Table Of ContentG ALILE O
1
AT WORK
HIS SCIENTIFIC
BIOGRAPHY
Stillman Dral^e
Galileo at age sixty, drawn by Ottavio Leoni at Rome in May
1624 and engraved by him the following year. (From an original
print in the author’s collection. Photo by University of Toronto THE UNIVERSITY OF CHICAGO PRESS
Photographic Services.) CHICAGO AND LONDON
a
With ambiguity but without
The University of Chicago Press, Chicago 60637 equivocation, this book is
The University of Chicago Press, Ltd., London
dedicated to
© 1978 by The University of Chicago
All rights reserved. Published 1978 Florence
Printed in the United States of America
82 81 80 79 78 5 4 3 2 1
Sttllman Drake is professor of the history of science at the University
of Toronto and one of the world’s foremost authorities on Galileo. He has
published English translations of Galileo's chief works, including Dialogue
Concerning the Two Chief World Systems and Two New Sciences, and is
the author of Discoveries and Opinions of Galileo, Galileo Studies: Per
sonality, Tradition, and Revolution, and Galileo Against the Philosophers.
A member of the International Academy of History of Sciences and twice
a Guggenheim Fellow, Drake has contributed more than fifty articles on
Galileo and his era to various American, Italian, and British scholarly
journals.
Library of Congress Cataloging in I^jblication Data
Drake, Stillman.
Galileo at work.
Includes bibliographical references and index.
1. Galilei, Galileo, 1564-1642. 2. Astronomers
—Italy—Biography. I. Title.
QB36.G2D69 520'.92'4 [B] 78-5239
ISBN 0-226-16226-5
4
CONTENTS
Preface
xiii
ONE
1564-89
Family and Education; First Essays; Dialogue on Motion;
Centers of Gravity; Music and Science
1
TWO
1589-92
Professor at Pisa; The Leaning Tower; De motu
18
THREE
1592-99
Professor at Padua; Mechanics; Copernicanism;
Tides; The Sector
33
FOUR
1600-1602
Method; Mechanics; Speeds of Planets; Inclined Planes;
Galileo’s Theorem; Force of Percussion
50
vii
1
Contents
Contents
ELEVEN
FIVE
1612-13
1603- 4
Scope of Science; Reply to Nozzolini; Satellite Forecasts;
Notes on Motion; Double-Speed Rule; Motion Experiments
Sunspot Letters
74
195
SIX
1604- 6 TWELVE
1613
Law of Free Fall; Logic of Acceleration; The New Star of 1604;
The Continuum; Answers to Critics; Copernicus and the Bible;
Polemics; Work on Motion
Letter to Castelli
91
214
SEVEN
1606-8 THIRTEEN
The Nova Again; Beam Strength; Hydrostatics; 1614-15
Speed Paradox Resolved; Inertial Experiment and Trajectory Weight of Air; Attack from the Pulpit; Letter to Dini;
117 Replies to Oppositions
231
EIGHT
1609- 10
FOURTEEN
Projected Treatise on Motion; The Telescope; 1616-18
Moon Observations; Satellites of Jupiter; Cosmogony Edict at Rome; Longitude Negotiations; Comet Controversy
134 252
NINE
FIFTEEN
1610- 11
1619-23
Starry Messenger; Move to Florence; Saturn and Venus;
Discourse on Comets; Jesuit Opposition; Lenses;
Floating and Sinking; Sunspots
The Microscope; The Assayer
157
267
TEN
SIXTEEN
1612
1624-26
Bodies in Water; Sunspots; Inertia and Rotation; Disputes;
Reply to Ingoli; Force and Resistance; The Dialogue Started
Longitude Project
289
177
IX
viit
Contents Contents
Notes
SEVENTEEN
1626-31 473
Work on Motion; Sunspot Paths; Dialogue Completed;
Licensing Problems; Hydraulics of Flood Control Index
306 509
EIGHTEEN
1631-33
Mathematics of Motion; Schciner; Galileo Ordered to Rome;
Trial and Sentencing
330
NINETEEN
1633-35
Siena; Strength of Materials; Eppur si Muove; Arcetri;
Indivisibles; Logic of Free Fall
353
TWENTY
1636-38
Theorems on Projectiles; Letter to Carcavy; Two New Sciences
Criticisms by Descartes
373
TWENTY-ONE
1639-40
Correspotidence on Science, Method, and Philosophy
394
TWENTY-TWO
1641-42
The Leaning Tower Again; Falsity of Systems;
Pendulum Escapement; New Dialogue Started; Death
413
Appendix: Biographies
437
x
PREFACE
Once some fathers came to see him, and he was working
in his garden and observing how the buds came out. He said,
"I am ashamed that you see me in this clown's habit; I'll
go and dress myself as a philosopher."
