German astronomer who discovered three laws of planetary motion had a mother imprisoned on witchcraft charges; in 1620 he put science on hold and penned a 128-page defense that freed her

Johannes Kepler (1571–1630) is remembered for discovering the three laws of planetary motion, helping establish the idea that planets move around the Sun in elliptical orbits. But during the height of his scientific career, Kepler faced a very different challenge on Earth: his elderly mother, Katharina Kepler, was accused of witchcraft.

For years, he would divide his attention between studying the heavens and defending his mother from a legal case that could have ended with her execution.

A witch-hunting panic reaches Kepler’s mother

The accusations against Katharina began in Leonberg, Germany, in 1615, when witch-hunting hysteria was a demonic practice across Europe, and accusations of witchcraft could lead to torture and execution. According to the American Physical Society(APS), local magistrate Lutherus Einhorn oversaw a series of witch trials in which 15 women were accused, and eight were executed. Katharina became caught up in the same atmosphere of suspicion.

A local woman, Ursula Reingold, accused Katharina of poisoning her with a potion. Katharina responded by suing Reingold for slander. Katharina's reputation did little to protect her. The APS described her as a local “wise woman” who prepared herbal potions, while her difficult temperament and family history contributed to suspicions about her.

But historian Ulinka Rublack's research presents a more complicated picture.

The woman behind the witchcraft accusations

Rublack examined the surviving records from the town where Katharina lived and found that some long-repeated stories about her were not supported by evidence. There was, she says, no evidence that Katharina had been raised by an aunt who was burned for witchcraft, despite this claim appearing in accounts of her life.

There was also no evidence that she made a living as a healer. She mixed herbal drinks for herself and sometimes helped others, but Rublack argues that this was very different from the image of a professional witch or magical healer.

Over time, however, accusations and rumors gathered around her. By 1620, the situation had become much more serious.

Kepler puts his scientific life on hold

In August 1620, Katharina was taken from her daughter's home and imprisoned on accusations of witchcraft. The American Physical Society says she faced 49 accusations. Rublack describes her as a woman in her late 70s who eventually spent more than a year chained to the floor of her prison.

Her son decided to intervene personally. Kepler hired lawyers and took over much of the defense. According to Rublack, he stored his books, papers and scientific instruments in boxes, moved his family to southern Germany and spent nearly a year trying to secure his mother's release.

This was not a minor interruption to his life. Kepler was then one of Europe's most important astronomers.

He had defended Copernicus's Sun-centered universe and discovered that planets move in ellipses. In 1619, he published Harmonices Mundi, which contained his third law of planetary motion. Yet while his scientific work was transforming astronomy, he was simultaneously fighting a legal battle to save his mother.

Kepler for the defense

Kepler's defense became the center of the case. The APS says he wrote most of a 128-page statement challenging the prosecution's arguments. Rublack calls his defense a “rhetorical masterpiece.”

He attacked inconsistencies in the accusations and argued that illnesses blamed on magic could instead be explained through medical knowledge and common sense.

The case lasted six years. Eventually, it reached the judicial college of the University of Tübingen. The judges found insufficient evidence to establish either guilt or innocence. But instead of simply releasing Katharina, they decided to frighten her into confessing.

Katharina refuses to confess

The authorities used a procedure known as territio verbalis. Katharina was shown instruments of torture, including hot irons, pincers, needles, the rack and a gallows and was given a graphic explanation of how they could be used.

She still refused to confess. The American Physical Society records her defiant response, “Do with me what you want. Even if you were to pull one vein after another out of my body, I would have nothing to admit.”

She then knelt and recited the Pater Noster. Katharina's refusal to confess ultimately helped bring the ordeal to an end.

The astronomer saves his mother

In the autumn of 1621, Katharina was finally released by order of the Duke of Württemberg. The six-year ordeal was over. She died the following year.

Kepler would live until 1630, leaving behind discoveries that transformed astronomy. But the story of his mother reveals another side of the famous astronomer.

He was not simply a man looking toward the stars. When his elderly mother faced imprisonment and the threat of torture, he put much of his scientific life aside and devoted himself to defending her.

His three laws of planetary motion changed humanity's understanding of the heavens. But during those same years, Kepler was engaged in another struggle, one much closer to home.