Humanities 3V. The Scientific Revolution

Lecture 21

Two “World Systems”

Outline

• The Significance of the Debate

• Two Systems: Ptolomeic vs. Copernican

• Dialogue Concerning the Two Chief WorldSystems, Ptolemaic and Copernican (1632)

• The Tower Argument

The Significance of the Debate

• Science versus the Religion/Bible: Whichhas authority to decide the truth aboutnature? (evolution, climate change)

• Rejection of an anthropocentric view of theuniverse (again compare evolution)

• How the debate gets settled: The nature ofscientific arguments

Ptolemaic World System• Based on Aristotle’s cosmology (geocentric, Earth

immobile)• Celestial and sublunary bodies have fundamentally

different properties (including different principles ofmotion)

• Aristotle’s picture is empirically false: planets arenot observed to move with constant circular motions

• In his Almagest, the Egyptian geometer Ptolemy (c.80-c. 170 AD) devised a system of mathematicalconstructions that “saved the phenomena”

Aristotle’sUniverse

• 55 concentricspheres

• Earth is at thecenter andimmobile

• outermost sphere isthe primum mobile,whose motionmoves the otherspheres

• for Christians,beyond this isheaven where theangels and blessedreside

Ptolemy, Epicycle Model ofRetrograde Planetary Motion

Ptolemaic motion showingeccentric (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 (sameprinciples of motion)

• Copernicus’ picture modified by JohannesKepler (1571-1630) based on observationaldata of Tycho Brahe (1546-1601)

Kepler’s Laws• First Law (1605): Planets

move in ellipses with the Sunat one focus

• Second Law (1602): Radiusvector describes equal areas inequal times

• Third Law (1618): Thesquares of the periodic timesare to each other as the cubesof the mean distances

Dialogue Concerning the TwoChief World Systems, Ptolemaic

and Copernican (1632)

Cast of Characters

• Salviati: Galileo’s spokesman, supposedlyan unbiased presenter of the two “worldsystems”

• Simplicio: defender of the authority ofAristotle and the Church

• Sagredo: “intelligent layman,” who appearsconvinced by the arguments on behalf of theCopernican system

Whether the Earth is Immovable• Galileo accepts that this question cannot be

decided on empirical grounds alone; it must besettled by reasoned argument.

• One kind of consideration: nature operates in thesimplest manner (least action). It is simpler forthe Earth to rotate daily west to east than for theentire heavens to move east to west.

• But this is met by what seems a decisiveobjection: if the Earth is rotating so quickly (!1000 mph), a stone dropped from a tower shouldfall behind it. But it does not….

Tower Objection: Why the Rotationof the Earth is Impossible

t1 t2

Stone released at t1. Itsnatural motion carries itstraight down to the base ofthe tower. Meanwhile theEarth’s rotation has carriedthe tower to a new positionat t2, so the stone must landbehind the tower. Since thisisn’t observed, the Earthmust be stationary.

Galileo’s Response• Empirical evidence cannot be taken at face value:

its significance has to be interpreted within atheory

• What an apparent motion (upward, downward)signifies depends upon the concepts we use toexplain motion

• Missing from Aristotle’s theory is the crucialdistinction between inertial and acceleratedmotion (or circular and downward motion)

Salviati

• “Therefore, its motion would be acompound of two, namely, one with whichit grazes the edge of the tower, and anotherone with which it follows the tower; theresult of this compound would be that therock would no longer describe a simplestraight and perpendicular line, but rather aninclined, and perhaps not straight, one” (p.223)

Galileo’s Response

t1 t2

Stone released at t1. At themoment of its release it ismoving in direction a with aninertial motion equal to themotion of the tower. It is alsosubject to an acceleratedmotion in direction b. Thesum of these two motionscarries the stone to the base ofthe tower in its new position att2.

b

a

Lessons for the Sciences

• Within science, “seeing is believing” is abad methodological rule

• Empirical evidence must be interpretedwithin a theory in order to assess itssignificance (sensory perception, opticaldistortion, 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 ofexperiment or men of dogmas. The men of experiment are like theant, they only collect and use; the reasoners resemble spiders, whomake cobwebs out of their own substance. But the bee takes amiddle course: it gathers its material from the flowers of the gardenand of the field, but transforms and digests it by a power of itsown. Not unlike this is the true business of philosophy; for itneither relies solely or chiefly on the powers of the mind, nor doesit take the matter which it gathers from natural history andmechanical experiments and lay it up in the memory whole, as itfinds it, but lays it up in the understanding altered and digested.Therefore from a closer and purer league between these twofaculties, the experimental and the rational (such as has never yetbeen made), much may be hoped.” (New Organon, I.95)

Denouement• Following its publication, the Dialogue is

condemned and burned, and Galileo again iscalled before the Inquisition

• This time under threat of torture, he abjures hiserrors and is sentenced to indefinite house arrest(only 7 of 10 inquisitors sign the sentence)

• He is allowed to return to his house outsideFlorence where he remains until his death (wherehe is visited by many admirers, including Hobbesand Milton)