"Why don't you have this work done by someone else?"
"No, no; I should lose the pleasure. If I thought it as much
fun to have things done as it is to do them, I'd be glad to."
This anecdote about Galileo in his old age at Arcetri was told
to Robert Southwell, later president of the Royal Society, when
as a young man he visited Vincenzio Viviani, Galileo’s last pupil
and first biographer. We are now so accustomed to seeing Galileo
dressed up as a philosopher that though the story has a delight
ful homey ring of authenticity, it is likely to sound made up.
Yet it is not the kind of story that would be made up in 1661,
especially not by an Italian talking to an English visitor. We
may take it as a fact that Galileo enjoyed doing things for him
self and profited from the opportunities thus afforded him for
making observations, recognizing all the while that such activi
ties were considered unsuitable to a philosopher.
There is no shortage of biographies of Galileo portraying him
as a profound scholar, holding a lofty place in the succession
of scientific pioneers from Aristotle through Ptolemy and Coper
nicus to Descartes and Newton. It is my purpose neither to con
tradict such accounts nor to add to their number, but rather to
show you Galileo in his working clothes, tending his scientific
garden and watching buds develop while his university colleagues
xiii
Preface Preface
were determining the necessary structure and ultimate purpose books, most of the material we have from Galileo's own hand is
of the entire universe. Galileo's fellow-gardeners are for the most undated. To put it all in serial order, and to date as nearly as
part unknown to fame, though they were closely linked to possible all documents relevant to his scientific career, has been
Galileo’s attitudes and his methods of investigation. Two of the the guiding objective in my selection ol things to be included
most acute were clerics; none were philosophers. Better than here. Until that has been done once, however tentatively in some
the professors, intelligent laymen caught the significance to respects, it is hardly possible to speak with confidence about
their own epoch of Galileo's labors. Finding those to their liking, Galileo’s own main interests, and hence about the probable
they gave him what encouragement he received during his life sources and motivations of his contributions to science.
time, furnishing also grist to his mill from their own observa It has been easy enough in the past to single out dramatic
tions and reflections. You will meet these supporters of Galileo aspects of Galileo’s life and relate those, not to his less cele
as well as scholars and theologians who vigorously opposed him. brated activities, but to the works of others before and after
Galileo was at once a lively participant in late Renaissance him and to the society in which he lived. His studies of motion,
Italian culture and an anachronism in its halls of academic his telescopic discoveries, and his clash with the church have
learning. If it was the fault of his nineteenth-century biograph been repeatedly so treated with little attention to the relations
ers to overemphasize the anachronism, it is perhaps a fault of between those justly esteemed accomplishments and Galileo's
his modern biographers to overemphasize cultural traditions own work in areas less dramatic—in the behavior of fluids, for
that surrounded him. Those have great statistical value, but they instance, or in the nature of continuous magnitude, in strength
tell us nothing about a particular individual human being. Just of materials, in the force of percussion, and the like. Yet even
as there might be no man of exactly average height, so there to name these suggests that their mutual relations had a certain
may have been no exactly typical scientist in Galileo's time. bearing on Galileo’s conception of science. Even less attention
From the thoughts of all his predecessors and contemporaries has ordinarily been given to Galileo’s musical background, his
we cannot confidently describe exactly his own thought; for respect for common sense, his mistrust of logical virtuosity, and
that, we need to know just how he spent his time. other matters which at first glance might seem insignificant or
Tedious indeed it must seem to plod year by year through the tangential to his science. The possibility remains that a better
life of a scientist. That is nevertheless the only sure way to find understanding of Galileo's scientific development will emerge
out what things actually shaped his thought, as distinguished from noticing recurrent themes within his own varied activities
from those that ought to have shaped it as determined by prob than by studying either the long-range emergence of a Scientific
able reasoning. In the case of Galileo, documents are plentiful— Revolution out of ancient mathematics and medieval philosophi
well over two thousand letters written by, to, or about him; cal speculation, or the temporary societal shock occasioned by
thousands of pages of manuscript notes and treatises; notarial Galileo's main achievements that have up to now received so
records, court proceedings, and account books. Together with much attention. The only way to determine whether that is so
his printed works and some by adversaries or friends, these fill is to put his varied activities in chronological order to see
twenty large volumes in the National Edition of Galileo's works. whether one led to another, and if not, what led to each.
