Bartsch’s Open Letter to Kepler, 1628

To the illustrious Academic University of Studies of Alsace at Strasbourg.

Greetings, and safety in every way, from God, thrice greatest and thrice best, the Founder, Mover and Preserver of the stars and the heavens.

It is now almost a year, gentlemen, since there came out of the heart of our Germany the long-desired work of the Rudolphine astronomical Tables: a work, if ever there was one, well worth preserving in cedar oil, and worth its weight in gold. So a fair judge of these things set above us will find, weighing in an even balance: the imperial and royal expense, and delight in this art, of its first promoters, the late Roman Emperor Rudolf II, after whom it is named, and Frederick II of Denmark, with whose support it was begun; the long and endless labours and costs of many, in instruments and in books alike, above all the thirty-eight years of night-watches, and the priceless diligence, of its first author, the most noble of the Danes, Tycho Brahe; the new discoveries in it, and the perfecting of its computations, drawn out over twenty-six years and repaid with interest, by the most famous of the Germans, Mr Johannes Kepler, Mathematician to three Emperors in turn,Rudolf II, Matthias and Ferdinand II. with his new hypotheses of physical causes and his edition of a calculation now complete and lasting; the longing and the outstanding approval of the other learned astronomers of our time; or finally, what the thing itself is, the accomplished restoration of the art of the stars, which everywhere threatened ruin, and the perfect, I might almost say final, reform of the errant celestial motions.

Moved by these attractions of reason, I too, in the many years in which I have handled astronomy mixed with medicine, weary of the old erroneous calculation, have always awaited these Tables eagerly; and since the time I received them as a gift from the Author himself, I have tried, so far as my slender powers allow, not only to learn the reasoning of the new physical hypotheses in the very motions of the planets, but also to practise the new and shorter method of logarithmic calculation. So it came about that during this past half-year (in which Mars greatly disturbed the public state of my country with soldiers, and Saturn my private state with my father’s death), putting other things aside for a time, I refreshed my mind with this new study of the heavens, and drew my eyes, intent on the orders above, away from the turmoil of the earth. In this busy business, entangled more and more in the one and freed little by little from the other, I tried one thing and another, and they came out happily as I wished; so that, beyond my first intention (by Hercules!), I caught on paper what is perhaps the first rough offspring born of the published Rudolphine Tables. Why, after licking it into livelier shape, I wanted to send it out in public, and to have it seen dressed in this new form, the twofold preface that follows will tell more fully and clearly.

I have chosen to call it Uraniburg, a metaphorical word, but to be understood modestly: for just as the observations of the celestial motions, and from them the foundations of the Rudolphine Tables, were taken from Uraniburg, Tycho’s seat of astronomy, which Brahe built at enormous expense for these purposes on the island of Hven in the Sound; so from this Keplerian Astro-poecilo-pyrgium, or Tychonic-Keplerian Ephemeris, even someone less practised in this kind of study can foresee, as if from the sky, the places of the stars observed there, assigned to definite times, and predict their configurations. And I chose to give it the surname of Strasbourg, because this Uraniburg, built (as I said) on the foundations of Tycho and Kepler, is set above all on your meridian as to the times of the aspects, and is shared under your name, without any display, with those who cultivate this art. …

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To the most excellent Imperial Mathematician, the famous Mr Johannes Kepler, etc., to be revered as a teacher.

What I often regret, most famous Sir, honoured patron, I suspect you have often wondered at: not so much that about a year ago, though our places were near each other, we had no chance to talk in person, as that for a year now, further apart and each not knowing where the other was, we have even stopped talking by letter altogether. But as I believe the latter will soon be made good, so I am glad that the former fell to my lot at Ulm three years ago. For at that time, on my way to Padua in Italy, I was allowed in passing a few words with you, few but welcome. But a year ago, on my return, my plan of completing Schiller’s posthumous Christian UranographyJulius Schiller's Coelum stellatum christianum (Augsburg, 1627), which renamed the constellations after Christian figures; Bartsch saw it through the press after Schiller's death. kept me at Augsburg, as you know, while the printing of Tycho’s work, the Rudolphine Tables, kept you busy at nearby Ulm in Swabia; until, when both our works were finished at the same time, your unexpected departure for Frankfurt and mine for home cut off every chance until now of talking about astronomy, whether by letter or in person. For though since then I have sent letters meant for you twice to Prague and once to Ulm, and have not neglected other chances of inquiring here and there, the letters came back to my hands from Prague with news, first of your absence, then of your departure, and from Ulm nothing; nor did I learn where you were living until a few days ago, when the most excellent Professor of Mathematics at Leipzig, the Licentiate Mr Philipp Müller, my teacher, whom I can never honour enough, having learned of it, told me that the graces of your Urania would from now on dwell in our Silesia and give it lustre. I congratulate you on this, and rejoice for myself, and wish us both every good. Meanwhile, if this piece of work of mine, such as it is, bearing your name, should reach your hands before my little letter, know that it is sent ahead not so much to tell you about me and my situation as about the reasoning of this work and undertaking of mine.

