Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker

Week 4.  Galileo.

excerpts from
Conversation with the Sidereal Messenger
Recently Sent to Mankind by Galileo Galilei, Mathematician of Padua (1610)
by Johannes Kepler (1571-1630)

Notice to the Reader

When many people asked my opinion about Galileo's "Sidereal Messenger," I decided to satisfy them all by adopting the short cut of circulating in printed form the letter which I sent to Galileo (and which was drafted in great haste, amidst unavoidable obligations, to meet a deadline).

But after it was printed, my friends cautioned me that it seemed a little too unconventional in conception.  One of them wanted the introduction done away with; another would have liked to soften certain expressions which, to short-sighted eyes, might appear to impute to my opponent views at variance with scholastic tradition; some also desired less extensive praise of Galileo, in order to make room for the verdict of famous men, whose attitude, they hear, differs from mine.

Therefore I resolved to advise the reader that everybody has his own preference.  Whereas most debaters get all heated up, I regard humor as a more pleasant tone in discussions.  Other authors strive for impressiveness in the exposition of philosophy by the weightiness of their assertions; yet they often prove amusing, unintentionally.  I seem by nature cut out to lighten the hard work and difficulty of a subject by mental relaxation, conveyed by the style....

I do not think that Galileo, an Italian, has treated me, a German, so well that in return I must flatter him, with injury to the truth or to my deepest convictions

Yet let no one assume that by my readiness to agree with Galileo I propose to deprive others of their right to disagree with him.  I have praised him, but all men are free to make up their own minds.  What is more, I have undertaken herein to defend some of my own views also.  I have done so with a conviction of their truth and with serious purpose.  Yet I swear to reject them without reservation, as soon as any better informed person points out an error to me by a sound method.


I may perhaps seem rash in accepting your claims so readily with no support from my own experience.  But why should I not believe a most learned mathematician, whose very style attests the soundness of his judgment?  He has no intention of practicing deception in a bid for vulgar publicity, nor does he pretend to have seen what he has not seen.  Because he loves the truth, he does not hesitate to oppose even the most familiar opinions, and to bear the jeers of the crowd with equanimity.

Does he not make his writings public, and could he possibly hide any villainy that might be perpetrated?  Shall I disparage him, a gentleman of Florence, for the things he has seen?  Shall I with my poor vision disparage him with his keen sight?  Shall he with his equipment of optical instruments be disparaged by me, who must use my naked eyes because I lack these aids.  Shall I not have confidence in him, when he invites everybody to see the same sights, and what is of supreme importance, even offers his own instrument in order to gain support on the strength of observations?

[W]ould it be a trifling matter for him to mock the family of the Grand Dukes of Tuscany, and to attach the name of the Medici to figments of his imagination, while he promises real planets?...

[Your] assertions about the body of the moon are made by others on the basis of mutually self-supporting evidence.  Their conclusions agree with the highly illuminating observations which you report on the same subject.  Consequently I have no basis for questioning the rest of your book and the four satellites of Jupiter.  I should rather wish that I now had a telescope at hand, with which I might anticipate you in discovering two satellites of Mars (as the relationship seems to me to require) and six or eight satellites of Saturn, with one each perhaps for Venus and Mercury....


...I move on with you to the new planets, the most wonderful topic in your little book....

I rejoice that I am to some extent restored to life by your work.  If you had discovered any planets revolving around one of the fixed stars, there would now be waiting for me chains and a prison amid Bruno's innumerabilities.  I should rather say, exile to his infinite space.  Therefore, by reporting that these four planets revolve, not around one of the fixed stars, but around the planet Jupiter, you have for the present freed me from the great fear which gripped me as soon as I had heard about your book ...

What Galileo recently saw with his own eyes, it had many years before not only proposed as a surmise, but thoroughly established by reasoning....  Who does not honor Plato's myth of Atlantis, Plutarch's legend of the gold-colored islands beyond Thule, and Seneca's prophetic verses about the forthcoming discovery of a New World, now that the evidence for such a place has finally been furnished by that Argonaut from Florence [Galileo]?  Columbus himself keeps his readers uncertain whether to admire his intellect in divining the New World from the direction of the winds, more than his courage in facing unknown seas and the boundless ocean, and his good luck, in gaining his objective.

In my own field too, the prodigies will naturally be Pythagoras, Plato, and Euclid.  Borne aloft by the pre-eminence of their reason, they argued that God could not have done otherwise than to arrange the world on the model of the five regular solids....  On the other hand, the plaudits of the average man will go to Copernicus who, equipped with a mind that was not average, yet drew a picture of the universe virtually as it is seen by the eye.  But he brought to light only the bare facts.

Trailing far behind the ancients will be Kepler.  From the visual outlook of the Copernican system he rises, as it were, from the facts to the causes, and to the same explanation as Plato from on high had set forth deductively so many centuries before.  He shows that the Copernican system of the world exhibits the reason for the five Platonic solids.

