At the same time (the 1320s and 1330s) that Des Murs was deeply involved, along with a small group of contemporaries, in adapting and implementing the innovative practices of Castilian astronomy for use in Paris, he was also engaged with a group of music theorists and composers, several of whom shared his interest in astronomy in implementing new methods for measuring time in polyphonic music. In this chapter, I consider two aspects of Des Murs’s writings on musica mensurabilis as they relate to his broader professional activity: (1) the implications of the revised chronology of Des Murs’s wider astronomical output for the datings of his music theory treatises; and (2) how the rhetoric articulated in Des Murs’s writings on astronomy, particularly the emphasis on the importance of observation and precise calculation, is also prominent in his work as a music theorist. Des Murs’s career is also illustrative of how the ars nova straddled two worlds: that of the professional (clerk, bureaucrat) working within the courtly environment (and one which prized novelty and subtilitas), and the scholarly environment of the Sorbonne with which Des Murs maintained an association for most of his life.