The Essential Galileo (46 page)

S
AGR.
    I am of the opinion that these better thinkers experienced what is now happening to me, too; that is, one cannot understand [487] how the three periods (annual, monthly, and daily) are entangled and how their causes appear to depend on the sun and moon without the sun and moon having anything to do with the water. For a full understanding of this business I need a longer and more focused application of my mind, which at the moment is very confused by its novelty and difficulty; but I do not despair of being able to grasp it if, in solitude and silence, I can return to chewing over what remains improperly digested in my mind.

Thus, the discussions of these four days provide strong indications in favor of the Copernican system. Among them, these three appear to be very convincing: first, the one taken from the stoppings and retrogressions of the planets and their approaching and receding from the earth; second, the one from the sun's rotation on itself and from what is observed about its spots; and third, the one from the ebb and flow of the sea.

S
ALV.
    Soon we could perhaps add a fourth one and possibly even a fifth one. That is, the fourth one would be taken from the fixed stars, if the most exact observations were to reveal in them those minute changes which Copernicus assumes to be imperceptible.
⁷⁸
There is now a fifth novelty from which one might be able to argue for the motion of the terrestrial globe. This refers to the extremely subtle things being discovered by the most illustrious Mr. Cesare Marsili,
⁷⁹
member of a very noble family of Bologna, and also a Lincean Academician; in a most learned essay he states that he has observed a constant though extremely slow motion of the meridian line. Having lately seen this essay with astonishment, I hope he sends copies of it to all students of the marvels of nature.

S
AGR.
    This is not the first time I have heard of this gentleman's refined learning and of his great concern to be a patron of all scholars. If this or some other work of his comes out, we can be sure that it will be a thing of distinction.

S
ALV.
    Now, since it is time to put an end to our discussions, it remains for me to ask you to please excuse my faults if, when more calmly going over the things I have put forth, you should encounter difficulties and doubts not adequately resolved. You should excuse me because these ideas are novel, my mind is imperfect, and the subject is a great one; finally, I do not ask and have not asked from others an assent which I myself do not give to this fancy, and I could very easily [488] regard it as a most unreal chimera and a most solemn paradox. As for you, Sagredo, although in the discussions we have had you have shown many times by means of strong endorsements that you were satisfied with some of my thoughts, I feel that in part this derived more from their novelty than from their certainty, and even much more from your courtesy; for by means of your assent you have wanted to give me the satisfaction which one naturally feels from the approval and praise of one's own creations. Moreover, just as I am obliged to you for your politeness, so I appreciate the sincerity of Simplicio; indeed, I have become very fond of him for defending his master's doctrine so steadfastly, so forcefully, and so courageously. Finally, just as I express thanks to you, Sagredo, for your very courteous feelings, so I beg forgiveness of Simplicio if I have upset him sometimes with my excessively bold and resolute language; there should be no question that I have not done this out of any malicious motive, but only to give him a greater opportunity to advance better thoughts, so that I could learn more.

S
IMP.
    There is no need for you to give these excuses, which are superfluous, especially to me who am used to being in social discussions and public disputes; indeed, innumerable times I have heard the opponents not only get upset and angry at each other, but also burst out into insulting words, and sometimes come very close to physical violence. As for the discussions we have had, especially the last one about the explanation of the tides, I really do not understand it completely. However, from the superficial conception I have been able to grasp, I confess that your idea seems to me much more ingenious than any others I have heard, but that I do not thereby regard it as truthful and convincing. Indeed, I always keep before my mind's eye a very firm doctrine, which I once learned from a man of great knowledge and eminence, and before which one must give pause.
⁸⁰
From it I know what you would answer if both of you are asked whether God with His infinite power and wisdom could give to the element water the back and forth motion we see in it by some means other than by moving the containing basin; I say you will answer that He would have the power and the knowledge to do this in many ways, some of them even inconceivable by our intellect. Thus, I immediately conclude that in view of this it would be excessively bold if someone should want to limit and compel divine power and wisdom to a particular fancy of his.