It is not an act of folly or jealousy to set the ancients above the moderns; the very nature of the subject demands it.  For the glory of the Creator of this world is greater than that of the student of the world, however ingenious.  The former brought forth the structural design from within himself, whereas the latter, despite strenuous efforts, scarcely perceives the plan embodied in the structure.  Surely those thinkers who intellectually grasp the causes of phenomena, before these are revealed to the senses, resemble the Creator more closely than the others, who speculate about the causes after the phenomena have been seen.

Therefore, Galileo, you will not envy our predecessors their due praise.  What you report as having been quite recently observed by your own eyes, they predicted, long before, as necessarily so.  Nevertheless, you will have your own fame.  Copernicus and I, as a Copernican, pointed out to the ancients the mistaken way in which they considered the five solids to be expressed in the world, and we substituted the authentic and true way....

[Giordano Bruno and others] thought that other celestial bodies have their own moons revolving around them, like our earth with its moon....  Moreover, they supposed it was the fixed stars that are so accompanied....  Now the weakness of his reasoning is exposed by your observations.  In the first place, suppose that each and every fixed star is a sun.  No moons have yet been seen revolving around them.  Hence this will remain an open question until this phenomenon too is detected by someone equipped for marvelously refined observations....  On the other hand, Jupiter is one of the planets, which Bruno describes as earths.  And behold, there are four other planets around Jupiter.  Yet Bruno's argument made this claim not for the earths, but for the suns....

It is not improbable, I must point out, that there are inhabitants not only on the moon but on Jupiter too....  But as soon as somebody demonstrates the art of flying, settlers from our species of man will not be lacking.  Who would once have thought that the crossing of the wide ocean was calmer and safer than of the narrow Adriatic Sea, Baltic Sea, or English Channel?  Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse.  Therefore, for the sake of those who, as it were, will presently be on hand to attempt this voyage, let us establish the astronomy, Galileo, you of Jupiter, and me of the moon....

Let the higher philosophy reflect, I repeat, and glance backward to some extent.  How far has the knowledge of nature progressed, how much is left, and what may the men of the future expect?....

There are in fact four planets revolving around Jupiter at different distances with unequal periods.  For whose sake, the question arises, if there are no people on Jupiter to behold this wonderfully varied display with their own eyes?  For, as far as we on the earth are concerned, I do not know by what arguments I may be persuaded to believe that these planets minister chiefly to us, who never see them.  We should not anticipate that all of us, equipped with your telescopes, Galileo, will observe them hereafter as a matter of course.

...Will some people regard our earthly astrology, or to speak technically, our doctrine of aspects, as false, because up to this very day we have erred regarding the number of planets constituting the aspects?....

[I]t becomes evident that these four new planets were ordained not primarily for us who live on the earth, but undoubtedly for the Jovian beings who dwell around Jupiter.

...For we see the moon so arranged therein that, as a planet revolving around the earth, it cannot appear to be intended for other globes, but only for the earth, which it encircles as it moves.  The diameter of its path is considered to be 1/20 of the diameter of the earth's "great circle" around the sun, but I believe it is hardly 1/30.  Hence, as seen from the sun, it subtends less than 3°, or on my figures, less than 2°.  Now Saturn is 10 times more remote, and Jupiter about 5.  Therefore, as seen from Saturn, our moon cannot diverge more than 18' or 12' from the earth; as seen from Jupiter about 36' or 24'.  Thus it is related to the inhabitants of Saturn and Jupiter precisely as Jupiter's satellites are to us, the creatures of earth....

The conclusion is quite clear.  Our moon exists for us on the earth, not for the other globes.  Those four little moons exist for Jupiter, not for us.  Each planet in turn, together with its occupants, is served by its own satellites.  From this line of reasoning we deduce with the highest degree of probability that Jupiter is inhabited.  Tycho Brahe likewise drew the same inference, based exclusively on a consideration of the hugeness of those globes....

Well, then, someone may say, if there are globes in the heaven similar to our earth, do we vie with them over who occupies the better portion of the universe?  For if their globes are nobler, we are not the noblest of rational creatures.  Then how can all things be for man's sake?  How can we be the masters of God's handiwork?

It is difficult to unravel this knot, because we have not yet acquired all the relevant information....

Geometry is unique and eternal, and it shines in the mind of God.  The share of it which has been granted to man is one of the reasons why he is the image of God.  Now in geometry the most perfect class of figures, after the sphere, consists of the five Euclidean solids.  They constitute the very pattern and model according to which this planetary world of ours was apportioned.  Suppose then that there is an unlimited number of other worlds....

[W]hat is the use of an unlimited number of worlds, if every single one of them contains all of perfection within itself?  Surely the situation is different with regard to the creatures which perpetuate themselves by a succession of generations.  Even Bruno, the defender of infinity, holds that each world must differ from the rest in the kinds of motion, although these are of like number.

