Johannes Kepler Defended His Mother in a Witchcraft Trial

The famous astronomer conducted his mother’s defense in an ordeal that lasted six years.

Witch trial, ca. 1598.

Johannes Kepler, the German mathematician who worked out the laws of planetary motion—one of the great achievements of renaissance science—became embroiled in his middle age in a battle between reason and superstition. It wasn’t his own ideas that were on trial, however.

In 1615, 24 witnesses accused Katharina Kepler, the astronomer’s mother, of being a witch. The evidence: She magically appeared through closed doors; paralyzed the schoolmaster with a drink of wine; and hit a young girl on the arm, causing inhuman pain. Even Katharina’s son Heinrich claimed that she had “ridden a calf to death and prepared him a roast dish from it, [and] he himself wanted to accuse her before the authorities.” Because of these charges and others, the elderly Katharina was chained to the floor of a prison cell, where she was watched by two guards.

Kepler, then the imperial mathematician to Emperor Rudolf II, took over his mother’s legal defense, not realizing the case would go on for six years. Katharina’s story is the subject of a new book, The Astronomer and the Witch (Oxford, 2015), by Cambridge professor Ulinka Rublack. 

Of the 73,000 men and women tried for witchcraft in Europe, nearly 50,000 were executed, according to Rublack. Of those, almost 25,000 “were executed within the boundaries of present-day Germany between 1560 and [1700]. Seventy-five percent of those accused in Germany were female.” The accusation of Katharina Kepler tore the family apart: Christoph, another of her sons, withdrew his support completely as the case went to trial. Katharina’s daughter, Margaretha, wasn’t able to leave her family to care for her imprisoned mother, leaving Katharina “lonely, and bereft of comfort.” Even Johannes expressed ambivalence, saying his mother was “the author of her own lamentable misfortune.” Because Katharina was a widow, more than 70 years old (nearly the oldest woman in her town), and had a sharp tongue, she made a likely suspect.  

Despite his doubts, Johannes helped his mother in various ways: He requested that she be removed from the prison and housed in the quarters of the civic guard, at her own expense, which improved her conditions greatly. Johannes uprooted his own family and put aside his work for nearly a year to defend Katharina during her trial. As befitted a mathematician, Johannes used logic and reason to combat the rumors and lies spread about his client.

The astronomer defended his mother well; after 14 months of incarceration, Katharina was absolved of all charges, but ill-will was running so high, she was forbidden to return to her village. She died six months later. Johannes never told his friends and colleagues why he had been called away from his scientific work; the stigma of a witchcraft accusation was too great. 

Rublack theorizes that Kepler’s intense feelings over Katharina’s death may have stemmed from remorse that he may have inadvertently caused his mother’s ordeal. In 1609 he wrote what would eventually be published as Somnium (The Dream). The book’s preface combined real and imagined elements, including a narrator whose mother is a witch who consults with a demon to learn how to travel to the moon. A copy of the manuscript circulated in 1611, and Kepler later wrote that when its subject matter “was taken up by senseless minds, it flared up into defamation, fanned by ignorance and superstition.” The book was finally published four years after Kepler’s death. Every sentence contained a footnote, says Rublack, ostensibly to show that the book’s plot had nothing to do with himself and his real-life mother.

Get the latest stories in your inbox every weekday.