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 t1. 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 t2, so the stone must land
behind the tower. Since this
isn’t observed, the Earth
must be stationary.
t1
t2
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 t1. 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
t2.
a
b
t1
t2
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)