The Ptolemy/Copernicus/Tycho EJS Model illustrates the relationships between the systems of planetary astronomy developed by Claudius Ptolemy, Nicholas Copernicus, and Tycho Brahe.  The model presents a simplified version of all three systems, showing the motions of the Sun/Earth and two planets (one inferior, one superior).  The model is initially set to model the planets Venus and Mars.  In addition, the apparent motion of the sun and two planets across the sky is displayed in another window.

This model can be used to illustrate the essential geometric equivalence between these three system.  In particular, it shows that the Earth's orbit (in the Copernican system) becomes the orbit of the sun in the Ptolemaic and Tychonic systems, and also appears as the deferent of Venus and epicycle of Mars in the Ptolemaic system.  The orbit of Venus about the sun (in the Copernican and Tychonic systems) becomes
the epicycle of Venus in the Ptolemaic system.  The orbit of Mars about the sun (in the Copernican and Tychonic systems) becomes the deferent of Mars in the Ptolemaic system.

Please note that this resource requires at least version 1.5 of Java (JRE).