We introduce the use of in situ interactive three-dimensional (3-d) figures in digital astronomy textbooks as a means of enhancing student learning. The recent 3-d extensions to the Adobe Portable Document Format (PDF), combined with simple JavaScript, provide new ways to present intrinsically 3-d models, data sets, and instructional diagrams in digital publications. This is an enhancement to the usual method of presenting static, two-dimensional views, or "comic strip" sequences, to indicate

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... anges in viewpoint. Interactive figures provide opportunities for students to undertake active learning while reading a textbook: they are able to explore and uncover the connections between viewpoint, orientation, and the 3d nature of models and data sets for themselves.