The Essential Galileo (7 page)
Salusbury, Thomas, trans. and ed. 1661â65.
Mathematical Collections and Translations
. 2 vols. London: Leybourne.
Santillana, Giorgio de, ed. 1953.
Dialogue on the Great World Systems.
Salusbury's translation revised by G. de Santillana. Chicago: University of Chicago Press.
âââ. 1955.
The Crime of Galileo
. Chicago: University of Chicago Press.
Schaff, Philip, and Henry Wace, eds. 1893.
A Select Library of Nicene and Post-Nicene Fathers
. 2nd series, vol. 6:
St. Jerome: Letters and Select Works
. New York: The Christian Literature Company.
Sosio, Libero, ed. 1970.
Dialogo sopra i due massimi sistemi
. [Same as Galilei 1970.]
Stafford Carlos, Edward, trans. and ed. 1880.
The Sidereal Messenger of Galileo Galilei and a Part of the Preface to Kepler's
Dioptrics. [Same as Galilei 1880.]
*Strauss, Emil, trans. and ed. 1891.
Dialog über die beiden hauptsachlichsten Weltsysteme
. [Same as Galilei 1891. Contains very useful notes.]
Tertullian, Quintus S. F. 1972.
Adversus Marcionem
. Trans. and ed. Ernest Evans. Oxford: Clarendon Press.
*Van Helden, Albert, trans. and ed. 1989.
Sidereus nuncius, or the Sidereal Messenger.
[Same as Galilei 1989. An excellent translation of the entire text, including very useful commentary and notes.]
*Wallace, William A. 1984.
Galileo and His Sources
. Princeton: Princeton University Press. [An excellent historical account of the connection between Galileo's work and that of his immediate Aristotelian predecessors.]
Westman, Robert S. 1986. “The Copernicans and the Churches.” In
God and Nature
, ed. David C. Lindberg and Ronald L. Numbers, pp. 76â113. Berkeley: University of California Press.
[55] To the Most Serene Cosimo II de' Medici, Fourth Grand Duke of Tuscany
[§1.1] There is certainly something very noble and humane in the intention of those who have endeavored to protect from envy the noble achievements of distinguished men, and to rescue their names, worthy of immortality, from oblivion and decay. This desire has given us the images of famous men, sculptured in marble, or fashioned in bronze, as a memorial of them to future ages; to the same feeling we owe the erection of statues, both ordinary and equestrian; hence, as the poet
2
says, has originated expenditure, mounting to the stars, upon columns and pyramids; with this desire, lastly, cities have been built, and distinguished by the names of those men, whom the gratitude of posterity thought worthy of being handed down to all ages. For the state of the human mind is such that, unless it be continually stirred by the likenesses of things obtruding themselves upon it from without, all recollection of them easily passes away from it.
Others, however, having regard for more stable and more lasting monuments, secured the eternity of the fame of great men by placing it under the protection, not of marble or bronze, but of the Muses' guardianship and the imperishable monuments of literature. But why do I mention these things? As if human wit, content with these regions, did not dare to advance further; whereas, since it well understood that all human monuments do perish at last by violence, by weather, or by age, it took a wider view and invented more imperishable signs, over which destroying Time and envious Age could claim no rights; so, betaking itself to the sky, it inscribed on the wellknown orbs of the brightest starsâthose everlasting orbsâthe names of those who, for eminent and god-like deeds, were accounted worthy to enjoy an eternity in company with the stars. Wherefore the fame of Jupiter, Mars, Mercury, Hercules, and the rest of the heroes by whose names the stars are called, will not fade until the extinction of the splendor of the constellations themselves.
But this invention of human shrewdness, so particularly noble and admirable, [56] has gone out of date ages ago, inasmuch as primeval heroes are in possession of those bright abodes and keep them by a sort of right. Into such company the affection of Augustus in vain attempted to introduce Julius Caesar; for when he wished that the name Julian should be given to a star that appeared in his time (one of those which the Greeks and the Latins alike name, from their hairlike tails, comets), it vanished in a short time and mocked his tooeager hope. But we are able to prophesize far truer and happier things for your highness, Most Serene Prince, for scarcely have the immortal graces of your mind begun to shine on earth, when bright stars present themselves in the heavens, like tongues to tell and celebrate your most eminent virtues to all time. Behold then, reserved for your famous name four stars, belonging not to the ordinary and less-distinguished multitude of the fixed stars, but to the illustrious order of the planets; like genuine children of Jupiter, they accomplish their orbital revolutions around this most noble star with mutually unequal motions and with marvelous speed, and at the same time all together in common accord they also complete every twelve years great revolutions around the center of the world, certainly around the sun itself.
