The Correspondence of Johannes Kepler

Primary Contributors:

Francesco Barreca, based on metadata collated by Adam Mosley

Portrait of Johannes Kepler, by an unknown artist. Copy of a lost original of 1610. (Stift Kremsmünster; source of image: Wikimedia Commons)

Johannes Kepler (1571–1630)

Johannes Kepler, a key figure in the development of modern cosmology and astronomy, is renowned most today for the three planetary laws bearing his name. He was born in Weil der Stadt on 27 December 1571, the son of a mercenary soldier who left the family when he was five years old and he spent his childhood living with his mother, Katharina Guldenmann, his grandparents, and his brothers and sisters. The family moved in 1576 to Leonberg, where Johannes attended the local Latin school. In 1584 he was admitted to the Lutheran Seminary of Adelberg and, on 17 September 1589, he enrolled in the University of Tübingen as a scholarship student with the intention of becoming a theologian and churchman.

After graduating from the Faculty of Arts, Kepler was recommended by the University for a position as teacher of mathematics in Graz. Albeit reluctantly, he accepted the post and in 1594 moved to Styria. While teaching mathematics and astronomy in Graz, he became convinced that God’s plan for the Universe was essentially geometrical and could be understood through a priori reasoning. ‘I wanted to become a theologian’, he wrote in 1595 to Michael Maestlin, the Tübingen professor of mathematics who had introduced him to Copernicanism, ‘and for a long time I was restless. Now however, behold how God is being celebrated in astronomy.’ The results of this line of research were summarised in Mysterium Cosmographicum, in which, by inscribing and circumscribing the orbits of the five Planets about the five Platonic solids (i.e. Tetrahedron, Cube, Octahedron, Dodecahedron, and Icosahedron), he argued that the structure of the Universe was inherently geometric.

Mysterium Cosmographicum, published in 1596, earned him fame as one of the most promising astronomers of his time, allowing him to get in touch with such mathematicians and astronomers as Giovanni Antonio Magini, Galileo Galilei, Raimarus Ursus, and, above all, Tycho Brahe, who was at that point about to resettle as Imperial Mathematician in Prague. Over the course of almost thirty years, Brahe had gathered a huge amount of extremely accurate astronomical observations, which, as Brahe kept them safely guarded, were coveted by astronomers all around Europe, with Kepler being no exception. ‘Brahe has abundant wealth’, he wrote to Maestlin in 1599. ‘Only, like most rich men, he does not know how to make proper use of his riches. Therefore, one must take pains to wring his treasures from him.’ Kepler met Brahe for the first time in February 1600, and in August was appointed as assistant. By that time, he had married Barbara Müller, with whom he had had two children, and, by refusing to convert to Catholicism, had been forced to leave Graz.

In Prague, far from being granted unrestricted access to ‘Tycho’s treasures’, Kepler was set to work on Mars’s orbit. It was only after Tycho’s unexpected death in 1601, and his subsequent appointment as the new Imperial Mathematician, that he was able to take full advantage of the data. During the twelve years of his service as Imperial Mathematician (1601–12), he published pivotal works such as Astronomiae Pars Optica (1604); De stella nova in pede Serpentarii (1604); Astronomia Nova (1609); and Dioptrice (1611). In Astronomia Nova, rejecting the widely accepted view of astronomy as a discipline committed to ‘saving the phenomena’, he demonstrated that the Planets move in elliptical orbits at varying speeds and assumed that a magnetic-like force emanating from the Sun carried them along their paths, laying down the groundwork for the development of modern astronomy. ‘I provide a philosophy, or rather a Celestial Physics, to replace celestial theology and Aristotle’s Metaphysics’, he wrote to physician Johann Georg Brennger in 1607. In 1610, he wrote Dissertatio cum Nuncio Sidereo, an open letter in which publicly he took Galileo’s side in the disputes that followed the publication of Sidereus Nuncius.

