This work is Weigel's main scientific work on astronomy. Its subject is spherical astronomy ('Sphaerica'), which, using the mathematical model of the sphere, deals with the coordinate systems for measuring the apparent sky. It forms the general part of astronomy, which is defined as the 'science of the quantity of phenomena as they appear to earthlings in the sky'. The work is the first to apply Weigel's Aristotelian-Euclidean method, which bases the universal claim of the 'mos geometricus' on a reinterpretation of Aristotelian analytics, to an individual science.