If observed from one night to the next, a planet appears to move from West to East against the background stars most of the time. Occasionally, however, the planet's motion will appear to reverse direction, and the planet will, for a short time, move from East to West against the background constellations. This reversal is known as retrograde motion.
retrograde motion
Earths faster motion makes Mars appear to be going backwards, the backwards motion, in fact, is what caused retrograde motion. --Ptloemy used Epicycles to explain how geocentrical models worked.
The epicycle in the Ptolemaic model was used to explain the retrograde motion of planets. It involved planets moving on small circular orbits within the larger orbit around Earth. By incorporating epicycles, Ptolemy was able to account for the observed motions of the planets in the night sky.
Retrograde motion is motion in the opposite direction. In the case of celestial bodies, such motion may be real, defined by the inherent rotation or orbit of the body, or apparent, as seen in the skies from Earth.
It offered more natural explanation for the apparent retrograde motion of planets in our sky.
All the models explain retrograde motion because it is such an obvious phenomenon. In Copernicus's model an outer planet goes into retrograde motion when the inner planet overtakes it so that it appears from the inner planet to be going backwards along the ecliptic.
The apparent westward movement of a planet is known as retrograde motion. This phenomenon occurs when a superior planet passes Earth in its orbit, making it appear to move backward in the sky relative to the stars.
Retrograde motion important in astronomy because it helped to explain that the planets revolved around the Sun. Retrograde motion is the ability of some objects to rotate in a direction that is opposite what is expected.
KEPLER
A major flaw of the Ptolemaic model was its complexity in explaining the retrograde motion of planets. Ptolemy introduced the concept of epicycles and deferents to account for this, which made the model overly complicated and not as accurate as later models, such as the heliocentric model proposed by Copernicus.
The observed retrograde motion of planets led Ptolemy to add epicycles to Aristotle's system. Epicycles were small circles that planets moved in on their larger circular orbits around Earth in order to explain the appearances of retrograde motion.
Earths faster motion makes Mars appear to be going backwards, the backwards motion, in fact, is what caused retrograde motion. --Ptloemy used Epicycles to explain how geocentrical models worked.