Accurata delineatio motuum stellae martis: How accurate is kepler’s pretzel diagram?

Abstract

In the first chapter of his Astronomia Nova, Kepler introduced a diagram that, he claimed, represents the path of Mars between 1580 and 1596, assuming a resting Earth. In the words of William H. Donahue, “the appearance of this diagram is a dramatic moment in the history of thought. Nothing like it had ever been published before.” It is unique mainly because it is the first diagram that offers what is explicitly asserted to be an accurate representation of the trajectory of a planet, the first diagram in which the path of a planet appeared clearly separated from the geometrical devices that produced it. This essay aims to offer an analysis of the accuracy of this diagram. The authors try to determine what tables, ephemerides, observations, or models are behind its construction and how accurately the diagram reflects the different elements of the trajectory of Mars. The outcome of this analysis raises the question of whether the diagram was made for the role it ultimately played in the Astronomia Nova or for some other purpose.