What is retrograde motion, and what did it tell astronomers

Answer and Explanation:

Retrograde motion refers to the phenomenon where a planet appears to move backwards in its orbit. However, this backwards movement of planets is not real (the planet doesn't actually change its movement direction), it only appears to change it movement because of its relative position from the Earth, and when the planet is exactly opposite the Sun in its elliptical orb. Planets go retrograde every few months when they are directly opposite the Sun. A Greek astronomer called Ptolemy in 150 AD discovered and explained retrograde motion of planets.