[489] S
ALV.
    An admirable and truly angelic doctrine, to which there corresponds very harmoniously another one that is also divine. This is the doctrine which, while it allows us to argue about the constitution of the world, tells us that we are not about to discover how His hands built it (perhaps in order that the exercise of the human mind would not be stopped or destroyed).
⁸¹
Thus let this exercise, granted and commanded to us by God, suffice to acknowledge His greatness; the less we are able to fathom the profound depths of His infinite wisdom, the more we shall admire that greatness.

S
AGR.
    This can very well be the final ending of our arguments over the last four days. Hereafter, if Salviati wants to take some rest, it is proper that our curiosity grant it to him, but on one condition; that is, when he finds it least inconvenient, he should comply with the wish, especially mine, to discuss the problems which we have set aside and which I have recorded, by having one or two other sessions, as we agreed. Above all I shall be looking forward with great eagerness to hear the elements of our Academician's new science of motion (natural and violent). Finally, now we can, as usual, go for an hour to enjoy some fresh air in the gondola that is waiting for us.

1.
Reprinted from: Galileo Galilei,
Galileo on the World Systems: A New Abridged Translation and Guide
, trans. and ed. by Maurice A. Finocchiaro, © 1997 by the Regents of the University of California. Published by the University of California Press.

2.
For the historical background, see the Introduction, especially §0.9.

3.
Galilei 1890–1909, 7: 29–31; translated by Finocchiaro (1997, 77–82). This preface was compiled jointly by Galileo and the Master of the Sacred Palace, or chief censor in Rome, a Dominican named Niccolò Riccardi (1585–1639).

4.
The antiCopernican Decree of the Index (5 March 1616); cf. §6.5.

5.
The Italian name of this philosopher is Simplicio, which is also a word with the connotation of
simpleton
. I have retained the name Simplicio in order to capture Galileo's double-entendre.

6.
Galilei 1890–1909, 7: 132.1–39.35; translated by Finocchiaro (1997, 117–28).

7.
That is, a fifth substance, besides the four terrestrial elements of earth, water, air, and fire; it was also called
quintessence
or
aether
.

8.
This disclaimer is only one of many similar ones interspersed throughout the
Dialogue;
it is part of Galileo's attempt to have this book interpreted as a legitimate hypothetical or critical discussion of the issues, rather than as an assertion or defense of Copernicanism, which would have violated the ecclesiastic restrictions placed upon him. Of course, his attempt was not entirely successful, as the trial of 1633 shows.

9.
This ellipsis is in Galileo's text.

10.
The Galenists were followers of Galen (c. 130–c. 200), Greek physician to the Roman emperor Marcus Aurelius, and writer of many treatises that made him the supreme authority on medicine until the 16th century. They held (correctly) that the nerves originate in the brain, whereas the Aristotelians held (incorrectly) that they originate in the heart; cf. Aristotle,
On the Generation of Animals
, V, 2, 781a20.

11.
Galileo uses the Latin phrase
ipse dixit
in his Italian dialogue, since the phrase has been adopted by other languages, including English. It derives from the traditional Latin version of the Bible, which uses the phrase in Genesis to refer to God's acts of creation. It literally means “he himself said it,” referring to someone who is regarded as an authority; it is a way of appealing to authority in the course of a discussion.

12.
Cf. Aristotle,
On the Generation of Animals
, V, 1, 780b21.

13.
Joachim of Floris (c. 1132–1202) was an Italian clergyman whose writings contain numerous vague and ambiguous prophecies, especially about the coming of a new age when the hierarchical structure of the Church and the separation between Christianity and other religions would no longer be needed.