If the worlds differ in their motions, then they must differ also in their distances, which determine the periods of the motions.  If they differ in their distances, then they must differ also in the arrangement, type, and perfection of their solids, from which the distances are derived.  Indeed, if you establish universes similar to one another in all respects, you will also produce similar creatures, and as many Galileos, observing new stars in new worlds, as there are worlds.

But of what use is this?  Briefly, it is better to avoid the march to the infinite permitted by the philosophers.  Since it is agreed that there is a limit to the regress in the direction of the smaller, why not also in the direction of the larger?  For example, take the sphere of the fixed stars.  One three-thousandth part of it, perhaps, is the sphere of Saturn.  Of this, in turn, 1/10th part is the sphere of the earth.  One three-hundred-thousandth part of the earth's diameter, again, is man.  A tiny part of man is the little pore beneath his skin.  Here we stop.  Nature goes no lower.

Now let us tackle the other horn of the dilemma.  Suppose those infinite worlds are unlike ours.  Then they will be supplied with something different from the five perfect solids.  Hence they will be less noble than our world.  Therefore it follows that this world of ours is the most excellent of them all, if there should be a plurality of worlds.

Let us now also indicate why the earth surpasses Jupiter and better deserves to be the abode of the predominant creature.

In the center of the world is the sun, heart of the universe, fountain of light, source of heat, origin of life and cosmic motion.  But it seems that man ought quietly to shun that royal throne.  Heaven was assigned to the lord of heaven, the sun of righteousness; but earth, to the children of man.  God has no body, of course, and requires no dwelling place.  Yet more of the force which rules the world is revealed in the sun (in the heaven, as various passages of Scripture put it) than in all the other globes.   Because man's house is otherwise, therefore, let him recognize his own wretchedness and the opulence of God.  Let him acknowledge that he is not the source and origin of the world's splendor, but that he is dependent on the true source and origin thereof.  Moreover, as I said in the "Optics," in the interests of that contemplation for which man was created, and adorned and equipped with eyes, he could not remain at rest in the center.  On the contrary, he must make an annual journey on this boat, which is our earth, to perform his observations.  So surveyors, in measuring inaccessible objects, move from place to place for the purpose of obtaining from the distance between their positions an accurate base line for the triangulation.

After the sun, however, there is no globe nobler or more suitable for man than the earth.  For, in the first place, it is exactly in the middle of the principal globes (if we exclude, as we should, Jupiter's satellites and the moon revolving around the earth).  Above it are Mars, Jupiter, and Saturn  Within the embrace of its orbit run Venus and Mercury, while at the center the sun rotates, instigator of all the motions, truly an Apollo, the term frequently used by Bruno.

Secondly, the five solids divide into two groups:  the three major bodies, cube, tetrahedron, and dodecahedron; and the two minor bodies, icosahedron and octahedron.  The earth's orbit separates the two groups like a partition, by touching the centers of the 12 faces of the dodecahedron above it, and the 12 vertices of the corresponding icosahedron below it.  Merely by its position amidst the solids, the sphere of the earth is more distinguished than the other spheres.

Mercury Venus Earth Mars Jupiter Saturn

Thirdly, we on the earth have difficulty in seeing Mercury, the last of the principal planets, on account of the nearby, overpowering brilliance of the sun.  From Jupiter or Saturn, how much less distinct will Mercury be?  Hence this globe seems assigned to man with the express intent of enabling him to view all the planets.  Will anyone then deny that, to make up for the planets concealed from the Jovians but visible to us earth-dwellers, four others are allocated to Jupiter, to match the four inferior planets, Mars, Earth, Venus, and Mercury, which revolve around the sun within Jupiter's orbit?


Let the Jovian creatures. therefore, have something with which to console themselves.  Let them even have, if it seems right, their own four planets arranged in conformity with a group of three rhombic solids.  Of these, one is the cube (a quasi-rhombic); the second is cuboctahedral; the third is icosidodecahedral; with 6, 12, and 30 quadrilateral faces, respectively.  Let the Jovians, I repeat, have their own planets.  We humans who inhabit the earth can with good reason (in my view) feel proud of the pre-eminent lodging place of our bodies, and we should be grateful to God the creator.

I have enjoyed this philosophical discussion with you about the new questions raised by your observations, Galileo....

It remains for me to make an urgent request of you, most illustrious Galileo.  Press on vigorously with your observations, and let us know at the very earliest opportunity what results your observations have attained.  Finally, forgive my diffuse and independent way of discussing nature. 


April 19, 1610.

Go to:
  • A Perfit Description of the Celestiall Orbs... (1576), by Thomas Digges (c.1546-1595)
  • Dialogue Concerning the Two Chief World Systems(1632), by Galileo Galilei (1564-1642)
Weekly Readings
Lecture Notes