Here's a quick simulation of
why the heliocentric Copernican model
predicts retrograde motion, as opposed to it being something accounted
for in the Greek geocentric models by epicycles. Just move the mouse over the area to control the Earth --- Mars moves automatically at the right (slower) rate, so it's easy to see what happens when we overtake it during our orbit around the Sun. The
apparent position of Mars relative to some background stars is at top. Mars
appears to move backwards at
opposition because of our relative positions and the
parallax of our sight-lines.
I've found over the last couple of semesters that students seem to "get it" much more quickly when they play with this for just a few seconds as opposed to looking at a static image as they'd usually see in a book.
This will ultimately be included in a longer article about orbits in general.
Copernican Simulator --- Retrograde Motion