But the Maker of the Stars himself seemed to direct me by clear reasons to assign these new planets to the famous name of Your Highness in preference to all others. For just as these stars, like children worthy of their sire, never leave the side of Jupiter by any appreciable distance, so who does not know that clemency, kindness of heart, gentleness of manners, splendor of royal blood, nobility in public functions, wide extent of influence and power over others (all of which have fixed their common abode and seat in Your Highness),â who, I say, does not know that all these qualities, according to the providence of God, from whom all good things do come, emanate from the benign star of Jupiter? Jupiter, I maintain, at the instant of the birth of Your Highness having at length emerged from the turbid mists of the horizon, and occupying the middle quarter of the heavens, and illuminating the eastern angle from his own royal house, from that exalted throne Jupiter looked out upon your most happy birth and poured forth into a most pure air all the brightness of his majesty, in order that your tender body and your mind (already adorned by God with still more splendid graces) might imbibe with your first breath the whole of that influence and power. But why should I use only probable arguments when I can demonstrate my conclusion with an almost necessary reason? It was the will of Almighty God that I should be judged by your most serene parents not unworthy to be employed in teaching mathematics to Your Highness, which duty I discharged, during the four years just passed, at that time of the year when it is customary to relax from more severe studies. Wherefore, since it fell to my lot, evidently by God's will, to serve Your Highness [57] and so to receive the rays of your incredible clemency and beneficence in a position near your person, what wonder is it if you have so warmed my heart that it thinks about scarcely anything else day and night, but how I, who am under your dominion not only by inclination but also by my very birth and nature, may be known to be most anxious for your glory and most grateful to you? And so, inasmuch as under your auspices, Most Serene Cosimo, I have discovered these stars, which were unknown to all astronomers before me, I have, with very good right, determined to designate them with the most august name of your family. And as I was the first to investigate them, who can rightly blame me if I give them a name and call them the
Medicean Stars
, hoping that as much consideration may accrue to these stars from this title as other stars have brought to other heroes? For, not to speak of your most serene ancestors, to whose everlasting glory the monuments of all history bear witness, your virtue alone, most mighty hero, can confer on those stars an immortal name. Similarly, who can doubt that you will not only maintain and preserve the expectations, high though they be, about yourself which you have aroused by the very happy beginning of your government, but that you will also far surpass them, so that when you have conquered your peers, you may still vie with yourself and become day by day greater than yourself and your greatness?
Accept, then, Most Clement Prince, this addition to the glory of your family, reserved by the stars for you. And may you enjoy for many years those good blessings, which are sent to you not so much from the stars as from God, the Maker and Governor of the stars.
Your Highness's most devoted servant, Galileo Galilei. Padua, 12 March 1610.
[59] Astronomical Message Containing and Explaining Observations Lately Made with the Aid of a New Spyglass regarding the Moon's Surface, the Milky Way, Nebulous Stars, an Innumerable Multitude of Fixed Stars, and Also regarding Four Planets Never Before Seen, Which Have Been Named Medicean Stars
[§1.2] In the present small treatise I set forth some matters of great interest for all observers of natural phenomena to look at and consider.
They are of great interest, I think, first, because of their intrinsic excellence; secondly, because of their absolute novelty; and lastly, also because of the instrument by the aid of which they have been presented to our senses.
The number of the fixed stars which observers have been able to see without artificial powers of sight up to this day can be counted. It is therefore decidedly a great feat to add to their number, and to set distinctly before the eyes other stars in myriads, which have never been seen before, and which surpass the old, previously known, stars in number more than ten times.
Again, it is a most beautiful and delightful sight to behold the body of the moon, which is distant from us nearly sixty radii
3
of the earth, as near as if it were at a distance of only two of the same measures. So the diameter of this same moon appears about 30 times larger, its surface about 900 times, and its solid mass nearly 27,000 times larger than when it is viewed only with the naked eye. And consequently anyone may know with the certainty that is due to the use of our senses that the moon certainly does not possess a smooth and polished surface, but [60] one rough and uneven, and, just like the face of the earth itself, it is everywhere full of vast protuberances, deep chasms, and sinuosities.
Then to have got rid of disputes about the galaxy or Milky Way, and to have made its essence clear to the senses, as well as to the intellect, seems by no means a matter that ought to be considered of slight importance. In addition to this, to point out, as with one's finger, the substance of those stars which every one of the astronomers up to this time has called nebulous and to demonstrate that it is very different from what has hitherto been believed, will be pleasant and very beautiful.
