I read that in the Ptolemaic model (or the geocentric system), for the retrogade motion to occur at opposition, the radii of the outer planets' epicycles have to be aligned with the earth-sun radius. For that reason the epicycles of Mars Saturn and Jupiter had to be exactly one solar year, so that it would align with the direction of the earth-sun radius. See for example here. This is one of the anomalies that couldn't be explained until Copernicus came along (namely, why the epicycles of the outer planets share the same time period as opposed to the inner planets), see here.
But I'm not sure I really understand why this is so. Can anyone explain this to me in simple terms (no math please)? A diagram would really help, but I wasn't able to find a diagram or a video that would specifically illustrate this. Most diagrams would just show the general alignement of the radii vectors of the outer planets to the sun's, but would not show how this applies to when Mars is at opposition, or why this wouldn't work if they weren't aligned (I do understand why the inner planets' deferents had to be a solar year though).