This simulator models the movement of planets around the sun in a simplified Ptolemaic model of the solar system, in which the Earth is motionless near the center.
In this system, the sun circles the Earth once per year. Planets move on a large loop around the Earth - the deferent - and upon a smaller loop called the epicycle. In this model, the rate of the planet's motion was set to match the average time it took to complete a full cycle through the zodiacal constellations (prograde motion), and the epicycle size and rotation rate were set to match the planet's observed occasional retrograde motion.
See how different motion rates and epicycle sizes were used to model the planets' apparent motion across the zodiac (displayed below).
The original simulation, along with related teaching materials, may be found at: https://astro.unl.edu
This simulator is part of the Foothill AstroSims project, which is aiming to reimplement, in HTML5, Flash-based simulations that are used in Foothill Astronomy courses.
Similar projects include work done by Nik Nyby at Columbia University (https://github.com/ccnmtl/astro-simulations) and Kevin Ditts at the University of New Mexico (https://physics.unm.edu/Courses/Rand/applets/).
Funding for Foothill AstroSims provided by an Equity & Innovation Grant from Foothill College.