Kepler, several months after departing Graz, has returned, now with the rest of his household, to Prague.

Through most of 1601, he is supported directly by Tycho, who has assigned him to analyzing planetary observations and writing a tract against Tycho's (by then deceased) rival, Ursus.

Tycho had in September secured him a commission as a collaborator on the new project he had proposed to the emperor: the Rudolphine Tables that should replace the Prutenic Tables of Erasmus Reinhold.

Two days after Tycho's unexpected death on October 24, 1601, Kepler is appointed his successor as imperial mathematician with the responsibility to complete his unfinished work.

The next eleven years as imperial mathematician are to be be the most productive of his life.