Fabricius's theory for Mars: The model that shocked Kepler

David Fabricius is recognized in the history of astronomy for his role in the discoveries of sunspots and the first variable star. As the around 50 letters between them show, he also played a significant role as Kepler's interlocutor when the latter was writing Astronomia Nova. One year before the publication of Astronomia Nova, Fabricius shared with Kepler a new model for Mars that he had developed. The model shocked Kepler because, using the traditional tools of circles and uniform motion, Fabricius had been able to find a model as accurate as Kepler's model involving his first two laws. The official story, based on the reconstruction of Fabricius's model that Apelt made in 1852, asserts that Kepler need not worry because, in fact, he misinterpreted the model. The true model that Fabricius proposed was not predictively accurate. In this paper I offer a new interpretation of Fabricius's model that shows that Kepler did indeed have reason to worry since Fabricius's model was extraordinarily accurate.