The Astronomer Who Defended His Mother From the Stake

On 7 August 1620, in the small town of Leonberg in the Duchy of Württemberg, the local Vogt — the ducal magistrate, a man named Lutherus Einhorn — issued an arrest warrant for the seventy-four-year-old widow Katharina Kepler on charges of witchcraft. The charges had been pending for five years. The complaint had originally been lodged in 1615 by a neighbor, a glazier’s wife named Ursula Reinbold, who claimed that Katharina had given her a hot drink that made her ill, and who had subsequently developed a chronic pain that local opinion attributed to magical injury. The Vogt had been gathering depositions from other neighbors for five years. The dossier ran to forty-nine charges.

Katharina Kepler was the mother of Johannes Kepler, then forty-eight years old, court mathematician to the Holy Roman Emperor and the most accomplished astronomer in Europe. Kepler heard about the arrest at his house in Linz, four hundred kilometers from Leonberg, several weeks after it happened. He took an unpaid leave from the imperial court, packed up his entire household — his wife, their seven children, and a sizeable mathematical library — and traveled to Württemberg to defend his mother.

He stayed for fourteen months. He wrote a 411-page legal defense. He argued the case in person before the ducal court at Stuttgart. His mother spent that winter chained to the floor of a cell in Güglingen, where she had been transferred for confinement, and was at one point led to the torture chamber and shown the instruments — a ducal procedure called territio verbalis, meant to break a confession without actually inflicting injury. She did not confess. She was 75 years old. She was acquitted on 4 October 1621. She died six months later.

This is, by reasonable measure, the most fully documented witchcraft trial in early modern Europe, and the most extraordinary thing about it is that the defendant won.

The forty-nine charges

The trial dossier, preserved in the Leonberg city archive, contains forty-nine specific accusations gathered from twenty-four witnesses between 1615 and 1620. They were not unusual for the genre. Katharina had cured a man’s foot with an unspecified ointment. She had asked a tailor to make her a glass dish for her drinks. She had touched a child on the cheek and the child had subsequently fallen sick. She had been seen at midnight near a well. She had ridden, on one occasion, a calf. She had been seen, by Ursula Reinbold, walking through walls.

Many of the accusations had a structural pattern that historians of European witch trials would recognize. Katharina had asked neighbors for small favors — a bowl of milk, the loan of a knife, a sip of wine — and the neighbors had refused, and shortly afterward the neighbor or someone in the neighbor’s family had become ill. The pattern is the standard maleficium-by-grudge model: the witch is the person you have insulted and who has therefore cursed you. Modern historians of witch-trial epidemics across Europe in the sixteenth and seventeenth centuries — Christina Larner, Robin Briggs, Brian Levack, more recently Ulinka Rublack — have shown that the pattern is socially predictable, particularly for elderly widows of unfortunate temperament living in small communities.

Katharina Kepler matched the social profile almost too perfectly. She was a widow (her husband, Heinrich, a mercenary soldier, had walked out on the family decades earlier and never returned). She was poor. She had a difficult personality — Johannes himself, in a private letter to a friend, described her as “small, thin, swarthy, gossiping, and quarrelsome, of a bad disposition.” She was the kind of person who would, when refused a small favor, hold a grudge audibly and at length. She had, over the years, made many enemies.

Her enemies, when the trial began, organized themselves into a coordinated complaint. The Vogt, Einhorn, was sympathetic to them — he disliked the Kepler family and had political reasons to want them humiliated. The case proceeded.

Why Kepler intervened

Kepler had been raised by this mother. She had taken him at age six to see the Great Comet of 1577 — a memory he later cited as the moment his interest in astronomy began. She had supported the family through her husband’s desertion, working as an herbalist and seller of small medicinal preparations. The forty-nine charges against her, when Kepler read them in 1620, were drawn substantially from the same herbalist practice that had fed him as a child.

He understood the legal situation precisely. Witch trials in Württemberg were governed by the Carolina, the imperial criminal code of 1532, which required physical or testimonial evidence and forbade the use of torture without prior corroboration. A determined defense, mounted by a literate person with legal training, could in principle force a court to acquit even in the presence of multiple accusers. Kepler had no legal training but he had access to one — his old university friend Christoph Besold, then a professor of law at Tübingen, who agreed to advise him on the case and to draft the formal legal motions.

The defense Kepler assembled, which survives as a 411-page manuscript in the Württemberg state library, addressed each of the forty-nine charges in turn. For most of them, the response was systematic and tedious. Each accusation was matched against the standards of the Carolina: was there physical evidence? Were there corroborating witnesses? Was the alleged supernatural effect explicable by ordinary causes? Where ordinary causes were plausible (a sick child who recovered, a temporary illness, a coincidence of timing), Kepler offered them. Where they were not, he argued that the accuser’s testimony was unreliable for specific documented reasons — Ursula Reinbold, for example, was suing the Keplers for unrelated financial debts, which Kepler showed had created a clear motive to fabricate.

The defense was not philosophical. Kepler did not argue that witchcraft did not exist. He believed, in the manner of educated Europeans of his time, that it might exist; that demons could in principle interact with humans; that the question of whether his mother was a witch was an empirical one to be resolved by examining the evidence. The evidence, he argued in document after document, did not support the accusation.

What the court did

The Stuttgart court, after fourteen months of confinement and several months of deliberation, accepted the defense. The ruling, dated 4 October 1621, found that the charges against Katharina Kepler had not been substantiated to the Carolina’s evidentiary standard. She was released. She walked out of the Güglingen prison on her own, unaided, and lived for another six months in her son’s house. She died on 13 April 1622, of what her son’s letters describe as “weakness.”

The Vogt Einhorn was not reprimanded. He continued in office. The accusers were not punished. Ursula Reinbold continued to live in Leonberg and continued to claim, until her own death several years later, that Katharina had ruined her health.

Kepler returned to his work in Linz with his family in late 1621, finished the third book of his Epitome of Copernican Astronomy, and published the Harmonices Mundi — the work in which the third law of planetary motion is stated — within a year of his mother’s death. He never wrote a separate account of the trial. He referred to it only in private correspondence and in the occasional dedication.

The 411-page manuscript was donated, after Kepler’s death in 1630, to the Württemberg ducal library by his second wife. It has been catalogued there since the seventeenth century. It is in clear, surprisingly cheerful Latin. Only one full English translation exists, and it is incomplete; the standard scholarly access remains Rublack’s 2015 monograph, which renders selected passages with commentary.

The exact spot in Güglingen where Katharina was held has not been identified. The prison no longer exists. There is no plaque.

In the church register of Heumaden, the small village outside Leonberg where she is buried, the entry for her death reads simply Catharina Keplerin, alt 76 Jahr — Katharina Kepler, age 76. There is no mention of the trial. There is no mention of the witnesses, the Vogt, or the man who saved her.