Ptolemy wrote the Planetary Hypotheses for both astronomers and instrument-makers. Most studies of this text concentrate on its meaning for the former, but there remain many questions surrounding its meaning for the latter.1 This article investigates the purpose of Ptolemy’s Planetary Hypotheses in light of what he says about instrument-making. It takes up the following questions: what kind of instrument does Ptolemy describe? And, could such an instrument have been constructed? I argue that he did not have one specific design (...) in mind, but instead he offered craftsmen options for the content, design, and operational methods. In addition to exhibiting his astronomical models and estimating planetary positions, the... (shrink)

The gravitational influence of Jupiter on Saturn produces, among other things, non-negligible changes in the eccentricity of Saturn that affect the magnitude of error of Ptolemaic astronomy. The value that Ptolemy obtained for the eccentricity of Saturn is a good approximation of the real eccentricity—including the perturbation of Jupiter—that Saturn had during the time of Ptolemy's planetary observations or a bit earlier. Therefore, it seems more probable that the observations used for obtaining the eccentricity of Saturn were done near Ptolemy’s (...) time, and rather unlikely earlier than the first century AD. Even if this is not quite a demonstration that Ptolemy used observations of his own, my argument increases its probability and practically discards the idea that Ptolemy borrowed values or observations from astronomers further back than the first century AD, such as Hipparchus or the Babylonians. (shrink)