As for, first, the occasion for computing this Ephemeris and the cause for publishing it, let me say this first: some time ago (if I remember rightly) I wrote to Ulm, or meant to write, among other things, that besides other little astronomical works especially suited to practical use, I was also planning, and, after various trials, already preparing, not the speculative Theories of the Planets, explained more fully and clearly by Peurbach, Mästlin, others and yourself, but their practical Spheres, if I may call them so, in which (whether they come to be called directories of the second movables, or spherical instruments of the second motions, or instrumental Equatoria of the Planets, or by some more fitting name) the planets’ own affections, that is, the proper motions of the second movables, can be shown to the eye and taught mechanically, roughly perhaps, but agreeing with the heavens to the degree, according to the astronomical hypotheses: no less than in the ordinary spheres or globes the common affections of the stars, which belong to them by reason of the first movable (whatever that may be), are commonly shown and can be set before the eyes even of the unlearned. For just as I know for certain, by experience, that many would neglect or fail to understand even the first motion of the heavens, which meets our eyes every day, unless it could be taught to the eye and led by the hand, as it were, through the rings of an armillary sphere or the circles of a globe; so, conversely, I believe that many more would study with greater zeal the motions of the second movables, which are less observable, indeed, but certainly more delightful, and bear witness to the wonderful wisdom of the First Mover, if they were shown in the same way, visibly and to the eye: if, I say, not only by numbers or diagrams, which show no change of motion unless redrawn many times, but also by spherical instruments (following both the old hypotheses of Ptolemy and the common ones of eccentrics, epicycles, equants and so on, and Tycho’s new ones of homocentric epicycles, or, as far as possible, yours of physical causes) there were set before the eyes of those less practised in this art, for each planet, first the mean motions of longitude, of the apogee or aphelion and of the eccentric, with their lines and equations, namely the eccentric, the equated or epicyclic, and those called the prosthaphaereses of the orb; then the true longitude and latitude, with the nodes; and finally the wonderful appearances of stations, retrogressions and direct motions, and of motion now faster, now slower; and, what is above all to be admired in Tycho’s system of motions, the most constant and most beautiful order of the other planets, always looking to the mean sun as their centre, like a leader or chorus-master, at fixed limits of conjunction and opposition.

With this said first, I do not deny that much help for this undertaking of mine comes from the reflections lately set out in public by the Licentiate Mr Müller, praised above, which aim at the same thing, and also, besides your Epitome of Copernican Astronomy, from Tycho’s Rudolphine Tables. For though, when they were sent to Augsburg, long desired as they were, still warm from the press, a gift beyond my deserts but not an unwelcome one, I did run through them in passing with great joy, yet I could neither read each part through properly, as you asked, nor answer your questions, prevented, as I touched on at the start, by the unexpected departure of each of us, you for Frankfurt and I for Leipzig, to join companions for the fair. There I afterwards left them for a time with the often-praised Mr Müller, who had not yet seen them, nor had any been brought there, and who received them all the more gladly; for I doubted I should try the calculation in my first days with my parents, whom I had not seen for eight years. And even afterwards, when I had got the Tables back at the beginning of this year, the unexpected and sad death of my dearly loved father held it up not a little.

But about half a year ago, or a little more, I took up the Tables again, eager partly to grasp your new physical hypotheses, partly to practise the new logarithmic calculation (ignoring even the mad din of Mars troubling my country), and began with much labour to work out, according to the precepts of the Tables, for certain days of the months of this present year and of the following year, 1629, for the five planets, which now wander most, first the mean motions of longitude, aphelion and node; then the mean, eccentric and equated anomalies; then, from the equations of the same names and of the annual orb, the true places themselves in longitude and latitude. But in the Moon, when I saw the difficult thorns of a longer calculation, namely two hypotheses, differing somewhat, especially in the matter of the variation, and when I also tested, on single days of some periods, what the difference between the two calculations was everywhere, and how great, it came about that more than half a year slipped by as I examined each point separately.

When this was done, since I had no chance of telling you of it, not knowing, in this corner of the land, where on earth you were living: at last, behold! barely two months ago, as the calculation went better and better under my hands, with most of the difficulties overcome, I decided to add the rest to complete the Ephemeris, and to give it to some bookseller to publish at his expense, so that, if in no other way sooner, I might at least bring you these public prefaces as soon as possible; which is also why I have not hesitated to set things out at length here. And just as a year ago I gladly, and rightly, offered you my small, ready service, so now I would have you firmly persuaded that I offer this year’s Ephemeris, first computed for my private use, to the public in such a way that I want your more splendid and worthier ones in no way hindered or disturbed, but rather (as I plainly confirm by this public testimony) helped, as far as my slender powers allow and as far as it serves your interest and mine; indeed, that it had to be published, so that you might know of my labour, such as it is, ready for you, and other experts in the stars might know of your lunar calculation and Tycho’s, never before set out together in your works. There are other private reasons for the publication, which I leave out now, since I shall explain them better some day in person (if God soon brings us together safe in Silesia); and I give notice of the rest in what remains. …

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This is what I have chosen to say to you beforehand, a second Tycho, about my present situation and undertaking, whatever may come of it; I hope soon to talk with you more of these and other things, by letter or in person. Meanwhile, read this with a fair and kindly mind, if you have time; support my studies, however small; and go on favouring hereafter, as before, the one who wishes you every health and strength,

Jac. Bartsch, the Author.