Retrograde motion occurs when a planet far from another planet and from which it is seen rotates at lower speed, creating the impression that it is "retrograding" with respect to the other planet.
Early astronomers get confused as they viewed the motion in the sky because they tried to explain it in terms of the geocentric theory.
Let suppose that we have a planetary system formed by a star and two planets rotating uniformly around the star. Besides, we assume that both planets are rotating at different velocities, and that planet A is more far away from the star than planet B, then the velocity of the planet A relative to planet B is:
[tex]\vec v_{A/B} = \vec {v}_{A}- \vec {v}_{B}[/tex] (1)
A retrograde motion of planet A occurs when [tex]\|\vec v_{A/B}\|\ne 0[/tex] and if [tex]\|\vec v_{A}\| < \|v_{B}\|[/tex] at least. In other words, retrograde motion exists when speed of planet A is less than speed of planet B, creating the impression that planet A is "retrograding" with respect to planet B. This question is easy to solve and to explain by considering the star as an inertial framework, base of the heliocentrical model.
Early astronomers were confused and unable to explain the phenomenon due to their adherence to geocentrical model, which assumed the Earth was the center of the solar system.
We kindly invite to check this question on retrograde motion: https://brainly.com/question/13586212
