Johannes Kepler: Life and Letters

Preface

The biography of Kepler which I herewith offer to the English reading public is the most detailed of this kind as yet presented. As far as I can see, even the modern German biographers have not yet based their sketches on a full evaluation of the standard edition of Johannes Kepler in seinen Briefen by Max Caspar and Walther von Dyck, München und Berlin 1930, 2 vols. Verlag R. Oldenbourg.

Apart from this basic work, use has been made of the uncompleted great new Kepler edition by von Dyck and the old still valuable edition by Christian Frisch. Further, all the rich literature available in the Library of Congress has been consulted, though not always relied on. For all the many services which the staff of the Congressional Library has rendered me during the long time needed for the preparation of this manuscript, I wish to express my special thanks.

I am much indebted to Mrs. Angelica Canfield for her kind, efficient help in proofreading and for very valuable comments.

My warmest thanks, also, to Dr. Herbert Weiss-berger of Washington Square College and the Heineman Foundation, for providing me with photostats of Kepler manuscripts hitherto unpublished and giving me the permission to show a page in the present work.

Last but not least, I owe special gratitude to Mr. Joseph Frank and Mrs. Rachel Frank for their never tiring help in correcting and remodelling the English of my manuscript.

Needless to say, I am particularly indebted to Albert Einstein who was kind enough to write an introduction to my book.

C

AROLA

B

AUMOARDT

Introduction

One can only feel grateful that the letters of this incomparable man have been made accessible to an English reading public by Mrs. Baumgardt’s translation. The letters extend over a period of time from 1596 to 1631. The selection is governed, primarily, by the necessity of communicating to the reader a picture of Kepler the man as a personality; no attempt is made to bring to the foreground his scientific achievements and their unique consequences. But a reader who knows the state of science at that time can also learn something valuable, in this respect, from these letters.

There we meet a finely sensitive person, passionately dedicated to the search for a deeper insight into the essence of natural events, who, despite internal and external difficulties, reached his loftily-placed goal. Kepler’s life was devoted to the solution of a double problem. The sun and the planets alter their apparent position, with relation to the background of the fixed stars, in a complicated manner, accessible to immediate observation; what had been observed and recorded with great diligence, therefore, was not actually the movements of the planets in space but the temporal alterations which the direction earth-planet undergoes during the passage of time. Since Copernicus had persuaded the small group of those competent to judge that the sun, in this process, should be considered as motionless, while the planets—including the earth—should be considered as moving around the sun, the first great problem that presented itself was: the determination of the true movements of the planets, including the earth, as it would look to an observer established on the nearest fixed star, and completely equipped with a stereoscopic double-telescope. This was Kepler’s first great problem. The second problem lay in the question: What are the mathematical laws controlling these movements? Clearly, the solution of the second problem, if it were possible for the human spirit to accomplish it, presupposed the solution of the first. For one must first know an event before one can test a theory related to this event.

Underlying Kepler’s solution of the first problem is an idea of true genius, which made possible the determination of the true pattern of the earth’s course. To be able to construct the earth’s course, one needs, besides the sun, a second stable point in planetary space; having such a point, one can—by employing it and the sun as fixed points for the measurement of angles—determine the true pattern of the earth’s course by the same method of triangulation generally used in drawing maps. Where, however, can such a fixed point be found, since all visible objects outside the sun, as a single object, execute unknown movements? Kepler’s answer: We know, with great exactitude, the ostensible movement of the planet Mars and the time it takes to circle the sun (Mars-Year). Each time that a Mars-Year passes, Mars should be at the same place in (planetary) space. If one limits oneself chiefly to the use of such points of time, the planet Mars represents for these a stable point in planetary space, which may be used as a fixed point in triangulation.

Using this principle, Kepler first determined the true movement of the earth in planetary space. And since the earth itself can be used at any time as a triangulation-point, he was also able to determine by observation the true movements of the other planets.

In this way, Kepler won the foundation for the determination of the three fundamental laws that will remain linked to his name for all time. How much inventive power, how much tireless, obstinate work was necessary to reveal these laws, and to establish their certainty with great precision—naturally, can hardly be evaluated by anyone.

This is what the reader must know, when he sees from the letters under what conditions of personal hardship Kepler completed his gigantic work. Neither by poverty, nor by incomprehension of the contemporaries who ruled over the conditions of his life and work, did he allow himself to be crippled or discouraged. In addition, he dealt with a field of knowledge that immediately endangered the adherent of religious truth. He belonged, nevertheless, to those few who cannot do otherwise than openly acknowledge their convictions on every subject. Nor was he one of those who derive instinctive pleasure from the battle with others, as, for example, was obviously the case with Galileo, whose heavenly maliciousness, even today, delights the understanding reader. Kepler was a pious Protestant, who made no secret of the fact that he did not approve all decisions of the Church; he was, for this reason, looked on as a sort of moderate heretic, and treated accordingly. This leads me to the internal difficulties already touched on that Kepler had to conquer. They are less easy to perceive than the external ones. His life work was possible only when he succeeded in freeing himself to a large extent from the spiritual tradition in which he was born. It was not only a question of religious tradition based on the authority of the Church, but of the general notions about the conditioning of events in the cosmos and in human life, as well as ideas about the relative importance of thought and experience in science.

