The Planetary Hypotheses (Ancient Greek: Ὑποθέσεις τῶν πλανωμένων, lit. "Hypotheses of the Planets") is a cosmological work, probably one of the last written by Ptolemy, in two books dealing with the structure of the universe and the laws that govern celestial motion.[40] Ptolemy goes beyond the mathematical models of the Almagest to present a physical realization of the universe as a set of nested spheres,[41] in which he used the epicycles of his planetary model to compute the dimensions of the universe. He estimated the Sun was at an average distance of 1,210 Earth radii (now known to actually be ~23,450 radii), while the radius of the sphere of the fixed stars was 20,000 times the radius of the Earth.[42]
The work is also notable for having descriptions on how to build instruments to depict the planets and their movements from a geocentric perspective, much like an orrery would have done for a heliocentric one, presumably for didactic purposes.[43]