This great mass of first-hand evidence affords a solid base for Some of those activities are already so well known that in
such conjectures as must be made concerning motives, methods order to make room for others less familiar, they will here re
of work, interests, and the like beyond those explicitly set forth ceive little more than bare mention. Some, especially among
by Galileo himself or bv men who knew him personally. Galileo’s early unpublished writings, are so little known as to
To the best of my knowledge, such a project has not been need, in my opinion, fairly full description. Thus the contents
previously undertaken. This one began as an attempt to put in of Galileo’s most famed books, easily accessible in English and
chronological order all of Galileo’s scientific work, from his stu presumably already read by most persons interested in his sci
dent days to the end of his life. Except for letters and published entific biography, will not be outlined herein; rather, so far as
xiv xv
Preface Preface
possible, dates will be identified at which various parts of those partisan character, some represented recollections of varying
books originated in Galileo's mind. This policy permits inclusion reliability, and some were written by persons not fully informed.
of translations of substantial parts of letters and treatises not Fortunately a good deal is known about many of the writers
previously seen in English. which is of assistance in assessing conflicts of evidence. Wrong
The result is—1 hope necessarily from the nature of my main judgments can be minimized only by considering all available
objective—an uneven narrative; episodes of great interest are original documents, while relatively few can be presented for
separated by periods of arduous labor or scientific idleness, at reasons of space. My opinions and conclusions have no other
least in appearance, though to omit either would render later claim to consideration than that they have been formed from
episodes less understandable. Even Galileo’s difficulties in hav all the documents. Where they may conflict with the opinions or
ing certain books published are not irrelevant. My intention is conclusions of others, I hope that further research will be stim
to give a fair overall view of Galileo's scientific career, which of ulated rather than that they will be rejected either as outmoded
course contains interludes of distraction from science by its or as bizarre. Views of early historians and biographers who
philosophical and religious adversaries. How the resulting pic lacked information now available to us were not always thereby
ture can be reconciled with others currently accepted is a ques rendered untenable, as some more recent writers have tended
tion I have preferred not to discuss. Modern portrayals appear to suppose. Neither is it likely that a completely correct histori
to me to employ certain selections of evidence from Galileo’s cal appraisal of Galileo’s scientific work has yet been reached.
writings mingled with thoughts of men before and after him in Hence I have not shrunk from supporting either seemingly out
order to show a smooth transition from ancient to modern sci moded or previously unventured opinions when the evidence
ence. In this book I am concerned rather with the transition be for them seemed strong to me. Documents here translated into
tween science as it was taught to Galileo and science as it was English for the first time present such evidence, perhaps over
later taught by him. Hence my scant references to other biog looked in the past.
raphies of Galileo does not mean that I have neglected to read Readers may wish to consider other accounts of Galileo's
them; rather, for the purpose of chronologically ordering life and work in which sharply contrasting overall views have
Galileo’s scientific work, it has been necessary to go back always been set forth. Since the literature is vast and multilingual, it
to original sources and it seemed best to cite those directly. may be useful to select a few items from it, mainly in English
The basic source of documentation is the National Edition of and of relatively recent date, that may profitably be consulted
Galileo's works directed and edited by Antonio Favaro: Le Opere as a sampling, with brief identifying remarks on each. An exten
di Galileo Galilei (20 volumes, Florence, 1890-1909, reprinted sive bibliography of books and articles since 1940 will be found
with additions in the 1930s and 1960s); References to this work in the work edited bv Ernan McMullin. Items listed here are
are given in the notes by volume and page number alone with arranged in order of their original publication.
out repetition of its title; when line numbers are required, they Alexandre Koyre: Etudes Galileennes (Paris, 1939; repr. 1966).
