2020-8-13 · (astronomy, historical) The outermost celestial sphere of the heavens in Ptolemaic astronomy, which was believed to cause all the inner spheres to rotate. [1814], Didacus Placidus de Titus [i.e., Placido Titi], "Theses, from the First Book of the Author''s ''Celestial Philosophy''", in John Cooper, transl., Primum Mobile, with Theses to the ...