14.
Astrology is the pseudoscience that tries to predict human behavior and future events based on the positions and configurations of the heavenly bodies. It should not be confused with the science of astronomy. But until Galileo's time, astronomy and astrology were usually practiced by the same persons, and the two terms were often used interchangeably. Galileo's criticism reinforced a preexisting trend critical of astrology that eventually resulted in the separation of the two. The relationship between astrology and astronomy is analogous to that between alchemy and chemistry.

15.
Alchemy was the ancient quest to turn base metals (such as iron) into precious ones (such as gold). Though this quest was impossible and alchemy was largely a pseudoscience, some aspects of it (such as its emphasis on experimental tinkering) made alchemy into a forerunner of the modern science of chemistry. As with astrology, Galileo's dismissal of alchemy was rare for his time and shows his remarkably modern outlook.

16.
The rest of this speech refers to figures of classical Greek and Roman mythology, and not to heavenly bodies. The moon was a goddess; Endymion was a young shepherd with whom she fell in love; and Actaeon was the unfortunate hunter who watched her bathe, for which she turned him into a stag, whereupon he was killed by his own dogs. Jupiter was the supreme god, Mercury was the messenger of the gods, and Pluto was the god who ruled the afterlife.

17.
Alexander of Aphrodisias, a Greek philosopher who lived around 200 A.D., best known for his commentaries on Aristotle, especially the one dealing with the book
On the Soul
. He interpreted Aristotle as implying that a person's soul is not immortal, and so was condemned at the Fifth Lateran Council in 1512 (Santillana 1953, 125 n. 8).

18.
According to classical Greek mythology, in one of his many exploits Hercules was condemned by the Delphic Oracle to be a servant to the queen of Lydia; she had him dress in women's clothes, live with her maids, spin wool like them, and make love to her.

19.
Galilei 1890–1909, 7: 139.36–150.12; translated by Finocchiaro (1997, 128–42).

20.
This sentence is the first of several statements in this selection of the principle of the relativity of motion, a basic principle of classical physics.

21.
This other motion is the annual motion, which Galileo discusses in Day III, whereas Day II deals with the diurnal motion; the Aristotelian and Ptolemaic arguments are presented in §8.4.

22.
Observationally speaking, at the time of Copernicus and Galileo, the earth's annual motion was “reflected” in the appearances of only the planets, not in those of the fixed stars. In fact, this lack of “reflection” constituted a key difficulty for Copernicanism; it was formalized in the objection from stellar parallax, which Galileo discussed in Day III but could not really refute because even his telescope did not reveal any stellar parallax.

23.
Cf. Aristotle,
Physics
, VIII, 4–5, 254b7–258b9; and
On the Motion of Animals
, 2, 698b8ff.

24.
This is the first of many statements of the principle of simplicity (or economy), which is a premise in the geokinetic argument of this selection; this argument may thus be called the simplicity argument for the earth's diurnal motion.

25.
I call this generalization Galileo's law of revolution. It is reminiscent of, and should be compared and contrasted with, Kepler's third law of planetary motion. The latter states that the planets revolve around the sun in such a way that the square of the period of revolution is proportional to the cube of the mean distance from the sun (that is, the period varies as the three-halves power of the distance).

26.
This refers to the celestial sphere or the orb of the Prime Mobile.

27.
This period refers to the precession of the equinoxes, namely, the apparent westward movement of the equinoctial points on the celestial sphere. Since the time of Copernicus, it has been estimated to have a period of 26,000 years, but ancient astronomers gave the higher figure of 36,000 years.

28.
Once again, this is a reference to the precession of the equinoxes.

29.
One of the issues in the Copernican controversy was whether the heavens are solid or fluid; that is, whether or not there exist crystalline spheres made out of impenetrable aether in which the various planets and fixed stars are embedded and whose rotation makes these heavenly bodies revolve.

30.
The orb of the Prime Mobile.

31.
In the Ptolemaic system, the diurnal motion is actually (as well as apparently) westward, whereas the individual planetary revolutions are actually (as well as apparently) eastward.