He had to free himself from an animistic, teleologically oriented manner of thinking in scientific research. He had to realize clearly that logical-mathematical theoretizing, no matter how lucid, could not guarantee truth by itself; that the most beautiful logical theory means nothing in natural science without comparison with the exactest experience. Without this philosophic attitude, his work would not have been possible. He does not speak about this, but the inner struggle is reflected in the letters. The reader should note the remarks on astrology. They show that the inner enemy, conquered and rendered innocuous, was not yet completely dead.

A

LBERT

E

INSTEIN

Princeton, New Jersey

Youth and Years of Apprenticeship

How little does the educated man of our time know about Johannes Kepler! Generally, even astronomers know only that Kepler, the German, was, probably, the most highly gifted among the four great founders of modern physics and astronomy—the heir and executor of the ideas of Copernicus, the Pole, the friend and collaborator of the great Italian Galileo Galilei and the most important of the forerunners of Isaac Newton, the first systematizer of modern science.

It is, however, hardly common knowledge that Kepler was not only one of the greatest scientific geniuses of all times, but also something of a poet and one of the most lovable and moving personalities of modern history. The nobility and superiority of character with which he endured a life full of hardship is still a shining example of inner spiritual strength in the midst of outward failure.

Max Brod, the fine poet, friend and editor of Franz Kafka, has written an exquisite novel dealing with the youth of Kepler and his relation to his teacher and collaborator, Tycho Brahe, whose astronomical teaching he was destined to outshine. The novel has been translated into English under the title The Redemption of Tycho Brahe. It gives a very valuable insight into the thoughts and feelings of the young Kepler.

But nowhere is full justice done to the man’s bitter, stirring, lifelong struggle with a grim fate, and the striking inner victories of his gay and warm-hearted nature over all the darkness of the circumstances that surrounded him. This fight for truth and personal integrity did not end until death came to him in his sixtieth year of life.

Everyone takes it for granted today that the paradoxical theory of Copernicus is scientifically superior to the common sense belief in the movements of the sun around the earth. We are no longer aware of how heretic, how absurd and how dangerous the Copernican theory was thought to be in the sixteenth and seventeenth centuries. We react as narrow-mindedly and unimaginatively as a daughter who found it too difficult to get the right marital partner while she believed it only too simple for her mother to marry her father—so well known to the mother and even the daughter herself.

The strange Copernican theory of the movements of the earth around the sun which received its most vital verification in Kepler’s life work was in his day generally considered as fantastic non-sense, and as wanton, completely unjustifiable contempt of Biblical teaching and common sense. Kepler had to spend the energies of his genius in an epic struggle for a most paradoxical and difficult truth; and although the subject matter may change from century to century, the life experience of the boldest discoverers of truth, unfortunately, even today, is too often extremely similar to that of Kepler.

Sir David Brewster, the eminent Scottish physicist, rightly counted Kepler among The great Martyrs of Science;¹ and Goethe rather reduced to banality the fate of Kepler by using him as an illustration for the axiom that the able man overcomes all obstacles in life.² Kepler’s martyrdom, however, took place despite the fact that hardly anyone could consider Kepler as an inferior mind during his lifetime, and that posterity was soon to recognize his full genius. Kant saw in him the most acute thinker ever born.³ Johann Gottfried von Herder,⁴ another of the German classicists of the eighteenth century, emphasized that Kepler was not only the discoverer of new astronomical truths but also the founder of whole new branches of sciences. And Alexander von Humboldt,⁵ himself a great explorer of nature, has pertinently pointed to that almost unparalleled combination of bold imaginative power with mathematical profoundness which characterizes the originality of Kepler’s mind.

Nor was Kepler himself unaware of the greatness of his achievements. In the preface of the fifth chapter of his Cosmic Harmonies Kepler concludes his humble apologies for the revolutionary and allegedly irreligious character in his teaching with the following proud and exalted sentence: "Eighteen months ago the first dawn rose for me, three months ago the bright day, and a few days ago the full sun of a most wonderful vision; now nothing can keep me back. I let myself go in divine rage. I defy the mortals with scorn by an open confession. I have stolen the golden vessels of the Egyptians to make out of them a holy tabernacle for my God, far away from