After Emperor Rudolf’s death in 1612, Kepler moved to Linz. His wife, Barbara, had died the previous year, and in 1613 he married Susanna Reutinger. Of the five children from his first marriage, only two survived into adulthood, and of the seven from his second marriage, only one survived. In Linz, Kepler, working as district mathematician, completed Epitome astronomiae Copernicanae (1618–21); Harmonices mundi (1619); a new edition of Mysterium Cosmographicum (1621); Tabulae Rudolphinae (1627); and Somnium (published posthumously in 1634). These were projects he had been working on from early in his career: Epitome was intended as a Copernican textbook; Harmonices mundi was a comprehensive treatise on harmony (in this treatise, Kepler enunciated the third law of planetary motions); Mysterium Cosmographicum revisited his geometrical ideas in the light of his more recent findings; Tabulae Rudolphinae, finally, marked completion of the work for which he had been appointed originally by Brahe. While in Linz, Kepler was called to Leonberg to defend his mother from charges of witchcraft.

As the Thirty Years War broke out in 1618, Kepler’s position in Linz grew increasingly uncomfortable and eventually the family was forced to leave the town in 1626. Two years later, Kepler was appointed official advisor by Albrecht von Wallenstein and he spent the following years with family at Sagan, busy preparing his late works for printing. He died in Regensburg on 15 November 1630. The epitaph he wrote for himself reads: ‘I measured the skies, now the shadows I measure / Skybound was the mind, earthbound the body rests’.

Partners and Additional Contributors

The metadata for this calendar of Kepler’s correspondence, based on the Gesammelte Werke (for further details, please see below), was compiled originally by Professor Adam Mosley in the course of his research on Tycho Brahe and was expanded prior to incorporation into EMLO by Dr Francesco Barreca of the Museo Galileo, Institute and Museum for the History of Science, Florence.

Thanks are extended to EMLO Digital Fellows Lucy Hennings and Callum Seddon for their work on the associated people and place name records, to Owen Hubbard and Charlotte Marique for their help to prepare the metadata for volumes 15 and 16, and to Charlotte Marique for her help with volume 17.

The Kommission zur Herausgabe der Werke von Johannes Kepler deserves most special thanks and credit for its achievement in providing useful resources for Keplerian studies.

Key Bibliographic Source(s)

Johannes Kepler. Gesammelte Werke, ed. Max Caspar, et al., currently 20 vols (Munich: C. H. Beck, 1938–).

Johannes Kepleri Astronomi opera omnia, ed. Christian Frisch, 9 vols (Frankofurti et Erlange: Heyder & Zimmer, 1858–1871).


When complete, this catalogue will feature 1,154 letters that range in date from 1590 to 1630 and are written mostly in Latin and German. The basic metadata have been derived and checked against volumes XIII–XVIII of Johannes Kepler Gesammelte Werke and full-text, searchable digital versions of these volumes may be found at Kommission zur Herausgabe der Werke von Johannes Kepler [under KGW Digital]; to aid consultation of the printed copies, a link is provided to the Kommission from each of Kepler’s letter records in EMLO.

Kepler’s correspondence offers an in-depth view of the astronomer’s life and work. Its contents vary greatly: there are letters dealing with family matters; others relate to astrological commitments and search for patronage, sometimes also touching upon political affairs and current events. The bulk, however, concern mathematical, astronomical, and philosophical topics. Correspondents include Michael Maestlin, Giovanni Antonio Magini, Herwart von Hohenburg, Galileo Galilei, Tycho Brahe, and Johannes Fabricius. Kepler was quite an elegant writer who liked to share and discuss his ideas with others; therefore, his correspondence represents a great resource for understanding the development of his researches. His correspondence with Frisian pastor and amateur astronomer David Fabricius, for example, offers precious insights into the elaboration of Astronomia Nova, and hints at the fact that he used letters as a means to procure feedback on his work. ‘I do not write letters to you, my dear Fabricius’, he wrote in 1605, ‘but the Commentaries [i.e. Astronomia nova] themselves, which I put off writing until so much time has elapsed that even I now scarcely grasp the sense of what I have written, unless I reread it carefully.’

Johannes Kepler’s signature. (Source of image: Wikimedia Commons)

Scope of Catalogue

Letters are numbered following the numeration in Johannes Kepler Gesammelte Werke. For each letter, a brief abstract, along with references to other letters, is provided. At present, the letters from volumes 13, 14, 15, 16, and 17 are contained in the catalogue; metadata for letters contained in subsequent volumes will be added in the course of 2019–20.


Further resources

Further bibliographic resources may be found at Kommission zur Herausgabe der Werke von Johannes Kepler.


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