Kepler is one of history’s most admired astronomers, who famously discovered that planets move in ellipses and defined the three laws of planetary motion. In 1615, at the height of his career, his widowed mother Katharina was accused of witchcraft; the proceedings led to a criminal trial that lasted six years. Kepler conducted his mother’s defence. The trial and the arguments advanced give a revealing picture of Europe on the cusp between the Reformation and the scientific revolution that was to follow.