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Lecture 11: Galileo, Telescopes, and Authority
© Darrin Durant 2004
Galileo, Telescopes, and Authority: the mathematician who wanted to
be a court philosopher
*** Early Career ***
 Pisa (1581-85): studied Aristotelian physics; private tutor in
practical mathematics (Ostilio Ricci and Nicolo Tartaglia)
 Florence (1585-89): unemployed.
 Pisa (1589-92): chair of mathematics (Grand Duke of Tuscany,
Ferdinand I of the Medici’s)
 Copernican influences & Clavius’ mathematics
 De Motu (1590): ‘cord connecting stones’ argument
‘sinking bodies’ argument
Different weights fall at same speed
No attempt to answer “why” questions
 Padua (1592-1610): chair of mathematics.
 Supplements income with tutoring & instrument making
 Job required public service (Venice a Republic)
 Freedom of enquiry required an absolute ruler.
 Cosmography (1597): on Ptolemaic system
 Letter to Kepler (1597): has “proofs” of Copernicanism
1604 nova supports Copernicus.
 Unpublished Mechanics: 1604 law of free fall (bodies fall
naturally downward; acceleration only at initial instant, then
constant; speed increases proportionate to spaces)
 Later: speed increases proportionate to times; no
constant speed after initial moment; acceleration
equal for all bodies, but is affected by resistance.
 Siderius nuncios (1610): the telescopic discoveries were
presented as emblems of Medici power and dynastic continuity
 Florence, court of Grand Duke Cosimo II (Medici), 1610 . . .
*** Galileo and Visual Astronomy ***
 Johannes Hevelius (1611-87): Selenographia (1642)
 Better telescopes & a better observer? Galileo cannot draw?
 Hevelius and others made astronomy a visual practice
 Galileo: uses pictorial representation, but values the written
word over the pictorial
 Why this preference?: Galileo’s social status & career trajectory
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Lecture 11: Galileo, Telescopes, and Authority
© Darrin Durant 2004
*** Representational Astronomy ***
 Pre-1609: diagrams, but not naturalistic representations (except
Leonardo da Vinci (1452-1519))
 Galieleo’s Siderius nuncius (1610) & Letters on Sunspots (1613)
 Use of pictures limited to these works
 Galileo uses pictures, but then he stops. Why?
 Mere mathematicians can use any resource they like
 Court philosophers do not mix with the mechanical arts
 Court Philosophers use mathematical symbols and words
 Renaissance arguments for naturalistic descriptions
1. Naturalism (art imitates nature, especially in botany & anatomy)
2. Linear Perspective (3-D on flat plane, Brunelleschi (1377-1446))
3. Printing: woodcuts and copper prints.
*** Astronomy versus other disciplines ***
 Recall Aristotelian metabasis
 Astronomy cannot compete with Physics, Theology
 Mechanical Arts cannot compete with Astronomy
 Astronomers can’t touch bodies. Bodies are divine.
E.g.: Peter Apian, 1532 comet book: anti-solarity via fanciful image
 Higher social status than the mechanical arts (painters, engravers)
 Hans Lipperhey, 1608 telescope: illiterate spectacle-maker
*** Galileo, Pictures, and the Court ***
 Verbal portrait describes changes over time (Sun & Moon)
 Text carries the real message
E.g.: ‘cavity’ of moon exaggerated = moon like earth valleys
 The setting of the court: do not mix mechanical & liberal arts
 Distancing strategy: recreate the origins of the telescope
 Responses to changes in telescopic design (concave and convex)
 Galileo’s concave ocular lens: erect image, but low
magnification (½ moon’s diameter)
 Later: convex ocular lens = inverted image, but higher
magnification
 Galileo dismissed convex designs as just showing things
‘bigger’. In 1638 he claimed ‘revelation’ for his discoveries
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Lecture 11: Galileo, Telescopes, and Authority
© Darrin Durant 2004
*** Galileo and the Telescope, post 1610 ***
 Standard of Proof = true conclusions deduced necessarily from true
premises
 The telescope offers the possibility of conclusive arguments?
 The mutual reinforcement of identity-formation (court philosopher)
and commitment to Copernicanism:
 If heliocentrism is true, then mathematicians are worthy
natural philosophers
 Siderius nuncios (1610): 4 parts . . .
1. Instrument description: the telescope
2. Moon just like the earth
3. Planets and stars less magnified than other objects
3a. Size v. brightness (determines angle)
3b. Does the telescope strip the stars & planets of light?
