copernicus picture modified by Johannes Kepler (1571-1630) based on observational data of Tycho Brahe (1546-1601)
Kepler s Laws First Law (1605): Planets move in ellipses with the Sun at one focus Second Law (1602): Radius vector describes equal areas in equal times Third Law (1618): The squares of the periodic times are to each other as the cubes of the mean distances
Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican (1632)
Cast of Characters Salviati: Galileo s spokesman, supposedly an unbiased presenter of the two world systems Simplicio: defender of the authority of Aristotle and the Church Sagredo: intelligent layman, who appears convinced by the arguments on behalf of the Copernican system
Whether the Earth is Immovable Galileo accepts that this question cannot be decided on empirical grounds alone; it must be settled by reasoned argument. One kind of consideration: nature operates in the simplest manner (least action). It is simpler for the Earth to rotate daily west to east than for the entire heavens to move east to west. But this is met by what seems a decisive objection: if the Earth is rotating so quickly (! 1000 mph), a stone dropped from a tower should fall behind it. But it does not.
Tower Objection: Why the Rotation of the Earth is Impossible Stone released at t 1. Its natural motion carries it straight down to the base of the tower. Meanwhile the Earth s rotation has carried the tower to a new position at t 2, so the stone must land behind the tower. Since this isn t observed, the Earth must be stationary. t 1 t 2
Galileo s Response Empirical evidence cannot be taken at face value: its significance has to be interpreted within a theory What an apparent motion (upward, downward) signifies depends upon the concepts we use to explain motion Missing from Aristotle s theory is the crucial distinction between inertial and accelerated motion (or circular and downward motion)
Salviati Therefore, its motion would be a compound of two, namely, one with which it grazes the edge of the tower, and another one with which it follows the tower; the result of this compound would be that the rock would no longer describe a simple straight and perpendicular line, but rather an inclined, and perhaps not straight, one (p. 223)
Galileo s Response Stone released at t 1. At the moment of its release it is moving in direction a with an inertial motion equal to the motion of the tower. It is also subject to an accelerated motion in direction b. The sum of these two motions carries the stone to the base of the tower in its new position at t 2. b a t 1 t 2
Lessons for the Sciences Within science, seeing is believing is a bad methodological rule Empirical evidence must be interpreted within a theory in order to assess its significance (sensory perception, optical distortion, the very small, the very large) Concepts without intuitions are empty. Intuitions without concepts are blind. (Kant, Hum 4)
Same Point in Bacon Those who have handled sciences have been either men of experiment or men of dogmas. The men of experiment are like the ant, they only collect and use; the reasoners resemble spiders, who make cobwebs out of their own substance. But the bee takes a middle course: it gathers its material from the flowers of the garden and of the field, but transforms and digests it by a power of its own. Not unlike this is the true business of philosophy; for it neither relies solely or chiefly on the powers of the mind, nor does it take the matter which it gathers from natural history and mechanical experiments and lay it up in the memory whole, as it finds it, but lays it up in the understanding altered and digested. Therefore from a closer and purer league between these two faculties, the experimental and the rational (such as has never yet been made), much may be hoped. (New Organon, I.95)
Denouement Following its publication, the Dialogue is condemned and burned, and Galileo again is called before the Inquisition This time under threat of torture, he abjures his errors and is sentenced to indefinite house arrest (only 7 of 10 inquisitors sign the sentence) He is allowed to return to his house outside Florence where he remains until his death (where he is visited by many admirers, including Hobbes and Milton)