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GALILEI, GALILEO (b. Pisa, Italy, 15 February
1564; d. Arcetri, Italy, 8 January 1642), physics,
astronomy.
or corrected by Galileo, the anonymous Jesuit was
subjected to criticism. The result was a direct attack
on Galileo by Grassi, under the pseudonym of
Lotario Sarsi, published in 1619.
Galileo replied, after much delay, with one of the
most celebrated polemics in science, Il saggiatore
(The Assayer). It was addressed to Virginio Cesarini,
a young man who had heard Galileo debate at Rome
in 1615-1616 and had written to him in 1619 to extol
the method by which Galileo had opened to him a
new road to truth. Since he could no longer defend
Copernicus, Galileo avoided the question of the
earth's motion; instead, he set forth a general scientific
approach to the investigation of celestial phenomena.
He gave no positive theory of comets, but
developed the thesis that arguments from parallax
could not be decisive concerning their location until
it was first demonstrated that they were concrete
moving objects rather than mere optical effects of
solar reflection in seas of vapor. No such proof appeared
to him to be available. In the course of his
argument, Galileo distinguished physical properties
of objects from their sensory effects, repudiated authority
in any matter that was subject to direct investigation,
and remarked that the book of nature, being
written in mathematical characters, could be deciphered
only by those who knew mathematics.
The Saggiatore was printed in 1623 under the
auspices of the Lincean Academy. Just before it
emerged from the press, Maffeo Barberini became
pope as Urban VIII. The academicians dedicated the
book to him at the last minute. Cesarini was appointed
chamberlain by the new pope, who had long
been Galileo's friend and was a patron of science and
letters. Galileo journeyed to Rome in 1624 to pay
his respects to Urban, and secured from him permission
to discuss the Copernican system in a book,
provided that the arguments for the Ptolemaic view
were given an equal and impartial discussion. Urban
refused to rescind the edict of 1616, although he
remarked that had it been up to him, the edict would
not have been adopted.
Dialogue on the World Systems.
The Dialogue
Concerning the Two Chief World Systems occupied
Galileo for the next six years. It has the literary form
of a discussion between a spokesman for Copernicus,
one for Ptolemy and Aristotle, and an educated layman
for whose support the other two strive. Galileo
thus remains technically uncommitted except in a
preface which ostensibly supports the anti-Copernican
edict of 1616. The book will prove, he says, that
the edict did not reflect any ignorance in Italy of the
strength of pro-Copernican arguments. The contrary
is the case; Galileo will add Copernican arguments
of his own invention, and thus he will show that not
ignorance of or antagonism to science, but concern
for spiritual welfare alone, guided the Church in its
decision.
The opening section of the Dialogue critically examines
the Aristotelian cosmology. Only those things
in it are rejected that would conflict with the motion
of the earth and stability of the sun or that would
sharply distinguish celestial from terrestrial material
and motions. Thus the idea that the universe has a
center, or that the earth is located in such a center,
is rejected, as is the idea that the motion of heavy
bodies is directed to the center of the universe rather
than to that of the earth. On the other hand, the
Aristotelian concept of celestial motions as naturally
circular is not rejected; instead, Galileo argues that
natural circular motions apply equally to terrestrial
and celestial objects. This position appears to conflict
with statements in later sections of the book concerning
terrestrial physics. But uniform motion in precise
circular orbits also conflicts with actual observations
of planetary motions, whatever center is chosen for
all orbits. Actual planetary motions had not been
made literally homocentric by any influential astronomer
since the time of Aristotle. Galileo is no exception;
in a later section he remarked on the irregularities
that still remained to be explained. Opinion today
is divided; some hold that the opening arguments of
the Dialogue should be taken as representative of
Galileo's deepest physical and philosophical convictions,
while others view them as mere stratagems to
reduce orthodox Aristotelian opposition to the earth's
motion.
Important in the Dialogue are the concepts of relativity
of motion and conservation of motion, both
angular and inertial, introduced to reconcile terrestrial
physics with large motions of the earth, in answer
to the standard arguments of Ptolemy and those
added by Tycho Brahe. The law of falling bodies and
the composition of motions are likewise utilized. Corrections
concerning the visual sizes and the probable
distances and positions of fixed stars are discussed.
A program for the detection of parallactic displacements
among fixed stars is outlined, and the phases
of Venus are adduced to account for the failure of
that planet to exhibit great differences in size to the
naked eye at perigee and apogee. Kepler's modification
of the circular Copernican orbits is not mentioned;
indeed, the Copernican system is presented
as more regular and simpler than Copernicus himself
had made it. Technical astronomy is discussed with
respect only to observational problems, not to planetary
theory.
To the refutation of conventional physical objections