3c. Stars & planets different = planets like moon & earth
4. Moons of Jupiter: analogous to heliocentric model
 Other observations, 1610-12
 Phases of Venus, Sunspots, 3-bodied shape of Saturn
*** How did Galileo “package” the discoveries? ***
 “Gifts” for Cosimo II = worthy of patronage
 Medici rule “written in the stars”
 “Medicean stars” were not generic, but distinctive of Medici
image and status = Galileo the “Philosopher & Mathematician”
*** How did Galileo bridge the “status gap”? ***
 Medici patronage elevates the social status of their client
 Astronomy/Mathematics required patronage to challenge philosophy
 Avoid metabasis prohibition by moving to a new venue – the Court
 Redrawing disciplinary boundaries = legitimate Copernicanism
 Did Galileo establish mathematical astronomy, or was there a trade
off for his personal success?
1. Exceptional discoveries rewarded by individual rulers
2. How to institutionalize personal patronage?
E.g.: Galileo’s replacement in Cosimo II’s court = Toricelli
in 1642, “Mathematician”
E.g.: Huygens ‘rings of Saturn’ discovery (Systema
Saturnium, 1659) dedicated to Leopold de’ Medici = no
response. Why?
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Lecture 11: Galileo, Telescopes, and Authority
© Darrin Durant 2004
*** Why was Galileo so aggressive? ***
 Courtly disputes: Florentine rhetorical duels of honour
 Inconclusive debates. No intervention without risk.
 Disputes not considered impolite
 No Treatises; just Dialogues, Discourses, and Letters.
 Format = safety valve for patrons v. “philosophical comedies”
 Authorial identity: Absolutist courts and court spectacles.
*** The Phases of Venus: unexpected? ***
 Kuhn (p. 100 of reader): phases = “direct” evidence for Copernicus,
and in Structure of Scientific Revolutions (1970) phases = “unsuspected”
(p. 154)
 Pre-1610 = phases required by Aristotelian cosmology
 Problem was to explain why they are not seen
 Post-1610 the phases contradict Aristotle-Ptolemy [A-P]. Huh?
 The problem: what are the sources of stellar illumination?
 Aristotle (all celestial bodies, except the moon)
 Scholastics (pseudo-Aristotle said ‘Sun’, in Liber de Elementis)
 Eg.: Albert of Saxony, Questiones…(1350)
 Copernicus, De Revolutionibus (1543), Book I, Ch. 10
refers to Albert of Saxony (stars and planets shine either
with their own light or that received from the sun)
 Scholastic explanation for not seeing the Phases
 Avicenna (defending Aristotle…) had said if planets/stars
received their light from the sun, phases would be visible,
varying according to distance from the Sun
 Albert of Saxony: (ad hoc defense of Aristotle…) replies
that we don’t see phases because Venus and Mercury are
transparent and absorb the light of the Sun
 Thus: phases question is neutral for Aristotelian cosmology
 Thus: Scholastic context for Galileo’s theories of the Phases of Venus
meant, at most, fixing the centre of Venus/Mars epicycles on the Sun
 Thus: why did Galileo argue the phases = disproof of A-P?
 Galileo had more important arguments for Copernican
cosmology than the Phases of Venus themselves
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Lecture 11: Galileo, Telescopes, and Authority
© Darrin Durant 2004
 Galileo claimed Copernicus could not refute A-P because…
 Venus should be 40 times as large in one position as in another
 Venus complete circle = farthest; crescent = closest & largest
 Mars should be 60 times as large in one as in another
 Thus: naked eye observations do not confirm Copernicanism
 Galileo says Copernicus was wise to maintain his view despite
the disconfirming observations
 Galileo’s line of argument
1. The “impediment of our eyes” causes bright distant objects to be
“festooned with adventitious and alien rays”
a. The telescope removes the “irradiation”, and reveals the
true light and true sizes
b. Galileo claims the telescope thus refutes A-P, but Kepler
disagreed
2. Old Cosmology (stars and planets are either luminescent or
receive their light from the Sun) versus New Cosmology (heavenly
bodies of two kinds: luminescent or non-luminescent (dark))
a. A-P cannot decide the matter, Galileo can . . .
b. Galileo example: moving telescope from Mars to Jupiter to
Saturn = decreasing brightness
c. Galileo example: no fixed star loses its luminosity
d. Thus: Sun & Stars luminous, Planets & Moon dark
 Galileo’s achievement?
 Not discovering the Phases of Venus
 Discovery that the telescope reveals the true light of the
heavenly bodies
 Galileo: the telescope, by revealing the ‘true light’, can arbitrate the
astronomical dispute and give a strict ‘disproof’ of A-P
*** What type of natural philosopher was Galileo? ***
 Do not search for essences or causes.
 Stick with knowing the primary properties of bodies (geometrical).
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