The Ejs Copernican System model illustrates Copernicus' system of planetary motions. The entire system is centered on the center of Earth's uniform, circular orbit. Sun is placed near, but not at, this center point. The orbit of each planet (other than Earth) consists of a deferent circle, centered on a point some distance from the center (at the eccentric point). Attached to this deferent is the center of a much smaller circle, the epicycle (or epicyclet). The radius of the epicycle is 1/3 the eccentricity of the deferent. The planet moves along the epicycle at a constant angular speed equal to twice the angular speed along the deferent. This model produces retrograde motion and changes in brightness that are always properly correlated with the location of Sun. In this simulation, the planet is assumed to move in the plane of the ecliptic, so its latitude is always zero. You can modify this simulation if you have Ejs installed by right-clicking within the plot and selecting "Open Ejs Model" from the pop-up menu item.

Ejs Copernican System model was created using the Easy Java Simulations  (Ejs) modeling tool.  It is distributed as a ready-to-run (compiled) Java archive.   Double clicking the ejs_astronomy_CopernicanSystem.jar file will run the program if  Java is installed.  Ejs is a part of the Open Source Physics Project and is designed to make it easier to access, modify, and generate computer models.  Additional Ejs models for astronomy are available.  They can be found by searching ComPADRE for  Open Source Physics, OSP, or Ejs.

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