This photo animation of Jupiter (brighter) and Saturn shows them each moving from west to east then appearing to change direction at nearly the same time and move from east to west (retrograde). Finally, they change direction again, again at nearly the same time and go back to their overall west to east motion. This coincidence -- that they change their apparent direction of motion at the same time -- is a strong argument in favor of the Copernican system. Why do they seem to change direction and why does the coincidence support the Copernican system?
Friedman, John L., "Time-lapse photos showing paths of Jupiter and Saturn 2000-2001" (2001). Physics Faculty Articles. Paper 1.