The Tübingen Astronomer Who Taught Kepler That Copernicus Was Right

Michael Maestlin (1550–1631) was the senior astronomer of the University of Tübingen — the flagship Lutheran university of the Duchy of Württemberg — for almost half a century. His scholarly output was modest by 16th-century academic standards: a handful of published treatises on specific astronomical observations, a heavily-used astronomical-mathematical textbook (Epitome Astronomiae, 1582), and a substantial body of unpublished correspondence and manuscript material. He never produced the kind of major synthesizing work that would have given him a first-rank reputation among his contemporaries.

He had two significant accomplishments. The first was his 1578 publication of the canonical observation of the Great Comet of 1577 — the work that, together with Tycho Brahe’s parallel publication, established that comets travelled above the moon and through the supposedly unchanging Aristotelian crystal spheres. The second, much more consequential, was that he taught the young Johannes Kepler that the Copernican heliocentric system was probably correct, in a Lutheran institutional context where openly Copernican teaching was professionally dangerous.

The Tübingen position

Maestlin had been a student at Tübingen in the late 1560s under the previous senior astronomer Philipp Apian. He took a Master’s degree in 1571, served briefly as a Lutheran pastor at Backnang (a small Württemberg town northeast of Stuttgart), and was appointed to the Heidelberg University mathematics chair in 1580. The Tübingen chair came open in 1583 and he returned to his alma mater, where he remained until his death in 1631.

The Tübingen mathematics-astronomy curriculum of the 1580s and 1590s was substantively conservative. The official teaching followed the Ptolemaic geocentric model — the standard university astronomy textbook for the entire German Lutheran higher-education system, Sacrobosco’s De Sphaera (originally written in the 1230s, with successive 16th-century commentaries), assumed a geocentric universe. The Lutheran theological-academic establishment was substantively hostile to Copernican teaching: Luther himself had reportedly called Copernicus “the fool who would turn the whole science of astronomy upside down”; the senior Lutheran theologians of the late 16th century treated heliocentrism as both empirically dubious and theologically problematic (it was difficult to reconcile with several specific biblical passages that referred to the sun’s motion).

Maestlin nonetheless privately considered Copernicus’s heliocentric system to be correct. He had read Copernicus’s De Revolutionibus (1543) carefully as a student in the late 1560s and had concluded — substantially on the strength of Copernicus’s mathematical-aesthetic argument that the heliocentric model produced a simpler and more harmonious account of planetary motion than the Ptolemaic alternative — that the heliocentric framework was the better physical theory.

He could not teach this publicly. The Tübingen Lutheran consistory had explicit oversight of the university curriculum; openly Copernican teaching would have triggered substantial institutional consequences. Maestlin therefore developed his characteristic dual teaching practice: the public lectures and the textbook taught the Ptolemaic geocentric system in the standard format; the private tutorials with selected advanced students (the brightest fraction of each year’s mathematics graduates) included the Copernican alternative as the substantively better theory.

Kepler

Johannes Kepler arrived at Tübingen as an undergraduate in 1589, aged 17. He was a scholarship student from the small town of Weil der Stadt, with a substantial mathematical aptitude that the local Lutheran pastor had spotted early. He took Maestlin’s basic astronomy course in his first year and was selected for Maestlin’s private advanced tutorial group in his second.

The intellectual transmission was substantively complete by the time Kepler graduated with his Master’s degree in 1591. He had been taught the Copernican heliocentric system as the better physical theory; he had been taught Maestlin’s specific argument about why the mathematical structure of the heliocentric model was preferable to the Ptolemaic alternative; he had been taught the specific astronomical-observational evidence (including Tycho Brahe’s 1572 supernova and the 1577 Great Comet) that had begun to dismantle the medieval Aristotelian cosmology that Ptolemy assumed.

Kepler was placed by the Tübingen consistory in a teaching position at Graz in 1594. His first major book, Mysterium Cosmographicum (1596), explicitly defended the Copernican system and articulated the geometric-Platonic theory of planetary spacing (the five Platonic solids nested between the six planetary orbits) that would dominate his thinking for the next thirty years. The book was dedicated to Maestlin and substantially organised around the Copernican-pedagogical framework that Maestlin had taught him.

The Tübingen Lutheran consistory’s response to the Mysterium Cosmographicum was negative: the Copernican content of the book made it institutionally awkward for the consistory to endorse. Maestlin substantially shielded Kepler from the worst of the consequential professional damage and continued, through the following two decades, to provide his former student with mathematical support and intellectual collaboration through their surviving correspondence.

The Württemberg cases

Maestlin’s later career included a involvement in the Württemberg witch trials — not as a witch-hunter, but as a partial defender of the accused. He provided expert mathematical-astronomical testimony in several specific cases in the 1610s and 1620s in which witchcraft accusations had relied on dubious astronomical-astrological calculations; his testimony substantially undermined the prosecution claims and produced several documented acquittals.

He was peripherally involved in the Katharina Kepler witch trial of 1620–1621 through his sustained professional relationship with Johannes Kepler and his Tübingen institutional position. He did not appear as a witness at Katharina’s trial — Kepler conducted the defence personally — but he provided advisory support through the proceedings and corresponded with the Tübingen lawyer Christoph Besold who was the formal legal advisor on the defence.

Maestlin died at Tübingen on 30 October 1631, aged 81. He had taught at the institution for 48 years. The Lutheran-Württemberg cultural-political environment that had forced him to teach the Copernican system in private tutorial rather than in public lecture had not significantly relaxed during his lifetime; his student Kepler had been forced out of Catholic Graz in 1600 by the Counter-Reformation, and most of his subsequent generation of central European astronomers had similar professional difficulties. The institutional acceptance of heliocentrism that Maestlin had spent half a century privately defending would not arrive in the German universities until the post-Newtonian period of the late 17th and early 18th centuries.

His unpublished correspondence and manuscript material — about 800 letters and several thousand pages of working notes — survives at the Stuttgart and Tübingen state archives. The Kepler-Maestlin correspondence, in particular, is one of the most consequential surviving German astronomical archives. Kepler’s Astronomia Nova (1609) and Harmonices Mundi (1619) — the two foundational works on the elliptical-orbit law and the equal-areas law of planetary motion — both contain extensive references to the mathematical-pedagogical framework that Maestlin had set up in the Tübingen tutorials thirty years earlier.

The student substantially eclipsed the teacher in the historical reputation. The teacher had nonetheless transmitted the framework on which the student would build.