PtolemyCopernicusTycho


General Description

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.

Note that the version of the Ptolemaic system shown has been scaled so that the deferent of the inferior planets, and epicycle of the superior planet, are the same size as the sun's orbit. The Ptolemaic system does not set the absolute size of the planetary orbits, just the relative sizes of the deferent and epicycle, so scaling the system in this way is mathematically legitimate (although Ptolemy never would have done it, since it causes the orbits of the planets to overlap with that of the sun).


Comparison Frame


Sky View Frame


Todd K. Timberlake (ttimberlake@berry.edu)