But that which will excite the greatest astonishment by far, and which indeed especially moved me to call it to the attention of all astronomers and philosophers, is this: I have discovered four wandering stars, neither known nor observed by any one of the astronomers before my time; they have their orbits around a certain important star of those previously known and are sometimes in front of it, sometimes behind it, though they never depart from it beyond certain limits, like Venus and Mercury around the sun.
All these facts were discovered and observed a short time ago with the help of a spyglass
4
devised
5
by me, through God's grace first enlightening my mind. Perchance other discoveries still more excellent will be made from time to time by me and by other observers with the assistance of a similar instrument. So I will first briefly record its shape and preparation, as well as the occasion of its being devised, and then I will give an account of the observations made by me.
[§1.3] About ten months ago a report reached my ears that a Dutchman had constructed a spyglass, by the aid of which visible objects, although at a great distance from the eye of the observer, were seen distinctly as if near; and some demonstrations of its wonderful performances were reported, which some gave credence to, but others contradicted. A few days later I received confirmation of the report in a letter written from Paris by a noble Frenchman, Jacques Badovere. This finally determined me to give myself up first to inquire into the principle of the spyglass, and then to consider the means by which I might arrive at the invention of a similar instrument. After a little while I succeeded, through deep study of the theory of refraction. I prepared a tube, at first of lead, in the ends of which I fitted two glass lenses, both plane on one side, but on the other side one spherically convex, and the other concave. Then bringing my eye to the concave lens [61] I saw objects satisfactorily large and near, for they appeared one-third of the distance and nine times larger than when they are seen with the natural eye alone. Shortly afterwards I constructed another more precise spyglass, which magnified objects more than 60 times. Finally, by sparing neither labor nor expense, I succeeded in constructing for myself an instrument so superior that objects seen through it appear magnified nearly 1,000 times, and more than 30 times nearer than if viewed by the natural powers of sight alone.
It would be altogether a waste of time to enumerate the number and importance of the benefits which this instrument may be expected to confer when used by land or sea. But without paying attention to its use for terrestrial objects, I betook myself to observations of the heavenly bodies. First of all, I viewed the moon as near as if it were scarcely two radii of the earth distant. After the moon, I frequently observed other heavenly bodies, both fixed stars and planets, with incredible delight; and, when I saw their very great number, I began to consider about a method by which I might be able to measure their distances apart, and finally I found one.
Here it is fitting that all who intend to turn their attention to observations of this kind should receive certain cautions. In the first place, it is absolutely necessary for them to prepare a most perfect spyglass, one that will show very bright objects distinct and free from any mistiness and will magnify them at least 400 times and show them as if only one-twentieth of their distance. Unless the instrument be of such power, it will be in vain to attempt to view all the things that have been seen by me in the heavens, and that will be enumerated below. Then in order that one may be a little more certain about the magnifying power of the instrument, one shall fashion two circles or two square pieces of paper, one of which is 400 times greater than the other; this will happen when the diameter of the greater is twenty times the length of the diameter of the other. Then one shall view from a distance simultaneously both surfaces, fixed on the same wall, the smaller with one eye applied to the spyglass, and the larger with the other eye unassisted; for that may be done without inconvenience at one and the same instant with both eyes open. Then both figures will appear of the same size, if the instrument magnifies objects in the desired proportion.
After such an instrument has been prepared, the method of measuring distances remains for inquiry, and this shall be accomplished by the following contrivance. For the sake of being more easily understood, let
ABCD
be the tube and E the eye of the observer. When there are no lenses in the tube, rays from the eye to the object
FG
would be drawn in the straight lines
ECF
and
EDG;
[62] but when the lenses have been inserted, the rays go in the bent lines
ECH
and
EDI
and are brought closer together, and those that originally (when unaffected by the lenses) were directed to the object
FG
will include only the part
HI.
Then, the ratio of the distance
EH
to the line
HI
being known, we shall be able to find, by means of a table of sines, the magnitude of the angle subtended at the eye by the object
HI
, which we shall find to contain only some minutes. Now, if we fit on the lens
CD
thin plates pierced some with larger and others with smaller apertures, by putting on over the lens sometimes one plate and sometimes another, as may be necessary, we shall construct at our pleasure different subtending angles of more or fewer minutes; by their help we shall be able to measure conveniently the intervals between stars separated by an angular distance of some minutes, within an error of one or two minutes. But let it suffice for the present to have thus slightly touched, and as it were just put our lips to these matters, for on some other opportunity I will publish the complete theory of this instrument.