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Solar System Models Lab — Instructor Resources


The NAAP Solar System Models Lab pedagogical objectives introduces the universe as envisioned by early thinkers culminating in a detailed look at the Copernican model.


Course Integration

Students working on the Solar System Models Lab need to have no additional knowledge beyond what they are expected to have when coming into the course. This segment works best during historical background portions of a course and before the introduction of Keplerian astronomy.


  • For Venus, the epicycle moves around the deferent such that the equant-epicycle line is fixed to be parallel to the earth-sun line.
  • Mercury is not shown as a preset because it involves a more complex system of circles. The Mercurial epicycle moves around a deferent which is a fixed distance not from the equant, but from a point on a sub-deferent with a diameter equal to the earth-equant distance. As with Venus, the equant-epicycle line is parallel to the earth-sun line.
  • For the Superior planets, the epicycle-planet line is fixed to be parallel to the earth-sun line.
  • The Moon was perhaps the most difficult to model and the Almagest presented more than one model. The final model is much like the Mercurial model but a zero-distance equant point and a sub-deferent of a fixed size which counter-rotated. The Moon is also not shown as preset.

It is important to note that the Ptolemaic system was revisited and revised by Arabic astronomers. In particular, they attempted to remove as much as possible the non-circular elements of the Ptolemaic model and to a large degree were successful. Copernicus, who particularly disliked the non-circular elements of the Ptolemaic model appears to have been aware of some of the Arabic methods and adopted an Arabic epicycle within epicycle model for his lunar model (the Ibn ash-Shatir model).