The executioner spread his instruments on the table before the old woman—thumbscrews, leg vices, the strappado rope—and her son, the greatest astronomer in Europe, could only watch.

The Astronomer Who Watched His Own Execution: Johann Kepler's Mother's Trial

When Europe's Greatest Scientist Fought to Save a Witch

Johannes Kepler, discoverer of planetary motion, spent fourteen months defending his elderly mother against witchcraft charges.

The cell stank of damp straw and human waste. Katharina Kepler, seventy-three years old and barely able to stand, pressed her back against the cold stone wall of the Güglingen tower as the executioner entered with his instruments. It was June 7, 1621, and the most feared moment of the witch trial had arrived: the territio verbalis, the formal threat of torture. The executioner laid out his tools before her eyes—thumbscrews, leg vices, the strappado rope—and demanded she confess to witchcraft.

She refused.

Her son Johannes, the Imperial Mathematician who had discovered the laws of planetary motion, had abandoned his work in Linz and traveled hundreds of miles to defend her. For fourteen months, he had filed legal briefs, cross-examined witnesses, and dissected the accusations with the same precision he applied to astronomical observations. The charges were absurd yet deadly serious: neighbors claimed Katharina had made a woman lame by touching her, killed infants with her evil eye, and brewed poisonous potions from herbs she gathered at midnight.

The prosecution had accumulated forty-nine witnesses against her. The case had begun six years earlier, in 1615, when a former friend n…

💡 Kepler's 128-page legal defense of his mother is considered one of the earliest systematic critiques of witch trial procedures and influenced later legal reforms.