follow the page number in parentheses. Since the Galilean cor This work set a new pattern for examination of Galileo's
respondence is arranged chronologically in volumes 10 to 18, science, presenting it as inspired by abstract thought rather
with few exceptions, letters will be usually identified only by than induction from experience, contrary to all past opin
year and month. English translations are cited in the notes by ion. Platonism and medieval philosophy were emphasized,
shortened titles that are identified in the first appearance of and Galileo’s name was in effect hyphenated with that of
each. Descartes. Further essays written by Koyre in 1943-53 were
Opinions expressed and conclusions drawn in this book are collected after his death in Metaphysics and Measurement
grounded to the best of my ability on careful weighing of all (London and Cambridge, Mass., 1968).
the available original documentary evidence. That does not Giorgio de Santillana: The Crime of Galileo (Chicago, 1955). A
mean that all documents agree, for obviously some Were of a biography centering on Galileo's conflict with the church.
xvi
XVII
Preface Preface
Santillana’s replies to my reading of the trial documents respects, the book suffers somewhat from neglect of work
will he found in an appendix to the book next listed. published abroad but presents an original position based
Ludovico Gevmonat: Galileo Galilei (Torino, 1957; S. Drake, tr., on careful study of Galileo's own principal works.
New York, 1965). William R. Shea: Galileo's Intellectual Revolution (New York
A biography stressing Galileo’s philosophy of science, criti and London, 1972).
cal of Koyre’s Platonist appraisal and sympathetic to the Contains papers dealing with Galileo's writings from 1610
view that Galileo took the instrumentalist position with to 1632, in which the role of experiment is regarded as sub
respect to mathematics. sidiary and the Archimedean model of nonempirical math
Arthur Kocstler: The Sleepwalkers (London and New York, ematical physics is stressed. Scant attention is paid to the
1959). In a long section of this book strongly opposing the role of arguments ad hotninem in Galileo's books of the
interpretation by Santillana, this author portrays Galileo’s period studied, so that a dogmatic rather than a scientific
work as an incorrect and irresponsible attack against the attitude is frequently attributed to him by the author.
social order, without any proper scientific foundation, which James MacLachlan: “A Test of an 'Imaginary' Experiment of
brought about his downfall and discredited science in the Galileo's," Isis 64 (1973): 374-79.
process. Experimental confirmation of a phenomenon described by
Thomas B. Settle: “An Experiment in the History of Science," Galileo which had been ridiculed by Koyre in 1960.
Science 133 (1961): 19-23. Winifred L. Wisan: “The New Science of Motion: A Study of
Corroboration of an experimental claim made by Galileo Galileo's De motu locali,” Archive for History of Exact Sci
which had been ridiculed in 1953 by Koyre on logical and ences 13 (1974):103-306.
historical grounds; for his arguments, see Metaphysics and The most detailed study of Galileo's work on motion yet
Measurement, listed above. published, emphasizing possible sources for it and propos
Pio Paschini: Vita e Opere di Galileo Galilei (Rome, 1964; repr. ing interpretations and datings for many of his working
1965). papers at variance with those which seem to me most
The most detailed modern biography, published by the probable.
Pontifical Academy of Sciences after the author’s death in R. H. Naylor: “Galileo and the Problem of Fall," British Journal
1962. The work had been completed in 1944, and though for History of Science 1 (1974): 105-34. One of several ar
some additions and corrections were made, much subse ticles by this writer opposing my interpretations of Galileo’s
quent research was left out of account. working papers on motion.
Eman McMullin (ed.): Galileo Man of Science (New York, Dudley Shapere: Galileo, A Philosophical Study (Chicago, 1974).
1967). A collection of studies by specialists in various dis A modern philosopher’s analysis of Galileo’s contributions,
ciplines written from different standpoints and reflecting a stressing the derivative character and general inadequacy
wide range of opinions. Of special value is the editor's intro of both his science and his philosophy.
ductory essay which presents a balanced view of Galileo's Basic differences of opinion among modern historians of sci
work in the light of research up to a decade ago. ence exist concerning (1) the sources employed by Galileo in
Maurice Clavelin: The Natural Philosophy of Galileo, first pub his investigations, and (2) the relations of Galileo’s science to
lished as La Philosophic Naturelle de Galilee (Paris, 1968). philosophy and religion at his time. As to (1), I take it as evident
English translation bv A. J. Pomerans (Cambridge, Mass., that Galileo had read Aristotle, Euclid, Archimedes, Ptolemy,
1974).
