Christoph Rothmann wrote a treatise on the comet of 1585 shortly after it disappeared. Though it was not printed until 1619, Rothman sent a copy of his treatise in 1586 to Tycho Brahe, decisively influencing the latter’s rejection of solid celestial spheres two years later. In his treatise, Rothmann joined the elimination of the solid celestial spheres to his concept of air as the substance filling the cosmos. He based his argument on the absence of refraction and the celestial location of the comet. The treatise also contained clear statements reflecting Rothmann’s adoption of Copernicanism. This first critical edition of the treatise is accompanied by an English translation and a thorough commentary. Some appendices with archival documents illustrate the genesis of Rothmann’s treatise.