This paper describes a series of activities in which students investigate and use the Ptolemaic, Copernican, and Tychonic models of planetary motion. The activities guide students through using open source software to discover important observational facts, learn the necessary vocabulary, understand the fundamental properties of different theoretical models, and relate the theoretical models to observational data. After completing these activities, students can make observations of a fictitious solar system and use those observations to construct models for that system. (Contains 1 table, 6 figures and 2 footnotes.)