and Copernicus, as well as that he made observations and mea
The most comprehensive study to date of Galileo’s phi surements of his own. Those sources would suffice to account
losophy and science, with the author's views of their prob for what he did in science, and he named them all many times.
able sources. Critical of Koyre's thesis in some important Other historians seek in addition to establish a continuity be
xviii xix
Preface
Preface
no appeal need be made beyond sense experiences and necessary
tween medieval and Galilean science. Yet characteristic enter
demonstrations. Such a domain; though very small, would be
prises of the late Italian Renaissance attempted to break from
exempt from the jurisdiction of philosophers and theologians.
the Middle Ages, and neither Galileo, nor his adversaries, nor
Even had Galileo not regarded that domain as ample for his
his supporters appealed to medieval writings in support of their
purposes and as meriting investigation for its own sake, his
positions. Since medieval physics and astronomy do not suflice
experiences with philosophers and theologians had convinced
to account for Galileo's contributions and are unnecessary in
him that to remain within it was the only way he might hope
explaining them, I consider the supposed continuity to be a still
for freedom of inquiry.
unverified hypothesis except in the trivial sense that medieval
Now it is true that very few, if any, scientists between the
concepts entered into Galileo's university education and were
time of Galileo and the nineteenth century were willing to pay
reflected in his unpublished early writings at Pisa.
such a price for freedom, and most (if not all) of them loved to
The relation of Galileo's science to philosophy and religion
play the philosopher and to pronounce also on religious issues.
emerges from contemporary records; philosophers and theolo
That does not constitute evidence, however, that Galileo himself
gians opposed him, the former unanimously and the latter not
wanted science to decide philosophical and theological ques
without a certain division of opinion at even the highest levels.
tions. Nor is it apparent to me, after a quarter-century of read
Whether or not physical science as conceived by the mature
ing what he wrote in letters, books, and notes, that Galileo's
Galileo was a threat to philosophy or religion is hard to say.
science owed anything whatever to the philosophers. No pre
Galileo believed that it was not. When the representative of
philosophy in his Dialogue feared its subversion by new sci cepts of Aristotle, Plato, the atomists, or the Stoics sufficed to
ences, Galileo's spokesman replied: lead him to the law of free fall or to his telescopic discoveries
in the sky; nor, so far as I see, were such precepts necessary for
Philosophy itself cannot but benefit from our disputes, for those achievements. The same is true for Galileo's science of
if our conceptions prove true, new achievements will be made; strength of materials, his hydraulics, his investigations of pen
if false, their rebuttal will further confirm the original doc dulums, and everything else that he regarded as scientific. Hence
trines. No, save your concern for certain philosophers; come
I consider past studies of Galileo’s philosophy, without dis
to their aid and defend them. As to science itself, it can only
respect to those who have industriously pursued them, as neither
improve.
necessary nor sufficient to convey a basic understanding of his
As to religion, Galileo wrote in his Letter to Christina: scientific career. Science, he said, enabled one to philosophize
better, not the other way round.
Copernicus never discusses matters of religion or faith, nor
It is not unreasonable to believe that the vast literature con
does he use any arguments that depend in any way upon
cerning Galileo will be able to absorb without serious damage
the authority of sacred writings which he might have interpreted
one book in which his scientific work is considered without also
erroneously. . . . He did not ignore the Bible, but he knew very
entering into possible philosophic implications. Yet since others
well that if his doctrine were proved, then it could not con
have read this book in typescript, I know I may be alone in that
tradict the Scriptures when they were rightly understood. . . .
In discussions of physical problems we ought to begifi not opinion, for none of them have failed to warn me against ne
from the authority of scriptural passages, but from sense glecting higher and more profound issues than, say, Eudoxian
experiences and necessary demonstrations; for the holy Bible proportion theory and mathematical paradoxes of the infinite.
and the phenomena of nature proceed alike from the divine Still, since that is as high as my own studies reach, and since
Word, the former as the dictate of the Holy Ghost and the latter others do so well at extending the domain of Galileo’s science
as the observant executrix of God’s commands. for him and then demolishing it with deeper philosophical in
sight, it seems to me that we should divide the labor according
These and similar passages strongly suggest that Galileo
to our respective tastes and interests. I am certain that in the
meant to restrict his science to the narrow domain within which
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