1 The Birth of Science - MSRI

318 11. The Age-Long Recovery
issue in his already-mentioned essay on the fixity of the heavens and the
motion of the earth (see note 37a); there was Paolo Sarpi, who introduced
the analogy (later taken up by Galileo) between the motion of the oceans
as a cause of tides and the nonuniform motion of a basin full of water. 103
Even Andrea Cesalpino, not at all a Copernican, maintained that tides are
caused by a motion of the earth, not rotation or revolution but a “small”
motion introduced ad hoc, and supposedly imparted onto earth from the
heavens. 103a His odd belief seems to confirm that the connection between
tides and earthly motion was suggested not by contemporary scientific
advances but by the reading of ancient sources.
All of this can help understand why Galileo, in the fourth and last day
of his Dialogue concerning the two chief world systems, “demonstrates” that
the cause of tides lies simply in the rotation of earth, thus obtaining the
longed-for proof of the earth’s motion. 104 He does this with many arguments
that have always seemed odd, to say the least, and whose origin
has hitherto been misunderstood. This was certainly the most unfortunate
mistake in Galileo’s scientific career; but considering that the literature at
his disposal on the subject of tides featured as star witnesses the passages
just cited, we must grant that his error was almost inevitable. Indeed, the
arguments of scientists (even scientists of the very first rank, like Galileo)
never spring unbidden from observations and experiments alone: mulling
over written sources is always a fundamental part of the process.
But though Galileo rejected the idea that tides can be explained through
solar and lunar actions, several others were aware of it and defended it. page 392
Thus it is that Galileo inveighs against “a certain prelate” — the same de
Dominis we met on page 303 — for having “published a little tract saying
that the moon, wandering through the skies, attracts and lifts to itself a
mass of water, which follows it around.” 105 The tract in question is named
Euripus, or the ebb and flow of the sea, 106 and in its few pages de Dominis
ascribes tides to the action of the moon and the sun (“Therefore we hold
that . . . the sun and the moon have a strong force, magnetic as it were. . . ”),
states that high tide occurs simultaneously at antipodal points, and shows
103 This appears in Sarpi’s Pensieri naturali, metafisici e matematici, contained in a manuscript of
1595; see especially thoughts 569, 570, 571, reported in the introductory essay to [Galileo/Sosio],
p. lxxvii.
103a A. Cesalpino, Peripateticarum quaestionum libri quinque, Venice, 1571, book III, question V.
104 This is the main theme of the book, which Galileo originally called Dialogue on the ebb and flow of
the tide. The title was changed in deference to the Inquisition’s stipulation (in allowing publication)
that heliocentrism be discussed therein solely as a hypothesis, and that no stress be laid on what
the author regarded as the physical proof of the earth’s motion.
105 Galileo, Dialogue concerning the two chief world systems, in [Galileo: Opere], vol. XIX, p. 415. We
shall see that he had good reason not to name de Dominis.
106 Marco Antonio de Dominis, Euripus, seu de fluxu et refluxu maris sententia. . . , Rome, 1624.
Revision: 1.11 Date: 2003/01/06 07:48:20