11/18/23, 2:12 PM
ASTR 100 B FA23: Lesson 2.1: Dawn of the Heliocentric Model: 6. Retrograde Motion in the Geocentric Model | Learn@Illinois
https://learn.illinois.edu/mod/lesson/view.php?id=8087864&pageid=1727870
2/2
You find the video above, which explains how ancient astronomers tweaked the simple geocentric model to account for the observed
retrograde motion of the planets. To make the model work, it assumes that the planets move in small circles called epicycles, and the
centers of the epicycles move around the Earth in orbits called deferents.
As you can see in the figure below, the combined motions of deferent and epicycle produce retrograde motion. The geocentric model
was saved! At least for now...
A geocentric model of a planet’s orbit around Earth (blue dot), showing the planet’s epicycle and deferent. Note the period of “backwards” motion at the bottom of the loop.
Richard W. Pogge, The Ohio State
University
