In Prague, Tycho worked closely with Johannes Kepler, his assistant. Kepler was a convinced Copernican, and considered Tycho's model to be mistaken, and derived from simple "inversion" of the Sun's and Earth's positions in the Copernican model. Together, the two worked on a new star catalogue based on his own accurate positions — this catalogue became the "Rudolphine Tables". Also at the court in Prague was the mathematician Nicolaus Reimers (Ursus), with whom Tycho had previously corresponded, and who, like Tycho, had developed a geo-heliocentric planetary model, which Tycho considered to have been plagiarized from his own. Kepler had previously spoken highly of Ursus, but now found himself in the problematic position of being employed by Tycho and having to defend his employer against Ursus' accusations, even though he disagreed with both of their planetary models. In 1600, he finished the tract "Apologia pro Tychone contra Ursum" (defense of Tycho against Ursus). Kepler had great respect for Tycho's methods and the accuracy of his observations and considered him to be the new Hipparchus, who would provide the foundation for a restoration of the science of astronomy.